Abstract:
396 Proc. Aust. Soc. Anim. Prod. Vol. 17 THE EFFECT OF POST-WEANING GROWTH ON THE FERTILITY OF BRAHMAN CROSS HEIFERS G. FORDYCE*, R.G. HOLROYD*_, T.A. JAMES*_ and D.J. REID** In the dry tropics of northern Australia heifers are generally weaned midyear at about six months of age and experience two dry seasons and a wet season prior to first mating at two years of age when only 60% are likely to conceive (Entwistle 1983). Pre-mating liveweight (PMLW) explains much of the variation in conception rate, but year effects explain further variation (Rudder et al. 1985). three (n=337) rates I rates F2 and later generation, l/2 and 3/4 Brahman cross heifers were mated for months as maiden 2-year-olds (n=674) and then as lactating 3-year-olds on Swan's Lagoon Research Station in the dry tropics. Seasonal growth between weaning and mating were analysed for their effects on conception using logistic models that also included PMLW, genotype and year. Only PMLW and year influenced conception rates (PCO.001). An example of the year effect can be seen from the 95% confidence interval of O-59-0.72 for the probability of conception for 250 kg two-year-old heifers. When year was ignored in the analyses an interaction between PMLW and growth rate in the first dry season (GRDl) became significant for two-year-old heifers (PCO.01); higher postweaning growth rate boosted the probability of conception particularly at low pre-mating live weights (Fig. 1). There was no evidence of this effect for threeyear-old lactating cows with only PMLW influencing conception rates (Fig. 1). Fig. 1. PMLW and GRDI effects on conception rates of Brahman cross heifers (i) Two years:- GRDl=O.lkg/day;--- GRDI =-O.lkg/day (ii) Three years:=-.. This finding suggests that the year effect on the probability of conception in two-year-old maiden Bos indicus cross heifers is explained to some degree by their growth in the dry season after weaning. Thus post-weaning dry season supplementation for heifers in the dry tropics may be important, not only for survival, but also for improved fertility as a two-year-old. ENTWISTLE, K.W. (1983). Australian Meat Research Committee Review No. 43. Aust. J. Exp. Agric. 25:489. RUDDER, T-H., SEIFERT, G.W. and BURROW, H.M. (1985). * Qld DPI, Swan's Lagoon, Millaroo, Via Ayr, Qld., 4807 ** Qld DPI, PO Box 1085, Townsville, Qld., 4810 11 Present address: Qld DPI, PO Box 6014, Rockhampton Mail Centre, Qld., 4702 _ Present address: Qld DPI, Brigalow Res Stn, MS 586, Theodore, Qld., 4719
