PSIX-20 Effect of Mepron on the reproductive performance of precocious Nellore heifers in a grazing system.

Abstract The study evaluated the potential of rumen-protected methionine (Mepron®, Evonik) to improve the reproductive performance of precocious heifers in tropical grazing systems. 84 Nellore heifers, 15 months old and 295 kg of initial body weight, grazed on Urochloa brizantha cv. Marandu, in a continuous grazing system in 18 paddocks (2 hectares each) from 15 January to 20 May 2024, during their first breeding season. The animals had ad libitum access to feed and water. In the beginning, all animals were randomly distributed according to initial body weight and reproductive tract score in six blocks designed with two treatments (42 animals per treatment, 7 heifers/paddock) in 12 paddocks. Every 28 days, animals were moved to another paddock to eliminate possible paddock effects. The heifers were submitted to a fixed-time artificial insemination (FTAI) protocol with three attempts using semen from the same sire. After insemination and subsequent ultrasound pregnancy diagnosis, the pregnant heifers were transferred to another 6 paddocks (3 blocks and 2 treatments) to maintain a similar stocking rate and grass availability. The treatments were: protein supplement without additive, CON; protein supplement + Mepron (4 g/day), MEP. Rump fat thickness (RFT) was measured by ultrasound at the beginning of the trial and the final diagnosis of pregnancy. Dry matter intake (DMI) was calculated using the indigestible neutral detergent fiber and Lipe® (external marker) to estimate total feces production. Pregnancy rate (PR), RFT, and DMI were analyzed using GLMMs in the R, with treatment means compared via Tukey’s test. Initial RFT was used as a covariate. Significance was determined at P≤0.05 for DMI and RFT, and P≤0.10 for PR. The average protein supplement intake for both groups was 0.7 kg/day. Results indicated that MEP increased the first (55 vs. 41%), second (55 vs. 27%), and final PR (81 vs. 67%), P=0.09, 0.02, and 0.08, respectively, compared to the CON group. There were no effects on DMI (P=0.36) and RFT (P=0.17). Mepron increased the fertility rate of heifers and can be used to improve reproductive performance in grazing systems.