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Scrotal insulation is frequently used to increase testicular temperature. In one study [29], scrota of B. indicus × B. taurus bulls were insulated for 48 hours. The nature and time (day 0, start of insulation) of the morphologically abnormal sperm that resulted were as follows: decapitated, days 6–14; abnormal acrosomes, days 12–23; abnormal tails, days 12–23; and protoplasmic droplets, days 17–23. Therefore scrotal heating affected sperm in the caput epididymis as well as spermatids. Although daily sperm production was not affected, epididymal sperm reserves were reduced by nearly 50% (9.2 vs 17.4 billion), particularly in the caput (3.8 vs 6.6 billion) and cauda (3.7 vs 9.5 billion), perhaps due to selective resorption of abnormal sperm in the rete testis and excurrent ducts. In another study [30, 31], scrota of six Holstein bulls were insulated for 48 hours (day 0, initiation of insulation). The number of sperm collected was not significantly different, but the proportion of progressively motile sperm decreased from 69% (prior to insulation) to 42% on day 15. The proportion of normal sperm was not significantly different from day −6 to day 9 (80%), decreased abruptly on day 12 (53%), and reached a nadir on day 18 (14%). Although there was considerable variation among bulls in both type and proportion of abnormal sperm, specific abnormalities appeared in a consistent chronological sequence: tailless, days 12–15; diadem, day 18; pyriform and nuclear vacuoles, day 21; knobbed acrosome, day 27; and Dag defect, day 30. When sperm were collected 3–9 days after insulation and examined immediately, their motility and morphology were similar to pre‐insulation values [30]. Compared to semen collected prior to insulation, following freezing, thawing, and incubation at 37 °C for three hours [30], there were significant reductions in the proportion of progressively motile sperm (46 vs 31%, respectively) and the proportion of sperm with intact acrosomes (73 vs 63%). Freezing plus post‐thaw incubation manifested changes that had occurred in sperm that were in the epididymis at the time of scrotal insulation.

In another study [32], scrotal insulation (four days) and dexamethasone treatment (20 mg/day for seven days) were used as models of testicular heating and stress, respectively. Some bulls seemed predisposed to produce sperm with a specific abnormality. Pyriform heads, nuclear vacuoles, microcephalic sperm, and abnormal DNA condensation were more common in insulated than dexamethasone‐treated bulls. Conversely, dexamethasone treatment resulted in an earlier and more severe effect on epididymal sperm, an earlier and greater increase in distal midpiece reflexes, and an earlier increase in proximal and distal droplets. Overall, types of defective sperm and the time of their detection were similar between treatments.