Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 22, 2011
Publication Date: March 1, 2012
Citation: Aiken, G.E., Goff, B.M., Witt, W.W., Kagan, I., Sleugh, B.B., Burch, P., Schrick, F.N. 2012. Steer and plant responses to chemical suppression of seedhead emergence in toxic endophyte-infected tall fescue. Crop Science. 52:960-969. Interpretive Summary: Tall fescue is a cool-season perennial grass extensively utilized for grazing and hay production on approximately 15 million ha of the United States, predominantly in a region east of the Great Plains and between the temperate Northeast and subtropical Southeast. A fungal endophyte infects tall fescue plants and produces ergot alkaloids that cause fescue toxicosis, with symptoms in cattle that include retention of rough hair coats, elevated body temperature, labored breathing, decreased prolactin concentrations, and poor performance. Cattle and horses selectively graze tall fescue seedheads, which can be problematic because ergot alkaloid concentrations in seedheads are greater than in leaf blades and sheaths. Chaparral herbicide applied early in vegetative growth suppresses seedhead emergence in tall fescue and may mitigate fescue toxicosis and enhance calf weight gain. A 2-yr grazing experiment was conducted to evaluate the effects of Chaparral herbicide on endophyte-infected tall fescue pastures on steer weight gains and physiology, plant ergot alkaloids concentrations, and pasture forage availability and nutritive values. Chemical suppression of seedheads reduced forage mass and did not affect ergot alkaloid concentrations in leaf blades and sheaths, but substantially reduced the density of reproductive tillers with seeds and stems. Daily weight gains were greater on herbicide treated pastures, which could be associated with a surge in consumed ergot alkaloids by steers grazing untreated pastures, and herbicide treated herbage having greater crude protein, water soluble carbohydrates, and in vitro digestibility. A biomarker of toxicosis, serum prolactin concentrations, indicated that the severity of toxicosis was reduced on treated pastures. Chemical suppression of tall fescue seedheads demonstrated to be effective management tool for use in backgrounding beef calves for the feedyard on endophyte-infected grass.
Technical Abstract: Chaparral® herbicide (Dow AgroSciences; Indianapolis, IN) has shown to suppress seedhead emergence in tall fescue [Lolium arundinaceum (Schreb.) Darbysh] and potentially mitigate the adverse effects of fescue toxicosis. A two-yr grazing experiment was conducted with steers grazed on endophyte-infected fescue-Kentucky bluegrass (Poa pratensis L.) pastures either treated or untreated with Chaparral herbicide to determine if seedhead suppression alleviates fescue toxicosis and increases average daily gain. Treatments were assigned to six, 3.0-ha pastures of endophyte-infected tall fescue-bluegrass pastures in a randomized complete block design with 3 replications. Pastures were grazed with 48 steers (2.7 steers ha-1) from 9 April to 1 July, 2009 and 6 April to 7 July, 2010. Reproductive tiller densities in treated pastures were low in both years (< 7 m-2) in comparison to higher densities in untreated pastures (113 and 69 reproductive tillers m-2 in 2009 and 2010, respectively). Ergopeptine (ergovaline & ergovalinine) concentrations were 3- to 6-fold greater in seeds than in leaf blades and sheaths of vegetative and reproductive tillers. Crude protein, water soluble carbohydrates, and in vitro digestible DM concentrations of whole tillers declined over time at a greater rate in untreated than in treated pastures. Steer average daily gain was greater on herbicide treated 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 indicated that Chaparral herbicide treatment can suppress reproductive development of tall fescue and subsequently increase weight gain and reduce the severity of toxicosis for grazing steers.