The analysis of the distribution of conception times in beef heifers

Abstract

A statistical model, developed by Bartlett for the analysis of grouped survival data, is presented as a method for analysis of data on times to conception. It is shown, for 458 15-month-old heifers, that conception times can be described by a Weibull distribution, the parameters of which depend strongly on breed, age and nutrition group. Liveweight at the start of joining accounted for almost all of the variation explained by the factor 'nutrition'. Conception rate was low and delayed in the lightest heifers (<181 kg at the start of joining), but Angus (A) heifers of this weight were more likely to conceive than were Angus x Friesian (AX F) or A x (A x F) heifers. Also, at a given liveweight younger heifers were less likely to conceive. Overall, conception rates were high for heifers > 210 kg at the start of joining. Graphs are presented relating probabilities of conception with breed, age and liveweight at the start of joining.