Reproductive Criteria of Beef Bulls during and after Exposure to Increased Ambient Temperature

Sixteen yearling Angus bulls were randomly assigned to one of two temperature-controlled chambers to determine the effects of elevated ambient temperature on body functions and semen characteristics. After 8 wk adjustment at 23 C, eight heat-stressed bulls were exposed to 35 ± 1 C for 8 h and 31 ± 1 C for 16 h during each 24-h period, and eight control bulls were maintained at 23 ± 1 C for 8 wk. Then all bulls were exposed to 23 C for 8 wk. Bulls were fed so that both control and stressed bulls gained at similar rates (.58 kg/d). Semen was collected with an artificial vagina twice weekly before, during and after heat stress. During treatment, the respiratory rate of stressed bulls was greater (P<.001) than that of control bulls (54.2 ± 1.5, 29.9 ± 1.5 breaths/min, respectively). Rectal temperatures were increased (P<.01) from 38.2 ± .1 to 38.7 ± .1 C and water consumption was increased by 35% in stressed bulls when compared with controls. Semen volume was not altered by treatment, but percentage of motile sperm decreased (P<.01) in stressed bulls by 2 wk after the start of heat treatment. Sperm motility of stressed bulls returned to normal values 8 wk after the end of heat treatment. Similarly, the percentage of aged acrosomes on sperm from stressed bulls increased (P<.01) by the second week of treatment and remained greater than that of controls throughout the stress period. Heat stress also resulted in more abnormal cells from the second week of treatment until 7 to 8 wk after heat stress. These data indicate that exposure of bulls to elevated ambient temperatures results in decreased semen quality as evidenced by a reduced percentage of motile sperm, reduced sperm output and an increased percentage of abnormal and aged sperm. Approximately 8 wk is required before semen quality returns to normal after heat stress of bulls.