Importance of body condition score and ovarian activity on determining the fertility in beef cows supplemented with long-acting progesterone after timed-AI.

The working hypothesis was that supplementation with progesterone (iP4) at early diestrus increases pregnancy rates of suckled beef cows with poor body condition score (BCS) and/or in anestrus when cows are submitted to timed insemination (TAI). The time of ovulation among suckled multiparous (n = 1270) and primiparous (n = 303) Nelore cows was synchronized using an estradiol/P4-based protocol for TAI (D0). At Day ‒10, visual evaluation of BCS was performed (scale = 1-5) and animals were classified according to an ovarian activity (OA) score: (OA1) absence of CL and presence of follicles ≥ 8 mm, (OA2) absence of CL and presence of follicles < 8 mm and (OA3) presence of corpus luteum (CL). On Day 4, animals were assigned randomly to receive 150 mg of injectable, long-acting P4 (iP4, n = 786) or non-iP4 (n = 787). Further, ultrasonic evaluations were performed on D0 and D4 for measurements of the largest follicle diameter (DF) and CL area, respectively. The BCS affected positively DF, CL area and OA. Supplementation with iP4 did not affect (P = 0.49) pregnancy rates and there was no significant interaction (P > 0.1) of P4 treatment with BCS or OA was detected for conception and pregnancy rates. Regardless of iP4, pregnancy rates of cows with moderate (2.75-3.25) (59.1%) and high (≥ 3.5) (57.8%) BCS were greater than those of cows with low (2.0-2.5) BCS (41.5%). Cows in OA2 had a greater P/AI (51.3%) than cows in OA1 (41.7%) or OA3 (41.9%). In conclusion, P4 supplementation after TAI did not improve P/AI and did not enhance the response to treatment of cows with a low BCS, regardless of OA.