Oral supplementation with melengestrol acetate incorporated in protein-energy blocks I - effect on pregnancy rates in primiparous cows and Nellore heifers in natural breeding and artificial insemination

Abstract We evaluated the effect of supplementation with MGA incorporated into nutritional blocks on pregnancy rates (PR) after natural mating and AI of heifers and primiparous cows (Nellore) raised in extensive management. In exp 1, 413 heifers were grouped: control; Block-EB-P4; Block-MGA ; and Bran-MGA, and animals submitted to natural mating. In exp 2, 301 heifers were grouped: control; Block-MGA; and Block-MGA-EC, with animals inseminated after estrus observation for 10 days and, afterwards, mated with bulls. In exp 3, 342 primiparous cows, were divided into 4 groups: Control, Block-MGA-EB-eCG;Block-MGA-EB-CalfRemoval and Block- MGA-CalfRemoval, next, cows were mated with bulls. In exp 1, PR at beggining of breeding season was higher in the Block-MGA (57.84%; P < 0.01) in relation to Control (40.8%) and Block-BE-P4 (36.4%) and it was similar to Bran-MGA (51.0%). In experiment 2, PR at beggining of the BS in the control (56.6%; P = 0.04) was lower than in the MGA-EC (74.4%) and similar to the MGA group (67.7%). In exp 3, at 70 days (P = 0.01) and at the end of the BS( P < 0.02), respectively, the control (55%; 60.4%) had a lower PR than the groups MGA-EB-eCG (72.3%;75.9%;), MGA-EB-CR (74.1%;82.7%) and MGA-CR (74.4%; 81.3%). The results showed that use of MGA added to nutritional blocks in primiparous and heifers provided higher pregnancy rates at the beginning of the breeding season, and also higher PR in primiparous at the end of the season, representing an alternative for reproductive management under extensive conditions.