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Concentrations of Progesterone during Early Follicular Development and Pregnancy Rate to AI in Beef Cows
The objective was to investigate if decreased progesterone (P4) concentrations earlier during follicular growth would impact fertility in beef cows. Crossbred (Angus x Hereford) cows (n = 261) received estradiol benzoate (EB; 1 mg, i.m.) and a previously used CIDR on d -7, to induce emergence of a new follicular wave approximately 3 days later (d -4). On d 0, all cows received 100 µg GnRH and were randomly assigned to one of the two treatments. In the high P4 (H; n = 131) treatment, the previously inserted CIDR was replaced with a new CIDR on d 0. In the low P4 (L; n = 130) treatment, 25 mg PGF was administered on d 0, and the CIDR previously inserted on d -7 remained. On d 5, blood samples to determine P4 concentrations were collected, all cows received two 25 mg PGF doses and CIDRs were removed. Estrous detection coupled with artificial insemination (AI) 12 h later (Estrus-AI) was performed for 60 h after PGF. Cows not detected in estrus within this period were bred by timed-AI (TAI) and received 100µg GnRH at 72 h. Pregnancy diagnosis was performed approximately 40 d after AI. P4 concentrations at CIDR withdrawal (d 5) were greater (P < 0.01) in the H (2.81 ± 0.10 ng/ml) than in the L (1.73 ± 0.05 ng/ml) treatment. Within the first 60 h after PGF, estrus response (82% vs. 85%) and estrus distribution (56.1 ± 0.7 h vs. 54.0 ± 0.7 h) did not differ between H and L treatments, respectively. Synchronized pregnancy rate was similar between H (77.1%) and L (82.3%) treatments. Across treatments, pregnancy rates were greater (P < 0.01) with Estrus-AI (82.9%) than TAI (63.6%). Concentrations of P4 on d 5 were negatively related (P< 0.01) with estrus response and time to estrus; across treatments. In conclusion, P4 concentrations during early follicular development did not influence synchronized pregnancy rate in beef cows.
Keywords: Beef cattle, Progesterone, Pregnancy rate