Some observations on the birth weight of beef cattle.

Abstract

SOME OBSERVATIONS ON THE BIRTH WEIGHT OF BEEF CATTLE G. W. SEIFERT* and J. F. KENNEDY* Summary Birth weights of 1908 purebred Herefords, Shorthorns, Brahmans and Africanders, and Fl and F2 generation Africander cross, Brahman cross and British cross calves, collected from 1954 to 1962, were analysed. Heterosis in the Fl was estimated as 13% for Brahman cross, 5% for Africander cross, and 3% for British cross. The mean birth weights of the F2 Zebu cross were less than those of the Fl, while in the British cross the reverse applied. In the Brahman cross, large sex differences appeared in the Fl, whereas no sex difference was found in the F2 generation. I. INTRODUCTION Crossbreeding of British bred cows and Zebu or Zebu cross bulls has increased rapidly in the beef raising areas of Northern Australia. The cross-breeding programme at the National Cattle Breeding Station, 'Belmont ', (Kennedy and Turner 1959) based on Africander, Brahman, Hereford and Shorthorn crosses, produces material which is comparable with many commercial undertakings. The first generation Zebu x British (Fl) have definite advantages over the British breeds. However little info,rmation is available on the merits of the second generation Zebu x British (F2). To compare post-natal growth between Fl 's and F2' the years would have s, to be directly comparable. Pre-natal growth is not affected by year variations and thus birth weight may be a useful indication of the true genetic potential of the cross-bred cattle. Effects of breed type and generation upon birth weight are examined in this paper. II. MATERIALS' AND METHOD S Calves representing the pure parental British breeds were born from 1954 to 1958, the Fl 's from 1954 to 1959 and F2' from 1957 to 1962. The analysis s showed that effect of year of birth is not significant. In the Fl `s, three year old heifers dropped calves 2.3 kg lighter than others and this has been corrected for in the data which are summarised in Table 1. No correction for sex has been applied because the sex effects differ in different groups; the means in Table 1 are based on data with sex ratios varying from 0.96 to 1.00. Birth weights for pure Brahmans and Africanders were few in number and were therefore not used. Heterosis in the crossbreds has been conservatively estimated by comparison with pure British calves, which represent the parent with higher birth weight and with the same maternal effect as Fl's (Ellis, Cartwright and Kruse 1965). * Division of Animal Genetics, C.S.I.R.O., National Cattle Breeding Station, 'Belmont', Rockhampton, Queensland. 257 TABLE 1 Mean birth weight a:nd coefficient of variation of breeds ad crosses III. RESULTS AND DISCUSSION Analysis of variance of Fl and F2 birth weights (Table 2) shows significant effects of sex, breed x generation, and breed x generation x sex. The breed x generation interaction is illustrated in Table 4. The Fl heterosis was high ( 13 % ) in Brahman cross, less (5% ) in Africander cross, and small (3 % ) in British calves. F2 Zebu crosses showed little advantage over pure British but birth weights of F2 British crosses were actually higher than Fl' (Table 1). s The breed x generation x sex interaction is seen in Table 5 which shows that the sex difference was lower in F2' than Fl' in all breeds but was strongly s s affected by generation in the Brahman cross. TABLE 2 Analysis of vmiwnce of birth weight j- The three 'breeds' are Africander cross, Brahman cross and British. * P<O.O5. **: P<O.Ol. 258 TABLE 3 t test of mean birth weights of directly colmpara:ble zebu cross Fl and F2 genIerations * P<O.Ol. ** P<O.OOl No. of animals in parenthesis. TABLE 4 Percentage advantage of Fl and F2 over mean of pure bred British calves (Heterosis), and advantage of Fl over F2 TABLE 5 Percentage advantage of male over female within generations ( To) Analysis of a portion of the data in which year of birth and age of dam s were strictly comparable in Fl 's and F2' led to the same conclusions as did analysis of the whole data (Table 3). The increase in the coefficient of variation from Fl to F2 seems negligible (Table 1) . The explanation for this may be that the effects of the genotype of the foetus and the maternal environment are confounded. These results indicate that the gain in birth weight of first generation Zebu crosses is large but is rapidly loxst in subsequent generations. This situation may be paralleled to some extent in other aspects of performance, and decline in heterosis must be counteracted by selection pressure. IV. REFERENCES E K , G. F. Jr., C ARTWRIGHT , T. C. and KRUSE, W. E. (1965). Heterosis for birth weight in Brahman Hereford crosses. Journal of Animal Science 24: 93. ENNEDY , J. F. and T URNER , H. G. (1959). A project on genetics of adaptation in cattle. Aims and methods. C.S.I.R.O., Australia, Division of Animal Health and Production, Divisional Report No. 8 Ser. (S.W.-3). LLIS 259