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 less than .001) than that of control bulls (54.2 +/- 1.5, 29.9 +/- 1.5 breaths/min, respectively). Rectal temperatures were increased (P less than .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 less than .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 less than .01) by the second week of treatment and remained greater than that of controls throughout the stress period.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of ambient temperature on prolactin concentrations in serum of Holstein and Brahman x Hereford heifers.

Four Holstein and four Brahman x Hereford heifers about 8 mo of age were used in a study to determine whether breed influences the effects of ambient temperature on concentrations' of prolactin in serum. Two heifers of each breed were stanchioned in each of two environmental chambers at 21 C for 7 d, after which chamber temperatures were changed to 7 or 31 C during 6 h. After 5 d at 7, 21 or 31 C, heifers were injected with 60 micrograms thyrotropin-releasing hormone (TRH). A switch-back design was used and each heifer was exposed to all treatments. Concentrations of prolactin in serum of heifers during exposure to 7, 21 or 31 C for 5 d were related to ambient temperature (9.0, 20.9 and 29.5 ng/ml, respectively; P less than .001), but the response was not influenced by breed. Heifers of both breeds responded similarly to treatment with TRH, and prolactin in serum increased (P less than .001) within 5 min from 7.0 +/- 3.2 to 45.7 +/- 8.2 ng/ml in heifers at 7 C, from 13.1 +/- 1.6 to 97.2 +/- 9.6 ng/ml in heifers at 21 C and from 18.2 +/- 3.5 to 96.2 +/- 11.3 ng/ml in heifers at 31 C. We conclude that concentrations of prolactin in serum of heifers are positively associated with ambient temperature and that the effects of temperature on basal and TRH-stimulated concentrations of prolactin do not differ significantly between Holstein and Brahman x Hereford heifers. Thus, differences in tolerance to heat were not related to differences in prolactin secretion.

Serum luteinizing hormone and testosterone in bulls during exposure to elevated ambient temperature.

Eight mature Angus bulls were used to evaluate the effects of exposure to 34 C on concentrations of serum luteinizing hormone (LH) and testosterone before and after treatment with gonadotropin releasing hormone (GnRH). After a 3-week period of adjustment at 22 C, the bulls were randomly assigned to either a control (22 +/- 1 C) or a heat stress ( 34 +/- 1C) treatment for 15 days. Blood was sampled via jugular cannula at 30-min intervals for 12 hr on days -2, 6 and 15 of treatment. After the 12 hr sampling period, 200 micrograms of GnRH were rapidly infused and blood samples were collected at 15-min intervals for 1 hr and then at 30-min intervals for 5 hours. Average serum LH concentrations, the areas under 12-hr LH profiles and the magnitude of episodic releases of LH were reduced (P greater than .05) in heat stressed bulls on day 6 of treatment, but only average LH concentration was reduced (P greater than .10) on day 15. Average serum testosterone concentrations, the areas under 12-hr testosterone profiles and the number and magnitude of testosterone secretory spikes were similar for heat stressed and control bulls on day 6 of treatment. However, serum testosterone concentrations were slightly reduced (P greater than .13) after 15 days of heat stress. Testosterone and LH responses to GnRH infusion were not influenced by heat stress. These results indicate that exposure of bulls to increased ambient temperatures causes a transitory reduction in serum LH concentrations after 6 days of exposure but that serum testosterone is not significantly altered.