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Comparison of rumen fermentation patterns and in situ

degradation of grazed herbage in Churra and Merino sheep

a ,

_*

a b b a

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´

M.J. Ranilla

, M.D. Carro , F.J. Giraldez , A.R. Mantecon , J.S. Gonzalez

a

´ ´ ´

Departamento de Produccion Animal I, Universidad de Leon, 24071 Leon, Spain

b

´ EAE-CSIC, Aptdo. 788, 24080 Leon, Spain

Received 7 September 1998; received in revised form 15 March 1999; accepted 26 March 1999

Abstract

The aim of this experiment was to study the effect of breed (Churra and Merino) and advancing season (middle June, late July and early October) on rumen fermentation in sheep grazing a continuously stocked grass / white clover pasture. Six mature sheep (three Churra and three Merino) fitted with rumen cannulae were used. During each grazing period, samples of the grazed herbage (obtained using three oesophageal-cannulated sheep from each breed) and grass hay were incubated in situ. Rumen pH, ammonia-N and volatile fatty acids concentrations were determined in ruminal samples at 11.00, 14.00, 17.00 and 21.00 h. Breed did not affect (P.0.05) any of the dry matter (DM) degradation parameters of grazed herbage during July, but potential DM degradability and effective DM degradability were higher (P,0.05) in Merinos than in Churras during June and lower (P,0.05) during October. On the contrary, there were no differences (P.0.05) between breeds in the effective degradability of either DM or neutral-detergent fibre from grass hay. Differences between breeds in rumen degradation parameters of grazed herbage are discussed in relation to seasonal changes in the composition of the grazed herbage and to changes in rumen parameters in both breeds.  2000 Elsevier Science B.V. All rights reserved.

Keywords: Rumen fermentation; Grazing season; Sheep breed; In situ degradation

1. Introduction breed or age. However, breed differences in terms of

digestibility and other aspects of food utilisation During the last decades, there has been a growing have been reported in cattle, sheep and goats under interest and speculation concerning interspecies and various conditions (Hunter and Siebert, 1985; Givens intraspecies differences in the feeding behaviour and and Moss, 1994; Silanikove et al., 1993). Differences digestive efficiency of domestic ruminants. Reports among ruminants may represent distinct feeding of differences between ruminant types or species strategies and behaviour adapted for survival in the have often neglected to account for such factors as natural environment, rather than for maximum

pro-duction in confinement (Van Soest, 1994).

Churra and Merino are the most common breeds *Corresponding author. Tel.: 134-987-291-240; fax:1

34-987-of sheep in north-west Spain. Recent studies with 291-311.

E-mail address: dp1mrg@unileon.es (M.J. Ranilla) both breeds have shown differences in rumen fer-0301-6226 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved.

mentation rates, digestibility and digesta flow kinet- of the experiment, with grass:clover (%) ratios of ics when the animals where kept in the same 86:11 and 85:10.

environment and received the same diet (Ranilla et This study was part of a larger grazing experi-al., 1997, 1998). Other studies have reported differ- ment. Plots were continuously grazed by non-ex-ences between Churra and Merino sheep in voluntary perimental sheep on a ‘put and take’ basis through-intake of forages (Amor, 1994), digestibility of fresh out the experiment to control sward heights.

Ex-´

herbage (Giraldez et al., 1994) and in diet selection perimental animals were moved to the plots at the when grazing hill shrub communities (Revesado et beginning of each experimental period. Sward sur-al., 1994). However, virtually no information is face height was measured three times each week available on rumen fermentation in Churra and using the Hill Farming Research Organisation sward Merino sheep grazing irrigated pastures. Sheep pro- stick (Barthram, 1986). Forty measurements were duction based on grazing irrigated systems could be taken at random in each plot. More details about an alternative, in many areas such as north-west pasture conditions and herbage mass are given by

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Spain, where current irrigated agricultural systems Mantecon et al. (1995). Three experimental periods ´

have led to surplus production (Lavın et al., 1994). were conducted during the grazing season: June 10 As ruminal conditions affect both the rate of fermen- to 22 (middle June), July 23 to August 4 (late July) tation and the rate of passage, and therefore the and September 28 to October 10 (early October). voluntary intake in grazing ruminants (Van Soest,

1994), such information would be valuable for 2.2. Animals establishing optimal grazing sheep production

sys-tems. The present study was conducted to examine Three oesophageal-cannulated Churra sheep (aver-fermentation rates and ruminal parameters (pH, age initial body weight (BW) 48.764.01), three ammonia and volatile fatty acid concentrations) of oesophageal-cannulated Merino sheep (average ini-Churra and Merino sheep grazing grass / white clover tial BW 43.7(3.60 kg), three rumen-cannulated Chur-irrigated pasture. ra sheep (average initial BW 42.7(4.73 kg) and three rumen-cannulated Merino sheep (average initial BW 40.1(1.25 kg) were used. All animals were healthy

2. Material and methods and between 2 and 3 years old. Oesophageal

fistula-tion procedure was carried out two months before the 2.1. Study area, treatments and sampling periods start of the experiment. Fistulation procedure and postoperative care, involving topical and intramuscu-´

Grazing trials were conducted at the Estacion lar antibiotic application for a period of 2 weeks after ´

Agrıcola Experimental of Consejo Superior de Inves- surgery, were outlined by McManus (1981). Rumen

´ ´

tigaciones Cientıficas (C.S.I.C.) located at Leon on fistulation was carried out by inserting a rumen north-west Spain (latitude 428 359 N; longitude 58 cannula of 35 mm diameter two months before the 439 E). The experiment was carried out over 5 start of the experiment. Throughout the experiment, months, from middle June to early October, on a sheep were given free access to water, shade and seven-year-old field sown with ryegrass (Lolium trace mineral salts. Animals were weighed at the

perenne), tall fescue (Festuca arundinacea) and beginning and end of each sampling period at about white clover (Trifolium repens) in a proportion of 20, 11.00 h.

15 and 1 kg per ha, respectively. The sward height

imposed was 3.5 cm. The field was divided in two 2.3. Experimental procedure plots of 0.15 ha that were randomly assigned to each

of the two breeds. Every year each plot received 45 Rumen-fistulated animals of each of the breeds kg / ha of each N, P and K, and 300 kg / ha of 33% were assigned randomly to one of the plots and then (NH ) NO . Each plot was irrigated every nine days4 3 remained on the same treatment across sampling for 24 h throughout the experiment. Both plots periods. Sampling periods consisted on a 7-day presented a similar botanical composition at the start adaptation phase and a 6-day collection period. On

day 8 of each period the oesophageal-cannulated obtained from each sheep through the rumen cannula at 11.00, 14.00, 17.00 and 23.00 h. Rumen fluid was sheep were used to collect representative forage

strained through four layers of cheese-cloth, its pH samples of the diet selected by each breed of sheep.

determined immediately and duplicate samples were Animals were fasted for 12 h overnight prior to

taken for volatile fatty acids (VFA) and ammonia collection of masticate samples. At about 08.00 h the

analyses. One ml of rumen fluid was added to 1 ml plug of the oesophageal-cannula was removed,

plas-of deproteinizing solution (0.10 plas-of metaphosphoric tic bags were attached to the neck of the fistulated

acid and 0.0006 crotonic acid; w / v) for VFA de-animals and the sheep allowed to graze until the bags

termination. Twenty ml of rumen fluid were acidified were filled (about 30 min). After collection, the

with 20 ml 0.5 M HCl for ammonia-N determination. fistulated sheep were administered salts and water,

Samples were stored at 2208C until analyses were the plug reinserted and sheep allowed to graze

undertaken. normally in the assigned plots the remainder of the

day. Samples were examined for contamination by

2.4. Analytical procedures regurgitated material (any contaminated material was

discarded) and composited within breeds across

DM was determined by drying at 1008C until sheep. Saliva was eliminated by gently squeezing

constant weight. Ash was determined by ashing and a portion of the samples was immediately deep

samples in a muffle furnace for 8 h at 500–5058C. frozen (2208C) and freeze-dried for dry matter

Nitrogen (N) was determined according to Associa-(DM) determination and stored for chemical

analy-tion of Official Analytical Chemists (AOAC, 1990). ses. The rest of sample was used to measure its

NDF, ADF and lignin analyses were carried out rumen degradability using the nylon bag technique

according to Goering and Van Soest (1970). Ammo-(Ørskov et al., 1980). The bags measured 12.5310

nia-N concentration was determined using an auto-cm and had a mean pore size of 40 mm. About 30 g

analyser (Kjeltec Auto 1030 Tecator) as described by of sample was weighed without further manipulation

McDonald et al. (1960). VFA were determined in into nylon bags, which were incubated in the rumen

centrifuged samples (1 ml) by gas chromatography, of each rumen-cannulated sheep for 3, 6, 12, 24, 48,

using a Shimadzu GC-14B gas chromatograph 72 and 96 h. One bag was incubated per time

equipped with an autosampler and a GP 60 / 80 interval per sheep beginning at about 11.00 h. As

Carbopack C / 0.3% Carbowax 20M / 0.1% H PO3 4 soon as the bags were removed from the rumen, they

column (Supelco Inc., Spain). were washed thoroughly under running cold water

for 2 min and then washed in the cold rinse cycle (20 _{2.5. Calculations and statistical analyses} min) of a washing machine. A further two bags per

grazed herbage sample received this washing treat- _{The values for disappearance of DM from grazed} ment alone (zero-time washout value). DM disap- _{herbage and grass hay were fitted to the exponential}

(2c t )

pearance was estimated from the loss in weight _{model y5a1b (12e ) (Ørskov and McDonald,} following oven drying at 608C for 48 h. The residues _{1979), whereas the values for NDF disappearance} were analysed for neutral-detergent fibre (NDF) to _{were fitted to the exponential model y5a1b (12}

2c( t2lag)

estimate fibre degradation. _{e ) (Dhanoa, 1988). Data were fitted with}

To evaluate rumen activity over the grazing _{time using the NLIN procedure of the SAS package} periods, samples from grass hay (crude protein (CP): _{(SAS, 1997).}

86 g, NDF: 693 g, acid-detergent fibre (ADF): 345 g _{Effective degradability (ED) was estimated in} and lignin: 56 g / kg DM) were also incubated in situ. _{each sheep by using the parameters a, b, c and lag} Samples of grass hay were ground using a hammer- _{and assuming a rumen particulate outflow rate (k ) of}

p 21

mill fitted with a 2 mm screen and about 5 g were _{0.03 h according to the equation proposed by} weighed into nylon bags, which were incubated in _{France et al. (1990):}

the rumen of each sheep following the same schedule

b3c 2k 3lag

described for grazed herbage samples. _ED5a1]]3es p d

c1kp On days 10 and 12 rumen fluid samples were

A lag time value of 0 was assumed for the heights were similar for Churra (3.8, 3.6 and 3.5 cm calculation of DM effective degradability (DMED). in June, July and October, respectively) and Merino In situ data were analysed by variance analysis as sheep (3.6, 3.4 and 3.5 cm in June, July and October, a split-plot design with breed as the main-plot respectively). Animals remained healthy through the treatment and sampling period as the subplot treat- experimental period and no variations in their BW ment (Steel and Torrie, 1980). Effects for breed, were observed. Marked changes in daily tempera-sampling period, breed3sampling period, and sheep tures were registered during the experimental within breed were included in the model. Breed periods, with minimum and maximum temperatures effects were tested using sheep within treatment as ranging from 6 to 248C, from 13 to 338C and from 5 the error term. The interaction breed3sampling to 188C for June, July and October, respectively period and the main effect of sampling period were (Table 1). No rain was recorded during any of the tested using residual error. When a significant (P, sampling periods.

0.05) breed3sampling period interaction was de- Changes in chemical composition of grazed her-tected, data were analysed within sampling period. bage through the experimental periods were similar Rumen pH, ammonia-N and VFA data (average for both breeds (Table 2). CP content increased from values of two sampling days) were analysed as a June through October for both breeds of sheep, split–split-plot design (Steel and Torrie, 1980) with whereas NDF, ADF and lignin contents declined. sampling time and the interactions breed3sampling Merino animals tended (P,0.10) to present a period3sampling time and breed3sampling time lower potential degradability of both DM and NDF added to the model. When significant (P,0.05) of grass hay than Churras (Table 3), but no signifi-breed3sampling period3sampling time, breed3 cant differences (P.0.05) were detected for ED of sampling period and breed3sampling time interac- DM and NDF (NDFED).

tions were detected, which precluded pooling pH, As significant breed3grazing period3sampling VFA and ammonia-N data across sampling time and time interactions (P,0.05) were detected for pH, sampling period, these variables were analysed for ammonia-N and VFA concentrations, mean values each individual sampling period and sampling time. for these parameters for each breed, grazing period The method of least significant difference was used and sampling time are presented in Figs. 1–3, to separate treatment means when a significant (P, respectively. In addition, molar proportions of the 0.05) F-test for treatment was detected. The GLM main VFA (mean values for the day) are presented in procedure of SAS (1997) was used for all statistical Table 4.

analyses. During June, Churras presented higher (P,0.05)

pH values at 11.00 and 17.00 h sampling than Merino sheep, and tended (P,0.10) to present a

3. Results greater acetate / propionate (Ac / Pr) ratio. No

differ-ences (P.0.05) between breeds were detected at any The target sward height (3.5 cm) was generally sampling time in pH values or total VFA con-achieved in all the sampling periods. Mean sward centration during July. During October, Churra sheep Table 1

Daily maximum and minimum temperatures (8C) for the three sampling periods

Sampling Day

periods

8th 9th 10th 11th 12th 13th

June min 10 8 6 6 9 9

max 23 24 22 16 16 14

July min 15 17 16 15 13 16

max 31 30 33 33 30 31

October min 6 8 5 7 7 6

Table 2

Chemical composition (g / kg dry matter) of grazed herbage (oesophageal extrusa) as affected by breed (Churra (CH) and Merino (ME)) and grazing season in sheep grazing grass-white clover pasture

Component and breed Sampling period

Middle June Late July Early October

Organic matter

CH 888 871 855

ME 890 893 872

Crude protein

CH 107 205 266

ME 124 210 285

Neutral-detergent fibre

CH 733 549 440

ME 715 523 442

Acid-detergent fibre

CH 355 235 189

ME 348 243 171

Acid-detergent lignin

CH 62.3 36.6 25.2

ME 59.1 37.5 28.6

Table 3

Influence of breed and advancing season on dry matter (DM) and neutral-detergent fibre (NDF) fractional rate of degradation (c), potential degradability (a1b), lag time (lag) and effective degradability (DMED and NDFED ) from grass hay incubated in the rumen of sheep grazing grass-white clover pasture

Breed Sampling period

Churra Merino s.e.d. Middle June Late July Early October s.e.d.

21 a b b b a

DM c (h ) 0.034 0.044 0.0110 0.044 0.040 0.032 0.0028

† a a b

a1b (g / kg) 726 681 20.6 697 696 718 8.5

DMED (g / kg) 490 496 15.1 504 494 483 11.0

21

NDF c (h ) 0.027 0.029 0.0017 0.031 0.028 0.025 0.0046

†

a1b (g / kg) 692 627 27.3 637 671 671 32.1

Lag (h) 4.9 2.3 0.56 3.7 2.9 4.3 1.60

b b a

NDFED (g / kg) 336 337 19.9 356 347 306 15.2

a,b

Means in a row with different superscript differ significantly (P,0.05).

†

P,0.10.

exhibited a higher (P,0.05) pH value and tended higher (P,0.05) potential degradability of both DM (P,0.10) to present a lower total VFA concentration and NDF than Churras, as well as a higher DMED at 23.00 h sampling, as well as a greater Ac / Pr ratio (588 vs. 541). On the contrary, no differences (P.

(3.37 vs. 3.02). There were no differences (P.0.05) 0.05) between breeds in any of the degradation between breeds in the ammonia-N values at any parameters were detected during July. During Oc-sampling time in any of the grazing periods. tober, Churras presented higher (P,0.05) values of Mean values of degradation parameters of grazed DMED and potential degradability of both DM and herbage for each breed and grazing period are given NDF, and tended (P,0.10) to present higher in Table 5. During June, Merino sheep presented a NDFED (775 vs. 739). In relation with the grazing

Fig. 2. Changes in ammonia-N concentrations in the rumen fluid Fig. 1. Changes in the rumen fluid pH of Churra and Merino of Churra and Merino sheep grazing a grass / white clover pasture. sheep grazing a grass / white clover pasture. Significance level: †

P,0.10; * P,0.05.

4. Discussion

period, rate of degradation, potential degradability Changes in chemical composition of grazed her-and ED of both DM her-and NDF were lowest (P,0.05) bage through the experimental periods were similar during June for both breeds. for both breeds. Similar increases in CP content and

position of the diet are the consequence of changes in both the quality of the herbage on offer and in the selection made by the animals. Thus, in a grazing study carried out on the same plots used here,

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Mantecon et al. (1995) reported an increase of the proportion of white clover and in the herbage regrowth produced as grazing period advanced, both of them being consequent with the changes observed in the chemical composition.

Regrowth of pasture swards is influenced by the frequency and intensity of previous grazing (Brougham, 1955) and, generally, its digestibility is reasonably constant, with little decline as the season progresses. In our experiment, the herbage grazed during July and October had a higher DMED than that grazed during June, thus reflecting the pasture regrowth produced during those grazing periods

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(Mantecon et al., 1995). The lower DM fractional rate of degradation observed for the herbage grazed in June (almost half of the estimated rate in July and October) is probably related to its higher cell wall content. Lignin is the major cell wall component limiting digestion of the cell wall polysaccharides in the rumen (Jung and Allen, 1995). The lower potential degradability of the NDF from the forage grazed during June (observed for the two breeds of sheep) was probably due to its higher cell wall lignification (mean value of 83.8 g / kg NDF) com-pared to that observed in July and October (69.2 and 61.0 g / kg NDF, respectively). In the same way, the rate of NDF degradation and its ED in grazed herbage were lower in June than in the later grazing periods.

Ruminal environment, as defined by its pH and fermentation end-products concentrations, affects the rate and extent of food rumen degradation. Previous studies in Churra and Merino sheep (Ranilla et al., 1997) suggest this could differ between the two breeds. When animals from both breeds were offered Fig. 3. Changes in total volatile fatty acids (VFA) concentrations

in the rumen fluid of Churra and Merino sheep grazing a grass / _{alfalfa hay at maintenance level, the Churras} ex-white clover pasture. Significance level: † P,0.10; * P,0.05. _{hibited a higher level of rumen microbial fibrolytic} activity than the Merinos, which was related to their ability to maintain pH within a range of values more favourable to fibre degradation (Ranilla et al., 1997). decreases in NDF content have been reported by Some of the differences in the in situ degradation of

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Valdes et al. (1995) for a grass-white clover pasture grazed herbage observed in our experiment could continuously stocked by Churra sheep from May to also be related to differences between breeds in their November. These changes in the chemical com- ruminal pH. Stewart (1977) showed that a reduction

Table 4

Influence of breed (Churra (CH) and Merino (ME)) and advancing season on molar proportions (mmol / mmol) of acetate, propionate, butyrate and isoacids (calculated as the sum of isobutyrate, isovalerate and valerate) and on the ratio acetate / propionate (Ac /Pr) in the rumen of sheep grazing grass-white clover pasture (data are the mean value for the day)

Sampling period

Middle June Late July Early October s.e.d.

Acetate

c b a

CH 63.2 61.3 59.3 0.52

b b a

ME 62.6 63.7 58.2 0.63

s.e.d. 1.84 0.91 0.72 –

Sig. NS † NS –

Propionate

CH 18.2 18.3 17.5 0.45

ME 19.9 19.2 19.2 0.68

s.e.d. 0.70 0.26 0.45 –

Sig. † † * –

Butyrate

a b c

CH 11.8 13.6 15.5 0.44

a a b

ME 12.1 11.8 14.8 0.77

s.e.d. 0.67 1.32 0.72 –

Sig. NS NS NS –

Isoacids

a b c

CH 5.3 6.5 7.7 0.22

a a b

ME 5.2 5.2 7.5 0.34

s.e.d. 0.28 0.25 0.40 –

Sig. NS ** NS –

Ac /Pr

CH 3.46 3.35 3.37 0.113

a,b b a

ME 3.14 3.31 3.02 0.101

s.e.d. 0.13 0.036 0.05 –

Sig. † NS ** –

a,b,c

Means in a row with different superscript differ significantly (P,0.05). Sig.: significance level: NS5non-significant; † P,0.10; * P,0.05; ** P,0.01.

of the rumen pH from 6.8 to 6.0 results in a presented shorter periods of low pH values. In moderate depression in fibre digestion, but a further addition, the greater Ac / Pr ratio found in the rumen decrease in pH to below 6.0 causes severe inhibition. of Churras compared to Merinos in October would The lower ED of the grazed herbage observed in also indicate increased fibre digestion.

October for Merinos compared to Churras could be However, it should be noted that when the grass related to the low rumen pH values (below 6.0) hay was incubated in the rumen of the grazing sheep, found at 17.00 and 23.00 h in Merino sheep (Fig. 1). no differences between breeds were detected in On the contrary, rumen pH in Churra did not drop either DMED or NDFED. Grazing ruminants rely below 6.0 at any sampling time. It has to be almost entirely on mastication to disrupt plant tissues remembered that it is not just the extent of pH that create physical barriers to digestion, whereas depression but also the period over this occurs what mechanical processing of feeds (i.e., ground) facili-reduces fibre degradation. With sampling times of tates microbial attack (McAllister et al., 1994). 17.00 and 23.00 h it is difficult to interpret which has Grazed herbage was incubated into the rumen of occurred in the interim, but it is possible that Churras sheep unprocessed, but the grass hay was ground

Table 5

Influence of breed (Churra (CH) and Merino (ME)) and advancing season on dry matter (DM) and neutral-detergent fibre (NDF) fractional rate of degradation (c), potential degradability (a1b), lag time (lag) and effective degradability (DMED and NDFED ) from grazed herbage in sheep grazing grass-white clover pasture

Sampling period

Middle June Late July Early October s.e.d. 21

DM c (h )

a b b

CH 0.055 0.095 0.109 0.0071

a b b

ME 0.046 0.091 0.108 0.0096

s.e.d. 0.0041 0.0069 0.0122 –

Sig. † NS NS –

a1b (g / kg)

a b c

CH 656 881 927 0.0

a b b

ME 754 883 904 11.2

s.e.d. 10.6 13.1 6.4 –

Sig. *** NS * –

DMED (g / kg)

a b c

CH 541 765 857 2.5

a b c

ME 588 762 821 13.1

s.e.d. 13.3 9.5 9.3 –

Sig. * NS *

-21

NDF c (h )

a b b

CH 0.059 0.117 0.115 0.0055

a b c

ME 0.048 0.105 0.131 0.0083

s.e.d. 0.0074 0.0081 0.0109 –

Sig. NS NS NS –

a1b (g / kg)

a b c

CH 671 821 887 11.0

a b b

ME 730 846 849 12.5

s.e.d. 9.4 18.1 7.5 –

Sig. ** NS ** –

lag (h)

CH 2.3 2.8 1.8 0.49

ME 3.7 2.4 2.7 0.53

s.e.d. 0.84 0.16 0.36 –

Sig. NS NS NS –

NDFED (g / kg)

a b c

CH 532 668 775 13.6

a b c

ME 528 674 739 18.6

s.e.d. 16.4 13.6 13.9 –

Sig. NS NS † –

a,b,c

Means in a row with different superscript differ significantly (P,0.05).

Sig.: significance level: NS5non-significant; † P,0.10; * P,0.05; ** P,0.01; *** P,0.001.

through a 2 mm screen, thus, reducing particle size The reasons why DMED and potential de-and increasing the surface area available for micro- gradability of the grazed herbage during June were bial attachment and enzymatic attack. This could higher in Merinos than in Churras are unclear. In explain the lack of differences between breeds when fact, rumen pH values during June were higher in grass hay was incubated, in spite of the differences Churra animals, although it must be stressed that at observed in rumen pH values. all sampling times both breeds showed values higher

than those expected to impair fibre degradation. The rate of OM degradation. Therefore, it can be as-in situ degradation depends on many factors, beas-ing sumed that ammonia-N concentrations were adequate one of them the characteristics of the feed incubated. for an optimal rumen fermentation in both Churra As suggested by Mertens (1977), the morphology of and Merino sheep over the three grazing periods. the plant, the crystalline structure of the fibre and However, the high concentrations observed for all other factors not related to the chemical composition periods (and specially during October) could be could affect the in situ degradability of the substrate indicative of substantial losses of dietary N before incubated. Herbage grazed by each breed could have the small intestine, as suggested by Beever et al. presented different characteristics which were not (1986). These authors reported that the high levels of reflected in its chemical composition, but affected its ammonia-N found in the rumen of cattle grazing in situ degradation. The fact that both breeds may white clover (values similar to the ones found in our have consumed a diet of different botanical com- study in October) were due to the readily soluble position, due to potential breed differences in their nature of the N constituents of the herbage, which ability for diet selection (Revesado et al., 1994), and gave rise to a supply of degraded N (ammonia-N in that the extent of mastication, may also differ particular), in excess of the capacity of the rumen between breeds, should also be considered. micro-organisms to assimilate the N into microbial Daily variations in rumen parameters (pH, VFA mass. Under these rumen conditions, the supply of a and ammonia concentrations) are mainly related to complementary feed (with a high energy content, but the pattern of intake. In grazing animals, the primary low in rumen degradable N) would help to maximise determinant of when animals graze is day length, microbial synthesis in both breeds.

with major grazing periods around dawn and dusk In relation to VFA values, both total VFA con-(Arnold, 1981). However, temperature and humidity centrations and molar proportions of the main VFA may alter when grazing periods begin and end. observed in our study were in the range of those

´

Grazing behaviour was not recorded in our experi- reported for other pastures (Garcıa et al., 1994: ment, but the daily evolution of the rumen parame- Olson et al., 1994).

ters agrees with the observations reported by other authors (Berggren-Thomas and Hohenboken, 1986).

During July, when the temperatures were high during 5. Conclusions

the day, rumen pH values were higher than 6.50

from 11.00 to 17.00 h, suggesting that sheep did not Some differences were noted in the rumen fermen-graze during this period. In fact, it was noticed that tation patterns in Churra and Merino sheep grazing at 14.00 and 17.00 h all sheep had sought for shade white clover / grass pasture. Season influenced the and no grazing was observed. The lower pH values composition of the diet grazed by both breeds of at 14.00 and 17.00 h observed in October compared sheep, and therefore, its estimated in situ degradation to June and July in both breeds of sheep seem to parameters. While the higher in situ degradability of indicate that grazing periods took place over this the grazed herbage in Churra animals in October time, in agreement with the idea that when daily could be related to its ability to maintain rumen pH maximum temperatures are lower than 158C most of within a range of values more favourable for fibre grazing is done during the day (Arnold, 1981). digestion, breed differences found during June did In agreement with other works (Cruickshank et al., not fit this explanation. It is possible that Churra and

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1992; Garcıa et al., 1994), ammonia-N concentra- Merino sheep may have selected different diets, tions increased with increased N content in the although this fact was not reflected in their chemical herbage as grazing season advanced. Rumen ammo- composition as determined by the analytical tech-nia-N concentrations were at all sampling times niques used in our study. Therefore, further research higher than the 50 mg / ml suggested by Satter and is required to assess differences between breeds in Slyter (1974) as optimal for efficiency of microbial the level of intake, ability to select the diet and growth, and also higher than the 200 mg / ml reported digesta flow kinetics under grazing conditions, as by Mehrez et al. (1977) as optimal for the maximum well as to explain the mechanisms involved.

Comparative digestibility of fresh herbage cut at two maturity

Acknowledgements

stages by two breeds of sheep. Anim. Sci. 58, 452–453 (abstr.). Givens, D.I., Moss, A.R., 1994. Effect of breed, age and body M.J. Ranilla gratefully acknowledges receipt of a _{condition of sheep in measurement of apparent digestibility of}

´

scholarship from the Ministerio de Educacion y dried grass. Anim. Feed Sci. Technol. 46, 152–162. Goering, H.K., Van Soest, P.J., 1970. Forage fiber analysis Ciencia of Spain. Financial support for this work

(apparatus, reagents, procedures and some applications). Agri-through a C.I.C.Y.T. Project (AGF94-0026) and by

cultural handbook no. 379. Agricultural Research Service, an European Union Project (AIR CT92-0646) is _{United States Department of Agriculture, Washington, DC.} gratefully acknowledged. Thanks are given to the Hunter, R.A., Siebert, B.D., 1985. Utilisation of low-quality

roughages by Bos taurus and Bos indicus cattle. 1. Rumen Meteorological Office from the Escuela Superior y

digestion. Br. J. Nutr. 53, 637–648.

´ ´

Tecnica de Ingenierıa Agraria of the University of

Jung, H.G., Allen, M.S., 1995. Characteristics of plant cell walls ´

Leon (Spain) for the kindly provision of the tempera- _{affecting intake and digestibility of forages by ruminants. J.} tures registered during the experimental period. _{Anim. Sci. 73, 2774–2790.}

´ ´ ´ ´ ´

Lavın, M.P., Mantecon, A.R., Giraldez, F.J., Mencıa, J.S., Dıez, P., 1994. Sheep production in north central Spain, current situation. In: Gibson, A., Plamant, J.C. (Eds.), The Study of

References _{Livestock Farming Systems in A Research and Development}

Framework, Pudoc, Wageningen, pp. 202–206.

´ ´

Amor, J.J., 1994. [Diurnal patterns of intake and rumination in Mantecon, A.R., Jaramillo, E., Frutos, P., Lavın, P., 1995. Effect sheep as affected by different factors]. Doctoral Thesis, Uni- of sward height and sheep breed on sward composition during

´

versity of Leon, Spain. summer grazing period. Anim. Sci. 60, 532 (abstr.). Arnold, G.W., 1981. Grazing Behaviour. In: Morley, F.H.W. (Ed.), McAllister, T.A., Bae, H.D., Jones, G.A., Cheng, K.J., 1994.

Grazing animals (World Animal Science, B1), Elsevier Sci- Microbial attachment and feed digestion in the rumen. J. Anim. entific Publishing Company, Amsterdam, pp. 79–104. Sci. 72, 3004–3018.

Association of Official Analytical Chemists, 1990. Official meth- McDonald, P., Stirling, A.C., Henderson, A.R., Dewar, W.A., ods for analysis of the Association of Official Analytical Stark, G.H., Davie, W.G., Macpherson, H.T., Reid, A.M., Chemists, 15th ed. Association of Official Analytical Chemists, Slater, J., 1960. Studies on ensilage. Technical Bulletin, No. Washington, DC. 24. Edinburgh School of Agriculture, pp. 1–83.

Barthram, G.T., 1986. Experimental techniques: the HFRO sward McManus, W.R., 1981. Oesophageal fistulation technique as an aid stick. Biennial Report. Hill Farming Research Organisation to diet evaluation of the grazing ruminant. In: Wheeler, J.L., 1984–85, pp. 29–30. Mochrie, R.D. (Eds.), Forage Evaluation. Concepts and Tech-Beever, D.E., Losada, H.R., Cammell, S.B., Evans, R.T., Haines, niques, Griffit Press Ltd, Australia, pp. 249–260.

M.J., 1986. Effect of forage species and season on nutrient Mehrez, A.L., Ørskov, E.R., McDonald, I., 1977. Rates of rumen digestion and supply in grazing cattle. Br. J. Nutr. 56, 209– fermentation in relation to rumen ammonia concentration. Br.

225. J. Nutr. 38, 437–443.

Berggren-Thomas, B., Hohenboken, W.D., 1986. The effects of Mertens, D.R., 1977. Dietary fiber components, relationship to the sire-breed, forage availability and weather on the grazing rate and extent of ruminal digestion. Federation Proc. 36, behaviour of crossbred ewes. Appl. Anim. Behav. Sci. 15, 187–192.

217–228. Olson, K.C., Caton, J.S., Kirby, D.R., Norton, P.L., 1994. In-Brougham, R.W., 1955. A study in rate of pasture growth. Austr. J. fluence of yeast culture supplementation and advancing season Agric. Res. 6, 804–812. on steers grazing mixed-grass prairie in the northern great Cruickshank, G.J., Poppi, D.P., Sykes, A.R., 1992. The intake, plains. II. Ruminal fermentation, site of digestion, and

micro-digestion and protein degradation of grazed herbage by early- bial efficiency. J. Anim. Sci. 72, 2158–2170.

weaned lambs. Br. J. Nutr. 68, 349–364. Ørskov, E.R., McDonald, I., 1979. The estimation of protein Dhanoa, M.S., 1988. On the analysis of dracon bag data for low degradability in the rumen from incubation measurements degradability feeds. Grass and Forage Sci. 43, 441–444. weighted according to rate of passage. J. Agricul. Sci.,

Cam-´

France, J., Thornley, J.H.M., Lopez, S., Siddons, R.C., Dhanoa, bridge 92, 499–503.

M.S., Van Soest, P.J., Gill, M., 1990. On the two-compartment Ørskov, E.R., Hovell, F.D.DeB, Mould, F.L., 1980. The use of the model for estimating the rate and extent of feed degradation in nylon bag technique for the evaluation of feedstuffs. Trop. the rumen. J. Theoret. Biol. 146, 269–287. Anim. Prod. 5, 195–213.

´ ´ ´ ´

Garcıa, M.A., Isac, M.D., Aguilera, J.F., Molina Alcaide, E., Ranilla, M.J., Carro, M.D., Valdes, C., Giraldez, F.J., Lopez, S., 1994. Rumen fermentation patterns in goats and sheep pastures 1997. A comparative study of ruminal activity in Churra and from semiarid Spanish lands unsupplemented or supplemented Merino sheep offered alfalfa hay. Anim. Sci. 65, 121–128.

´ ´ ´

with barley grain or barley grain-urea. Livest. Prod. Sci. 39, Ranilla, M.J., Lopez, S., Giraldez, F.J., Valdes, C., Carro, M.D., 81–84. 1998. Comparative digestibility and digesta flow kinetics in

´ ´ ´ ´

´ ´

Revesado, P.R., Mantecon, A.R., Frutos, P., Gonzalez, J.S., 1994. Statistical Analysis System Institute, 1997. SAS companion for Comparative studies of diet selection by Churra and Merino the Microsoft Windows Environment, version 6. SAS Institute genotypes grazing on a hill shrub community. In: Lawrence, Inc., Cary, NC.

T.L.J., Parker D.S., Rowlison, P. (Eds.), Livestock Production Steel, R.G.C., Torrie, J.H., 1980. Principles and Procedures of and Land Use in Hills and Uplands. Occasional publication of Statistics, second edition, McGraw-Hill, New York.

the British Society of Animal Production, no 18, pp. 109–110. Stewart, C.S., 1977. Factors affecting cellulolytic activity of Satter, L.D., Slyter, L.L., 1974. Effect of ammonia concentration rumen contents. Appl. Environ. Microbiol. 33, 497–502.

´ ´ ´ ´

on rumen microbial protein in vitro. Br. J. Nutr. 32, 199–208. Valdes, C., Mantecon, A.R., Giraldez, F.J., Bermudez, F.F., 1995. Silanikove, N., Tagari, H., Shkolnik, A., 1993. Comparison of rate Herbage intake by Churra ewes grazing at two different sward

of passage, fermentation rate and efficiency of digestion of heights. J. Anim. Feed Sci. 4, 1–9.

high fiber diet in desert Bedouin goats compared to Swiss Van Soest, P.J., 1994. Nutritional Ecology of the Ruminant, Saanen goats. Small Ruminant Res. 12, 45–60. second edition, Cornell University Press, Ithaca, New York.

position of the diet are the consequence of changes in both the quality of the herbage on offer and in the selection made by the animals. Thus, in a grazing study carried out on the same plots used here,

´

Mantecon et al. (1995) reported an increase of the proportion of white clover and in the herbage regrowth produced as grazing period advanced, both of them being consequent with the changes observed in the chemical composition.

Regrowth of pasture swards is influenced by the frequency and intensity of previous grazing (Brougham, 1955) and, generally, its digestibility is reasonably constant, with little decline as the season progresses. In our experiment, the herbage grazed during July and October had a higher DMED than that grazed during June, thus reflecting the pasture regrowth produced during those grazing periods

´

(Mantecon et al., 1995). The lower DM fractional rate of degradation observed for the herbage grazed in June (almost half of the estimated rate in July and October) is probably related to its higher cell wall content. Lignin is the major cell wall component limiting digestion of the cell wall polysaccharides in the rumen (Jung and Allen, 1995). The lower potential degradability of the NDF from the forage grazed during June (observed for the two breeds of sheep) was probably due to its higher cell wall lignification (mean value of 83.8 g / kg NDF) com-pared to that observed in July and October (69.2 and 61.0 g / kg NDF, respectively). In the same way, the rate of NDF degradation and its ED in grazed herbage were lower in June than in the later grazing periods.

Ruminal environment, as defined by its pH and fermentation end-products concentrations, affects the rate and extent of food rumen degradation. Previous studies in Churra and Merino sheep (Ranilla et al., 1997) suggest this could differ between the two breeds. When animals from both breeds were offered Fig. 3. Changes in total volatile fatty acids (VFA) concentrations

in the rumen fluid of Churra and Merino sheep grazing a grass / _{alfalfa hay at maintenance level, the Churras} ex-white clover pasture. Significance level: † P,0.10; * P,0.05. _{hibited a higher level of rumen microbial fibrolytic} activity than the Merinos, which was related to their ability to maintain pH within a range of values more favourable to fibre degradation (Ranilla et al., 1997). decreases in NDF content have been reported by Some of the differences in the in situ degradation of

´

Valdes et al. (1995) for a grass-white clover pasture grazed herbage observed in our experiment could continuously stocked by Churra sheep from May to also be related to differences between breeds in their November. These changes in the chemical com- ruminal pH. Stewart (1977) showed that a reduction

Table 4

Influence of breed (Churra (CH) and Merino (ME)) and advancing season on molar proportions (mmol / mmol) of acetate, propionate, butyrate and isoacids (calculated as the sum of isobutyrate, isovalerate and valerate) and on the ratio acetate / propionate (Ac /Pr) in the rumen of sheep grazing grass-white clover pasture (data are the mean value for the day)

Sampling period

Middle June Late July Early October s.e.d.

Acetate

c b a

CH 63.2 61.3 59.3 0.52

b b a

ME 62.6 63.7 58.2 0.63

s.e.d. 1.84 0.91 0.72 –

Sig. NS † NS –

Propionate

CH 18.2 18.3 17.5 0.45

ME 19.9 19.2 19.2 0.68

s.e.d. 0.70 0.26 0.45 –

Sig. † † * –

Butyrate

a b c

CH 11.8 13.6 15.5 0.44

a a b

ME 12.1 11.8 14.8 0.77

s.e.d. 0.67 1.32 0.72 –

Sig. NS NS NS –

Isoacids

a b c

CH 5.3 6.5 7.7 0.22

a a b

ME 5.2 5.2 7.5 0.34

s.e.d. 0.28 0.25 0.40 –

Sig. NS ** NS –

Ac /Pr

CH 3.46 3.35 3.37 0.113

a,b b a

ME 3.14 3.31 3.02 0.101

s.e.d. 0.13 0.036 0.05 –

Sig. † NS ** –

a,b,c

Means in a row with different superscript differ significantly (P,0.05). Sig.: significance level: NS5non-significant; † P,0.10; * P,0.05; ** P,0.01.

of the rumen pH from 6.8 to 6.0 results in a presented shorter periods of low pH values. In moderate depression in fibre digestion, but a further addition, the greater Ac / Pr ratio found in the rumen decrease in pH to below 6.0 causes severe inhibition. of Churras compared to Merinos in October would The lower ED of the grazed herbage observed in also indicate increased fibre digestion.

October for Merinos compared to Churras could be However, it should be noted that when the grass related to the low rumen pH values (below 6.0) hay was incubated in the rumen of the grazing sheep, found at 17.00 and 23.00 h in Merino sheep (Fig. 1). no differences between breeds were detected in On the contrary, rumen pH in Churra did not drop either DMED or NDFED. Grazing ruminants rely below 6.0 at any sampling time. It has to be almost entirely on mastication to disrupt plant tissues remembered that it is not just the extent of pH that create physical barriers to digestion, whereas depression but also the period over this occurs what mechanical processing of feeds (i.e., ground) facili-reduces fibre degradation. With sampling times of tates microbial attack (McAllister et al., 1994). 17.00 and 23.00 h it is difficult to interpret which has Grazed herbage was incubated into the rumen of occurred in the interim, but it is possible that Churras sheep unprocessed, but the grass hay was ground

Table 5

Influence of breed (Churra (CH) and Merino (ME)) and advancing season on dry matter (DM) and neutral-detergent fibre (NDF) fractional rate of degradation (c), potential degradability (a1b), lag time (lag) and effective degradability (DMED and NDFED ) from grazed herbage

in sheep grazing grass-white clover pasture

Sampling period

Middle June Late July Early October s.e.d.

21

DM c (h )

a b b

CH 0.055 0.095 0.109 0.0071

a b b

ME 0.046 0.091 0.108 0.0096

s.e.d. 0.0041 0.0069 0.0122 –

Sig. † NS NS –

a1b (g / kg)

a b c

CH 656 881 927 0.0

a b b

ME 754 883 904 11.2

s.e.d. 10.6 13.1 6.4 –

Sig. *** NS * –

DMED (g / kg)

a b c

CH 541 765 857 2.5

a b c

ME 588 762 821 13.1

s.e.d. 13.3 9.5 9.3 –

Sig. * NS *

-21

NDF c (h )

a b b

CH 0.059 0.117 0.115 0.0055

a b c

ME 0.048 0.105 0.131 0.0083

s.e.d. 0.0074 0.0081 0.0109 –

Sig. NS NS NS –

a1b (g / kg)

a b c

CH 671 821 887 11.0

a b b

ME 730 846 849 12.5

s.e.d. 9.4 18.1 7.5 –

Sig. ** NS ** –

lag (h)

CH 2.3 2.8 1.8 0.49

ME 3.7 2.4 2.7 0.53

s.e.d. 0.84 0.16 0.36 –

Sig. NS NS NS –

NDFED (g / kg)

a b c

CH 532 668 775 13.6

a b c

ME 528 674 739 18.6

s.e.d. 16.4 13.6 13.9 –

Sig. NS NS † –

a,b,c

Means in a row with different superscript differ significantly (P,0.05).

Sig.: significance level: NS5non-significant; † P,0.10; * P,0.05; ** P,0.01; *** P,0.001.

through a 2 mm screen, thus, reducing particle size The reasons why DMED and potential de-and increasing the surface area available for micro- gradability of the grazed herbage during June were bial attachment and enzymatic attack. This could higher in Merinos than in Churras are unclear. In explain the lack of differences between breeds when fact, rumen pH values during June were higher in grass hay was incubated, in spite of the differences Churra animals, although it must be stressed that at

than those expected to impair fibre degradation. The rate of OM degradation. Therefore, it can be as-in situ degradation depends on many factors, beas-ing sumed that ammonia-N concentrations were adequate one of them the characteristics of the feed incubated. for an optimal rumen fermentation in both Churra As suggested by Mertens (1977), the morphology of and Merino sheep over the three grazing periods. the plant, the crystalline structure of the fibre and However, the high concentrations observed for all other factors not related to the chemical composition periods (and specially during October) could be could affect the in situ degradability of the substrate indicative of substantial losses of dietary N before incubated. Herbage grazed by each breed could have the small intestine, as suggested by Beever et al. presented different characteristics which were not (1986). These authors reported that the high levels of reflected in its chemical composition, but affected its ammonia-N found in the rumen of cattle grazing in situ degradation. The fact that both breeds may white clover (values similar to the ones found in our have consumed a diet of different botanical com- study in October) were due to the readily soluble position, due to potential breed differences in their nature of the N constituents of the herbage, which ability for diet selection (Revesado et al., 1994), and gave rise to a supply of degraded N (ammonia-N in that the extent of mastication, may also differ particular), in excess of the capacity of the rumen between breeds, should also be considered. micro-organisms to assimilate the N into microbial Daily variations in rumen parameters (pH, VFA mass. Under these rumen conditions, the supply of a and ammonia concentrations) are mainly related to complementary feed (with a high energy content, but the pattern of intake. In grazing animals, the primary low in rumen degradable N) would help to maximise determinant of when animals graze is day length, microbial synthesis in both breeds.

with major grazing periods around dawn and dusk In relation to VFA values, both total VFA con-(Arnold, 1981). However, temperature and humidity centrations and molar proportions of the main VFA may alter when grazing periods begin and end. observed in our study were in the range of those

´

Grazing behaviour was not recorded in our experi- reported for other pastures (Garcıa et al., 1994: ment, but the daily evolution of the rumen parame- Olson et al., 1994).

ters agrees with the observations reported by other authors (Berggren-Thomas and Hohenboken, 1986).

During July, when the temperatures were high during 5. Conclusions

the day, rumen pH values were higher than 6.50

from 11.00 to 17.00 h, suggesting that sheep did not Some differences were noted in the rumen fermen-graze during this period. In fact, it was noticed that tation patterns in Churra and Merino sheep grazing at 14.00 and 17.00 h all sheep had sought for shade white clover / grass pasture. Season influenced the and no grazing was observed. The lower pH values composition of the diet grazed by both breeds of at 14.00 and 17.00 h observed in October compared sheep, and therefore, its estimated in situ degradation to June and July in both breeds of sheep seem to parameters. While the higher in situ degradability of indicate that grazing periods took place over this the grazed herbage in Churra animals in October time, in agreement with the idea that when daily could be related to its ability to maintain rumen pH maximum temperatures are lower than 158C most of within a range of values more favourable for fibre grazing is done during the day (Arnold, 1981). digestion, breed differences found during June did In agreement with other works (Cruickshank et al., not fit this explanation. It is possible that Churra and

´

1992; Garcıa et al., 1994), ammonia-N concentra- Merino sheep may have selected different diets, tions increased with increased N content in the although this fact was not reflected in their chemical herbage as grazing season advanced. Rumen ammo- composition as determined by the analytical tech-nia-N concentrations were at all sampling times niques used in our study. Therefore, further research higher than the 50 mg / ml suggested by Satter and is required to assess differences between breeds in Slyter (1974) as optimal for efficiency of microbial the level of intake, ability to select the diet and growth, and also higher than the 200 mg / ml reported digesta flow kinetics under grazing conditions, as by Mehrez et al. (1977) as optimal for the maximum well as to explain the mechanisms involved.

Comparative digestibility of fresh herbage cut at two maturity

Acknowledgements

stages by two breeds of sheep. Anim. Sci. 58, 452–453 (abstr.). Givens, D.I., Moss, A.R., 1994. Effect of breed, age and body M.J. Ranilla gratefully acknowledges receipt of a _{condition of sheep in measurement of apparent digestibility of}

´

scholarship from the Ministerio de Educacion y dried grass. Anim. Feed Sci. Technol. 46, 152–162. Goering, H.K., Van Soest, P.J., 1970. Forage fiber analysis Ciencia of Spain. Financial support for this work

(apparatus, reagents, procedures and some applications). Agri-through a C.I.C.Y.T. Project (AGF94-0026) and by

cultural handbook no. 379. Agricultural Research Service, an European Union Project (AIR CT92-0646) is _{United States Department of Agriculture, Washington, DC.} gratefully acknowledged. Thanks are given to the Hunter, R.A., Siebert, B.D., 1985. Utilisation of low-quality

roughages by Bos taurus and Bos indicus cattle. 1. Rumen Meteorological Office from the Escuela Superior y

digestion. Br. J. Nutr. 53, 637–648.

´ ´

Tecnica de Ingenierıa Agraria of the University of

Jung, H.G., Allen, M.S., 1995. Characteristics of plant cell walls ´

Leon (Spain) for the kindly provision of the tempera- _{affecting intake and digestibility of forages by ruminants. J.} tures registered during the experimental period. _{Anim. Sci. 73, 2774–2790.}

´ ´ ´ ´ ´

Lavın, M.P., Mantecon, A.R., Giraldez, F.J., Mencıa, J.S., Dıez, P., 1994. Sheep production in north central Spain, current situation. In: Gibson, A., Plamant, J.C. (Eds.), The Study of

References _{Livestock Farming Systems in A Research and Development}

Framework, Pudoc, Wageningen, pp. 202–206.

´ ´

Amor, J.J., 1994. [Diurnal patterns of intake and rumination in Mantecon, A.R., Jaramillo, E., Frutos, P., Lavın, P., 1995. Effect sheep as affected by different factors]. Doctoral Thesis, Uni- of sward height and sheep breed on sward composition during

´

versity of Leon, Spain. summer grazing period. Anim. Sci. 60, 532 (abstr.).

Arnold, G.W., 1981. Grazing Behaviour. In: Morley, F.H.W. (Ed.), McAllister, T.A., Bae, H.D., Jones, G.A., Cheng, K.J., 1994. Grazing animals (World Animal Science, B1), Elsevier Sci- Microbial attachment and feed digestion in the rumen. J. Anim. entific Publishing Company, Amsterdam, pp. 79–104. Sci. 72, 3004–3018.

Association of Official Analytical Chemists, 1990. Official meth- McDonald, P., Stirling, A.C., Henderson, A.R., Dewar, W.A., ods for analysis of the Association of Official Analytical Stark, G.H., Davie, W.G., Macpherson, H.T., Reid, A.M., Chemists, 15th ed. Association of Official Analytical Chemists, Slater, J., 1960. Studies on ensilage. Technical Bulletin, No.

Washington, DC. 24. Edinburgh School of Agriculture, pp. 1–83.

Barthram, G.T., 1986. Experimental techniques: the HFRO sward McManus, W.R., 1981. Oesophageal fistulation technique as an aid stick. Biennial Report. Hill Farming Research Organisation to diet evaluation of the grazing ruminant. In: Wheeler, J.L.,

1984–85, pp. 29–30. Mochrie, R.D. (Eds.), Forage Evaluation. Concepts and

Tech-Beever, D.E., Losada, H.R., Cammell, S.B., Evans, R.T., Haines, niques, Griffit Press Ltd, Australia, pp. 249–260.

M.J., 1986. Effect of forage species and season on nutrient Mehrez, A.L., Ørskov, E.R., McDonald, I., 1977. Rates of rumen digestion and supply in grazing cattle. Br. J. Nutr. 56, 209– fermentation in relation to rumen ammonia concentration. Br.

225. J. Nutr. 38, 437–443.

Berggren-Thomas, B., Hohenboken, W.D., 1986. The effects of Mertens, D.R., 1977. Dietary fiber components, relationship to the sire-breed, forage availability and weather on the grazing rate and extent of ruminal digestion. Federation Proc. 36, behaviour of crossbred ewes. Appl. Anim. Behav. Sci. 15, 187–192.

217–228. Olson, K.C., Caton, J.S., Kirby, D.R., Norton, P.L., 1994.

In-Brougham, R.W., 1955. A study in rate of pasture growth. Austr. J. fluence of yeast culture supplementation and advancing season

Agric. Res. 6, 804–812. on steers grazing mixed-grass prairie in the northern great

Cruickshank, G.J., Poppi, D.P., Sykes, A.R., 1992. The intake, plains. II. Ruminal fermentation, site of digestion, and micro-digestion and protein degradation of grazed herbage by early- bial efficiency. J. Anim. Sci. 72, 2158–2170.

weaned lambs. Br. J. Nutr. 68, 349–364. Ørskov, E.R., McDonald, I., 1979. The estimation of protein Dhanoa, M.S., 1988. On the analysis of dracon bag data for low degradability in the rumen from incubation measurements degradability feeds. Grass and Forage Sci. 43, 441–444. weighted according to rate of passage. J. Agricul. Sci.,

Cam-´

France, J., Thornley, J.H.M., Lopez, S., Siddons, R.C., Dhanoa, bridge 92, 499–503.

M.S., Van Soest, P.J., Gill, M., 1990. On the two-compartment Ørskov, E.R., Hovell, F.D.DeB, Mould, F.L., 1980. The use of the model for estimating the rate and extent of feed degradation in nylon bag technique for the evaluation of feedstuffs. Trop. the rumen. J. Theoret. Biol. 146, 269–287. Anim. Prod. 5, 195–213.

´ ´ ´ ´

Garcıa, M.A., Isac, M.D., Aguilera, J.F., Molina Alcaide, E., Ranilla, M.J., Carro, M.D., Valdes, C., Giraldez, F.J., Lopez, S., 1994. Rumen fermentation patterns in goats and sheep pastures 1997. A comparative study of ruminal activity in Churra and from semiarid Spanish lands unsupplemented or supplemented Merino sheep offered alfalfa hay. Anim. Sci. 65, 121–128.

´ ´ ´

with barley grain or barley grain-urea. Livest. Prod. Sci. 39, Ranilla, M.J., Lopez, S., Giraldez, F.J., Valdes, C., Carro, M.D.,

81–84. 1998. Comparative digestibility and digesta flow kinetics in

´ ´ ´ ´

´ ´

Revesado, P.R., Mantecon, A.R., Frutos, P., Gonzalez, J.S., 1994. Statistical Analysis System Institute, 1997. SAS companion for Comparative studies of diet selection by Churra and Merino the Microsoft Windows Environment, version 6. SAS Institute genotypes grazing on a hill shrub community. In: Lawrence, Inc., Cary, NC.

T.L.J., Parker D.S., Rowlison, P. (Eds.), Livestock Production Steel, R.G.C., Torrie, J.H., 1980. Principles and Procedures of and Land Use in Hills and Uplands. Occasional publication of Statistics, second edition, McGraw-Hill, New York.

the British Society of Animal Production, no 18, pp. 109–110. Stewart, C.S., 1977. Factors affecting cellulolytic activity of Satter, L.D., Slyter, L.L., 1974. Effect of ammonia concentration rumen contents. Appl. Environ. Microbiol. 33, 497–502.

´ ´ ´ ´

on rumen microbial protein in vitro. Br. J. Nutr. 32, 199–208. Valdes, C., Mantecon, A.R., Giraldez, F.J., Bermudez, F.F., 1995. Silanikove, N., Tagari, H., Shkolnik, A., 1993. Comparison of rate Herbage intake by Churra ewes grazing at two different sward

of passage, fermentation rate and efficiency of digestion of heights. J. Anim. Feed Sci. 4, 1–9.

high fiber diet in desert Bedouin goats compared to Swiss Van Soest, P.J., 1994. Nutritional Ecology of the Ruminant, Saanen goats. Small Ruminant Res. 12, 45–60. second edition, Cornell University Press, Ithaca, New York.