Chaparral Herbicide Application for Suppression of Seedhead Emergence in Tall Fescue Pastures and Possible Alleviation of Fescue Toxicosis

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/yd. 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. Introduction: Tall fescue is well adapted and widely used for cattle grazing in a region commonly referred to as the “fescue belt”, which lies east of the Great Plains and between the temperate Northeast and subtropical Southeast. A fungal endophyte (Neotyphodium coenophialum) infects tall fescue plants and produces ergot alkaloids that induce a toxicosis. Symptoms of the ‘fescue toxicosis’ include retention of rough hair coats, elevated body temperature, labored respiration, and decreased prolactin _________________________ 1 This research is funded by Dow AgroSciences LLC. 2 Mention of trade names or commercial products in the article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA 3 Research Animal Scientist/Agronomist, USDA-ARS Forage-Animal Production Research Unit; Professor, Department of Plant and Soil Sciences, University of Kentucky; Plant Physiologist, USDA-ARS Forage-Animal Production Research Unit, Lexington, KY 40546, respectively. concentrations (Schmidt and Osborn, 1993). A major problem for cattle exhibiting toxicosis is vulnerability to severe heat stress in the presence of high ambient temperature and humidity (Hemken et al., 1981). Poor BW gain efficiency and diminished market value of beef calves exhibiting symptoms of toxicosis have limited the use of tall fescue for commercial stocker production (Hoveland, 1993). Therefore, the grass is primarily used for cow-calf production even though ergot alkaloids can reduce both reproductive performance and milk production. Rottinghaus et al. (1991) ranked plant parts in ergovaline concentrations as seed heads > stems & leaf sheaths > leaf blades, with concentrations in seed heads sometimes 3to 5-fold greater than those in leaf blades. High ergot alkaloid concentrations in seed heads can be problematic because cattle may selectively graze seed heads (Aiken et al., 1993). Chaparral herbicide (Dow AgroSciences LLC, Indianapolis, IN 46268) application to tall fescue pastures has potential to enhance calf weight gain efficiency and alleviate fescue toxicosis by suppressing emergence of seedheads. A 2-yr grazing experiment is being conducted to evaluate effects of Chaparral herbicide on seed head suppression and determine if the suppression positively affects average daily gain (ADG) and two physiological measures, rectal temperature, and serum prolactin. All pastures were previously encroached by Kentucky bluegrass; therefore, a second objective is to determine if competitiveness of fescue with bluegrass is affected by the suppression of seedheads. This paper reports results from the first year of data collection. Materials and Methods: The grazing experiment is being conducted at the University of Kentucky Animal Research Center in Woodford County. The experimental protocol was reviewed and accepted by Institution’s Animal Care and Use Committee at the University of Kentucky. With or without Chaparral herbicide treatments were assigned to 6, 7.5-ac pastures of endophyte-infected tall fescue as a randomized complete block design with three replications. The herbicide was sprayed on 4 Apr., 2009 at 2 oz/ac. Polycoated urea (ESN; Agrium Inc., Calgary, Alberta) was applied at a rate of 60 lb N/ac. Forty-eight crossbred steers [initial BW = 614 ± 48 (s.d.)], of primarily Angus breeding, were blocked by BW for random assignment to 6, 7.5-ac pastures of endophyte-infected tall fescue (8 steers/pasture). Cattle were weighed following a 12to 14-h fasting from feed and water on the initial (9 April) and final (1 July) days of grazing (83 d). Rectal temperature was recorded and jugular blood was collected for each steer prior to fasting on the final day of grazing. Rectal temperatures were recorded in the late afternoon following daily maximum ambient temperatures. Blood was centrifuged for 15 min at 10,000 x g and serum was stored frozen (-20°C) for assaying prolactin following procedures of Bernard et al. (1993). Forage mass was estimated at 2-wk intervals with a falling disk meter. Disk meter height was recorded for 50 randomly chosen locations within each pasture. Calibration samples for regressing sample dry weight over mean disk meter height to estimate forage mass were collected on 25 May and 20 June by clipping forage below the disk meter plate to the soil surface at 5 random locations per pasture. Single tillers from 25 randomly chosen plants were clipped at the crown in each pasture on 5 June in 2007 to estimate endophyte infection percentages using immunoblot test kits (Agrinostics Ltd. Co., Watkinsville, GA). At 2-wk intervals, single tillers were clipped at the crown from 25 randomly chosen plants within each pasture. A second set of tillers were collected on 20 May for hand-separation into leaf blade, sheath, and seedhead components, with vegetative tillers being collected in treated and untreated pastures, and reproductive tillers being collected in untreated pastures. Whole tillers and tiller components were analyzed for ergovaline and ergovalanine ergopeptines using a modified HPLC florescence procedure (Yates and Powell, 1988). Seed heads per m was determined for each pasture on 30 June by counting reproductive culms within a 0.17 m ring placed in 10 randomly chosen sites. Bluegrass-tall fescue percentages in the stands were estimated on 19 May using point-transects (n = 500 points/pasture). Whole tillers and tiller parts were analyzed for N using a Leco FP-215 N Analyzer (CP = %N x 6.25). Water-soluble carbohydrates were extracted by agitating 250 mg tissue in 25 mL water for 3 hours at room temperature. Extracts were filtered, brought to 25 mL with water, and assayed colorimetrically for WSC, using a potassium ferricyanide assay based on the protocols of Dairy One (Ithaca, NY). All responses were statistically analyzed using PROC MIXED of SAS (SAS Inst. Inc., Cary, NC). Sampling date was evaluated as a repeated measure in analyses of CP and WSC. Results and Discussion: Endophyte infection percentages of tall fescue tillers in pastures averaged 89.3 ± 3.0 (SEM) %. Dry matter percentage of tall fescue was not affected by herbicide treatment and averaged 56.0 ± 5.7%. Although there was considerable bluegrass, it matured 2 to 3 wk earlier than fescue and cattle were observed to selectively graze fescue, particularly after bluegrass had set seed. Seed heads/yd were greater in untreated (94 ± 6 seed heads/yd) than treated (6 ± 2 heads/yd) pastures, which clearly indicated that the herbicide treatment suppressed seed head emergence of tall fescue. Mean forage availability was less (P < 0.10) in treated (3027 lb DM/ac) than untreated (3780 lb DM/ac) pastures. A portion of the higher availability in untreated pastures was due to the higher quantity of stems and seed heads. It was uncertain if herbicide treatment adversely affected vegetative growth rate. Ergot alkaloid concentrations in whole tillers did not differ (P > 0.10) between treated and untreated pastures, but seeds produced in untreated pastures had 4-fold greater ergovaline concentrations than leaf blades (Table 1). Reproductive culms counted on 30 June were practically void of seed heads, with only stems remaining from apparent selective grazing of seed heads by cattle. Therefore, cattle on the untreated pastures consumed high concentrations of ergot alkaloids when selectively grazing seed heads. Whole tillers in treated pastures were higher in CP (P < 0.10) and WSC (P < 0.10) than in untreated pastures (Table 2). Leaf blades and sheaths from whole tillers that were collected on 20 May, were analyzed to determine if lower CP and WSC in untreated pastures was due to a dilution effect of low-quality stems in whole tillers. Leaf blades in treated pastures were higher (P < 0.10) in CP and WSC than those in untreated (vegetative and reproductive tillers combined) pastures. Leaf sheaths from treated pastures were higher (P < 0.10) in CP, but there was no treatment effect on WSC of sheaths. Suppression of seed heads reduces translocation of N and carbohydrates from leaves to seeds conserving CP and WSC concentrations in leaves. Steer ADG in treated pastures was 64% greater (P < 0.05) than in untreated pastures (Table 3). Rectal temperature with treated pastures was 1.2°F lower than with untreated pastures. The elevation in rectal temperature of steers grazing treated pastures could be partially attributed to fescue toxicosis, but a major part of it was likely due to the measurements being recorded in the late afternoon after the maximum ambient temperature had reached 96°F. Serum prolactin concentrations with treated pastures were 6.5-fold greater than those on untreated pastures to suggest at least a reduction in the severity of toxicosis in steers grazing herbicide treated pastures. Results from this first year of data collection indicate that toxic endophyteinfected fescue pastures can be treated with Chaparral herbicide to substantially boost steer weight gain and reduce the severity of toxicosis by minimizing ingestion of seedheads with high alkaloid content. However, a second year of data will be needed to verify these results.