Effects of early weaning on the reproductive performance of suckled Nelore cows in the subsequent breeding season.

This study aimed to evaluate the effects of early weaning on body composition, hormone concentrations and metabolites, and reproductive performance of Nelore cows in the subsequent breeding season (BS). Suckled cows that became pregnant by timed-AI (TAI) in the 2020-BS were exposed in 2021 to early weaning (EW) at 150 days (27 primiparous [PRI] and 74 multiparous [MUL]) or conventional weaning (CW) at 240 days post-partum (30 PRI and 77 MUL). Body weight and condition score (BCS) were determined at 2020-BS, EW, CW, prepartum, and 2021-BS. Blood samples were collected at EW, CW, prepartum (54.75±0.56 days prepartum), and 2021-TAI and assayed for IGF-I, NEFA, and BHB concentrations. In 2021 BS, cows were exposed to a P4/E2-based protocol for TAI at Day 0 (D0), and a second TAI was performed at D22 in females detected with luteolysis (D20) by Doppler ultrasound. The presence of corpus luteum (CL) on D-10, estrous expression and dominant follicle (DF) diameter, and blood perfusion (BP) on D-2 and D0 were determined. Data were analyzed by ANOVA or logistic regression of SAS as a 2×2 factorial with main factors of parity (PRI or MUL) and weaning strategy (EW or CW). An interaction of parity and weaning strategy was not observed (P>0.1), but the weight (kg) and BCS were greater (P<0.05) in MUL cows at the five timepoints, and EW cows were heavier than CW at the moment of CW (541 vs. 493 kg; and 5.3 vs. 4.3), prepartum (551 vs. 506 kg; and 5.2 vs. 4.4) and 2021-BS (475 vs. 450 kg; and 4.5 vs. 3.7). Plasma urea concentration at 2021-BS was greater (P=0.01) for PRI than for MUL. A parity by time interaction was observed (P≤0.05) for concentrations of IGF-I, NEFA, and BHB. PRI cows had greater (P≤0.05) concentrations of IGF-I at EW and greater (P≤0.05) prepartum concentrations of NEFA and BHB than MUL cows. The proportion of cows with CL at D-10 was not affected (P>0.1) by weaning but was greater (P<0.05) in MUL than in PRI cows (40.4 vs. 15.7%). The diameter of DF and proportion of BP on D0 were greater (P<0.05) in EW cows than in CW cows. The pregnancy rate (P/AI, %) at the first TAI was greater (P<0.05) in EW cows (60% vs. 45%), whereas no difference (P>0.1) was observed at the second TAI. Cumulative P/AI (first and second TAIs) was greater (P<0.05) in EW cows (81% vs. 63%). In conclusion, weaning at 150 days in Nelore cattle is a strategy to successfully recover the parous cow's body condition and to improve pregnancy success in the next BS, regardless of the cow's parity order.