H . Valk et al. Livestock Production Science 63 2000 27 –38
35
late summer cows on N produced significantly
were smaller. One of the factors that influence the
150
less milk than the cows in the other treatments 21.2 magnitude of the effects is stage of maturity. This is
on N versus 24.6 and 24.4 kg milk on N
and emphasised in A
where the differences in chemical
150 450
92
N , respectively. When compared within each
composition and nutritive value were much larger
300
experiment, milk yield on treatment N was sig-
caused by a substantial difference in growing days
150
nificantly P , 0.05 lower than on N Table 4,
for 150N grass Table 2. Also Salette 1982 and
450
except in S . In S cows on N
produced Peyraud and Astigarraga 1998 stated that NDF
91 92
300
significantly less milk than cows on N 21.5
content and d value of grass are more influenced
450 OM
versus 23.7 kg milk. In late summer, cows on N by stage of maturity than by N fertilization.
300
produced significantly more milk than cows on N ,
Nutritive value decreased with decreasing levels of
150
but no difference between N and N
was N application. The reduction of DVE and OEB with
300 450
observed. Yields of FPCM, fat and protein followed decreasing N fertilizer reflected the decrease in CP
the differences in milk yield. Except in S and A
content. The reduction in DVE ranged between 11
92 93
where milk protein content between treatments was and 18 g kg DM and was much smaller than the
significantly affected in a non-consistent way, no reduction in OEB, which decreased between 54
differences in milk constituents between treatments A and 75 S g kg DM. This agrees with
93 91
were observed. Mean liveweight LW of cows in estimates based on nylon bag studies Valk et al.,
S fed N
was significantly higher than of those in 1996; Van Vuuren et al. 1991. The reduction in VEM
91 300
the other groups. In spring, live weight changes were content with decreasing N fertilizer was mainly
positive in contrast to experiment A where live
caused by the reduction in digestible CP which is an
93
weight changes were tended to be negative. arithmetical component in the VEM equation Van
Es, 1978. In A , also the reduction in OM di-
92
gestibility attributed to the decrease in calculated
4. Discussion VEM content.
4.1. Effect of N fertilizer on chemical composition 4.2. Effect of N fertilizer on grass intake and
and nutritive value of grass digestibility
The consequences of the use of older grass in S Voluntary intake is a result of many interactions
92
compared with S was that contents of grass CP,
between the feed, the animal and its environment
91
DVE, OEB, VEM and d were lower and contents
e.g., Minson, 1990; Ketelaars and Tolkamp, 1992.
OM
of grass NDF and sugar were higher in S . The In our experiments, VI of cows fed grass fertilized
92
differences in chemical composition and nutritive with different N levels was compared at the same
value between A and A
were more related to time and place. So, within each experiment, VI was
92 93
differences in growing conditions due to drought, in influenced by differences in the chemical and nutri-
agreement with the research of Deinum 1966. tive value of grass due to differences in level of N
Differences in chemical composition and nutritive fertilizer. Between experiments, VI could also be
value between grass harvested in spring and late affected by animal, climatic or seasonal factors.
summer are usually caused by blooming of grass in In spring, level of N fertilizer had no marked
spring early summer, which does not occur in late effect on VI of cows fed grass, harvested at nearly
summer in temperate regions Minson, 1990. In our similar stages of maturity. This is in agreement with
experiments, no large differences between spring and the literature reviewed by Minson 1990 and
late summer grass were observed probably caused by Peyraud and Astigarraga 1998. Although, d
OM
our strategy of grassland management with a high differed significantly between treatments in a range
frequency of defoliation at a relatively young stage. between 76.6 to 79.9 Table 3, VI was not
The observed effects of a reduction in N fertilizer affected. This agrees with Conrad et al. 1964 who
on chemical composition and d were in agreement
found no relationship between d and VI at d
OM OM
OM
with the results of Deinum 1966 and Wilman and levels higher than 70.
Wright 1983, but the effects in our experiments In late summer, the reduction in N fertilizer from
36 H
. Valk et al. Livestock Production Science 63 2000 27 –38
450 and 300 to 150 kg N ha per year decreased VI. sufficient, even though the OEB intake was negative.
This was observed both when grass was harvested at A negative OEB suggest a deficiency of undegrad-
different stage of maturity A and when grass was able protein in the rumen for optimum microbial
92
harvested at more or less similar stage of maturity synthesis which could reduce rate of OM degradation
A . Similarly to the spring experiments, these and consequently VI Forbes, 1995. However, this
93
differences in VI could not be explained by the negative OEB intake had no effect on VI probably
observed differences in d and d
. Forbes 1995 because DVE intake exceeded the requirement by
OM NDF
states that the rate of degradation of NDF is a better 20 resulting in high N recycling from the blood
predictor for intake than digestibility. During our urea pool to the NH -N pool in the rumen Tamm-
3
experiments subsamples of grass were incubated in- inga et al., 1994.
situ Valk et al., 1996. From these results it is concluded that a reduction in N fertilizer decreased
4.3. Effect of N fertilizer on milk production the rate of NDF degradation in both spring and late
summer. We can not explain why the positive Most of the variation in FPCM between treatments
relationship between VI and rate of NDF degradation within each experiment could be related to variation
observed in late summer was not observed in the in grass kVEM intake which was calculated by
spring experiments. It can only be speculated if in multiplying grass DM intake with VEM content of
spring a possible negative influence on VI attributed grass. Due to the large number of experiments and
to the decrease in the rate of degradation, was the variation within each experiment, it can be
compensated by the markedly increase in sugar demonstrated how FPCM reacts on differences in
content of 150N improving palatability Peyraud and kVEM intake caused by differences in DM intake
Astigarraga, 1998. This phenomenon was probably and or VEM content of grass. Therefore, the relative
not observed in late summer due to the fact that contribution of DM intake intake effect and grass
sugar content was only slightly increased compared VEM content quality effect to the difference in
to the increase in spring. kVEM intake from high to low N treatment was
Based on the critical level of 140 g CP kg DM in calculated. For example, if DM intake decreased
grass for dairy cows to maintain VI Peyraud and from 17 to 16 kg d and grass VEM content de-
Astigarraga, 1998, the low CP content of 150N creased from 950 to 900, the difference in kVEM
grass in S 131 g kg DM was probably just
intake was 1.75 5 17 3 0.95 2 16 3 0.90. The
92
Fig. 2. The amount of FPCM produced per difference in kVEM intake in relation to the relative contribution of VEM content of grass in the difference in kVEM intake between treatments within each experiment.
H . Valk et al. Livestock Production Science 63 2000 27 –38
37
relative contribution
of DM
intake was
51 Acknowledgements
[ 5 17 2 16 3 0.900 1.75 3 100] and the contribu- tion of VEM content was 49 5 100 2 51.
The authors acknowledge the assistance of a large In spring, the reductions in kVEM intake from
number of students who participated in the experi- high to low N treatment were mainly due to a
ments, the farm staff for taking care of the animals, reduction in VEM content Table 2, whereas in late
and the laboratory staff who performed a large summer these differences were mainly caused by a
numbers of chemical analyses. The experiments were reduction in DM intake Table 3. The relative
financed by the Ministry of Agricultural, Nature contributions of VEM content to the reductions in
Management and Fisheries and by Dutch Fund for kVEM intake between treatments were related to the
Manure and Ammonia Research FOMA. observed difference in FPCM production expressed
as kg FPCM per kVEM difference Fig. 2. This figure clearly demonstrates a decrease in the re-
References
sponse on FPCM when the relative contribution of VEM content to the difference in kVEM intake
Conrad, H.R., Pratt, A.D., Hibbs, J.W., 1964. Regulation of feed
increases. So, the calculated difference in VEM
intake in dairy cows. 1. Change in importance of physical and
content of high and low fertilized grass must be
physiological factors with increasing digestibility. J. Dairy Sci.
smaller based on the observed response in FPCM
47, 54–62. Deinum, B., 1966. Climate, nitrogen and grass. I. Research into
production.
the influence of light intensity, temperature, water supply and
Within each experiment DVE intake increased at
nitrogen on the production and chemical composition of grass.
higher levels of N fertilizer. However, for all treat-
Meded, L.H. Ed., Wageningen, pp. 1–91, 66–11.
ments in all experiments, DVE intake was always
Forbes, J.W., 1995. Voluntary Food Intake and Diet Selection in
more than 20 above DVE requirement Tamminga
Farm Animals, CAB International, Wallingford. Genstat 5 Committee, 1993. Genstat 5 Release 3 Reference
et al., 1994. So, it cannot be expected that differ-
Manual, Oxford University Press, Oxford.
ences in DVE intake influenced milk yield or milk
Ketelaars, J.J.M.H., Tolkamp, B.J., 1992. Toward a new theory of
composition. It seems also unlikely that the negative
feed intake regulation in ruminants. 1. Causes of differences in
OEB of 150N grass in S influenced milk per-
voluntary feed intake – critique of current views. Livestock
92
formance because also for this treatment DVE was
Prod. Sci. 30, 269–296. Meijs, J.A.C., 1981. Herbage intake by grazing dairy cows. In:
fed above requirement and DM intake was main-
Agric. Res. Rep. Vers. Landbouwk. Onderz, Pudoc, Wagen-
tained.
ingen, p. 909. Minson, D.J., 1990. Forage in ruminant nutrition. Academic Press,
San Diego, CA, 483 pp.
5. Conclusions
