Abstract
Heat stress strongly negatively affects reproductive traits in dairy cattle. The purpose of the present study was to investigate the effect of heat stress in superstimulated Thai-Holstein crossbreds under tropical climate conditions. Data included 75 records from 12 superovulated Thai-Holstein crossbreds between 2018 and 2020. Cows were superstimulated with conventional treatment. The mean temperature-humidity index (THI) was evaluated for three data collection periods: during the 9, 21 and 42-day periods before the insemination day to determine the period in which THI mostly affected superstimulation responses. The THI levels/thresholds were determined and interpreted together with the superovulatory response. A significantly negative correlation was obtained for the THI during the period 9 days before insemination. Negative effects on the number of large follicles and corpora lutea began at a THI of 72 and were apparently severe after a THI of 77, similar to the ovulation rate, fertilized ova and transferable embryos (p < 0.05). Meanwhile, more degenerated embryos were found with THI values (p < 0.05). The superovulatory response in Thai-Holstein crossbreds under tropical climate conditions is highly affected by heat stress starting at a THI of 72 and becomes more severe at a THI higher than 77.
Highlights
Heat stress is such an important issue for dairy production because of its negative effects on depressed milk production [1] and on reduced reproductive performance [2,3]
Some studies have inferred that cows exposed to chronic heat stress for at least two estrous cycles showed impaired conception rates [16], as the growth of small antral follicles to preovulatory follicles in the cows requires about two estrous cycles [17]
We evaluated the impact of heat stress as measured by temperature-humidity index (THI) on superovulation parameters in Thai-Holstein crossbreds under tropical climate conditions
Summary
Heat stress is such an important issue for dairy production because of its negative effects on depressed milk production [1] and on reduced reproductive performance [2,3]. In vitro cultures have been used to study the adverse effects of elevated temperature on incomplete oocyte maturation, fertilization and early embryonic development. The direct exposure of oocytes at the germinal vesicle stage to 41 ◦ C for 6–12 h showed reduced ability to undergo nuclear maturation and embryo development [7]. The heat sensitivity of embryos is considered to be stage-dependent, in which early-stage embryos are more susceptible to elevated temperatures than advanced-stage embryos, such as morulae or blastocysts [9,10,11]. These effects result in low fertility during the hot season
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