Factors Affecting Growth Curve Parameters of Hereford and Brangus Cows

Abstract

Brody’s model was used to fit individual growth curves to 74 Hereford and 83 Brangus cows, born in 2 consecutive years and located in two distinct New Mexico range environments. The effects of year of birth, ranch, ‘breed, pregnancy status at 43 months of age and their interactions on growth curve parameters, weights and weight estimates were analyzed. Hereford cows were later maturing (P<.01) to slightly heavier mature weights than Brangus cows. Year (P<.05) and ranch (P<.01) affected mature weight in Herefords but not in Brangus, suggesting a greater susceptibility of Hereford mature cows to changes in environment. Rate of maturing was affected (P<.01) by ranch and breed, but not by year. Pregnancy status did not affect any of the growth curve parameters. Goodness of fit of the serial weight measurements varied with the year, ranch and breed. Phenotypic coefficients of variation in body weight decreased in both breeds until a certain age, near first parturition. Herefords were more uniform in weight near maturity than Brangus. Correlations among weights at diverse ages within breeds were comparatively higher for the breed with the slowest growth rate at the given age interval. A phenotypic antagonism between mature weight and rate of maturing was found in both breeds, indicating that earlier maturing heifers would have lighter mature weights than late maturing females.