ARS | Publication request: Chaparral Herbicide Application for Suppression of Seedhead Emergence in Tall Fescue Pastures and Possible Alleviation of Fescue Toxicosis

Submitted to: American Forage and Grassland Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 23, 2010
Publication Date: June 21, 2010
Citation: Aiken, G.E., Witt, W.W., Kagan, I. 2010. Chaparral Herbicide Application for Suppression of Seedhead Emergence in Tall Fescue Pastures and Possible Alleviation of Fescue Toxicosis. American Forage and Grassland Conference Proceedings.

Interpretive Summary: A fungal endophyte infects most tall fescue plants and produces ergot alkaloids that can induce a toxicosis. Growing cattle exhibiting “fescue toxicosis” have elevated core body temperatures, low prolactin concentrations, and poor weight gain efficiency. Chaparral® herbicide (Dow AgroSciences LLC) applied to endophyte-infected (E+) tall fescue suppresses emergence of seedheads, which have 3- to 4-times greater alkaloid concentrations than leaf blades. A 2-yr grazing experiment is being conducted with E+ tall fescue to determine if herbicide suppression of seedheads can mitigate the negative effects of ergot alkaloids on steer weight gain and physiology. In the first year of the experiment, steers grazing herbicide-treated pastures had 64% greater average daily gain, reduced rectal temperatures, and a 6-fold increase in serum prolactin concentrations. Herbicide application also increased both crude protein and water soluble carbohydrates by two percentage units. Results of the first year of the experiment indicated that application of Chaparral herbicide to E+ tall fescue can suppress seedhead emergence to improve steer weight gain and reduce the severity of toxicosis. A 2nd year of data is needed to verify these results, but chemical treatment of endophyte-infected tall fescue could show to be a feasible option in improving weight gain and condition of fescue cattle.

Technical Abstract: Chaparral® herbicide has shown in small-plot experiments to suppress seed head emergence in tall fescue. A two-yr grazing experiment is being conducted with steers grazed on endophyte-infected tall fescue pastures that are either treated or untreated with Chaparral® herbicide. The objective of the experiment is to determine if suppression of seed head emergence and maturity can increase average daily gain (ADG) and alleviate fescue toxicosis. In 2009, herbicide treatments were assigned to six, 7.5-acre 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). Seed head concentrations were practically void in treated pastures, whereas untreated pastures had 94 seed heads/yd2. Ergot alkaloid (ergovaline plus ergovalanine) concentrations were 4-fold greater in seed than in leaf blades and sheaths. Whole tillers and leaf blades in treated pastures had higher (P < 0.10) crude protein (CP) and water soluble carbohydrates (WSC) than those in untreated pastures. Average daily gain of steers grazing treated pastures was higher (P < 0.05) than those grazing untreated pastures. Steers on treated pastures also had lower rectal temperatures and greater serum prolactin concentrations. Results in 2008 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. The grazing experiment is being repeated in 2010 to verify results of the first year.