Influence of Rough Hair Coats and Steroid Implants on the Performance and Physiology of Steers Grazing Endophyte-Infected Tall Fescue in the Summer1

Sixty-two steers were assigned to six 3- ha pastures of toxic tall fescue to study the effects of steroid implants and winter hair coat retention on weight gain and body heat dissipation. Either 10 clipped or 10 unclipped steers were assigned to pastures as the main plot treatment. Groups of steers in each pasture were implanted with either Synovex-S (200 mg progesterone, 20 mg estradiol) or Compudose (25 mg estradiol) and evaluated as subplot treatments. An area approximately 41cm2 was clipped over the shoulder to measure hair length growth (mm/d) and sweating rate (g/m2 per h). Responses were measured at 28, 56, 84, and 104 d of grazing. Frequency of grazing behavior was periodically measured during the experiment. Hair growth rate averaged 0.28 mm/d and was unaffected (P > 0.10) by implant treatments. Sweating rate declined (P < 0.001) over the grazing period and was less (P < 0.10) with the progesterone-estradiol implant. Rectal temperatures were lower (P < 0.05) in clipped cattle (39.3°C) compared with unclipped cattle (39.5°C) on d 84 when the highest ambient temperature (33°C) of the study was recorded. At increased ambient temperatures, percentage of steers actively grazing was negatively correlated (-0.82, P < 0.10) with ambient temperature. Cumulative ADG declined (P < 0.05) over the last 2 weigh dates when there were higher ambient temperatures. Results indicated that rough hair coat retention and continuous hair growth into the summer caused impaired thermoregulatory ability at higher ambient temperatures and that heat stress was exacerbated by reduced sweating.