Impacts of body condition score at beginning of fixed-timed AI protocol and subsequent energy balance on ovarian structures, estrus expression, pregnancy rate and embryo size of Bos indicus beef cows

Abstract

These two studies evaluated the effects of body condition score (BCS) at beginning of fixed-timed artificial insemination (FTAI) protocol and subsequent BCS change on ovarian structures, estrus expression, pregnancy rate and embryo size of multiparous Nellore beef cows. In Experiment 1, 240 cows, distributed over four years (n = 40, 20, 82 and 98 cows, for years 1, 2, 3 and 4, respectively) were assigned to FTAI protocol from d -11 to 0. Cow BCS (d -11 and 40), body weight (BW; d -11 and 40), dominant follicle (d 0) and corpus luteum (CL; d 7 and 14) size, plasma progesterone (d 7 and 14), pregnancy rate (d 40) and embryo size (d 40) were evaluated. In Experiment 2, 2443 cows (single year) were assigned to FTAI protocol from d -11 to 0. Cow BCS (d -11 and 30), BW (d -11 and 30), estrus expression (d 0), and pregnancy rate (d 30) were evaluated. BCS scale used ranged from 1 (extremely thin) to 9 (extremely fat) and the cows were categorized in groups (BCS ≥ 5 and < 5 and in lost, maintained and gained BCS). Experiment 1, cows with BCS ≥ 5 at beginning FTAI protocol tended (P ≤ 0.10) to have greater CL diameter (d 7), CL volume (d 7 and 14) and had greater (P ≤ 0.05) plasma concentration of progesterone (d 14) and embryo size (d 40), compared to cows with BCS < 5. Further, cows that gained BCS from d -11 to 40 tended (P = 0.10) to have greater dominant follicle size and cows that gained and maintained BCS had greater (P = 0.02) plasma concentration of progesterone (d 14), compared to cows that lost BCS. The embryo size was greater (P < 0.01) for cows that gained, intermediary to cows that maintained and lower to cows that lost BCS. In Experiment 2, cows with BCS ≥ 5 had greater (P = 0.02) estrus expression, compared to cows with BCS < 5, and the pregnancy rate increased linearly with BCS (P < 0.01; 20.0 to 69.1% for BCS 2 and 9, respectively). Further, cows that gained had greater (P < 0.01) estrus expression compared to cows that maintained and lost BCS, and cows that gained and maintained BCS had greater pregnancy rate compared to cows that lost BCS (P < 0.01; 62.2, 59.7 and 49.3 ± 10.3%, respectively). Thus, cows with greater BCS at beginning of FTAI protocol or that gained or maintained BCS during synchronization and after AI had greater ovarian structures development, estrus expression, pregnancy rate and embryo size.