Abstract

A model is constructed to give the probability of conception conditional on the previous calving date in a herd of Angus beef cows. The parameters of the model are the anoestrous period following calving, the length of the oestrous cycle (both independent random variables having normal probability distributions), and the conception rate at each oestrus occurring within the mating period. The anoestrous period is assumed to depend linearly on the calving date. The observed calving distributions of 10 consecutive years in a beef cow herd were used in the model to estimate the optimum herd conception rate for the parameters "anoestrous period" and "regression between anoestrous and calving date". A conception rate at each oestrous during mating of 0.7 was assumed. Anoestrous period was constrained by experiment to between 50 and 90 days. The regression between anoestrous and calving date was constrained to between 0 and -0.5 days per day. These constraints constituted a non-linear programming problem. The optimum values for the anoestrous period under these conditions lay between 60 and 70 days for the 10 years of data analysed. It is shown that the anoestrous period has very little effect on herd in calf rate below 70 days in the calving distributions considered. It is suggested that resources should not be used to reduce the anoestrous period below 70 days. The anoestrous period tended to have a greater effect on herd conception rate if less than 90% of the herd had calved before day 40 of the calving period, and the anoestrous period was longer than 70 days. Beginning mating 7 days earlier generally increased the herd conception rate by about 1% over the 10 years considered.

DF, Waldron, JN Clarke, and AL Rae

Proceedings of the New Zealand Society of Animal Production, Volume 51, , 405-410, 1991