Chemical Suppression of Reproductive Development In Endophyte-Infected Tall Fescue for Improving Cattle Weight Gain and Physiology.

Small-plot experiments have shown Chaparral herbicide (Dow AgroSciences; Indianapolis, IN) to suppress the reproductive development of tall fescue.  A two-yr grazing experiment was conducted with steers grazed on endophyte-infected tall fescue pastures either treated or untreated with Chaparral^® herbicide to determine if suppression of maturity can increase average daily gain (ADG) and alleviate fescue toxicosis.  Herbicide treatments were assigned to six, 3.0-ha pastures of endophyte-infected tall fescue pastures in a randomized complete block design with 3 replications.  Pastures were grazed with 48 steers (8 per pasture) from 9 April to 1 July, 2009 and 6 April to 7 July, 2010.  Concentrations of reproductive culms in treated pastures were extremely low in both years (< 7 culms m^-2), whereas untreated pastures had 113 and 68 reproductive culms m^-2 in 2009 and 2010, respectively.  Ergovaline concentrations were 6-fold greater in seed than in leaf blades; however, ergovaline concentrations in blades and sheaths of vegetative tillers were similar (P > 0.10) between treated and untreated pastures in both years. In 2010, approximately 76% of the seedheads in untreated pastures had been removed via grazing by 17 June.  Crude protein, water soluble carbohydrates, and in vitro true digestible DM concentrations in whole tillers were greater (P < 0.10) for treated pastures over most of the grazing season in both years.  Average daily gain on treated pastures was 39% greater (P < 0.05) than on untreated pastures.  Steers on treated pastures had lower (P < 0.10) rectal temperatures and 2-fold greater (P < 0.01) serum prolactin concentrations.  Results indicate that Chaparral^® herbicide treatment can suppress reproductive development of tall fescue to increase weight gain and mitigate the adverse effects of fescue toxicosis.