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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 Author(s): G. Stakelum Source: Irish Journal of Agricultural Research, Vol. 25, No. 2 (Aug., 1986), pp. 179-189 Published by: TEAGASC-Agriculture and Food Development Authority Stable URL: http://www.jstor.org/stable/25556147 . Accessed: 12/06/2014 18:26 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . TEAGASC-Agriculture and Food Development Authority is collaborating with JSTOR to digitize, preserve and extend access to Irish Journal of Agricultural Research. http://www.jstor.org This content downloaded from 195.34.79.49 on Thu, 12 Jun 2014 18:26:38 PM All use subject to JSTOR Terms and Conditions

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

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Page 1: 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

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 .

Accessed: 12/06/2014 18:26

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

TEAGASC-Agriculture and Food Development Authority is collaborating with JSTOR to digitize, preserve andextend access to Irish Journal of Agricultural Research.

http://www.jstor.org

This content downloaded from 195.34.79.49 on Thu, 12 Jun 2014 18:26:38 PMAll use subject to JSTOR Terms and Conditions

Page 2: 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

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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Page 3: 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

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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Page 4: 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

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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Page 5: 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

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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Page 6: 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

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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Page 7: 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

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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Page 8: 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

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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Page 9: 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

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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Page 10: 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

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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Page 11: 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

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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Received December 13, 1985

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