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* Dairy Production Department, Teagasc, Moorepark Production Research Center, Fermoy, Co. Cork, Ireland
Department of Animal Science, Faculty of Agriculture, University College Dublin, Belfield, Dublin 4, Ireland
INRA, UMR Production du Lait, 35590 St. Gilles, France
Corresponding author:
J. Kennedy; e-mail:
jkennedy{at}moorepark.teagasc.ie.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: genetic merit • concentrate supplementation • grass dry matter intake
Abbreviation key: CI = concentrate intake, DMD = dry matter digestibility, GDMI = grass dry matter intake, GOMI = grass organic matter intake, HC = high concentrate, HM = high merit, LC = low concentrate, LN = lactation number, MC = medium concentrate, MADF = modified acid detergent fiber, MM = medium merit, MP = measurement period, OMD = organic matter digestibility, PBW = preexperimental BW, PD = predicted difference, PI = pedigree index, PMF = preexperimental milk fat, PMP = preexperimental milk protein, PMY = preexperimental milk yield, TDMI = total dry matter intake, TOMI = total organic matter intake
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Ulyatt and Waghorn (1993) and Muller and Fales (1998) emphasized that the limitations to cow productivity in pasture-based dairy systems arise from low voluntary intake of herbage and from nutrient levels that differ from those required by lactating cows. The limitation to intake is especially apparent in early lactation, when cows approach peak lactation and grazed herbage is low in DM and often has high CP content. Milk production responses to concentrate supplementation at pasture depend mainly on grass supply and quality, season of year, stage of lactation, and level and type of supplementation (Leaver et al., 1968; Journet and Demarquilly, 1979; Stakelum et al., 1988). The latter reviews quoted average responses of between 0.4 and 0.5 kg of milk per kilogram of concentrate DM. Milk yield responses were generally much less than would be expected because of substitution of concentrate for grazed grass. However, a number of more recent studies have shown higher responses with higher yielding cows (Bargo et al., 2000; Delaby et al., 2001). Since the mid-1980s, the rate of genetic improvement for milk production in Irish dairy herds has increased rapidly through the importation of North American and European Holstein-Friesian genetics. The objective of the present study was to quantify the effect on nutrient intake and utilization, of increasing early and midlactation concentrate supplementation of first-, second-, and third-lactation Holstein-Friesian dairy cows of medium and high genetic merit.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Animal Groups
Two genetic groups, containing 48 cows each, were used in 1998 (yr 1). The genotypes were selected from two groups of 60 animals each. All cows were in their first lactation in yr 1. First-lactation cows of similar genetic merit replaced cows culled at the end of lactation in 1998 (18) and in 1999 (12). Cows not in calf at the end of the 13-wk breeding season accounted for 90% of cows culled in each of the 2 yr. Other diseases such as digestive disorders, mastitis, and lameness accounted for the remaining 10% of cows culled. The resulting dataset contained 96 first-lactation animals in yr 1, 78 second-lactation and 18 first-lactation animals in 1999 (yr 2) and 72 third-lactation, 12 second-lactation, and 12 first-lactation animals in 2000 (yr 3). Sixty-six animals had records for all three lactations on the same feeding system.
The high merit (HM) cows were selected based on their superior pedigree index (PI) for milk production, while the medium merit (MM) cows were selected on their lesser pedigree index for milk production. Mean PI (and SD) for the HM group were +276 kg of milk (100), +8.9 kg of fat (4.75), +9.7 kg of protein (3.19), -0.03 g of fat/kg (0.086) and +0.01 g of protein/kg (0.035), and the corresponding values for the MM group were +81 kg of milk (95), +3.8 kg of fat (4.95), +4.3 kg of protein (2.59), +0.013 kg of fat/kg (0.099), and +0.031 of protein/kg (0.036). The PI for each cow was calculated as 0.50 x sire predicted difference (PD) + 0.25 x maternal grand sire PD. The PD of the sires and maternal grand-sires were those from the August 2001 international evaluations of the Animal Center, Uppsala, Sweden using the MACE (multiple-trait across-country evaluation) technique, with no Moorepark records included. In 2001, the sires of the cows in both the HM and MM groups had at least 75 daughters in at least 70 Irish herds contributing to these evaluations with a sire reliability of greater than 90%. The average proportion of Holstein Friesian genes in the HM and MM cows were 75 and 65%, respectively; the remaining genes were Friesian.
All animals were on a similar feeding system from calving (mid-January) until mid-April in yr 1, when cows were on average 71 DIM. This consisted of ad libitum grass silage while indoors, ad libitum grass silage by night plus grass by day from early February onwards and grass by day and night from mid-March onwards. Concentrate supplementation was 6 kg/cow per day in two equal feeds. In mid-April, cows were grouped into blocks of three within each genetic group, on the basis of calving date and milk yield, and randomly assigned to one of three levels of concentrate feeding. Mean preexperimental milk yield (PMY) and (SD) values were 28.5 kg of milk (2.17) and 0.9 kg of protein (0.07) for the HM group, and were 23.8 kg of milk (3.10), and 0.8 kg of protein (0.07) for MM group.
Over the total lactation in yr 1, 2, and 3; 436, 348, and 345 kg of concentrate/cow per year, as fed, were offered in the low concentrate feeding system (LC); 806, 781, and 844 kg in the medium concentrate feeding system (MC); and 1422, 1558, and 1641 kg in the high concentrate feeding system (HC), respectively. The MC was the industry norm for seasonal spring calving herds in Ireland (Buckley et al., 2000). No concentrate supplementation occurred in July, August, and September, because grass supply is normally adequate and cows are a later stage in lactation. Supplementation is reintroduced in October, because of decreasing grass supply and reduction in forage intake, the lower DM content, increasing proportion of wastage, fouling, and reduced grazing time. The concentrate supplementation strategy is outlined in Table 1. 
Table 2 shows the chemical composition of the concentrate offered to the cows during each measurement period. The ingredient content of the concentrate (kg/tonne, as fed) was as follows: barley 250, corn gluten 250, beet pulp 250, soybean meal 100, rapeseed meal 100, tallow 10, and mineral plus vitamins 40.
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The cows were offered grass silage ad libitum during the winter indoor period before calving. Postcalving and before turnout to pasture (late-February), all cows were offered grass silage ad libitum and the level of concentrate specified by the feeding system. Postturnout animals were grazed on a rotational management system (Dillon et al., 1995). Pregrazing yields (> 4 cm) were maintained at between 1800 and 2100 kg of DM/ha. A flexible approach to pasture allocation was adopted, with the aim of providing a postgrazing height of 6 to 8 cm for all three feeding levels. Pasture allowances (> 4 cm) (SD) on average were 23.6 (1.81), 22.3 (2.30), and 20.0 (2.32) kg of DM/cow per day for the LC, MC, and HC feeding systems respectively during measurement period 1 (MP1). The corresponding pasture allowances were 24.9 (1.21), 24.6 (2.42), and 23.6 (1.49) kg of DM/cow per day in measurement period 2 (MP2) for the LC, MC, and HC feeding systems, respectively. This was facilitated through weekly monitoring of farmlet grass cover (O’Donovan, 2000).
All concentrate feeding was offered in individual stalls in the milking parlor in two equal feeds each day, except before turnout to pasture, when concentrate was offered in three equal feeds to the HC cows. Cows were milked twice daily at 0700 and 1600 h over the 3 yr of the study. First-lactation cows were given a 10-wk dry period; 8 wk was considered adequate in subsequent lactations.
Animal Measurements
Individual intakes of all cows were estimated on two occasions while at pasture. MP1 was in mid-June in yr 1 and in mid-May in yr 2 and 3; MP2 was in early September in the 3 yr. The intake estimates in MP1 correspond on average to d 138, 97, and 87 of lactation for yr 1 to 3, respectively. In MP2 the intake estimates correspond to d 187, 209, and 206 of lactation, respectively. Individual animal intakes were estimated using the n-alkane technique (Mayes et al., 1986) as modified by Dillon and Stakelum (1989). In yr 1 and 2, each cow was dosed 8 d before fecal collection with slow release CAPTEC n-alkane CRC n-dotriacontane (C32), assay 198, wt/wt, and linear density 145.2 mg/mm (CAPTEC), (NZ Ltd., Auckland). After the 8-d equilibration period, fecal grab samples were collected twice daily, after morning and evening milkings, in the following 6 d. In yr 3, the cows were dosed twice daily (after milking) for a 12-d period with paper filters or bungs (Carl Roth, GmbH and Co. KG, Karlesruhe, Germany) containing approximately 500 mg of C32 each. Fecal grab samples were collected twice daily from each cow (after milking) in the last 6 d. The fecal samples from each cow for each 6-d period were bulked for analysis. Herbage samples were collected manually to represent herbage grazed (following close observation of the grazing animal) after both the morning and evening milking on d 6 to 11 of each measurement period. The ratio of herbage C33 (tritriacontane) to dosed C32 was used to estimate intake. The dietary DM and OM digestibility coefficients were estimated from the concentration of the C35 (pentatriacontane) n-alkane (Dove and Mayes, 1991), assuming a fecal recovery of 0.90 (Dillon, 1993). The n-alkane concentration of the dosed pellets, feces, herbage, and concentrate were determined as described by Dillon (1993). Estimates of daily herbage intake were calculated as follows:
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where Fi, Ci, and Hi are the concentrations (mg/kg of DM) of the natural odd-chain n-alkanes in feces, concentrate, and pasture, respectively, Fj, Cj, and Hj are the concentrations (mg/kg of DM) of the even-chain n-alkane in feces, concentrate and pasture, respectively, Di is the dose rate (mg/d) of the even-chain n-alkane, and Ic is the daily concentrate intake (kg DM/d). Estimates of diet DMD were calculated also from the ratio of feed and fecal concentrations of the natural odd-chain n-alkane C35 as follows:
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where Hi and Fi are the concentrations (mg/kg of DM) of the natural C35 alkane in feed and feces, respectively. The recovery of C35 in the feces was assumed to be 0.90, based on the results reported for dairy cows fed pasture (Dillon, 1993).
During MP1, the LC, MC, and HC cows were offered 0.0, 3.0, and 6.0 kg of concentrate as fed daily, respectively. During MP1, the LC group received 0.5 kg of a high calcined magnesite supplement to prevent grass tetany. During MP2, HC cows were offered 4.0 kg of concentrate as fed daily; LC and MC cows were on grass only. Individual milk yields were recorded on 5 consecutive days per week (Monday to Friday) in the 3 yr. Milk fat, protein, and lactose concentrations were determined in successive morning and evening milk samples once each week using a Fos-let instrument (AS/N Foss Electric; Hillerød, Denmark). Milks were analyzed for total protein (International Dairy Federation, 1993). BW was recorded weekly and BCS (Lowman et al., 1976) once every 3 to 4 wk.
Sward Measurements
Pregrazing herbage yield (above the 4-cm horizon) was determined in each grazing paddock based on four strips of grass (0.95-m wide; 4.5 to 5.5-m long) cut with an Agri mower. The grass from each strip was weighed and sampled, and a sub-sample was dried overnight at 90°C for DM determination. The remaining herbage from the four samples from each paddock were bulked and a subsample taken. This subsample (approximately 100 g) was freeze-dried and used for chemical analyses.
The chemical analyses were in vitro OM digestibility (OMD) (Morgan et al., 1989), DM digestibility (DMD), CP based on Kjeldahl N, modified acid detergent fiber (MADF) (Clancy and Wilson, 1966) and NDF (Van Soest, 1966). A total of 30 pre- and postgrazing sward surface heights (Hutchings, 1991) were recorded in each paddock immediately prior to grazing and immediately post-grazing.
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STATISTICAL ANALYSIS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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where:
| Yijklm | = | observation for each trait on animali,
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| Gi | = | genetic group (i= 1,2),
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| Lj | = | lactation number (j= 1,2,3),
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| Bk(Gi*Lj) | = | effect of blockjwithin genetic groupiby lactation numberjinteraction, (k = 1,2,3,...16),
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| Fl | = | concentrate feeding level (l= 1,2,3),
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| eijklm | = | residual error term.
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Genotype, lactation number, and genotype x lactation number were tested for significance using block (genotype x lactation number) as the error term; feed, genotype x feed, lactation number x feed, and genotype x lactation number x feed were tested using the residual as the error term.
In MP1, linear and quadratic orthogonal polynomial contrasts, embedded in the treatment structure, were used to test for linear and curvilinear trends with concentrate supplementation level (Snedecor and Cochran, 1980). In MP2, concentrate intake (CI) was similar for both LC and MC. Orthogonal contrasts were used to test the effect of the concentrate level of supplementation ((LC+MC)/2 vs. HC) and the carryover effect of MC supplementation in early lactation (MC vs. LC).
An overall multiple regression analysis was carried out on data for the 66 cows, which remained on the study (in the same feeding systems), for the 3 yr to examine the effects on milk production due to PMY, grass DM intake (GDMI), CI, DIM, and lactation number (LN). The effects on GDMI of PMY, CI, pre-experimental BW (PBW) and LN were also analyzed by multiple regression analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The chemical compositions of the herbage selected were similar for all three concentrate-feeding levels; the herbage selected was of high quality, as indicated by the high OMD (organic matter digestibility) and CP values, and the low MADF (modified acid detergent fiber) values (Table 4).
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shows the effect of genotype, concentrate feeding level, and the genotype x concentrate feeding level interaction on milk production, intake and BW in MP1. The HM cows produced significantly (P< 0.001) higher milk yield (+4.4 kg/d), SCM (+3.0 kg/d) (Tyrell and Reid, 1965), fat (+0.16 kg/d), protein (+0.13 kg/d), and lactose (+0.19 kg/d) yields than the MM cows. The MM cows produced milk with significantly higher fat (+2.0 g/kg;P< 0.01), protein (+0.8 g/kg;P< 0.05) and lactose (+0.7 g/kg;P< 0.001) concentrations than the HM cows. The HM cows had significantly (P< 0.001) higher grass OM intake (GOMI) (+1.01 kg/d), total OM intake (TOMI) +0.91 kg/d), grass DM intake (GDMI) (+1.04 kg/d) and total DM intake (TDMI) (0.92 kg/d) than the MM cows. Genotype had no significant effect on diet digestibility coefficients or BW. However, DMI per 100 kg of BW (results not shown), was higher (P< 0.01) for the HM than the MM cows (3.29 vs. 3.14%). Milk, protein, and lactose yields increased linearly with increasing concentrate, and the genotype responses did not differ significantly. The responses of fat and SCM yields to increasing level of concentrate depended on genotype. The response was linear for the HM cows, and it was curvilinear for the MM cows. The effect of concentrate level on milk fat concentration depended on genotype; the response was curvilinear for the MM genotype, and there was no effect with the HM genotype. Milk protein concentration increased linearly with increasing concentrate level; the response did not differ significantly between genotypes. The increase in milk lactose concentration with level of concentrate was significantly greater with the MM than with the HM genotype. The responses of GOMI, GDMI, OMD, DMD, and BW to concentrate level were linear and negative and did not differ significantly between genotypes. Similarly, the responses of TOMI and TDMI to concentrate level were linear but were positive and did not differ significantly between genotypes. Parity had a significant (P< 0.001) effect on milk production and TDMI (Figure 1). The average daily milk yields were 23.6, 31.4, and 34.4 kg/cow, while the average TDMI were 14.9, 17.7 and 20.9 kg/cow per day for LN 1, 2 and 3 respectively.
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shows multiple regression equations predicting milk production and GDMI from a combination of independent variables associated with the 66 cows which remained on the study for the 3 yr. The partial regression coefficient of PMY, CI, GDMI, DIM, and LN on milk, protein, and fat yields were significant. The proportion of the variation explained by the equations ranged from 82% (RSD = 0.082) for protein yield to 71% (RSD = 0.132) for fat yield. The partial regression coefficient relating daily milk yield to an increase of 1 kg of PMY was 0.676. The values were 0.688 and 0.305 for preexperimental milk protein (PMP) and preexperimental milk fat (PMF), respectively. The partial regression coefficients relating daily milk yield to an increase of 1 kg of CI was +1.10 kg of milk, +0.038 kg of protein and +0.032 kg of fat. Similarly, for each kilogram of GDMI, milk yield increased by 0.335. Table 6 also shows the equation predicting GDMI from independent variables PMY, PBW, CI, DIM, and LN. The equation accounted for 68% (RSD = 1.865) of the variation in the GDMI. The partial regression coefficients relating daily GDMI to an increase in 1 kg of PMY, PBW and CI were 0.123, 0.006 and -0.54, respectively.
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shows the effect of genotype, concentrate feeding level, and the interaction concentrate feeding level x genotype on milk production, intake, and BW in MP2. As in early lactation, the HM cows produced significantly (P< 0.001) higher milk (+3.2 kg/d), SCM (+2.2 kg/d), fat (+0.07 kg/d, protein (+0.09 kg/d) and lactose (+0.12 kg/d) yields than the MM cows. The MM cows produced milk of significantly higher fat (+1.5 g/kg;P< 0.01), protein (+0.6 g/kg;P< 0.05) and lactose (+0.7 g/kg;P< 0.001) concentration than the HM cows. The HM cows had significantly (P< 0.01) higher GOMI (+0.92 kg/d), TOMI (+0.91 kg/d), GDMI (+1.01 kg/d), and TDMI (+0.99 kg/d). Genotype had no significant effect on diet digestibility coefficients or BW. Similar to MP1, the DMI per 100 kg of BW was higher (P< 0.01) for the HM than the MM cows (3.24 vs. 3.07%).
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). The average daily milk yields were 20.8, 22.4, and 23.2 kg/cow, while the average TDMI’s were 16.5, 17.4, and 19.8 kg/cow for LN 1, 2, and 3, respectively. Table 8 shows equations predicting milk, protein, and fat yields from independent variables PMY, CI, GDMI, DIM and LN. The proportion of the variation explained by the equations ranged from 72% (RSD = 0.069) for protein yield to 41% (RSD = 0.105) for fat yield. The partial coefficient relating daily milk yield to an increase of 1 kg of PMY was 0.440. The values were 0.428 and 0.251 for PMP and PMF, respectively. The partial regression coefficients relating daily milk yield to an increase of 1 kg of CI was +0.94 kg of milk, +0.037 kg of protein and +0.025 kg of fat. Similarly for each kg of GDMI, milk yield increased by 0.497. Table 8 also shows the equation predicting GDMI from independent variables PMY, PBW, CI, DIM, and LN. The equation accounted for 57% (RSD = 1.788) of the variation in GDMI. The partial regression coefficient relating daily GDMI to an increase in 1-kg of PMY, PBW, and CI were 0.190, 0.007 and -0.444, respectively.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The higher milk production achieved with the HM cows is similar to that previously reported (Bryant and Trigg, 1981; Veerkamp et al., 1994; Buckley et al., 2000). Other studies comparing HM and MM cows show that increases in milk production were not accompanied by a reduction in the milk fat, protein and lactose contents (Bryant and Trigg, 1981; Grainger et al., 1985; Veerkamp et al., 1994). The milk constituent’s reduction in the present study probably reflects the genetic predisposition of the animals to produce milk of differing composition as indicated in their pedigree index. The significantly higher GDMI of the HM cows is similar to that reported previously (Buckley et al., 2000). The selection of cows for higher milk production leads to higher feed intake as a consequence of the genetic correlation between the traits (Van Arendonk et al., 1991). The nonsignificant effect of genotype on OMD and DMD is expected (Holmes, 1988).
The experimental milk yields were a function of PMY (average milk production in lactation wk 5, 6, 7, and 8 of yr 1, when all animals were on the same feeding system), CI, GDMI, DIM, and LN. The response to an increase of 1 kg of PMY was 0.676 kg and 0.440 kg milk daily in MP1 and MP2, respectively. The higher value in MP1 than in MP2 is probably due to the PMY covariable measurement time, being closer to MP1 and also earlier in lactation.
The average milk yield response in MP1 of 1.10 kg of milk per kilogram of concentrate DM intake is higher than that reported previously (Leaver et al., 1968; Journet and Demarquilly, 1979; Stakelum et al., 1987), and this response was linear up to the highest supplementation level. However, more recent studies have shown similar higher milk production responses (Robaina et al., 1998; Bargo et al., 2000; Reis and Combs, 2000; Delaby et al., 2001). Bargo et al. (2000) obtained responses to concentrate supplementation of 1.36 and 0.96 (kg of milk/kg concentrate) for cows grazing at low and high pasture allowances. Robaina et al. (1998) obtained responses ranging from 0.81 to 1.56 accompanied by a linear improvement in protein concentration of 0.36 g/kg per kilogram of concentrate DM varying from 1.8 to 6.7 kg of DM per day. Reis and Combs (2000) obtained responses to concentrate supplementation of 1.0 and 0.86 (kg of milk/kg of concentrate DM) for cows supplemented with 5 and 10 kg of concentrate daily. Walker et al. (2000) concluded that response of FCM of 1.1 kg/kg of cereal based concentrate DM can be achieved when fed twice daily and at rates up to 3 kg of DM/day to dairy cows grazing restricted amounts of paspalum-type pasture in Autumn. Delaby et al. (2001) obtained an average response of 1.04 kg of milk per kilogram of concentrate DM, and this response was linear from a supplementation level of between 4 kg and 6 kg/d. The slightly lower response in MP2 (0.94 kg of milk per kilogram of concentrate DM) is expected because of cows’ lower energy requirement at this time (Journet and Demarquilly 1979). Because the reviews by Leaver et al. (1968) and Journet and Demarquilly (1979), the genetic merit of dairy cows has increased appreciably, and this is evident in the difference in the level of milk production in the studies. Consequently, it is much more difficult to meet the energy requirements of the high yielding dairy cows at pasture.
The negative coefficient for DIM was expected, as maximum milk yield normally occurs between wk 5 and 7 of lactation (Bines, 1976). The magnitudes of the coefficients are very much in line with those expected with normal lactation profiles. The effect of parity was large, especially in early lactation and was similar to that observed previously (Buckley et al., 2000).
In the present study the observed GDMI was a function of PMY, PBW, CI, DIM, and LN. Higher yielding cows have a greater nutrient demand, and this is reflected in increased grass intake (Grainger et al., 1985; Gordon et al., 1995; Buckley et al., 2000). The partial regression coefficients of 0.12 and 0.19 kg increase in GDMI per kg of PMY obtained in the present study are similar to the 0.16 obtained by Bines et al. (1977) and the 0.13 obtained by Conrad et al. (1964). The partial regression coefficient for experimental milk yield varies from 0.43 to 0.47 kg of DM per kilogram of FCM (Stakelum and Connolly, 1987) to 0.25 kg of OM per kilogram of milk yield (Peyraud and Gonzalez-Rodrigez, 2001). Stakelum and Connolly (1987) stated that the theoretical energy requirement for milk production is 0.4 kg of DM per kilogram of FCM. However, Ostergaard (1979) considered milk yield an output factor and did not include milk yield for predicting DM intake. Milk yield is not known at the time of prediction. Consequently Peyraud et al. (1996) prefer to use preexperimental milk yield or potential milk yield for predicting DMI.
The increases of 0.6 and 0.7 kg of GDMI per 100 kg of PBW are lower than that reported previously for experimental BW (Conrad et al., 1964; Stakelum and Connolly, 1987). Models for predicting DMI with primiparous and multiparous cows generally have the lowest change in intake due to difference in BW (Coulon et al., 1989; Kristensen and Kristensen, 1987; Rook et al., 1991). This may be explained by the relatively lower intake capacity of primiparous cows (Ingvartsen, 1994). The intake capacity in the first part of lactation of primiparous cows calving at an age of 2 yr and weighing 500 kg is only around 80% of that of multiparous cows, and only about two thirds of this difference can be explained in BW and milk yield (Jarrige, 1986). If parity is not taken into account, the unexplained part of the difference may be incorporated into the parameter for BW, which is then increased.
The substitution rates, 0.54 and 0.44 kg of grass per kilogram of concentrates, obtained in the present study is lower than that reported previously (Leaver et al., 1968; Stakelum, 1988). This is particularly significant because previous results showed that substitution rates are highest when GDMI and digestibility are high (Stakelum et al., 1988). However more recent studies have shown lower substitution rates (Robaina et al., 1998; Bargo et al., 2000; Reis and Combs, 2000). Robaina et al. (1998), using the n-alkane technique obtained substitution rates of 0.3 and 0.6 kg of DM reduction in pasture intake/kg of DM grain consumed at low and high pasture allowances. Bargo et al. (2000) using the Cr2O3 as fecal indicator obtained substitution rates of 0.26 and 0.55 kg of DM reduction in pasture/kg of concentrate DM consumed at low and high pasture allowances. Faverdin (1991), showed that the substitution rate is lower with high yielding dairy cows when the energy requirements of the animal are not being met. In these situations, concentrate only slightly reduces grass intake and appreciably influences the energy balance of the animals, which results in increased animal performance (Coulon and Remond, 1991). The most limiting nutrient in most grazing situations is energy supply (Muller and Fales, 1998). Concentrate supplementation increases energy intake and thereby increases rumen fermentation and microbial protein synthesis, which in turn contributes to optimize DMI. The positive effect of DIM in early lactation and the negative effect in late lactation are expected given that maximum intake is achieved at about 100 d postcalving (Brown et al., 1977). Bines (1979) observed mean intake values of 13.7, 15.8, and 16.7 kg for first-, second-, and third-lactation cows, respectively. The relative values for first-lactation cows were much larger in the present study especially in early lactation. The probable reason for the difference is that the age at first calving was approximately 24 mo, while the age at first calving is greater in most other systems of milk production.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication February 13, 2002. Accepted for publication April 3, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSSTATISTICAL ANALYSISRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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J. R. Roche, D. P. Berry, and E. S. Kolver Holstein-friesian strain and feed effects on milk production, body weight, and body condition score profiles in grazing dairy cows. J Dairy Sci, September 1, 2006; 89(9): 3532 - 3543. [Abstract] [Full Text] [PDF]
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A. Sairanen, H. Khalili, J. I. Nousiainen, S. Ahvenjarvi, and P. Huhtanen The Effect of Concentrate Supplementation on Nutrient Flow to the Omasum in Dairy Cows Receiving Freshly Cut Grass J Dairy Sci, April 1, 2005; 88(4): 1443 - 1453. [Abstract] [Full Text] [PDF]
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B. Horan, P. Dillon, P. Faverdin, L. Delaby, F. Buckley, and M. Rath The Interaction of Strain of Holstein-Friesian Cows and Pasture-Based Feed Systems on Milk Yield, Body Weight, and Body Condition Score J Dairy Sci, March 1, 2005; 88(3): 1231 - 1243. [Abstract] [Full Text] [PDF]
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J. R. Roche, S. Petch, and J. K. Kay Manipulating the Dietary Cation-Anion Difference via Drenching to Early-Lactation Dairy Cows Grazing Pasture J Dairy Sci, January 1, 2005; 88(1): 264 - 276. [Abstract] [Full Text] [PDF]
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F. J. Mulligan, P. Dillon, J. J. Callan, M. Rath, and F. P. O'Mara Supplementary Concentrate Type Affects Nitrogen Excretion of Grazing Dairy Cows J Dairy Sci, October 1, 2004; 87(10): 3451 - 3460. [Abstract] [Full Text] [PDF]
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—
) in early lactation (a, b, c, P 
0.001).