|Mc Clanahan, L - KY COOP EXT. SERVICE|
|Dougherty, C - UNIVERSITY OF KENTUCKY|
Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 8, 2008
Publication Date: May 22, 2008
Citation: Mc Clanahan, L.K., Aiken, G.E., Dougherty, C.L. 2008. Case Study: Influence of Rough Hair Coats and Steroid Implants on the Performance and Physiology of Steers Grazing Endophyte-Infexted Tall Fescue in the Summer. Professional Animal Scientist. 24:269-276. Interpretive Summary: Tall fescue is the predominant forage utilized for cattle grazing in a region commonly referred to as the “fescue belt”. Unfortunately, most plants of tall fescue are infected with an endophyte that produces toxic alkaloids that can inflict fescue toxicosis. Symptoms of the malady include poor weight gain, winter hair coat retention in the summer, low serum prolactin, and constricted peripheral blood flow. Cattle grazing endophyte-infected fescue often suffer from heat stress in the summer months. Winter hair coats not shed by fescue cattle in the late spring and summer likely insulate body heat in warmer ambient temperatures, but this effect has not been documented. Vascular constriction of blood flow to peripheral tissues in cattle grazing toxic fescue also reduces dissipation of body heat. It is possible, though, that implantation with steroids could affect vascular circulation because research in both humans and livestock has shown that estrogen induces vasodilation while progesterone can cause vasoconstriction. A grazing experiment evaluated the effects of hair coat retention and implant progesterone on weight gain and physiologic measures associated with body heat dissipation in beef cattle exhibiting fescue toxicosis. Results of this experiment indicated that rough hair coat retention can elevate body temperature in the presence of higher ambient temperature and that implanted progesterone can reduce sweating. Physiological mechanisms that allow cattle to adjust to summer heat are compromised in cattle exhibiting fescue toxicosis and, therefore, summer grazing of endophyte-infected tall fescue by beef calves should be avoided.
Technical Abstract: Sixty-two steers were assigned to six, 3.0-ha pastures of toxic tall fescue to study the effects that implant progesterone and winter hair coat retention during the summer, a symptom of fescue toxicosis, has on weight gain and body heat dissapation. Either ten clipped or ten unclipped steers were randomly assigned to pastures as the main plot treatment. Five steers in each pasture were implanted with Synovex-S™ (200 mg progesterone-20 mg estradiol) and five were implanted with Compudose™ (25 mg estradiol) as subplot treatments. A small area over the shoulder was clipped to measure hair length growth (mm/day) and sweating rate (g/m2/h). Responses were measured at 28, 56, 84, and 104 days of grazing. Grazing frequencies were periodically measured during the experiment. Hair growth rate averaged 0.28 mm/day 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 (39.5 vs. 39.3°C) only on day 84 when the highest ambient temperature (33°C) of the study was recorded. At high environmental temperatures, percentage of steers actively grazing was negatively correlated (P < 0.10) with ambient temperature. Cumulative ADG declined (P < 0.05) over the last two 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.