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Herbage Intake of Grazing Dairy Cows: 3. Effects of Herbage Mass, Herbage Allowance andConcentrate Feeding on the Herbage Intake of Dairy Cows Grazing on Mid-Summer PastureAuthor(s): G. StakelumSource: Irish Journal of Agricultural Research, Vol. 25, No. 2 (Aug., 1986), pp. 179-189Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25556147 .
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Ir. J. agric. Res. 25: 179-189, 1986
Herbage Intake of Grazing Dairy Cows
3. Effects of herbage mass, herbage allowance and concentrate feeding on the herbage intake of dairy cows grazing on mid-summer pasture
G. Stakelum
An Foras Taluntais, Moorepark Research Centre, Fermoy, Co. Cork
Abstract
Forty spring-calving dairy cows were allocated to four grazing treatments (two levels of daily
herbage allowance with and without concentrate feeding). The treatments were imposed for
16 days in August 1984. Daily herbage allowances based on sampling to ground level were
24 and 16 kg dry matter/head. Concentrates were individually fed once daily to two groups at 4.5 kg/head. Daily intake of concentrate organic matter was 3.8 kg/head. Daily herbage
intake was measured by a pasture sampling technique.
Herbage mass increased from 3,394 to 4,319 kg organic matter/ha during the experiment. This was associated with significantly increased herbage intakes of 0.26 and 0.11 kg/100 kg increase in herbage mass for the high and low herbage allowance groups, respectively. The increased herbage intake at higher herbage mass was associated with more efficient grazing as sward stubble mass was unaffected. The effect of concentrate feeding on daily herbage intake was associated with reduced grazing efficiency and increased sward stubble mass.
Concentrate feeding reduced herbage intake by 0.68 and 0.33 kg/kg of concentrate organic matter fed. Increased herbage allowance increased herbage intake by 0.17 and 0.50 kg per
kg of allowance at low and high herbage mass, respectively. There was no interaction between herbage allowance and concentrate feeding on the
production of milk or milk protein. Herbage allowance had no effect on either the production
of milk or milk protein. Concentrate feeding increased milk and milk protein by 0.50 and 0.02 kg/kg of concentrate organic matter fed, respectively.
Introduction It is generally agreed that concentrate feeding
may be economically justified in situations where herbage intake is restricted. Herbage
intake restrictions are likely to occur where
herbage is scarce due to slow growth or where
intensive grazing pressures are utilised. There
is little information in the literature on whether concentrate feeding might
supplement rather than substitute for herbage in these situations.
The substitution of concentrate for herbage is influenced by the digestibility of the
herbage (Holmes and Jones, 1964; Leaver,
179
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180 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 25, NO. 2, 1986
1976), type of supplement (Umoh and
Holmes, 1974), the season of the year
(Leaver, Campling and Holmes, 1968) and
the level of concentrate feeding (Hijink et al,
1982). Additionally, the level of herbage intake has a major influence on the
substitution of concentrate for herbage (Meijs and Hoekstra, 1984). Previous experiments
with grazing dairy cows in the spring and autumn found that the effect of concentrate
feeding on herbage intake did not interact
significantly with herbage allowance
(Stakelum, 1986a, b). Additionally, increased
herbage mass was found to be significantly
associated with increased herbage intake and
a significant interaction was found between
herbage mass and herbage allowance
(Stakelum, 1986b). The present experiment was carried out on
a mid-summer aftermath sward to study the
effects of concentrate supplementation and
herbage mass at different herbage allowances
on the substitution rates of concentrate for
herbage. The investigation of these effects in
mid-summer was considered necessary as
sward structure in summer is often very
different from that existing in spring and also the autumn investigation (Stakelum, 1986a)
was carried out on a sward which was very
high in herbage mass (5100 kg organic matter/ha). Herbage mass was lower during
the earlier part of the experiment and had
increased substantially for the latter part. The
effects of herbage allowance and concentrate
supplementation could therefore be followed
at the different herbage masses.
Experimental Experimental design
Two levels of daily herbage allowance of 16
(L) and 24 kg (H) dry matter/head and 0 (U) and 3.9 kg (S) of concentrate dry matter/head/
day were compared in a 2 x 2 factorial
design. The daily herbage allowances were
measured by cutting herbage to ground level.
The concentrate was fed individually to cows
in one feed daily immediately after the
morning milking. The treatments were
applied for 16 days starting on August 10 and
finishing on August 26.
Animals
Forty spring-calving dairy cows were divided
into ten blocks of four cows each, on the basis
of their previous 3 weeks' milk yield, calving date and lactation number. Animals were
allocated at random from each block to the
four treatments. No first parity animals were
used and they were selected from a larger
group of cows which were rotationally grazed
around paddocks without supplementary feeds. The principal characteristics of the four
groups are shown in Table 1 for the 3 weeks
prior to the start of the experiment.
TABLE 1: Main pre-experimental characteristics of the four treatment groups grazing mid-summer herbage at 24
(H) or 16 kg (L) of herbage dry matter allowance and supplemented with 0 (U) or 3.8 (S) kg of concentrate
dry matter_
Treatment
_UH_UL_SH_SL_SED
Fat (3.6%) corrected milk Yield (kg/head/day) 14.3 15.3 13.9 14.2 0.60 Mean calving datea Jan 26 Jan 28 Jan 31 Feb 6 8.1 Lactation number 4.2 4.6 6.2 5.3 0.88 Condition score 2.6 2.6 2.7 2.6 0.14
Body weight (kg)_549_515_532_536_18.2
aBased on number of days from January 1 to calving date
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STAKELUM: HERBAGE INTAKE OF COWS, 3 181
Feed The concentrate feed was composed of 95 %
barley and 5% molasses. The botanical
composition of the standing herbage was 68 % Lolium perenne, 14% Agrostis stolonifera, 8% Dactylis glomerata, 4% other grasses
(Phelum pratensis, Holcus lanatus and
Festuca rubra) and 6% assorted weeds. The
sward was a regrowth following a silage cut
on May 21 and a series of condition cuts taken between July 3 and July 20 in an attempt to
keep herbage mass more or less constant.
Nitrogen was applied for silage in early April at the rate of 120 kg/ha. After silage, nitrogen was applied at 60 kg/ha and at 45 kg/ha for
subsequent grazing as well as 8 and 33 kg/ha of phosphorus and potassium, respectively.
Grazing management and herbage sampling
Grazing management and herbage sampling for yield before and after grazing were the same as previously reported (Stakelum,
1986b). The mean height of tillers adjacent to a ruler in the sward remaining after grazing
was taken during day 9 to day 13 of the trial.
Forty heights were taken at random in each
plot per day. On day 14 grass was cut to 4.7 and 5.5 cm with an Agria-mowing machine
ahead of the grazing cows. The grass was
frozen and later fed to four male castrate
sheep at the maintenance level of feeding. The
feeding and sampling of herbage and faeces were the same as previously reported
(Stakelum, 1986b).
Animal measurements
Cows were weighed once weekly for 3 weeks before the experiment, three times weekly
during the experiment and once weekly for
3 weeks after the experiment. Milk yield was
recorded on 5 days per week, morning and
evening, for the entire lactation. Milk was
sampled on a successive evening and morning
for each week of the lactation.
Statistical analysis
Analysis of variance was used to examine the
effects of treatments and their interactions.
The daily estimates of herbage intake, concentrate intake and herbage allowance was
used. Regression analysis of daily herbage
intake and herbage mass was carried out and
the effect of herbage allowance and
concentrate feeding on the constants and
slopes of the regression lines was examined.
Results Growth conditions were very poor following the conditioning cuts due to a drought.
However, rainfall occurred shortly before and
during the first few days of the experiment and growth rates improved. There was 3394
kg of herbage organic matter/ha available for
TABLE 2: Composition (g/kg) of the plucked samples taken to stimulate grass grazed by the four treatment groups
during the night and day grazings
Treatment
_UH UL_SH_SL
Night grazing Ash 83.4 84.6 80.0 84.8
Crude protein 198.5 198.2 191.8 180.4
MAD-fibre 184.4 197.0 187.8 175.5
In vitro OMD 758.0 744.0 757.0 757.0
Day grazing Ash 80.2 93.4 73.2 87.4
Crude protein 105.6 95.7 108.8 102.7
MAD-fibre 262.9 274.4 264.5 259.0
In vitro OMD_646.0_656.0_639.0_658.0
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182 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 25, NO. 2, 1986
grazing during the first 8 days (Ml) of the
experiment. This had increased to 4319 kg/ha for the second 8 days (M2).
The composition of the concentrate on a dry matter basis was 31 g ash, 125 g crude
protein, 50 g crude fibre and 829 g of in-vitro
digestible organic matter per kg. The concentrate contained 865 g of dry matter per
kg fresh weight. The composition of the
herbage samples taken from the top of the swards was different between the night and
day grazings (Table 2). Crude protein content decreased and modified acid detergent fibre content increased. In-vitro organic matter
digestibility decreased between grazings by approximately 100 g/kg digestible organic
matter. The four groups of cows selected
herbage of a similar composition. The
composition of the herbage fed to the sheep indoors is shown in Table 3. The two
herbages were almost similar in composition and similar to those in Table 2. The in vivo
digestibility co-efficients were about 35 g/kg
higher than the in vitro digestibility co efficients.
There were no concentrate refusals by any cows during the experiment. Intake of
concentrate organic matter was 3.8
kg/head/day. Realised daily herbage organic matter allowances were 21.9 and 14.6 (SE
0.04) for the high and low herbage allowance
groups, respctively, and showed no variation
over the 16 days. Table 4 shows the significances of the main
effects of the treatments and their interactions.
Increased herbage allowance and concentrate
feeding decreased grazing efficiency when assessed as the proportion of herbage harvested. Increased herbage allowance
increased herbage and total intake while concentrate feeding reduced herbage intake
but increased total intake. Increased herbage mass increased grazing efficiency and herbage and total intake. Sward stubble mass and
sward stubble height were increased by increased herbage allowance and concentrate
TABLE 3: Composition (g/kg) of the herbage cut at 5.5 and 4.7 cm and fed indoors to four male castrate sheep
Cutting height (cm)
_515_4/7_SED
Ash 88.0 91.9
Crude protein 175.4 174.6 MAD-fibre 255.9 247.9
In vitro OMD 726.0 721.0 In vivo
OMD_752.7_765.1_4j*_ In vivo values are not significantly different (p<0.05)
TABLE 4: Significances of the main effects of treatments and their interactions
Main effects3 Interactions
HA CS_HM HAXCS HAxHM CSxHM
Sward stubble mass *** *** NS * NS NS Sward stubble heightb
*** ** - NS - -
Grazing efficiency *** *** *** NS ** NS
Herbage intake *** *** *** * *** NS Total
intake_***_***_***_*_***_NS
aHA = herbage allowance; CS = concentrate feeding; HM =
herbage mass
bAt high herbage mass (M2) only
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STAKELUM: HERBAGE INTAKE OF COWS, 3 183
TABLE 5: Effect of herbage allowance and concentrate
feeding on sward stubble height at high herbage mass
_(cm)_
Herbage Concentrate allowance feeding SED
H 5.8 U 4.8 0.19 L
4.3_S 5.3_
feeding. Herbage mass had no effect on sward
stubble mass. There was no three-way
interaction between the effects of the
treatments. But there was a strong tendency
for the effects of herbage allowance and
herbage mass on sward stubble mass to
interact with one another (p<0.13).
Table 5 shows the effects of herbage allowance and concentrate feeding on sward
stubble height. Only the main effects of
herbage allowance and concentrate feeding are shown as these effects did not interact with
one another. The low herbage allowance
groups grazed 1.5 cm tighter than the high allowance groups while the unsupplemented
groups grazed 0.5 cm tighter than the
supplemented groups. The herbages cut at 5.5
and 4.7 cm, the composition of which is shown in Table 3, simulated the herbages removed in grazing by the H + S and L +
U treatments, respectively. Table 6 shows the individual treatment
means for sward stubble mass and grazing
efficiency at high and low herbage mass. The SH group left the highest amount of herbage after grazing. As sward mass increased, the
H groups increased their grazing efficiency while the L groups did not. Supplementing the H group at low herbage mass (Ml) increased sward stubble mass and reduced grazing
efficiency. However, at high herbage mass
(M2), supplementation increased sward stubble mass and reduced grazing efficiency for both the H and L groups.
Herbage and total intakes for the treatment
groups are shown in Table 7. Increased sward
mass had its greatest effect in increasing
TABLE 6: Effect of treatment on sward stubble mass (kg organic matter/ha) and grazing efficiency (percentage sward mass consumed) at low and high herbage mass
_Treatment_
_ UH_UL SH_SL_SED
Herbage mass Sward stubble mass
Ml 1240b 757de 1686a 841de 100.1 M2 ll^1* 668e 152 la 953cd
Grazing efficiency Ml 62b 79de 50a 72d 2.96
M2_76^_85^_65^_75^_ Values with dissimilar superscripts are significantly different from one another (p<0.05)
TABLE 7: Effect of treatment on herbage and total intake (kg organic matter/head/day) at low and high herbage mass
Treatment
_UH_UL_SH_SL_SED
Herbage mass Herbage intake Ml 13.6^ 11.6de 11.0e 10.6e 0.60
M2 16.6a 12.5cd 14. lb 11.0e
Total intake Ml 13.6cd 11.6e 14.8C 14.3C 0.60
M2_16^_12.5de_17^_14/7;_ Values with dissimilar superscripts are significantly different from one another (p<0.05)
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184 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 25, NO. 2, 1986
herbage and total intake at high herbage allowance. Supplementation reduced the
herbage organic matter intake of the H groups
by 2.6 and 2.5 kg at low and high herbage mass, respectively. The reduction was 1.0 and
1.5 kg for the L groups at low and high herbage mass, respectively, with only the
reduction at high herbage mass being significant. However, the effect of
concentrate on herbage intake was not
different at the two herbage masses (see Table
4). Supplementation increased total intake of the H group significantly at high herbage mass
but increased total intake of the L group at both herbage masses.
Table 8 shows the regression analysis of
daily herbage intake against herbage mass
corrected to an average mass of 4000 kg
organic matter/ha. The relationship between
intake and herbage mass was different
between the allowance groups as indicated by
the difference between the two slopes. The absence of an interaction between concentrate
supplementation and herbage mass is
confirmed by the common slopes of the
equations for each of the allowance groups.
The effect of concentrate feeding on herbage intake, as indicated by the difference between
each pair of constants, was 0.55 and 0.17 kg
of herbage intake reduction per kg of concentrate consumed for the high and low
herbage allowance, respectively. The
regression analysis explained 69% and 64% of the variation in daily herbage intake for the
high and low groups, respectively. For each
100 kg organic matter increase in herbage
mass, daily herbage intake increased by 0.26
and 0.11 kg organic matter for the high and low herbage allowance groups, respectively.
There was no interaction between the
effects of herbage allowance and concentrate
feeding on milk or protein production.
Therefore, only the main effects are shown
(Table 9). Herbage allowance had no effect on the production of either milk or milk
protein for any period. Concentrate feeding
significantly increased the production of milk; and there was a substantial carry-over (2.1 kg/
head/day) of this increased milk production
during the 3-week post-trial period (2) when
feeding of concentrates had been withdrawn.
Milk protein production was also significantly increased by concentrate feeding but dropped
sharply when concentrates were withdrawn.
The milk yield response to concentrates was
0.5 kg/kg of concentrate organic matter fed.
This response increased to 1.23 kg over the
two periods due to the carry-over effect.
Discussion
The coefficient of variation of herbage
intake was 8.2%. This corresponded to a
standard error of the estimate of daily
herbage intake of 1.03 kg organic matter per
cow. This variation is comparable to other
values with previously cut swards (Walters
and Evans, 1979). Meijs (1981) reported that
TABLE 8: Regression analysis of daily herbage intake (kg organic matter/head) as the dependent variable against herbage mass (kg organic matter/ha-4000) as the dependent variable
Treatment_Constant_Slope_SE of
Slope_RSD_r_
UH 15.12 0.0026*** (?0.0005) 1.57 0.83*** SH 13.03
UL 11.93 0.0011*** (?0.0003) 0.72 0.80***
_SL_11.30_
The constants in the H and L equations are significantly different at the 1 % and 5% levels of probability, respectively; the slopes of both equations are significantly different at the 1 % level
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STAKELUM: HERBAGE INTAKE OF COWS, 3 185
TABLE 9: Main effects of herbage allowance and concentrate feeding on fat-corrected (3.6%) milk and protein
yield (kg/head/day) during the experimental (1), post-experimental (2) and combined experimental and post
experimental periods (3)
Herbage allowance Concentrate feeding
_H_L_SED_U_S_SED
Period Fat-corrected milk yield
1 13.0 13.3ns 0.58 12.2 14.1** 0.61
2 12.6 12.5ns 0.63 11.5 13.6** 0.66
3 12.8 12.8ns 0.56 11.8 13.8** 0.59
Protein yield
1 0.43 0.43ns 0.012 0.40 0.46*** 0.012
2 0.41 0.40ns 0.028 0.39 0.42ns 0.028
3_0A2_0.41ns_0.01?_(U9_0.44*_0.019
Superscripts refer to the significance of the difference between the value indicated and its relevant pair
average group intakes can be estimated with
a coefficient of about 6% when aftermath
or previously topped grazed swards are used.
The digestibility of the herbage in the period of low herbage mass (Ml) in this trial would have been equal to or slightly higher than the
values indicated in Tables 2 and 3. The increase in intake for the groups on the high allowances associated with the increased
herbage mass was due to increased grazing
efficiency rates while sward stubble mass
remained unaffected. The association of
increased intake with increased herbage mass
for both allowances is only slightly higher than that previously reported (Stakelum, 1986b). The regression coefficients of 0.0021 and 0.0007 in spring compare with 0.0026 and 0.0011 in the present study for the high and low allowance, respectively.
Jamieson (1975) and Hodgson, Rodriguez Capriles and Fenlon (1977), working with calves in strip grazing experiments, concluded
that intake was not likely to be markedly affected by the mass of herbage per unit area
(after correction for digestibility effects). Combellas and Hodgson (1979) found that intake of cows was lower at high than at low
herbage mass at comparable levels of
digestibility over the range 3790 to 5770
kg/ha. However, the study of intake and
herbage mass in the present trial was not
carried out simultaneously at the two
contrasting herbage masses and therefore the
effect is confounded with time. Prevailing weather was dominated by anti-cyclonic
conditions during the trial. Zero rainfall was recorded for all but 2 days when 21 mm fell.
The absence of any effect of herbage mass
on sward stubble mass in conjunction with the
substantially increased rates of grazing efficiency indicates that the high allowance
groups grazed to a similar sward height or
residue as herbage unit area on offer increased
and thereby increased their intake of herbage. The low allowance groups grazed at high rates of efficiency during both periods of the trial and increased their intake of herbage by a
smaller amount. This indicates that these
groups, due to the severe grazing regime, were grazing close to maximum defoliation
at all times.
As in the previous trial (Stakelum, 1986b) the height and density of the sward pre
grazing were not measured. It is, however,
reasonable to assume that the height of the sward increased with increasing mass of
herbage and that this factor rendered the
herbage more easily accessible to the grazing animal. Meijs (1981) found no effect of higher levels of herbage mass on intake of herbage
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186 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 25, NO. 2, 1986
by cows. By varying the lengths of rest
periods he produced herbages of differing masses and digestibilities; he concluded that
grazing efficiency was unaffected by
increasing herbage mass at constant allowance
levels and that therefore residual sward mass
and the total herbage (kg/ha) removed during grazing were proportional to the initial sward mass. Stockdale (1985) showed that intake of
grazing dairy cows was positively related,
among other variables, to herbage mass. He
concluded that where herbage mass was not
related to herbage quality one would expect
increased intake with increases in herbage mass as cows have access to a greater
proportion of harvestable material, assuming
that herbage below a constant level is
inaccessible regardless of herbage mass.
In the present study, the sward increased
in digestibility from the lower horizons to the
upper horizons unlike the primary spring sward used in the previous study (Stakelum, 1986b). In that study increased sward mass
was associated with increased sward stubble
mass and grazing efficiency (629 vs 710
kg/ha; p<0.05; 77.4 vs 79.8%, p<0.06). The structure of the present sward was quite different from the primary spring sward of the
previous study. The digestibility of the sward increased from 65.0 to 75.4% OMD from the
bottom to the top of the grazed horizon. The
digestibility was higher and uniform
throughout the profile of the spring sward.
The presence of poorly digestible material in the lower part of the present sward may have
inhibited the animals from grazing lower into the profile.
Concentrate feeding depressed herbage intake by 0.68 and 0.33 kg/kg of concentrate consumed at the high and low herbage allowance, respectively. The regression
analysis adjusted these substitution rates to
0.55 and 0.17 kg/kg of concentrate consumed at the high and low herbage allowance,
respectively. Although low herbage intakes were found at the lower herbage masses, there
was no interaction between concentrate
feeding and herbage mass on herbage intake
from either the analysis of variance (Table 4) or the regression analysis (Table 8); this
agrees with previous findings (Stakelum,
1986b).
Figure 1 shows a regression line relating
herbage intakes at zero concentrate intake to
herbage intake at a concentrate intake in the
range of 3 to 4 kg from the present study and other published works (Stakelum, 1986a, b).
Within the range of herbage intakes studied
(11-17 kg), the substitution rates are higher than those reported with grazing cows (Meijs and Hoekstra, 1984) and stall-fed cows
(Hijink et al, 1982) where comparable levels of concentrates were fed. In the latter case,
where concentrate was fed at 6.3 kg, the
substitution rates were comparable to those
predicted from this regression line. Jennings and Holmes (1983) reported an increase in
herbage intake of 1 kg per kg of concentrate
17
| 16 c
-2 * Autumn herbage J= 15 - ? c Spring herbage /
c a Summer herbage jf
I U '
/ T 13 - X CO /
g 12~
x
I ,,- A -
I -o X
r. ...... . 10 11 12 13 14 15 16 17
Herbage intake (kg) at zero concentrate intake CX)
Fig. 1: The effect of concentrate feeding on
herbage intake (Y = 0.80 (? 0.08) X + I.U) RSD = 0.5L R2 = 0.92***
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STAKELUM: HERBAGE INTAKE OF COWS, 3 187
organic matter fed with continuously stocked
dairy cows. No adjustment was made for the
effect of supplement on herbage digestibility. However, in a second trial, small substitution
rates of 0.08 and 0.23 were reported
(Jennings and Holmes, 1984) after correction for the associative effects on herbage
digestibility of concentrate feeding. The lower
substitution rate was related to high daily
yields (> 25 kg), and the higher rate to lower
daily milk yields (> 18 kg), although there was an increase of almost 1 kg of daily concentrate organic matter consumed in the
latter case.
Other workers have reported substitution
rates with grazing dairy cows with
comparable levels of concentrate
supplementation. MacLusky (1955) and Corbett and Boyne (1958) found substitution rates of 0.39 and 0.42-0.58 kg/kg concentrate
dry matter fed respectively. Umoh and Holmes (1974), working with grazing beef cattle supplemented with sugar beet pulp, and Sarker and Holmes (1974), working with dry cows supplemented with concentrates, found
substitution rates of 0.52 and 0.54,
respectively. The substitution rates predicted from the regression analysis of herbage mass
and herbage intake (Table 8) in the present study was 0.55 at the high allowance of
herbage which agrees very well with the above values. The substitution rate of 0.17
predicted for the low herbage allowance
agrees with the value of 0.11 predicted by Meijs and Hoekstra (1984) at low herbage allowance.
The effect of herbage allowance on daily herbage intake was 0.17 and 0.50 kg/kg increase in herbage allowance for low and
high herbage mass, respectively. The value
0.50 agrees well with the previously reported values of 0.45 and 0.50 (Stakelum, 1986a, b). The value of 0.17 falls within the range of those reported by Greenhalgh et al (1966),
Greenhalgh, Reid and Aitken (1967), Combellas and Hodgson (1969) and Le Du et al (1979). A similar strong effect of
herbage allowance on herbage intake has been
reported (Meijs, 1981, 1983, 1984; Mott, 1974). In the experiments of Meijs,
allowances were measured above 4.5 cm and
grazing periods of 3-4 days were utilised. In those of Mott, cutting height was above 4 cm
but the swards were repeatedly grazed and
therefore allowance effects were confounded
with contamination effects. Direct comparison of the effects of herbage allowance on herbage
intake with other experiments is difficult due to a variety of factors such as the cutting
machinery used, milk yield of the cows,
previous nutrition of the cows and the grazing
system employed.
The absence of any effect of herbage allowance on milk or milk protein yield could be due to the short duration of the experiment and to the fact that the high allowance groups (SH and UH) had reduced intakes during the
Ml period. The effect of concentrate feeding on milk yield was substantial (0.50 kg of
milk/kg concentrate organic matter fed).
However, when concentrates were withdrawn
at the end of the experiment, the milk yield differential maintained itself, giving a
response of 1.24 kg of milk per kg concentrate
organic matter fed. These response figures contrast sharply with the mean response rates
of 0.33 and 0.40 kg milk/kg concentrate fed
reported by Leaver, Campling and Holmes
(1968) and Journet and Demarguilly (1979). Gleeson (1980) has reported mean response rates of 0.6 kg of milk in mid-summer and
overall response rates across the grazing season 0.81 kg of milk for strategic supplementary feeding under rotational
grazing. On continuously stocked pastures at
high grazing pressures, Jennings and Holmes
(1983, 1984) have reported responses of 1.1, 0.60 and 0.53 kg of milk per kg concentrate.
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188 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 25, NO. 2, 1986
In these experiments very small substitution
rates occurred with the concentrates.
The results indicated a beneficial effect of increased sward mass on herbage intake.
Concentrate feeding reduced herbage intake
and this reduction was consistent with
previous experiments in this series. The
magnitude of the reduction depended on the initial level of herbage intake. Concentrate
feeding at current prices of 16p/litre of milk
(3.6% fat) and a barley nut at 13p/kg was
profitable when the carryover response rate
of 1.24 kg/kg of concentrate organic matter
fed was used. Returns of around ?600 worth of milk over the 6-week period would accrue
from an investment of around ?500 in the concentrate for a 50 cow herd.
Acknowledgements
The author thanks Mr. Jim Cronin and Mr.
Patrick Dillon for field supervision of animals and sampling and Mr. Joseph O'Dwyer for
laboratory analyses of feed samples. The
author also thanks Dr. John Connolly for
statistical analysis of the data.
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