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Reproductive Behavior of Bos Indicus Females in a Subtropical Environment. III. Calving Intervals, Intervals from First Exposure to Conception and Intervals from Parturition to Conception¹

Florida Agricultural Experiment Station, Gainesville^,4

Abstract

Records were obtained on 3,938 Brahman calves from four ranches with limited breeding seasons in Florida to evaluate reproductive performance of their dams. The adjusted mean for 2,924 calving intervals was 409.9±2.2 days, and the effect of the dam's age when her first calf was born, the dam's age at calving, sex of the calf and location were highly significant (P<.01). The location x age at calving interaction was significant (P<.05). The repeatability of 2,778 calving intervals of 613 cows was 0.08.

When long intervals, resulting from the cow's failure to calve in consecutive years, were excluded the adjusted mean for 2,527 intervals was 374.7±1.0 days with highly significant differences due to age at calving and location. The location x age at calving interaction was also highly significant. The repeatability of 2,346 intervals of 566 cows was 0.03.

Location differences appeared to be due to differences in the length of the breeding season. Age differences were caused by the tendency of the intervals between calvings in consecutive years to shorten with increasing age of the cow. Four- to 5-yr.-old cows were more likely to fail calving than older cows.

The interval from exposure to conception was greater the longer the breeding season lasted. Heifers conceived 64.5±1.7 days after first exposure to a bull, while cows which had calved at least 1 month before the breeding season started conceived after 45.5±1.1 days, the difference being highly significant (P<.01).

The mean interval from parturition to conception was 65.3±1.3 days for 911 calvings which took place during the breeding season. There was a highly significant (P<.01) difference between the four herds, and seemed to be related to the length of the breeding season.

The results suggest that selection for reproductive performance during a short breeding season will decrease the average calving interval, and thus increase overall reproductive efficiency.

¹ Florida Agricultural Experiment Stations Journal Series No. 2477.

² Present address: Facultad de Ciencias Veterinarias, Universidad Central de Venezuela, Maracay, Venezuela.

³ The authors express their appreciation to Mrs. R. Madsen for technical assistance with the processing of the data, and to the staff of W. H. Stuart Ranch, U. S. Sugar Corp., Norris Cattle Co. and A. Duda & Sons for furnishing the records for this study.

⁴ Department of Animal Science.