Supplementation with melengestrol acetate (MGA) post TAI improves fertility in suckled Nelore cows

The objective of this study was to evaluate if oral supplementation of Melengestrol Acetate (MGA®, Zoetis, SP, Brazil) after TAI and the supplementation period increases pregnancy rate in Nelore cows submitted to TAI protocols with different gonadotropin stimulation (Calf removal (CR) or eCG). A total of 2301 multiparous suckled Nelore cows was enrolled to the experiment, distributed in 24 lots with similarly days post partum, and access to pasture of Braquiaria and water ad libitum. All cows were synchronized with the protocol: 2.0 mg of Estradiol Benzoate im (2.0 ml of Gonadiol®, Zoetis, SP, Brazil) + intravaginal progesterone device (CIDR®, Zoetis, SP, Brazil) in d-11; 12.5 mg of PGF2α im (2.5 ml of Lutalyse®, Zoetis, SP, Brazil) in d-4; Removal of CIDR® + 0.5 mg of Estradiol Cypionate im (0.3 ml of ECP®, Zoetis, SP, Brazil) in d-2; AI in d0. In d-2 15 lots (Farm 01) received 300 IU of eCG im (1.5 ml of Novormon®, Zoetis, SP, Brazil) and 9 lots (Farm 02) received CR. After insemination each lot of animals were randomly distributed in three treatments: Control- Animals did not receive MGA; MGA 6- MGA supplementation during 6 days, from day 13 to day 18 after TAI; MGA 14- MGA supplementation during 14 days, from day 5 to day 18 after TAI. Melengestrol Acetate dose utilized was 0.5 mg/ day (2.28 g of MGA®/day) supplied with mineral salt every day. Mineral salt intake was evaluated for 3 days before the start of supplementation. Pregnancy diagnosis was performed by ultrasound 30 days after insemination. Pregnancy per AI (P/AI) was determined by dividing the number of pregnant cows by the number of inseminated cows. Was used the PROC GLIMMIX of SAS program for data analysis, random variables were included in the model and withdrawals (criterion of Wald ) when P>0.2. Two contrasts were used to determine the effect of MGA supplementation [Control vs. MGA (MGA 6 + MGA 14)] and to determine the effect of MGA supplementation duration (MGA 6 vs. MGA 14). Significant differences were considered when P≤0.05. There was no interaction between BCS or stimulus (CR and eCG) on pregnancy rate. Cows supplemented with MGA had higher P/AI [62.7% (983/1568)] compared to control group [56.2% (412/733)] independent of gonadotropin stimulation. The duration of supplementation, independent of gonadotropin stimulation, did not change pregnancy rates [MGA 6: 61.9% (520/840) vs. MGA 14: 63.6% (463/728)]. We concluded that MGA supplementation post TAI increased pregnancy rates in suckled Nelore cows, independent of gonadotropin stimulation and duration of supplementation.