Abstract
This study was designed to test the efficacy of melengestrol acetate (MGA) in combination with prostaglandin F 2α (PGF 2α) in synchronizing estrus in cyclic and noncyclic heifers. One hundred thirty-one cyclic and prepubertal crossbred heifers were randomly assigned to three treatment groups: Controls (n = 43); MGA (0.5 mg/d for 7 d) and PGF 2α (25 mg i.m. on Day 7; n = 44); and PGF 2α (25 mg i.m. on Day 7; n = 44). Observations for estrus were made at 6-n intervals throughout the 7-d treatment period followed by a 34-d artificial insemination breeding season. A greater percentage (P < 0.05) of MGA-PGF 2α noncyclic heifers showed behavioral estrus (91%) than did Control (67%) or PGF 2α heifers (61%) during the 34-d artificial insemination period. There was no difference (P > 0.05) between synchronization rates of the MGA-PGF 2α heifers and PGF 2α heifers 7 d after PGF 2α administration. The percentage of control animals in estrus during the first 25 d of the breeding season did non differ from the synchronized rates of MGA-PGF 2α and PGF 2α heifers (P > 0.05). Conception rates (heifers pregnant/heifers inseminated) did not differ (P > 0.05) for cyclic or prepubertal heifers among Control, MGA-PGF 2α or PGF 2α heifers. Though conception rates did not differ, there was a trend toward lowered conception rates in MGA-PGF 2α heifers.
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