A single dose of FSH or hCG during a split-time AI program did not enhance follicular growth or pregnancy per artificial insemination in beef heifers.

The objective of this study was to evaluate the effect of a low dose of hCG or FSH on follicle growth and pregnancy per artificial insemination (AI) in crossbreed beef heifers subjected to a split-time AI program. All heifers (n = 386) were subjected to a 7-d Co-synch protocol and an intravaginal controlled internal drug release (CIDR) device (GnRH and CIDR insertion; 7 days later PGF2α, CIDR removal, and application of a tail head estrus detection patch; 36 hours later AI of heifers detected in estrus, 24 hours later timed AI of remaining heifers). At CIDR removal, heifers were randomly assigned to receive either 150 IU of hCG, 20 mg of FSH, or remained as untreated controls. The diameter of the preovulatory follicle and the pregnancy per AI did not differ among treatments. A smaller (P = 0.035) proportion of heifers in the hCG group were detected in estrus compared with control. Additionally, the incidence of premature ovulation was greater (P < 0.01) among heifers treated with hCG compared with control. In conclusion, using a low dosage of hCG or FSH at the time of device removal did not affect pre-ovulatory follicle growth or the proportion of pregnant heifers. [ABSTRACT FROM AUTHOR]