| Abstract Fifteen bulls (5-Charolais, 5-Limousin, 5-Holstein) were used in experiment to research the influence of bull heat stress on semen quality. At the same time the mechanism was studied with the varied serum biochemical index, seminal plasma biochemical index, respiratory rate and rectal temperature. Depending upon the climatic conditions, the whole year was divided into four periods梥ummer (June to August) autumn (September to November) , winter (December to February) and spring (March to May) . Summer was heat stress period and the other three seasons were non-heat stress period. Results showed that: Heat stress made a significant reduction on bull semen quality, as evidenced by a reduced semen motility (14.3%), semen concentration (30.6%) , percentage of motile sperm (20.9%) and percentage of intact acrosome (19.4%); but the rate of abnormal sperm was increased by 42.3% in summer (P<0.01). The bull productivity was effected significantly by heat stress. The quantity of frozen semen was decreased from 23679 count in April to 5114 count in July(P<0.01 ). The total semen quality of Holstein was better than that of Limousin and Charolais. In heat stress period, The respiratory rate and rectal temperature were at the highest level (P0.05) .ln summer, the level of blood serum CK (48.88U/L) was decreased by 34.9% (P<0.01) and 48.4% (P<0.0 1) respectively compared with spring and autumn, but higher than that of winter (P>0.05)...
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