Chemical Suppression of Seedhead Emergence in Endophyte-Infested Tall Fescue for Improving Steer Weight Gain and Physiology.

Chaparral^® herbicide has shown in small-plot experiments to suppress seedhead emergence in tall fescue.  A grazing experiment was conducted with steers grazed on endophyte-infected tall fescue that were either treated or untreated with Chaparral^® herbicide.  The objective of the experiment was to determine if suppression of seedhead emergence and maturation can increase average daily gain (ADG) and alleviate fescue toxicosis.  Herbicide treatments were assigned to six, 3.0-ha pastures of toxic tall fescue pastures in a randomized complete block design with 3 replications.  Pastures were grazed from 9 April to 1 July, 2009 with 48 steers (8 per pasture).  Treated pastures were practically void of seedheads, whereas untreated pastures had seedhead concentrations of 113 m^-2.  Concentrations of ergovaline plus its epimer, ergovalanine, were 4-fold greater in seedheads than in leaf blades. Whole tillers and leaf blades in treated pastures had higher (P < 0.10) crude protein and water soluble carbohydrates than those in untreated pastures.  Mean average daily gain of steers grazing treated pastures (0.91 kg d^-1) was higher (P < 0.05) than those grazing untreated pastures (0.55 kg d^-1).  Rectal temperatures measured in the late afternoon were lower in steers grazing treated pastures than those grazing untreated pastures (40.5 vs. 41.2°C, respectively).  Further, serum prolactin concentrations were higher in steers grazing treated pastures than those grazing untreated pastures (58 vs. 9 ng mL^-1, respectively).  Results in 2009 indicated that Chaparral^® herbicide treatment suppressed seed head emergence and maturity of tall fescue pastures to increase weight gain and reduce the severity of fescue toxicosis, if not alleviate the malady.