Characterising lactation curves in heifers of different breeds 1st calved in moderate or low condition

Abstract

Proc. Aust. Soc. Anim. Prod. 1994 Vol. 20 CHARACTERISING LACTATION CURVES IN HEIFERS OF DIFFERENT BREEDS FIRST CALVED IN MODERATE OR LOW CONDITION G.J. SAWYERA, E.J. SPEIJERSB R.J. MORRISA and F.J. COUPAR ADept of Agriculture, Cattle Industries Branch, Bunbury, W.A. 6230 BDcpt of Agriculture, Biometrics Section, South Perth, W.A. 6151 CDcpt of Agriculture, Vassc Research Station, Busselton, W.A. 6280 C SUMMARY Milk yields were measured every 6 weeks from 21220 days after calving in either moderate or low conditioned groups of Angus, Angus x Friesian or Simford first-calf heifers for 3 successive years. Day of peak yield, peak yield and total 220 day yield were calculated by fitting 3 published equations to data on each of the breed x year x calving condition combinations. All equations ranked breed and calving condition combinations similarly for 220 day milk yield, but peak yield and day of peak yield were more variable. The Jenkins equation consistently accounted for more variation in milk yield data (58%) compared with the Wood and Wood-weighted milk curves. Calf weight at 200 days and estimated 220 day milk yield (Jenkins) were highly correlated (r = 0.59, P c 0.001). Keywords: lactation curves, first-calf beef heifers. INTRODUCTION Milk production of beef cattle is economically important because of its direct influence on calf weaning weights, efficiency and costs of production and maintenance (Montano-Bermudez and Nielsen 1990). Data on the milk yield of beef cattle under southern Australian grazing conditions are relatively few (Morgan 1991). Overseas studies have either provided a 'snapshot' view for relative milk yields of cows of various breeds or nutritional treatments (Dawson et al. 1960) or have used lactation curve equations, similar to those for dairy cattle (Hohenboken et aZ. 1992). Few studies have reported the milk yield of first-calving beef heifers though weaning weights of their progeny are often lo-15% less than those of mature cattle. This study evaluates how well 3 published equations fit lactation curves to periodic measurement of milk yield in first-calving beef heifers and examines the influence of breed and the condition at calving. The correlation of milk production ranking with calf growth was also determined. MATERIALS AND METHODS Animals, husbandry, pasture available and supplementary feeding This work forms part of a larger experiment conducted at Vasse Research Station, near Busselton, Western Australia, between 1986 and 1988. The annual rainfall averages 750 mm (range 550-950 mm) with an 8 month growing season for annual pastures (April-November). Angus, Angus x Friesian F2 and Simford F2 heifers were allocated on the basis of liveweight and managed at pasture during spring (about the second trimester of pregnancy) so that there were 2 liveweight/condition groups (moderate and low) within each breed prior to calving. Over all years the weight and condition score (CS, O-5 scale) at first calving for moderate-conditioned Angus, Angus x Friesian or Simford heifers was 401 kg and 2.6, 439 kg and 2.2 and 472 kg and 2.4, respectively. Heifers in low condition were about 40 kg and 0.7 CS units less at calving. Cow and calf groups were rotated around paddocks at a stocking rate of 1 cow-calf unit/l-l ha. Animals were weighed, condition scored and pasture on offer was assessed by a visual technique calibrated by 0.25 rn2 quadrat cuts (Sawyer et al. 1993), every 3 weeks throughout the experiment. Most heifers calved in March before the onset of significant seasonal rainfall (April). Pasture on offer ranged from 850-1500 kg JIM/ha during the months of March-May, increased to llOO1500 kg JIM/ha for the next 3 months, then was consistently above 2000 kg DM/ha during the spring flush in 1986 and 1988. In 1987 pasture on offer was always 30-80% above that in the other 2 years, particularly after the break of season. Lactating first calvers were fed high quality silage at 6-O-10.5 (Mar&April), 5.0-8.0 (May), 3.0-7.0 (June) and O-3.0 kg DM/hd.day (July). The quality of the silage (63-70% DM digestibility and 17.5% crude protein) ensured most animals only sustained minor weight loss during early lactation (range O-O.4 kg/day). Milk assessment Each year 7-10 animals of each breed and from each calving condition group were selected at random 21-35 days after calving for milk assessment, and 5 further measurements were taken, using these 128 Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 animals, every 35-42 days until calves were 180-220 days old. Milk production weigh-suckle-weigh technique (Dawson et al. 1960). First-calvers were separated the late morning before assessment. Calves suckled their dams at 1700 hours, were weighed and allowed to suckle their dams at 0600 hours; weighed again and milk 24 hour production. was measured by a from their calves in separated overnight, weights converted to Lactation curves and analysis The Wood (Wood 1967) Wood-weighted (Rowlands et al. 1982) and the Jenkins (Jenkins and Ferrell 1984) equations were fitted to the milk production data for each breed by condition group in each year. Each model also included a term for individual cow variation. Analysis of variance was used to examine the effects of breed, condition at calving, year and their 2-way interactions on the Jenkins' estimates of peak yield, day of peak yield and total 220 day yield. The year effect was significant, therefore data was not combined across years. The correlation between the Jenkins' estimate of total 220 day yield for each dam and the growth rate of their calf was also determined. RESULTS Lactation curves are presented for Simford heifers calved in moderate condition in 1986 as representative of all curves generated (Figure 1). The shapes of the Wood and Wood-weighted curves differed markedly accordin g to the variability of milk yield measurements for the breed x calving condition x year combinations, whereas the Jenkins equation was a more consistent shape as the curve is forced through the origin (Figure 1). The Jenkins equation fitted the milk yield data more consistently than either of the 2 other equations tested. It accounted for an average of 57.5% of variance (range 3180%) and the significance of the linear regression (log form of the equation) in all cases was P c 0.001. In contrast the Wood and Wood-weighted equations accounted for less than 27% of variance in milk yield data and significant linear regressions were only derived for half the breed x calving condition x year combinations. Descriptive statistics characterising lactation curves for all first-calvers in 1986 are presented in Table 1. The Jenkins equation repeatedly produced a peak in lactation at about day 70 in 1986 whereas peak day of lactation varied more widely for the Wood and Wood-weighted equations. In subsequent years peak day of lactation was later (90-200 days). Peak yield and estimated total 220 day milk yield were generally greater in all combinations analysed if the Wood-weighted equation was used (Table 1). Across all breeds and calving condition groups the Jenkins equation estimated milk yield to 220 days ranging from 648 kg (Angus 1986) to 1496 kg (Angus x Friesian 1987, Table 2). Within any year all equations described a similar ranking in total 220 day production across breed x calving condition cells with Angus first-calvers producing the least milk and Angus x Friesians the most. 129 Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 With the Jenkins curve, breed (P c 0.001) and year (P < 0.001) significantly influenced total 220 day milk yield, the day and level of peak milk yield (Table 2). Condition at calving significantly affected only the total milk yield (P c 0.05). The only significant interactions were for both breed (P < 0.001, shown in Table 2) and condition at calving with year (P c 0.04) on total yield. In 1986 and 1988 heifers calved in low condition produced 9% and 28% less milk respectively, than those calved in moderate condition. In 1987, when feed on offer was 30-80% more, this difference was not apparent. The correlation between weight of the calf at 200 days and total milk yield (220 days) was significant (r = 0.59, P c 0.001) and resulted in a similar ranking of cow productivity based on either calf 200 day weight or 220 day milk production as estimated by the Jenkins equation. Table 1. Milk production variables estimated from lactation curves for all heifers calved in 1986 Table 2. Effects of breed and year on milk production estimated using the Jenkins equation DISCUSSION Under the conditions of this study we found that published equations, several of which were originally used on dairy cattle (Wood 1967; Rowlands et al. 1982), could characterise the lactation curves of firstcalving beef heifers. This enables maximum use of research data which may be included in models of beef production under grazing and supplementary feeding situations, and in breeding and production efficiency studies. The Jenkins equation consistently fitted the milk yield data better than the Wood or Wood-weighted equations. This result contrasts with that of Hohenboken et al. (1992) who used autumn calving Angus and Angus x 1/4Holstein beef cows. They found that all equations ranked cows similarly for milk production and the Wood-weighted equation characterised the lactation curves best, but no 130 Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 details of nutritional management were supplied. The shape of the curves we fitted was not 'typical' of milk production curves most commonly used in the dairy industry. The Jenkins equation showed peak yields at least 10 weeks into lactation, considerably later than peak dairy cow yields, and the general shape of the curve was flatter. Suck@ young calves may not be able to consume all the milk produced in the period studied, and this could account for the better fit of the Jenkins curve to our data. Other work using injections of oxytocin and either machine or hand milkin, Oave higher estimates of production in early-mid lactation cattle, similar 0a to the Wood equations (Morgan 1991, Hohenboken et al. 1992). The 220 day total yield derived by integration of the curve was highly correlated with calf 200 day weight, confirming the suitability of the Jenkins equation to characterise lactation curves in beef cattle run in a mediterranean environment. A feature of this work was the considerable variability encountered both among animals and between breeds and years. Though the same relative rankings for the various breed x calving condition x year combinations were maintained no matter what lactation curve equation was used, there was almost a 3fold difference between highest and lowest 220 day milk yield. Under these conditions average estimates of peak and total milk yield usin, the Jenkins equation were always associated with less variation than 0 with either of the Wood equations. There was a marked effect of year, reflecting excellent annual pasture orowth in 1987 which increased total milk yield by about 50% compared with other years. This was despite supplementary feeding with good quality silage which kept early lactation liveweight loss to a minimum in all years. Pasture quantity in 1987 over-rode any effect on subsequent milk yield from reduced body reserves in heifers of all breeds calved in low condition (CS 1.5-1.7). This finding emphasises how responsive this class of beef breeder can be to good nutrition and confirms their requirement for preferential treatment, especially quality nutrition, for optimum production. REFERENCES DAWSON, W.M., COOK, A.C. and KNAPP, B. JNR. (1960). J. Anim. Sci. 19: 502-8. HOHENBOKEN, W-D., DUDLEY, A. and.MOODY, D.E. (1992). Anim. 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