Submitted to: American Society of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: February 23, 2006
Publication Date: July 9, 2006
Citation: Aiken, G.E., Looper, M.L., Kirch, B.H. 2006. Physiologic changes in heifers following grazing of toxic or non-toxic tall fescue. American Society of Animal Science. 89(1):101. Technical Abstract: Rectal temperature and serum prolactin in yearling heifers were monitored for 22-d following a grazing trial that compared BW gains between 'Kentucky-31' and 'MaxQ' tall fescues (Festuca arundinaecea). Response variables were used to compare physiological changes between heifers removed from Kentucky-31 and those removed from non-toxic MaxQ (control). Heifers were stratified by BW and pregnancy status into 4 groups of 6 heifers and groups were randomly assigned to pastures. Grazing was initiated on 1 March, 2005 and terminated on 21 June, 2005. At termination of grazing, the heifers were maintained as a single group, and grazed on a 0.4-ha pasture of bermudagrass (Cynodon dactylon) and offered free choice bermudagrass hay. Following termination of fescue grazing, rectal temperatures were determined and blood samples were collected at 0830 on d 1, 3, 6, 8, 10, 14, 16, 20, and 22. Carryover effects from the fescue treatments were evaluated using pasture as the experiment unit. Rectal temperature was not affected (P > 0.01) by pregnancy status, but there was a cubic (P < 0.05) relationship between rectal temperature and days on bermudagrass. Rectal temperatures for heifers grazed on Kentucky-31 were maximum (39.8°C) in 6 d, but declined and stabilized at 39.2°C in approximately 14 d. Serum prolactin had a cubic (P < 0.001) relationship with days on bermudagrass, but the trends differed (P < 0.001) between the two fescue cultivars. Prolactin was initially low (15.0 ng/mL) in Kentucky-31 heifers, but increased nonlinearly and stabilized at approximately 113.8 ng/mL in 10 d. Although prolactin tended (P < 0.10) to be highest for pregnant heifers grazed on MaxQ, prolactin for pregnant and open heifers grazed on MaxQ were initially high (pregnant = 292.8 ng/mL; open = 280.3 ng/mL), but decreased nonlinearly and stabilized (pregnant = 160.1 ng/mL; open = 127.3 ng/mL) in 10 d. Results indicated that heat stress can be alleviated for pregnant and open heifers grazed on toxic tall fescue by removing them from fescue and providing non-toxic diets for 10 to 14 days.