Effects of stocking rate, season, and changes in fungal endophyte level on performance of steers grazing tall fescue

A study was conducted to evaluate steer performance on tall fescue (Festuca arundinacea Schreb.) pastures with different Neotyphodium coenophialum [(Morgan-Jones and Gams) Glenn, Bacon, and Hanlin] endophyte infestation levels ranging from < 5 to > 80% and three stocking densities (SD). Eighteen 1.2-ha pastures were grazed with newly weaned Angus (Bos taurus L.) and Angus cross steers. Each pasture was assigned one of four initial endophyte (E+) levels of about 0, 20, 40, and 80% infestation. Steer and available forage weights were measured at 21-d intervals. Average daily gain (ADG) and beef production for each season were 335 g d^-1 and 100 kg ha^-1 for fall/winter, 480 g d^-1 and 155 kg ha^-1 for spring, and 260 g d^-1 and 38 kg ha^-1 for summer, respectively. Seasonal ADG for fall/winter, spring, and summer were 475, 556, and 297 for low SD; 286, 496, and 317 for medium SD; 242, 389, and 166 g d^-1 for high SD. Increasing SD from low to high resulted in decreased ADG due to low amounts of available forage at high SD. Decreases in ADG and beef production per unit of E+ level increase were largest at high SD, implying that E+ tall fescue plants increased in number more at high SD than at low or medium SD, and that reduced gains may have been due to increased proportions of E+ tall fescue ingested by steers as a result of E+ level shift. Performance of steers grazing tall fescue pastures changing in E+ level over four years was affected by SD and season interactions and varied with treatment combinations.