Postpartum weight and body condition loss and performance of fall-calving cows

A total of 166 mature fall-calving Hereford cows were used to study the effects of weight and body condition losses on the reproduction of cows and on the performance of their calves. This study was conducted before and during the breeding season over a 3-yr period. The cows calved in a good body condition and were assigned to one of the following treatment groups: the MM group to maintain weight from calving (September - October) through breeding (December - January), the LM group to lose up to 10% of the postpartum weight from calving to the beginning of breeding and to maintain the weight during breeding, and the ML group to maintain weitht from calving to the beginning of breeding and to lose 10 - 15% of the postpartum weight during breeding. Weight losses were adjusted by altering daily amounts of protein supplement and forage. Cows in the LM treatment group lost 3, 17 and 6% of their postpartum weight before breeding in yr 1, 2, and 3, respectively, and they had a 10-d longer (P <0.05) interval from calving to first estrus than MM or ML group cows. The percentage of cows with ovarian activity at the beginning of breeding was reduced (37 vs 64%, P <0.01) for LM compared with MM cows. Treatment-by-year interactions were significant for pregnancy rate and ovarian activity at the end of breeding. In Year1, ML cows lost 14% of postpartum weight during breeding and had reduced (P <0.01) pregnancy rates compared with MM or LM cows (50 vs 79 and 88%). Only 41% of ML cows in Year 1 had luteal activity after estrus compared to 93% for MM and 79% for LM cows. During Year 2, LM cows lost 17% of postpartum weight before breeding, while ML cows lost 6% of their postpartum weight before breeding and 11% during breeding. Pregnancy rates tended to be reduced for both LM and ML cows compared to MM cows (53 and 65 vs 87%, P >0.10). The percentage of cows with ovarian activity at the end of breeding was reduced (P <0.05 for the LM treatment group and tended (P >0.10) to be lower for ML than for MM cows. In Year 3, pregnancy rates were greater (89, 84 and 85% for MM, LM and MM, respectively) for all groups. However, the percentage of cows with luteal activity after estrus was greater (P <0.05 for MM (94%) than for LM and ML (64 and 67%). Calf weaning weights (205 d in April) were not significantly influenced by treatment. We conclude that even if cows have adequate energy reserves at calving (good body condition), optimal reproductive efficiency of fall-calving range cows cannot be ensured.