Stocking Rate and Implantation Effects on Weight Gain and Recovery from Heat Stress for Yearling Steers Grazed on Endophyte-Infected Tall Fescue

Authors: Aiken, Glen; Looper, Michael; Tabler, Samuel

Submitted to: Crop Science Society of America Monograph
Publication Type: Abstract Only
Publication Acceptance Date: 6/28/2005
Publication Date: 11/6/2005
Citation: Aiken, G.E., Looper, M.L., Tabler, S.F. 2005. Stocking Rate and Implantation Effects on Weight Gain and Recovery from Heat Stress for Yearling Steers Grazed on Endophyte-Infected Tall Fescue. Crop Science Society of America Monograph. Nov 8,2005:10.

Interpretive Summary: Implantation with anabolic steroids can improve weight gain of beef cattle on endophyte-infected (E+) tall fescue (Lolium arundinacea), but the effect has not been determined over a range of stocking rates. A 2-yr grazing experiment evaluated combinations of four stocking rates (3.0, 4.0, 5.0, and 6.0 steers/ha) and implantation treatments (with or without) for yearling steers grazed on E+ ‘KY-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 serum prolactin were monitored for 8 d to assess recovery from toxicosis-related heat stress. There was an implantation x stocking rate interaction (P < 0.05) on ADG. Trends in the regressions indicated ADG response to implantation was strongest for the light-stocking rate, but response to implantation decreased with increases in stocking rate. 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 post-grazing monitoring phase. Rectal temperatures decreased to temperatures consistent with a healthy, unstressed animal between 6 and 8 d. There was a non-linear increase in serum prolactin that did not stabilize by d 8. Results indicated that implantation increases ADG with lighter stocking rates but the benefits of implantation diminish with increases in grazing intensity, and that heat stress of steers exhibiting symptoms of toxicosis can be alleviated in 6 to 8 days.

Technical Abstract: Implantation with anabolic steroids can improve weight gain of beef cattle on endophyte-infected (E+) tall fescue (Lolium arundinacea), but the effect has not been determined over a range of stocking rates. A 2-yr grazing experiment evaluated combinations of four stocking rates (3.0, 4.0, 5.0, and 6.0 steers/ha) and implantation treatments (with or without) for yearling steers grazed on E+ ‘KY-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 serum prolactin were monitored for 8 d to assess recovery from toxicosis-related heat stress. There was an implantation x stocking rate interaction (P < 0.05) on ADG. Trends in the regressions indicated ADG response to implantation was strongest for the light-stocking rate, but response to implantation decreased with increases in stocking rate. 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 post-grazing monitoring phase. Rectal temperatures decreased to temperatures consistent with a healthy, unstressed animal between 6 and 8 d. There was a non-linear increase in serum prolactin that did not stabilize by d 8. Results indicated that implantation increases ADG with lighter stocking rates but the benefits of implantation diminish with increases in grazing intensity, and that heat stress of steers exhibiting symptoms of toxicosis can be alleviated in 6 to 8 days.