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Although heating seems to affect Sertoli and Leydig cell function, germ cells are the most sensitive [35]. All stages of spermatogenesis are susceptible, with degree of damage related to the extent and duration of the increased temperature [35]. Spermatocytes in meiotic prophase are killed by heat, whereas sperm that are more mature usually have metabolic and structural abnormalities [36]. Heating testes usually decreases the proportion of progressively motile and live sperm, and increases the incidence of morphologically abnormal sperm, especially those with defective heads [37]. Increased testicular temperature caused a lack of chromatin protamination and subtle changes in head shape of bull sperm [38]. Despite considerable variation among bulls in the nature and proportion of defective sperm, order of appearance of specific defects is relatively consistent [31, 32]. Unless spermatogonia are affected, the interval from cessation of heating to restoration of normal sperm in the ejaculate corresponds to the interval from the beginning of differentiation to ejaculation [37]. Following scrotal insulation in bulls, blastocyst rate (in vitro system) was more sensitive than cleavage rate [39]. Even though sperm morphology has returned to normal, their utilization may result in decreased fertilization rates and an increased incidence of embryonic death [40].