STAGE OF LACTATION EFFECTS, REPEATABILITIES AND INFLUENCES ON WEANING WEIGHTS OF YIELD AND COMPOSITION OF MILK IN BEEF CATTLE

Abstract

Yield and composition of milk was measured on 270 cows with up to three parities each. Several combinations of breeds were included with all first parity females nursing Angus-sired calves and second and third parity females nursing Simmental-sired calves. Milk yields and compositions were obtained at 6, 14 and 22 wk postcalving. Yields were measured by both calf-nursing and machine-milking techniques. Milk yields were similar throughout lactation in first parity but declined over stages of lactation in parities two and three. No interactions between breed group of cow and stage of lactation were found. Milk fat percentages were found to be highest in early lactation, while protein percentages increased during lactation and lactose percentages remained constant. Milk yields increased from first to second parity, but were similar for second and third parities. Composition of milk was similar in all parities. Cows classified as being of dairy breeding had the highest milk yields, Jersey crosses were intermediate, while Charolais, British breeds and their crosses were lower and similar in milk yield. Jersey crosses were highest in milk fat percentage. Milk yield estimates were higher for the calf-nursing method than for the machine-milking method. Estimated repeatabilities of total milk yield, milk fat percent, milk protein percent and milk lactose percent were 0.40, 0.36, 0.58 and 0.52, respectively. Milk yields had a significant positive effect on weaning weights of calves. The inclusion of either milk fat percent or milk protein percent as covariates did not increase the percent of variation explained in weaning weights. Key words: Milk yield, beef weaning weights, milk composition, repeatabilities