Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/18/2006
Publication Date: 3/1/2006
Citation: Aiken, G.E., Tabler, S.F., Looper, M.L., Brauer, D.K., Strickland, J.R., Shrick, F.N. 2006. Influence of stocking rate and steroidal implants on growth rate of steers grazing toxic tall fescue and subsequent physiological responses. Journal of Animal Science. 84:1626-1632. Interpretive Summary: Tall fescue is the predominant perennial grass utilized for grazing in the transition zone of the United States. It is widely adapted and persistent under heavy grazing, but cattle grazing tall fescue can consume toxic alkaloids that are produced by a cohabitating fungal endophyte. Toxic alkaloids can result in a malady known as fescue toxicosis. Cattle exhibiting toxicosis have elevated body temperatures, rough winter haircoats that are retained in the spring and summer months, and poor weight gain. A two-year grazing experiment was conducted to determine if plantation with anabolic steroids (progesterone and estradiol benzoate) affects severity of toxicosis, and evaluate if stocking rate (3.0, 4.0, 5.0 and 6.0 steers/ha) interacts with implantation in affecting daily weight gain and subsequent recovery from toxicosis-related heat stress. Differences in average daily weight gain were greatest for implanted steers on the lightest stocking rate, but differences between the two treatments diminished as stocking rate increased. Frequency of rough haircoats was similar between implanted and non-implanted steers when grazing was terminated in late June, but rectal temperatures were slightly higher for implanted steers. Following termination of grazing, the steers were placed on fescue-free diets, rectal temperatures of both implanted and non-implanted steers declined to those considered stabile and healthy in 8 to 10 days. Implantation with steroid hormones can improve weight gain on tall fescue but it also can induce some additional heat stress. However, results further indicated that toxicosis-related heat stress for implanted and non-implanted cattle can be alleviated in 8 to 10 days following removal from endophyte-infected tall fescue pasture and placement on a fescue-free diet.
Technical Abstract: A 2-yr grazing experiment evaluated interactions between stocking rate and implantation with steroid hormones on weight gain and symptoms of toxicosis for yearling steers grazing endophyte-infected (E+) tall fescue (Festuca arundinacea). Combinations of four stocking rates (3.0, 4.0, 5.0, and 6.0 steers/ha) and ear implantation treatments (200 mg progesterone, 20 mg estradiol benzoate) were randomly assigned to eight, 1-ha pastures of E+ Kentucky-31 tall fescue. At the conclusion of grazing in the second yr (22 June), steers were placed on a bermudagrass [Cynodon dactylon (L.) Pers.] pasture and rectal temperature and concentrations of serum prolactin were monitored for 10 d to assess carry-over effects of stocking rate and implantation on recovery from toxicosis related heat stress. Average daily weight gain response to implantation was strongest for the light-stocking rate, but response to implantation diminished as stocking rate increased. Stocking rate did not influence (P = 0.89) post-graze rectal temperature, but the regression intercept for implanted steers was 0.4oC higher (P < 0.05) than for non-implanted steers, and the difference was consistent during the 10-d post-grazing monitoring phase. Concentrations of prolactin during the 10-d post-graze phase increased, but trends differed due to an implantation x stocking rate interaction (P < 0.05). Results indicate that implantation with progesterone/estradiol benzoate increases ADG with lighter stocking rates but the benefits of implantation diminish with increases in stocking rate. Although implantation increased rectal temperatures, post-graze rectal temperature and serum prolactin for implanted and non-implanted steers returned to values indicative of a stable and healthy status in a 192 to 240 (8 to 10-d) period.