332 Use of an electronic activity detection system to characterize estrus activity in crossbred beef heifers differing in follicle number.

Increased numbers of antral follicles have been associated with decreased calving day, increased fertility, increased serum estradiol concentrations, increased serum progesterone concentrations, and increased estrus behavior in cattle. In addition, cows with increased fertility have been shown to have greater activity during behavioral estrus when using electronic activity detection systems. Therefore, the objective of the current study was to evaluate an electronic estrus detection system to determine if differences in estrous cycle characteristics could be detected among beef heifers differing in antral follicle number. The hypothesis was that the percent of heifers demonstrating behavioral estrus and peak activity would be decreased in heifers with low numbers of antral follicles. Estrus activity collars were placed on crossbred beef heifers (n = 359) at 12.0 ± 0.1 mo of age to monitor estrus activity, and heifers were submitted for ultrasonographic evaluation of antral follicle number at 13.5 ± 0.1 mo of age, immediately before the start of the breeding season. During a 21 d period, heifers were artificially inseminated 12 h after estrus was diagnosed by the electronic system. Following the period of artificial insemination, heifers were placed with bulls for an additional 42 d, and submitted for ultrasonographic pregnancy diagnosis 45 d after removal of the bulls. Heifers were classified as low ( < 16 follicles), medium (16–25 follicles) or high ( > 25 follicles). The percent of heifers demonstrating behavioral estrus was analyzed using the GLIMMIX procedure of SAS with antral follicle group as the fixed effect with a binomial distribution and a logit link. Length of the estrous cycle before insemination and peak activity at insemination were analyzed using the MIXED procedure of SAS with antral follicle group as a fixed effect. The percent of heifers demonstrating behavioral estrus was lower in heifers with low numbers of antral follicles compared to heifers with medium or high numbers of follicles (P = 0.02; 46.2 ± 4.2 vs. 57.5 ± 2.4%). Peak activity and estrous cycle length; however, did not differ among antral follicle groups (P > 0.1). Combined with our histological and hormonal data, these results indicate that greater numbers of antral follicles contribute to increased behavioral estrus due to increased serum estradiol concentrations; however, this does not result in a difference in peak activity calculated by the electronic system during behavioral estrus. USDA is an equal opportunity provider and employer.