Extending melengestrol acetate feeding from 14 to 18 days during an estrus synchronization and artificial insemination protocol for March-calving beef heifers



      Our objective was to determine whether or not extending the length of melengestrol acetate (MGA) feeding beyond the recommended 14-d MGA estrus synchronization protocol would alter the proportion of heifers that expressed estrus during the estrus detection period following administration of prostaglandin F (PG), and resulted in increased conception rates to AI, or both.

      Materials and Methods

      In 2020 and 2021 March-calving heifers from 2 separate herds were allocated to pens and randomly assigned to a 14-d MGA protocol (E14) or an 18-d MGA protocol (E18, Figure 1). Timing of estrus after MGA withdrawal was recorded in the 2021 replicate only. In both years, estrus expression at AI was recorded using estrus detection aids (Estrotect) and a patch score system (1 = <25% removed, 2 = 25 to 50% removed, 3 = >50% removed, 4 = patch missing). Patch scores of 3 and 4 were considered to be expressing estrus. Pregnancy was confirmed with ultrasound.

      Results and Discussion

      Heifers exhibiting estrus earlier after MGA withdrawal also exhibited estrus earlier after PG administration (P < 0.01), but there was no effect of treatment on timing of estrus. Estrus after MGA withdrawal also coincided with estrus after PG administration (P < 0.01), but there was no effect of treatment on estrus expression. Heifers exhibiting estrus were more likely to become pregnant (P < 0.01), but conception rates to AI did not differ between treatments.

      Implications and Applications

      A longer MGA protocol did not significantly alter timing of estrus expression or pregnancy rates resulting from AI.

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