Directory UMM :Data Elmu:jurnal:E:European Journal of Agronomy:Vol13.Issue2-3 July2000:

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Soil – plant – animal relations in nutrient cycling: the case of

dairy farming system ‘De Marke’

H. van Keulen *, H.F.M. Aarts, B. Habekotte´, H.G. van der Meer,

J.H.J. Spiertz

Plant Research International,P.O. Box 16,6700 AA Wageningen, The Netherlands

Received 19 February 1999; received in revised form 12 July 1999; accepted 23 October 1999

Abstract

Forage and ruminant production in Western Europe have increased significantly since World War II. However, in the last decade the livestock production sector has come under increasing pressure as the European Union introduced the milk quota system, effectively curbing total national and individual farm production volume, and national governments increasingly took measures to reduce the losses of nutrients from these systems to the environment. In the late 1980’s in the Netherlands a project was initiated with the objective to design, test and further develop a farming system that can serve as a starting point for the development of dairy farms on dry sandy soils with average milk production (system ‘De Marke’) using the method of ‘prototyping’. First, a number of farming systems have been identified that, in theory, meet the formulated objectives. From this theoretically acceptable set one of the technically and economically most attractive and, from a research point of view, most interesting was implemented at the experimental farm ‘De Marke’ in 1992. The functioning of this system is monitored in quantitative terms by measuring flows of dry matter and nutrients. The results sofar suggest that on dry sandy soils strict environmental standards for losses of nitrogen and accumulation of phosphorus can be attained within a short time while maintaining the current milk quota. Grass, maize and fodder beets could be grown in such a way that the norms for nitrate losses could be met. Improved utilization of animal manure and lower manuring levels allowed a reduction in the use of fertilizer-N of 74% compared to current practice. The input of P in feed and the output in milk and meat could be more or less balanced: P-surplus was only 18% of that on the current farm. To obtain reliable results the research will be continued for a longer period of time because the system has to stabilize, the soil nutrient and organic matter stores and soil fertility react slowly to changes in management, and weather conditions are always variable. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Sustainable farming systems; Nutrient balances; Nutrient management; Environmental impact; Prototyping

www.elsevier.com/locate/eja

This paper is an elaborated version of the presentation to the 5th ESA-Congress by Spiertz et al. (1997). * Corresponding author: Tel.: +31-31-7475955; fax: + 31-31-7423110.

E-mail address:[email protected] (H. van Keulen).

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1. Introduction

Forage and ruminant production in Western Europe have increased significantly since World War II under, on the one hand, the influence of the Common Agricultural Policy of the European Union that guaranteed high and stable product prices (De Wit et al., 1987; De Wit, 1988) and on the other, relatively low prices of inputs, particu-larly inorganic fertilizers and concentrates

(Kete-laars and van der Meer, 1998). Inorganic

fertilizers are, in general, a more reliable and easier-to-manage source of nutrients than either legumes or animal slurries, while concentrates serve as a ready means to supplement the unpre-dictable quantity and quality of roughage supply and allow intake levels necessary to sustain high levels of production per animal.

Dairy farming is the major sector of Dutch agriculture, using about 64% of the cultivated area and serving as the main source of income for about 35% of the farmers (CBS, 1998). In the sixties and seventies these dairy farming systems strongly specialised and intensified by increased inputs of chemical fertilisers and purchased feeds. Although introduction of the milk quota system in 1984 reduced milk production by 15% the average is still high, i.e. between 1987 and 1997

about 12 250 kg/ha on the specialised dairy

farms. Especially, systems in the sandy regions in the East and South of the Netherlands are inten-sive, mainly because of the high prices of agricul-tural land.

In the last decade the livestock production sec-tor has come under increasing pressure as the European Union introduced the milk quota sys-tem, effectively curbing total national and

individ-ual farm production volume, and national

governments increasingly took measures to reduce the losses of nutrients from these systems to the environment (Van Boheemen, 1987; Schro¨der, 1992; van der Meer et al., 1997). Intensification has led to a serious imbalance between inputs of nutrients in purchased fertilisers, concentrates and roughage and atmospheric deposition, and out-puts in milk and meat, as illustrated in Table 1 for nitrogen at the national level in the Netherlands. At farm level for the period 1983 – 1986 outputs were on average only 14% of the input for nitro-gen (N), 32% for phosphorus (P) and 17% for potassium (K).

So far most attention has been paid to pollu-tion of water and soil resources by leaching and run-off of N, P and organic matter and accumula-tion of phosphorus. The N surplus contributes to

environmental pollution through ammonia

volatilisation, run off, leaching of nitrate and

production of N2O during denitrification (Aarts et

al., 1992). Ammonia volatilization has received particular attention in areas with high animal densities, where it contributes to high levels of atmospheric deposition. This high N-load causes eutrophication of terrestrial ecosystems, increased susceptibility of trees to stress, soil acidification and excessive nitrate leaching from affected ecosystems (van der Meer et al., 1997).

Table 1

Nitrogen balance of the Dutch dairy farming sector (106_kg)a

Year Imports Exports Surplus NUEb

Fertilizer Concentrates Milk/Meat

42 36

8 70

1950 0.46

138

1960 25 45 118 0.28

1970 277 52 56 273 0.17

1980 356 117 77 396 0.16

379 153

1985 83 449 0.16

351

303 138 90 0.20

1989

a_{Source: van der Meer et al. (1997).}

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Table 2

Maximum levy-free (see text for explanation) nutrient sur-pluses in the Netherlands (kg/ha, deposition and fixation by clover not included)

2000 2002 2005 2008 1998

Grassland 300 275 250 200 180 150 125 110

175 100

Arable land

15 13

17 11

Grassland 9

15 13

Arable land 17 11 9

for farming systems were defined (Table 2). These threshold values are temporary (political) com-promises between environmental demands and

agricultural possibilities (Dekker and van

Leeuwen, 1998).

Surpluses above the levels presented in Table 2 will be levied. However, uncertainty exists among dairy farmers on the impact of a reduced surplus of P (through lower inputs of fertilizer or concen-trates) on soil fertility and — as a consequence — on crop production. Moreover, reducing the P surplus may lead to high costs because it limits the application of slurry, which then has to be exported or additional land has to be bought. This legislation stimulates more efficient nutrient management as farmers try to avoid fines while maintaining production.

These measures have resulted in reduction in the surpluses on commercial farms (Aarts et al., 1999), but further reductions are necessary.

One of the goals of agricultural research is to develop guidelines for farmers to design and im-prove their farms, taking into account their spe-cific circumstances such as soil type, and both short- and long-term objectives. In addition, the government needs information on the practical feasibility of measures aimed at reducing the envi-ronmental impact of dairy farming. This paper describes the results of farming systems research focusing on dairy farming on sandy soils and long-term objectives with respect to nutrient sur-pluses, and, hence on improving nutrient manage-ment.

2. Objective dairy farming system ‘De Marke’ A dairy farm can meet the environmental objec-tives of the government by extensification, i.e. by producing less milk per hectare (Weissbach and Ernst, 1994). For a given milk quota at farm level more land will then be needed, which, especially in the sandy areas, is scarce and expensive. In the Netherlands land taken out of production is mainly used for nature development, recreation, housing, industry and extension of infrastructure. Extensification (i.e. reducing milk production per unit area) without area expansion leads to re-Surplus P mainly accumulates in the soil, in

addition to some surface flow, but continued ac-cumulation will lead to saturation and leaching. Hence, the quality of the Dutch environment is threatened and one of the world’s most vital and scarce minerals is wasted (Tisdale et al., 1985).

Since the middle of the eighties the Dutch government is making efforts to reduce the nega-tive impact of farming on the environment by formulating norms for nutrient surpluses. For ni-trogen this implies that groundwater may contain only 50 mg nitrate per litre — on light sandy soils

it is now often more than 200 mg/l (Boumans and

van Drecht, 1995) — and that ammonia emission (from faeces and urine) should be limited to 30% of the average level in 1980, i.e. an annual loss of

30 kg NH3-N/ha at most. Field data suggest a

required P supply in excess of crop uptake of 10 – 30 kg/ha per year to maintain an optimal soil fertility level from an agricultural point of view. Based on average precipitation surplus and Dutch groundwater quality standards (maximum of 0.40 mg/l P-total), a loss of about 1 kg P2O5/ha per year would be acceptable, compared to about 30 in the mid-eighties (Oenema and van Dijk, 1994). Hence, to meet the standards for the year 2008 the P surplus has to be reduced by 72%.

To realize those norms, first the use of animal manure was restricted on the basis of P load. As a result animal manures were better distributed over the country, but surpluses of N and P de-creased by only 14% (Brouwer et al., 1997) and, thus, remained unacceptably high. Subsequently in 1997 maximum permitted surpluses of N and P

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duced milk production and, hence, a strong de-crease in income. Moreover, it is doubtful whether the growing world population can be provided with enough high-quality food in the future with a reduction in the area of cultivated land and more extensive land use. In the coming decades doubling of food production is expected to be necessary. Hence, the project ‘De Marke’ aims at reducing environmental problems by fo-cusing on the question to what extent technologi-cal options to improve nutrient management on dairy farms offer opportunities to maintain milk production per hectare.

Characteristic for the dairy farm is the combi-nation of plant and animal production. Animal manure and roughage form the links between the animal and the plant components. Nutrients not exported — in milk and animals — or lost, pass through a cycle: part of the nutrients from fodder is excreted by the animals in faeces and urine. This manure is being used in crop cultivation, its nutrients are taken up by the crop and then again consumed by the cattle. Nutrients exported in milk and animals and lost in the cycle are com-pensated for by the input of nutrients in fertilizers and purchased roughage and concentrates.

Nutrient flows can be influenced by manage-ment, aiming at reducing losses. For instance, at most farms cattle take up much more nitrogen than is necessary physiologically. Hence, a reduc-tion in the nitrogen content in the rareduc-tion will hardly affect milk production, but will lead to less nitrogen in the urine and, consequently, lower losses. It is important that, wherever possible, measures in the different parts of the cycle be mutually synergystic, and geared to the specific possibilities and limitations of the farm. Cost minimization is an important criterion.

The objective of this project is to design, test and further develop a farming system that can serve as a starting point for the development of dairy farms on dry sandy soils with average milk

production (about 12 000 kg/ha). The farming

system should meet the strict environmental norms derived from policy papers as indicated in Section 1. To avoid desiccation the use of ground-water should be limited. Hence, irrigation is ap-plied, but restrictively. Water stress may have

negative effects on utilization of already applied fertilizers (environmental considerations) and may require purchase of larger quantities of fodder (economic and environmental considerations). Manure production is restricted to the amount that can reasonably be applied at the farm.

3. Research methodology

3.1. Prototyping

In this research project the method of ‘proto-typing’ is used (Vereijken, 1992, 1997). Objectives of the farming system in terms of environmental criteria have been identified. This was followed by establishment of input – output relationships for the plant and animal components of the farm, applying existing and especially developed and adapted calculation programmes. This refers, among others, to the relationship between crop yield and the required amounts of fertilizers and water, and that between daily milk production per animal and the required amounts of energy, protein and phosphorus (Aarts et al., 1992; Biewinga et al., 1992). On the basis of these relationships a number of farming systems have been designed that, in theory, meet the formulated objectives.

From the theoretically acceptable set of systems one of the technically and economically most attractive and, from a research point of view, most interesting was implemented at the experi-mental farm ‘De Marke’ in 1992. Subsequently, it was further developed with the aim to more fully realize the objectives. The functioning of this sys-tem — also called ‘De Marke’ — is monitored as completely as possible in quantitative terms by measuring, wherever possible, flows of dry matter and nutrients. Moreover, detailed observations are carried out at a number of fixed sites, to increase insight in the processes taking place in the soil, mainly with respect to nitrogen, phospho-rus, water and organic matter.

The selected research method has some impor-tant advantages. The scientific approach to pro-duction systems concentrates at the farm level, the level at which management operates. The

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inte-grated effect of processes is illustrated and the transfer of the knowledge acquired is facilitated as the functioning of the system can be directly observed. By building a bridge between theory and experimental testing the danger is avoided that, as in desk-studies, too much attention is paid to elements that appear less relevant in practice or — the other way around — that insufficient attention is paid to issues that prove important later. Prototyping also has some disadvantages. The experimental system has been selected, rather subjectively, from a number of options and reli-able comparison with other systems is impossible, since only one system can be implemented and even that system is continuously developing. Therefore, the results can not be tested statisti-cally. These disadvantages can (at least) partly be overcome by: (i) using an appropriate monitoring programme, (ii) additional (inter)disciplinary re-search aimed at determination of causal relation-ships that explain the behaviour of the system and can be used to investigate the consequences of alternatives, and (iii) combine the experimental research at the prototype farm with modelling to explore alternative possibilities (Van de Ven and van Keulen, 1996). Prototyping is also expensive: Implementation of the experimental system at ‘De Marke’ required an investment of 10 million guilders and the annual budget amounts to about 1 million guilders.

3.2. Experimental methods

For grass cut for conservation, silage maize and beets, dry matter yield was determined on the weighing bridge. It has been assumed that during conservation and feeding an additional 7% is lost. Intake of pasture grass has been estimated by visually estimating standing dry matter at the beginning and the end of the grazing period and

assuming a daily growth rate of 50 kg dry matter/

ha.

The quantities of N and P in faeces produced in the stable have been determined as volume times contents. The quantities in faeces and urine ex-creted during grazing have been estimated from the input in fodder by subtracting the output in milk and meat and the production in the stable.

This calculation procedure may lead to relatively large inaccuracies in the partitioning of nutrients between stable and pasture.

4. Experimental farm ‘De Marke’

To test the selected farming system 55 ha of land were bought in De Achterhoek, the eastern sandy part of The Netherlands. Hence, the exper-imental farm is almost twice the size of the aver-age farm in the sandy area. This was considered necessary for research reasons: to obtain suffi-ciently reliable results more animals are needed than on the average farm.

The land comprising the experimental farm ‘De Marke’, reclaimed from heather around the turn of last century, consists of an upper layer of 25 – 30 cm with an average organic matter content of 4.9%, overlying practically humusless sand. The groundwater table is at most places too deep for water to reach the root zone. The water holding capacity is, therefore, less than 50 mm on 60% of the cultivated land and for the remainder varies between 50 and 100 mm. Hence, the land belongs to the driest 10% of the sandy soils in the country.

The plots of ‘De Marke’, acquired from various landowners, had a different history with respect to the application of animal manure, resulting in considerable differences in soil fertility, especially for phosphorus (Schoumans, 1995).

The total land area comprises the ‘home plot’, close to the farm buildings, that can be irrigated, if necessary, and is preferentially used for grazing, and the ‘field plot’ more distant, with no irriga-tion possibilities and mainly used for silage making.

The farming system of ‘De Marke’ operates through very careful management of crops, live-stock and manure.

4.1. Crops

’De Marke’ consists of 31 ha of grassland and 24 ha of maize (until 1995 18 ha and 6 ha of fodder beets). About one third of the grassland is permanent, the remainder in rotation with maize.

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Most of the maize is used as whole crop silage and about 10 ha for ground ear silage (home-grown concentrate). Annual N application (avail-able N in the slurry+fertiliser-N) does not exceed

250 kg/ha on grass and 100 kg/ha on maize.

Actual rates of application take into account residual effects of preceding crops, particularly ploughing-in a grass sward or a cover crop. Maize is always followed by a cover crop to absorb residual inorganic N. In recent years no fertiliser P has been applied. Irrigation is applied only to enable grazing during part of the day (because of reasons of animal welfare) or to prevent early dying of crops due to drought.

4.2. Li6estock

Milk production per cow is high at 8150 kg per lactation, with almost 4.5% fat and 3.5% protein. Young stock is kept only for replacement. This explains the low N and P output in animals sold compared to the current farm. Much attention is

paid to avoid protein and P surpluses in the ration of the cows. Grazing is restricted, particu-larly in late summer and autumn to avoid large N losses from urine patches. From April to Septem-ber cows are stabled during part of the afternoon and at night, and supplemented with maize silage.

4.3. Animal manure

’De Marke’ has a free-stall barn, with a closed coated floor, sloping towards a central longitudi-nal urine drain. Alleys are equipped with a scraper to remove faeces and urine to a com-pletely enclosed storage silo. All slurry produced is applied to the farm; on grassland by injection with open slits between early March and 15 Au-gust, and on maize land by injection just before sowing.

The main characteristics of the experimental farming system are summarized in Tables 3 and 4. A comparison is being made between the expected values of these characteristics (prognosis) — the

Table 3

N and P balances of the average specialized dairy farm in the middle of the eighties, the prognoses at the start of ‘De Marke’ and the values realized in the years 1993/1994 and 1994/1995 (kg/ha)

‘Average’ 1983/1986

Characteristic ‘De Marke’ prognoses 1993/1994 1994/1995

N P N P N P N P

Input

67 6.0 52 1.8

Fertilizer 330 15.0 96 0.0

41 5.9 82 15.0

Feed 182 32.0 84 11.5

0.9 49

0.9

Deposition 49 1.0 49

30 0.0 12 0.0 5 0.0

N-fixation clover 0 0.0

7 0.0 5 0.0

Various 4 0.0 4 0.0

48.0 17.7 238 12.4

568 199

Sum 192 12.8

Output

68 12.0

Milk 62 10.6 65 10.5 64 10.6

8 2.2

Meat 13 4.0 10 3.0 9 2.7

0 0.0 0 0.0

Feed 8 1.2 0 0.0

Mutation

0 0.0 0 0.0

Number of animals 0 0.2 0 −0.1

0.0

0 −6.4

Manure storage 0 0.0 11 −1.0 −50

0.8 17

−2.3

0 −35

Feed stock 0.0 0 0.0

Sum 81 16.0 70 12.8 59 11.6 40 7.6

Input–outputa ₄₈₇ _32.0 ₁₂₂ _0.0 ₁₄₀ _6.1 ₁₉₈ _4.8

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Table 4

Characteristics of the cattle component of the average specialized dairy farm in the middle of the eighties, the prognoses at the start of ‘De Marke’ and the characteristics realized in the years 1993/1994 and 1994/1995

‘De Marke’ ‘Average’ 1983/1986

Characteristic 1993/1994 1994/95

prognoses

11 890

Realized milk production (kg/ha) 12 798 12 047 11 664

12 487 12 681

13 161

FPCMa _(kg_/_ha) _{12 288}

2.31

Milking cows/ha 1.47 1.45 1.43

8495

FPCM/cow (kg) 5697 8720 8467

0.57 0.81

0.76

Young stock/cow 0.70

4047

Purchased concentrate (kg DM/ha) 1377 1544 1542

0 251

Purchase of roughage (kg DM/ha) 2136 693

11 240 12 726

16 158

Feed intake cattle (kg DM/ha) 11 495

1.23

Feed intake cattle (kg DM/kg FPCM) 0.90 1.00 0.94

Nutrients (kg/ha) N P N P N P N P

Input

74.0 278 40.1 336

Feed 496 47.1 330 43.6

Output

12.0 62 10.6 65 10.5 64 10.6

Milk 68

4.0 8 2.2 10

13 3.0

Meat 9 2.7

Mutation cattle 0 0.0 0 0.0 0 0.2 0 - 0.1

Input–output

58.0

(=Faeces+urine) 415 208 27.3 261 33.4 257 30.6

a_{Fat and protein corrected milk.}

and 0 kg P, respectively. The N surplus in the year 1993/1994 (140 kg N/ha) was close to the expected

value, but it was clearly higher (198 kg N/ha) in

the subsequent year, due to a higher fertilizer application rate and the reduction in stocks of slurry. The P surplus exceeded the expected value in both years, but if the changes in stocks of slurry are disregarded it is slightly negative in

1994/1995, i.e. slightly more P was exported from

the farm in milk and cattle than imported in fertilizers and feed. As the import of P in feed appeared higher than expected application of fer-tilizer P was discontinued from the spring of 1994 onwards. Because of autonomous developments (especially legislation on low-emission application

techniques and increasing milk production/cow)

N surplus at the current farm has decreased by

about 80 kg/ha (17%) to about 400 kg/ha (Van

Eck, 1995), while the P surplus was hardly

af-fected (30 kg/ha; Oenema and van Dijk, 1994).

results of the model calculations at the basis of the farming system design — and the results realized in the (financial, 1 May to 1 May) years 1993/1994 and 1994/1995, the first two years that the system functioned completely. The ‘average’ farm from the middle of the eighties is taken as reference (henceforth referred to as ‘the current farm’) as a discontinuity occurred at that time, because of the introduction of milk quota and of environmental legislation.

5. Results

5.1. The farm system

The N and P balances of the farm are presented in Table 3. The prognosis was that the surplus at farm level would decrease from 487 kg N and 32

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5.2. The cattle component

The milk quota of ‘De Marke’ was slightly lower than that of the current farm, but reduc-tions in the second half of the eighties have re-sulted in comparable quota during the reporting period. Model calculations (Table 4) suggest that the feed requirements for a given milk quota decrease when milk production per cow increases, as per kg of milk less energy is needed for mainte-nance of the animals and less young stock is needed for replacement (at a similar life span). Hence, the aim at ‘De Marke’ was to realize a considerably higher milk production per cow than the current farm, which was realized and has resulted in a much lower stocking rate. The higher milk production has not resulted in the expected savings in feed. The energy intake (kVEM (kVEM is the abbreviation of the Dutch (kilo) digestible energy for milk production (Tamminga et al., 1994))) of the dairy cattle appeared substantially higher than expected on the basis of the feeding standards, without the animals showing signs of fattening (Meijer, 1994). The models were origi-nally developed for a production range up to

about 6500 kg/cow (personal communication, A.

Meijer, Experimental Station for Cattle, Sheep and Horse Husbandry (PR)), and it appears, therefore, that extrapolation to the realized levels at ‘De Marke’ is not without problems.

The number of young stock per milking cow was higher than anticipated and, on average, practically similar to that on the current farm. The main reason is that in the reported period the herd was in its building-up phase, requiring maxi-mum possibilities for selection. In practice, in the sandy areas, the number of young stock per milk-ing cow increased to 1 (Leneman et al., 1999), while at ‘De Marke’ it has decreased to 0.7. Because of the high energy requirements of the dairy cattle and the high number of young stock, dry matter intake per unit of milk production during the first year was 11% higher than pre-dicted, but considerably below that at the current farm. In the second year it was only 4% higher than predicted.

In formulating the ration, including grazing management, the aim is to minimize its protein

and phosphorus contents. At ‘De Marke’ animals are taken out of pasture about 1 month earlier than at the current farm and daily grazing time is restricted. In summer the cows are stabled during the afternoon and the night and are supplemented with maize. In the first year the cows were stabled at night only, but the alternate periods of availability of protein-rich (daytime grazing) and protein-poor (maize at night) feed appeared too long and led to digestive problems in the highly productive cattle. The dairy cows are moved to a new plot every 2 days, after which the pasture is further grazed by young stock. Restricting grazing time results in less urine and manure patches, and in a larger part of the faeces and urine being produced in the stable. Nutrients are thus utilized much more efficiently, as less N is lost from pasture, and application of slurry is synchronized to crop demand.

As manure is spread on the land only between early March and mid-August, the required storage capacity at ‘De Marke’ is substantially higher than at the current farm, that did not face any restrictions on manure application in the middle of the eighties. However, the time during which manure can be spread on sandy soils has been limited by legislation to the period 1 February – 15 September, hence the storage capacity at the cur-rent farm has also increased.

The difference between input and output in Table 4 represents excretion by the animals. At the current farm it exceeded 400 kg N/ha, indicat-ing that only 16% of the N consumed by the animals was converted into milk and meat. The anticipated reduction at ‘De Marke’ (50%) was based on increased utilization efficiency at animal level to 25% through a low-protein ration, high milk production per cow and a reduction in the number of young stock. The realized utilization efficiency of feed N was 22% due to the higher protein content in the ration — protein intake exceeded the value expected on the basis of the CVB-standards (Meijer, 1994) by more than 18% — and a higher number of young stock. P-utiliza-tion efficiency was also lower than expected, but the difference was smaller than for N.

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5.3. The manure component

Manure production at pasture was in agree-ment with the prognosis (Table 5) and nutrients in faeces and urine produced in excess of what was anticipated appear to have ended up in the stable. By restricting grazing time to about one third of current (assuming day-and-night grazing) excre-tion of nutrients at pasture was on average only 22 (N) and 17% (P) of the total, compared to 50% at the current farm. In the eighties, on a substan-tial part of the farms, dairy cattle were stabled during the night so that, on average, a smaller part of the nutrients in faeces and urine than given in Table 5, will have been excreted at pas-ture. Presently, on more than half of the farms on sandy soils dairy cattle are stabled at night.

The difference between input and output of nitrogen in the manure represents volatilization of ammonia from the stable, at pasture, during stor-age and following application. Ammonia emission had to be estimated as it was not measured at ‘De Marke’ during the period treated here, so that the results of the model, in relative terms, are repro-duced. In absolute terms the losses were 3 – 4

kg/ha higher as more nitrogen was excreted by the

animals. As at the current farm N-excretion is much higher and manure was surface-applied,

N-loss as ammonia was 105 kg/ha. Due to the

compulsory injection of manure, ammonia

emis-sion at current farms will have decreased at this

moment by about 40 kg N/ha (Lekkerkerk et al.,

1995). Of the total loss of more than 20 kg N/ha

– from faeces and urine — 4 kg was lost during grazing (volatilization 7.5% of the N-excretion, Vertregt and Rutgers (1988)), 4 kg following ap-plication at pasture (5% of the ammonia-N), 0 kg after injection on arable land (1.25% of the am-monia-N) and 14 kg from the stable (7% of the N-excretion), which was, therefore, the main source of ammonia. (Ketelaars et al., 1995). At the moment, as more measured data for ‘De Marke’ are available, the impression is that, even under careful management, volatilization losses are somewhat higher than assumed here (Van der Schans et al., 1999).

5.4. The soil/crop component

The area of the different forage crops is deter-mined on the basis of a compromise between the production capacity of the crops under the condi-tions of ‘De Marke’, the value of their products in the ration and the possibilities to use animal manure efficiently.

The proportion of grass in the rotation at ‘De Marke’ is lower than at the current farm and the proportion of maize consequently higher (Table 6). The main reason is the aim to reduce output of N in urine and faeces, demanding energy-rich feed Table 5

Nutrient flows of the manure component for the average specialized dairy farm in the middle of the eighties, the prognoses at the start of ‘De Marke’ and the values realized in the years 1993/1994 and 1994/1995 (kg/ha)

‘De Marke’

‘Average’ 1983/1986 1993/1994

Characteristic 1994/1995

prognoses

N P N P N P N P

Input

55 7.0 52

26.0 7.7

191 62 3.3

Production faeces and urine at pasture

Production faeces and urine in the stable 224 32.0 151 20.2 209 24.7 194 27.1 Output

164 26.0 51 7.0 48 7.7 57

Faeces+urine at pasture (after volatilization) 3.3

33.5 227

25.8 181

20.2 Applied slurry (after volatilization) 146 32.0 137

−6.4

Change in manure stock 0 0 0 0 11 −1.0 −50

Input–output

−0.1 21

0 18

0.0

105 0.0

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Table 6

Characteristics of the crop component of the average specialized dairy farm in the middle of the eighties, the prognoses at the start of ‘De Marke’ and the values realized in the years 1993/1994 and 1994/1995

‘De Marke’ prognoses 1993/1994 1994/1995 Characteristic ‘Average’ 1983/1986

Area(%of total)

56 55 60

Grassland 90

33 34

10 32

Maize

11 11 8

Fodder beets 0

Artificial fertilizer(kg/ha)

67 52 96

N 331

6 2

15 0

25 38

K 30 0

30 12

N-fixation clover (kg/ha) 0 5

Net yield(kg DM/ha)a

9276 9409

9942 8792

Grass

Maize 10 274 11 167 10 657 9276

14 133 16 583

– 10 064

Beets (leaf included)

10 436 10 615

Average of farm 9975 9046

a_{Net yield is the value after subtraction of grazing, harvesting, conservation and feeding losses, and thus equals animal intake.}

with a low N content, to compensate for the high nitrogen contents in the grass products in the ration. Supplementary feeding with maize serves as an energy buffer and, moreover, limits the risk of grass tetany during the grazing season. Maize silage has a higher energy content than grass silage, reducing the need for concentrate supple-ment in the winter period. Moreover, maize and fodder beets are much more efficient in terms of water and nutrient requirements per unit har-vestable dry matter than grass (Aarts and

Grashoff, 1993). At ‘De Marke’ moisture

availability is generally the most limiting factor for crop production and although the aim is to

minimize water use for irrigation, because

groundwater is a scarce resource, and because it requires energy and labour, the area of grassland exceeds that of arable land. Important reasons are the higher yields of N and P (reducing import of these elements in purchased feeds), the possibili-ties for grazing and the greater opportunipossibili-ties to use animal manure. Moreover, grassland stimu-lates soil organic matter build-up and, therefore, indirectly soil moisture supply. In recent years the proportion of maize in the rotation at current

farms has also increased (in 1992/93 it comprised

about 25% of the area at dairy farms in the sandy

areas (Leneman et al., 1999)) and the situation at ‘De Marke’ is not an exception anymore.

Fodder beets are hardly grown in actual prac-tice, but at ‘De Marke’ they were intended to replace part of the concentrates. Moreover, the yield of dry matter and energy per hectare is as a rule very high, and its nitrogen uptake capacity exceeds that of maize, so that beets as first crop after the grassland period was considered safer. As large amounts of beets in the ration for highly productive dairy cattle appeared to lead to diges-tive problems, after the first year the proportion of beets in the ration in winter was reduced, and part of the beets was ensiled with maize, so that they could be incorporated in the summer ration. However, the beets then have to be harvested early, hence at a lower dry matter yield, and cultivation of a catch crop is necessary (to take up the nitrogen mineralized after harvest). Despite incorporation of the beets in the summer ration the original area of beets (6 ha) appeared too large for judicious feeding practice, and was re-duced to 4 ha in 1994, and eventually discarded after 1995.

At ‘De Marke’ part of the maize is harvested as ground ear silage (GES), very high in energy and mainly used as concentrate replacer for the most

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productive animals. Maize stover is also harvested and used as an absorbing bottom layer for the rather wet grass silage in autumn, which is then covered with beet leaves. This silage appeared very suitable for feeding young stock and dry cows. In current practice GES is also increasingly used, but the stover is not (yet) harvested, and less autumn grass is ensiled, as grazing generally con-tinues for 1 month longer.

The total grassland area is divided into 22 ha for ley farming and 9 ha for permanent pasture. Continuous arable farming leads to lower soil organic matter content than ley farming, eventu-ally resulting in increased drought sensitivity. Yield of maize under ley farming is higher than under continuous cropping and the risk of build-up of weed populations (by selection or building up of resistance) is lower. Moreover, during the grass period herbicides, used on the arable crops,

can be decomposed. Grass/clover mixtures were

always sown immediately after harvest of the maize, hence clover entered the winter period in the seedling stage and appeared very sensitive to frost, leading to low clover densities after most winters. Consequently, nitrogen fixation by clover in grassland, calculated as 4.5% of clover dry matter yield (Biewinga et al., 1992), was substan-tially lower than expected. Sowing in spring may help to avoid this problem. The grass sward, after 3 years, is broken up in early spring. Subse-quently, fodder beets are grown — because they can use the nitrogen mineralized from the decom-posing grass very efficiently — followed by two (home plot) or four (field plot) years of maize. Additional research at ‘De Marke’ has shown that cultivation of maize after 3 years of grass is also possible when no artificial fertilizer is applied: mineral nitrogen in the soil did not reach alarm-ingly high levels, and the quality of the groundwa-ter was no different from that under other plots. Fertilizer application is plot-specific and based on the uptake capacity of the crops, taking into account soil moisture supplying capacity and the place of the crop in the rotation. For phosphorus the principle of ‘equilibrium fertilization’ is ap-plied, i.e. no more is applied than taken up by the crop. However, during the grassland period fertil-izer application exceeds crop uptake, during the

arable period it is lower. Under ley farming a larger part of the N-requirement of grassland can be covered by slurry because on grassland P in slurry is more restrictive than N and on leys higher quantities of P are allowed. On perma-nently arable land N in the manure is, as a rule, restrictive for the quantity of slurry that can be applied and adding fertilizer P may be necessary. In the ley system phosphorus requirements of the crops can almost completely be covered by animal manure. On average, during the 2 years, only 74

kg N/ha from fertilizer were required, i.e. about

one quarter of that on the current farm.

In 1993 and 1994 permanent grassland was manured on average with 47 tons of cow slurry and 136 kg fertilizer-N per ha (in total 237 kg of mineral N). Leys received 75 tons of slurry and 123 kg fertilizer-N per ha (in total 289 kg of mineral N). Maize and beets were fertilized with

27 and 34 tons of slurry/ha, respectively (65 and

85 kg of mineral N). On both grassland and arable land slurry was applied by injection, on grass with open slits.

Between the rows of maize Italian ryegrass was sown in June, to take up the nitrogen mineralized during the ripening phase and after harvest of the maize. In the spring of 1995 the grass was grazed by young stock, in 1994 it was ploughed in.

Grassland was irrigated when necessary to con-tinue grazing in dry periods or to avoid re-sowing. Beets and maize were irrigated only when there was a risk of premature death. The field plot of the farm — 29% of the total area with a relatively high proportion of arable land — is never irri-gated. During dry years — as in 1994 — on

average about 50 mm/ha total farm area was

applied, about 90% on grassland and 10% on maize land. The growing season 1993 was charac-terized by a warm and dry spring and a wet summer and on average only 9 mm of irrigation water was applied on grassland. The spring of 1994 was cold and wet and was followed by a very dry summer.

The differences in yield among crops appeared in accordance with the expectations. In both years realized net dry matter yield of grassland was close to the prognosis for an average year. Yield of maize in 1993 was slightly below the expected

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value. The very low yield of beets in 1994 was due to drought, an attack of rhizoctonia and early harvest, as part of the beets were ensiled with maize. Although the yields of grass and maize were on average lower than those on current farms, average farm yield is almost equal, because of a higher proportion of fodder beets and maize in the rotation. The feeding value of the products was, on average, slightly higher than that of the average of samples from practice.

The amount of N in slurry — after correction for ammonia volatilization — was higher than at the current farm because of the strong reduction in the proportion of manure excreted at pasture and to the emission-reducing measures for slurry. In both years slurry was the main source of nutrients at ‘De Marke’ (Table 7).

In 1993/1994 total nutrient supply to the soil

was similar to the prediction (355 kg N and 37.2 kg P/ha); in 1994/1995 it was substantially higher because more slurry and fertilizer were applied to grassland, as a different calculation method for fertilizer requirements was used. Moreover, the correction for lower yields due to drought was insufficient. Therefore, the store of slurry

de-creased in 1994/1995. The contribution of clover

to the N-supply of the farm was lower than expected as explained earlier.

N and P contents of the harvestable crop (gross yield) were in both years almost equal to the predictions, but considerably lower than those in practice. The relatively much higher nitrogen

pro-duction at the current farm — 398 kg N/ha

versus, on average, 276 at ‘De Marke’ — reflects the combined effect of a higher proportion of grassland and a much higher fertilizer level on grassland. On average, during the 2 years, 135 kg N and 4.7 kg P per ha of the nutrients applied were not recovered in the harvested products. These quantities must thus have accumulated in the soil, leached, lost in surface runoff (Sharpley and Withers, 1994), or denitrified (only N). Mod-ifications in the soil store can only be identified in the long run because of its large size: the soils at ‘De Marke’ contain on average more than 7000 kg of N and more than 3100 kg of P per ha.

5.5. The feed component

The proportion of nutrients from both har-vestable crop and purchased feed ingested by the animals was in accordance with the predictions and, therefore, higher than at the current farm. Hence, nutrients in feed are used more efficiently, as a result of lower grazing losses — due to short grazing periods per plot, stabling at night and a

Table 7

Nutrient flows of the soil/crop component of the average specialized dairy farm in the middle of the eighties, the prognoses at the start of ‘De Marke’ and the values realized in the years 1993/94 and 1994/95 (kg/ha)

Characteristic ‘Average’ ‘De Marke’ 1993/1994 1994/1995

prognoses 1983/1986

N P

N P P N P

Input

164

Faeces and urine at pasture (after volatilization) 26.0 51 7.0 48 7.7 57 3.3 227 Slurry application (after volatilization) 146 32.0 137 20.2 181 25.8 33.5

52 1.8 96 0

15.0

Fertilizer 330 67 6.0

49 0.9 49 0.9 1.0

Deposition 49 49 0.9

– 5 – 12 –

N-fixation clover 0 – 30

18 2.5 20 2.6 6.0

Harvest loss 60 21 3.1

Output

34.5

398 270

Harvestable crop 48.0 276 37.3 275 35.2

85 -0.1 79

32.0 351

Input-outputa _3.5 ₁₈₄ _5.8

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Table 8

Nutrient flows for the feed component for the average specialized dairy farm in the middle of the eighties, the prognoses at the start of ‘De Marke’ and the values realized in the years 1993/1994 and 1994/1995 (kg/ha)

‘De Marke’ prognoses 1993/1994 1994/1995 Characteristic ‘Average’ 1983/1986

P N

N P N P N P

Input

48.0 276 37.3 275

398 35.2

Harvestable crop 270 34.5

32.0 41 5.9 82 15.0

Purchased feed 182 84 11.5

Output

74.0 278

Feed intake 496 40.1 336 47.1 330 43.8

0 0 0 8 1.2 0

Feed sold 0 0

0 0 0 −35

0 −2.3

Mutation feed stock 17 0.8

6.0 39.0 3.1 48 4.2

Input–outputa ₈₄ ₇ _1.4

a_{Grazing, harvesting, conservation and feeding losses.}

short grazing season — careful silage making, and utilization of maize straw and beet leaves, that at the current farm are left in the field. Inputs of N and P with feed from outside the farm and from the feed stocks were substantially higher than predicted because of a larger herd size (Table 8). Moreover, more N and P was imported with feed than necessary according to the feeding stan-dards. Fodder with ‘adapted’ nutrient contents is rather expensive and it is cheaper to limit the input of N and P on the farm through restricted fertilizer purchases. Differences between years in the balance of inputs and outputs are partly due to inaccuracies in determining the feed stocks.

6. The fate of the nutrient surplus

6.1. Nitrogen

The nitrogen surplus of the farm (Table 3) is the sum of ammonia volatilization, denitrification, surface run-off, leaching and modifications in the soil store. Surface run-off does not play a role, as there is hardly any surface water at ‘De Marke’. Only at extremely high rainfall will water be removed through ditches. About 22 kg of N per ha was lost as ammonia from manure and an estimated 2 kg from crop residues. Total loss of ammonia-N thus equals 24 kg plus an unknown quantity from ensiled products.

Through denitrification a maximum of some

tens of kg N/ha were lost annually from the top

soil or the grass sward (Corre´, 1996). Denitrifica-tion can be important in parts of the plots where water stagnates (lack of oxygen) and there is ample supply of easily decomposable organic mat-ter. Denitrification from deeper layers is consid-ered unimportant at ‘De Marke’ because of the deep groundwater level, the low contents of or-ganic matter in the subsoil and the limited amount of nitrate that leaches from the top soil. It has been assumed that during both years deni-trification was 25 kg N/ha.

In Fig. 1 the time course of nitrate content in the upper groundwater is given, measured by RIVM (National Institute of Public Health and Enviromental Protection), averaged for the farm.

Fig. 1. The concentration of nitrate in the upper groundwater of ‘De Marke’.

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was expected (Goossensen and Meeuwissen,

1990). No clear differences were found among crops, nor an effect of irrigation. On the basis of precipitation surplus and the measured nitrate contents in the groundwater annual leaching is

estimated at 50 kg N/ha.

The sum of N-losses through ammonia

volatilization, denitrification and leaching of ni-trate equals 99 kg/ha. The average surplus at farm level is 169 kg N/ha. The difference between these

two values (70 kg N/ha) must be attributed to

measuring errors, losses from ensiled feed and changes in the store of soil organic-N. On aver-age, the store of soil organic-N appeared to have

increased annually by 17 kg/ha between 1990 and

1995, but obviously that estimate is not very accurate.

At ‘De Marke’ net mineralization showed large variations among plots and among years. Gener-ally, the results suggest that the more humid the conditions during the growing season the higher net mineralization (Aarts, 1996a,b). This could imply that — if the soil store is in equilibrium in the long run — it decreases in relatively wet years and increases during dry years.

6.2. Phosphorus

The P-content in the upper groundwater was

only 0.13 mg/l when sampled in October 1994, i.e.

much lower than the target value for sandy soil

(0.4 mg/l; Willems and Fraters (1995)). As hardly

any P leached it is most likely that the small P-surplus accumulated in the soil. This is in agree-ment with the observation that the total amount of P in the soil did not change in the period 1989/1990 – 1995/1996 (Habekotte´ et al., 1998). However, despite a slightly positive P-surplus, the

P-status of the soil in terms of plant-available P

(P-Al andPw; Van der Paauw (1956) and Van der

Paauw (1971)) decreased by some 2 – 4% annually. This might be the expected response to the instan-taneous reduction in the input of external P (slurry and chemical fertilizer). Because of the high inputs of P prior to implementation of ‘De Marke’, the labile pool of P in the upper layer of the soil is relatively high compared to the stable pool, which results in net transfer from the former to the latter (Janssen et al., 1987; Wolf et al., 1987). This process continues until the reverse

flow is in equilibrium again. As Pwand P-Al are

most closely related to the size of the labile pool, these soil fertility indicators may be expected to further decrease in the (near) future.

The decrease in Pw and P-Al, — in

combina-tion with the low P-fertilizer level — caused problems in a number of plots during the early growth of maize (Aarts, 1995). Soil analyses indi-cate that the solubility of P has decreased,

espe-cially where the soil contained high

concentrations of iron and aluminum (Schou-mans, 1995). Until now, the significantly lower P-status of the soil and the associated slower early growth of maize have not resulted in lower yields (Habekotte´ et al., 1998).

7. Discussion and conclusions

The results from ‘De Marke’ so far suggest that on dry sandy soils strict environmental standards for losses of nitrogen and accumulation of phos-phorus can be attained within a short time, while maintaining the current milk quota. The nutrient flows realized within the farming system agree well with the values calculated on the basis of existing knowledge. Intake of N and P by cattle was, however, higher than expected as the N- and P-contents of the ration were above the feeding standards and the number of young stock higher than anticipated. Nevertheless, input of N and P in purchased feed was only 50% of that of the current farm. The higher intake of N and P in feed resulted in higher than expected excretion in faeces and urine. This did not lead to problems with respect to the application of manure on the

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farm, but the (calculated) ammonia emission was higher than expected (mainly from the stable). However, ammonia emission did not exceed the official norm (a reduction of 70% compared to 1980). To compensate for the relatively high input of N and P in purchased feed the input of artifi-cial fertilizers had to be limited. From the spring of 1994 onwards, therefore, at ‘De Marke’ no P fertilizer was used.

by 75% until just below the standard of 50 mg/l,

with only small differences among crops. After breaking up 3-year old grassland, fodder beets and maize could absorb the nitrogen released from the decomposing sods. Hence, that allowed introduction of a ley farming system comprising maize and grass only, so that fodder beets (com-prising originally 10% of the farm area) could be taken out of the rotation.

The contribution of clover to the nitrogen sup-ply of grassland was smaller than expected as a result of winter (frost) damage to the clover seedlings after sowing in autumn. Clover without frost damage could be easily maintained in the sward. Shifting the moment of sowing to the spring may allow further savings on fertilizer ni-trogen in the future.

When calculating the annual nitrogen surplus of the farm, in principle, the change in soil store would have to be taken into account However, annual changes can not be measured accurately enough because of the large quantities involved. When calculating the (average) surplus over a longer period the change in soil store can be included. Indications suggest that the rate of change in the soil store strongly depends on mois-ture supply (Hassink et al., 1996). Under more favourable soil moisture conditions the rate of decomposition of organic material is apparently higher, resulting in a higher rate of mineraliza-tion. When the change in soil nitrogen store is not considered, a high nitrogen surplus in a dry year

can be less damaging to the environment than a lower surplus in a wet year. The experimental period is still too short for estimation of the influence of the farming system on the ‘equi-librium-level’ of the soil organic nitrogen store. That value is, however, important in determining the environmental impact of the farm surplus over a longer time period.

The input of P in feed and the output in milk and meat could be more or less balanced: P-sur-plus was only 18% of that on the current farm so that phosphorus availability for the crop de-creased. Although symptoms appeared during early growth of maize in a number of plots, no effects on production could be established (Aarts et al., 2000). There are no indications that in the future strict environmental standards for phos-phorus, formulated by the Dutch government (Table 2), would limit production possibilities on sandy soils (Aarts et al., 2000). Even the stricter environmental standards applied at ‘De Marke’ are not expected to lead to negative consequences for crop yields and quality in the coming decades. For more reliable results the research must be continued for a much longer period of time be-cause the system has to stabilize, the soil nutrient and organic matter stores and soil fertility react slowly to changes and weather conditions are always variable. However, there are no reasons to expect major problems in the near future.

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. .

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In 1993/1994 total nutrient supply to the soil was similar to the prediction (355 kg N and 37.2 kg P/ha); in 1994/1995 it was substantially higher because more slurry and fertilizer were applied to grassland, as a different calculation method for fertilizer requirements was used. Moreover, the correction for lower yields due to drought was insufficient. Therefore, the store of slurry de-creased in 1994/1995. The contribution of clover to the N-supply of the farm was lower than expected as explained earlier.

N and P contents of the harvestable crop (gross yield) were in both years almost equal to the predictions, but considerably lower than those in practice. The relatively much higher nitrogen pro-duction at the current farm — 398 kg N/ha versus, on average, 276 at ‘De Marke’ — reflects the combined effect of a higher proportion of grassland and a much higher fertilizer level on grassland. On average, during the 2 years, 135 kg N and 4.7 kg P per ha of the nutrients applied were not recovered in the harvested products. These quantities must thus have accumulated in the soil, leached, lost in surface runoff (Sharpley and Withers, 1994), or denitrified (only N). Mod-ifications in the soil store can only be identified in the long run because of its large size: the soils at ‘De Marke’ contain on average more than 7000 kg of N and more than 3100 kg of P per ha. 5.5. The feed component

Table 7

Characteristic ‘Average’ ‘De Marke’ 1993/1994 1994/1995

prognoses 1983/1986

N P

N P P N P

Input

164

Faeces and urine at pasture (after volatilization) 26.0 51 7.0 48 7.7 57 3.3

227

Slurry application (after volatilization) 146 32.0 137 20.2 181 25.8 33.5

52 1.8 96 0

15.0

Fertilizer 330 67 6.0

49 0.9 49 0.9

1.0

Deposition 49 49 0.9

– 5 – 12 –

N-fixation clover 0 – 30

18 2.5 20 2.6

6.0

Harvest loss 60 21 3.1

Output

34.5

398 270

Harvestable crop 48.0 276 37.3 275 35.2

85 -0.1 79

32.0 351

Input-outputa _3.5 ₁₈₄ _5.8

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Table 8

‘De Marke’ prognoses 1993/1994 1994/1995 Characteristic ‘Average’ 1983/1986

P N

N P N P N P

Input

48.0 276 37.3 275

398 35.2

Harvestable crop 270 34.5

32.0 41 5.9 82 15.0

Purchased feed 182 84 11.5

Output

74.0 278

Feed intake 496 40.1 336 47.1 330 43.8

0 0 0 8 1.2 0

Feed sold 0 0

0 0 0 −35

0 −2.3

Mutation feed stock 17 0.8

6.0 39.0 3.1 48 4.2

Input–outputa ₈₄ ₇ _1.4

a_{Grazing, harvesting, conservation and feeding losses.}

6. The fate of the nutrient surplus

6.1. Nitrogen

Through denitrification a maximum of some tens of kg N/ha were lost annually from the top soil or the grass sward (Corre´, 1996). Denitrifica-tion can be important in parts of the plots where water stagnates (lack of oxygen) and there is ample supply of easily decomposable organic mat-ter. Denitrification from deeper layers is consid-ered unimportant at ‘De Marke’ because of the deep groundwater level, the low contents of or-ganic matter in the subsoil and the limited amount of nitrate that leaches from the top soil. It has been assumed that during both years deni-trification was 25 kg N/ha.

In Fig. 1 the time course of nitrate content in the upper groundwater is given, measured by RIVM (National Institute of Public Health and Enviromental Protection), averaged for the farm.

Fig. 1. The concentration of nitrate in the upper groundwater of ‘De Marke’.

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The nitrate contents strongly decreased over time and were, on the last reported date, on average slightly below 50 mg/l, the requirement for drink-ing water. Nitrate contents under current plots close to ‘De Marke’ measured by the Drinking Water Company Oostelijk Gelderland, in the au-tumn of 1993, were almost four times as high. Apparently, the adapted management results in realisation of the objective ‘clean groundwater’ within a few years, where a much longer period was expected (Goossensen and Meeuwissen, 1990). No clear differences were found among crops, nor an effect of irrigation. On the basis of precipitation surplus and the measured nitrate contents in the groundwater annual leaching is estimated at 50 kg N/ha.

The sum of N-losses through ammonia volatilization, denitrification and leaching of ni-trate equals 99 kg/ha. The average surplus at farm level is 169 kg N/ha. The difference between these two values (70 kg N/ha) must be attributed to measuring errors, losses from ensiled feed and changes in the store of soil organic-N. On aver-age, the store of soil organic-N appeared to have increased annually by 17 kg/ha between 1990 and 1995, but obviously that estimate is not very accurate.

6.2. Phosphorus

The P-content in the upper groundwater was only 0.13 mg/l when sampled in October 1994, i.e. much lower than the target value for sandy soil (0.4 mg/l; Willems and Fraters (1995)). As hardly any P leached it is most likely that the small P-surplus accumulated in the soil. This is in agree-ment with the observation that the total amount of P in the soil did not change in the period 1989/1990 – 1995/1996 (Habekotte´ et al., 1998). However, despite a slightly positive P-surplus, the

P-status of the soil in terms of plant-available P (P-Al andPw; Van der Paauw (1956) and Van der

most closely related to the size of the labile pool, these soil fertility indicators may be expected to further decrease in the (near) future.

The decrease in Pw and P-Al, — in

combina-tion with the low P-fertilizer level — caused problems in a number of plots during the early growth of maize (Aarts, 1995). Soil analyses indi-cate that the solubility of P has decreased, espe-cially where the soil contained high concentrations of iron and aluminum (Schou-mans, 1995). Until now, the significantly lower P-status of the soil and the associated slower early growth of maize have not resulted in lower yields (Habekotte´ et al., 1998).

7. Discussion and conclusions

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Grass, maize and fodder beets could be grown in such a way that the norms for nitrate losses could be met. Improved utilization of animal ma-nure and lower manuring levels allowed a reduc-tion in the use of fertilizer-N of 74% compared to current practice. The nitrate concentration in the upper groundwater decreased within a few years by 75% until just below the standard of 50 mg/l, with only small differences among crops. After breaking up 3-year old grassland, fodder beets and maize could absorb the nitrogen released from the decomposing sods. Hence, that allowed introduction of a ley farming system comprising maize and grass only, so that fodder beets (com-prising originally 10% of the farm area) could be taken out of the rotation.