ABSTRACT
Objective
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 F2α (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.
Key words:
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Article Info
Publication History
Accepted:
July 20,
2022
Received:
April 29,
2022
Footnotes
The authors have not declared any conflicts of interest.
Identification
Copyright
© 2022 American Registry of Professional Animal Scientists. Published by Elsevier Inc. All rights reserved.
