Abstract:
Animal Production in Australia REPEATABILITY OF WEANING WEIGHTS AND WEIGHT GAINS IN BEEF CATTLE G.W. SEIFERT*, D.C. NICOL** and H. HIGGS*** SUMMARY Repeatabilities of weaning weights from a Belmont Red herd ranged from zero to 0.286 according to the correction factors applied to the data. Repeatability was highest when the data were corrected for sex of calf, year of birth, day of birth and sire. Repeatability of day of birth was 0.309. Correlations between weaning weight and post-weaning gain were very low and ranged from r = -0.085 to r = 0.005. INTRODUCTION Culling cows and calves on the basis of the calves' weaning weight is recommended (Daly 1977). The National Beef Recording Scheme (NBRS) provides its members with estimates of the most probable producing ability (MPPA) of cows based on weaning weight adjusted by the number of calves and a repeatability factor of 0.4 (NBRS Recording Manual). By inference beef producers are encouraged to retain those cows that have high MPPA ratios. Experience with producers has been that they are receptive to this approach. Published estimates of repeatabilities for weaning weights for tropical breeds, or from Australian herds generally are not available. Overseas estimates are therefore used. This paper examines repeatabilities of weaning weight in a small, intensive, well-managed Belmont Red stud herd. MATERIALS AND METHODS Data were collected from the 'Bangalla' to 1980. 'Bangalla' is situated on the Mary Queensland. Mean annual rainfall is 1200 mm April. Mean daily maximum temperatures vary with an annual mean of 26.3OC (Anon 1975). purebred Belmont Red herd from 1977 River, 10 km south of Maryborough, with 70% falling during November to from 29.8OC (January) to 21.2OC (July) Improved tropical pastures stocked at 2.26 beast per ha provided the major portion of the grazing. During winter 4 ha of irrigated rye grass were grazed for one hour daily to supplement the tropical pastures. Calves that had a mean day of birth of October 3 + 37 days were weaned at an average age of 204 days usually in mid-April. Final live weights at 510 days were recorded for 32 bulls, Weaning weights were analysed by least squares (Harvey 1960) to obtain correction factors for years, sex, sire and interactions and linear and quadratic effects for day of birth. The weaning weight data were adjusted for dam age before analyses using the NBRS adjustinents because not all dam ages were represented. Repeatabilities of 117 weaning weights of calves from 61 cows were obtained from intra-class correlations. Estimates of the variance components were obtained from hierarchial analyses of variance by equating the mean squares to the expected mean squares. Correlations between weights and gains of the bulls were obtained by computing covariances and variances within years and pooling these for the overall correlation. * Tropical Cattle Research Centre, CSIRO, Rockhampton, Qld 4701. ** QDPI, Bundaberg, Qld 4670. *** 'Bangalla', Maryborough, Qld 4650. 392 Animal Production in Australia RESULTS AND DISCUSSION The average weaning weight for age (WWA) corrected for dam age was 1.01 + 0.169 kg per day of age. The effects of sire, sex, and years were significant(P < 0.01) while the first order interactions were not. Both linear (bl = 2.9927 x 10V3WWA/day of birth, P < 0.01) and quadratic (b2 = -1.318 x 10B5 WWA/(day of birth)2 P < 0.05) day of birth effects were significant. The repeatabilities of weaning weight following various adjustments are given in Table 1. The repeatabilities of weaning weight and WWA, uncorrected for known fixed effects or corrected for only sex, year or dam agewerevery low. Corrections for sex, year and dam age simultaneously improved repeatability, but the inclusion of day of birth adjustments lowered the repeatability. TABLE 1 Repeatabilities (+ standard error) of weaning weights following various corrections and the repeatability of day of birth The repeatability of day of birth of 0.309 was higher than the repeatability of weaning weight. The significant repeatability of day of birth indicates that cows tend to maintain their order of calving within the calving season. The significant effect of day of birth on weaning weight, its relatively high repeatability and the lowering of the repeatability estimate of weaning weight when the dataarecorrected for day of birth, 'indicates that the repeatability of weaning weight unadjusted for significant day of birth effects is in part due to the repeatability of day of birth. Cows culled on the weaning weight of their calves may therefore in part be culled for day of birth effects. In the bulls, weaning weight for age was very poorly correlated with postweaning gain irrespective of the type of correction. Correlations ranged from r = -0.085 for raw weaning weight to r = 0.005 for WWA corrected for dam age. As would be expected, weaning weight was highly correlated with final weight (r = 0.881, P < O.Ol)andalsoWWAwith final weight for age (r = 0.697, P < 0.01). These correlations were very similar to those reported by Seifert (1975). These limited data indicate that repeatabilities for tropical cattle may be lower than those reported for temperate breeds, but more estimates are required. Therefore the use of weaning weight as a criteria for culling cows may not be very reliable and culling on other production criteria may be more worthwhile. The variability of estimates from the same data also point to the need for adjustments for known environmental effects if cows are to be culled on the weaning weight of their calves. ACKNOWLEDGEMENTS The authors wish to thank Mrs. H. Burrow for her assistance with the analyses. 393 Animal Production in Australia REFERENCES ANON (1975). Climatic Averages, Queensland Department of Affairs,Bureau of Meteorology, Australian Governmen Canberra, 1975. DALY, J.J. (1977). Beef Cattle Husbandry Tech. Bull. No. HARVEY, W.R. (1960). USDA, Agricultural Research Service KILKENNY, J.B. (1968). Anim. Prod. -- (4): 483. 10 SEIFERT, G.W. (1975). Aust. J, Agric: Res. 22: 393. Science and Consumer t Publishing Service, 7, QDPI, 75. , No AR%20-8.