Influence of Heat Stress on Male Fertility

Introduction Testicular temperature of bulls must be 2 to 6° C cooler than body temperature to allow normal spermatogenesis and maturation of sperm. Exposure of bulls to increased ambient temperatures results in reduced semen quality (Casady et al., 1953; Skinner and Louw, 1966; Meyerhoeffer et al., 1985). Local heating of the scrotum of bulls for a few days causes a reduction in the number of sperm ejaculated (Austin et al., 1961; Gerona and Sikes, 1970). Heat stress has minimal effects on testicular endocrinology in bulls (Minton et al., 1981, Barth and Bowman, 1994) although heat stressed alters steroidogenesis in boars (Wettemann and Desjardins, 1979). Increased testicular temperature, whether caused by exposure to elevated ambient temperature or fever, reduces semen quality (Waites, 1970). Breeding seasons in many locations in the U.S. occur during months when bulls are exposed to elevated ambient temperatures, and semen is collected for artificial insemination during all months of the year. Potential fertility of an individual bull is more important than of an individual cow because one bull can influence pregnancy rate of 30 or 40 cows with natural mating, or thousands of cows with artificial insemination. Exposure of bulls to optimal ambient temperatures during the two months before breeding or semen collection, and during the breeding season, without even one or two days of exposure to heat stress, is essential to maximize reproductive performance.