Endophyte infection level of tall fescue stockpiled for winter grazing does not alter the gain of calves nursing lactating beef cows.

We examined the effect of endophyte infection level of tall fescue (Festuca arundinacea Schreb.) used for stockpiled forage on the performance of lactating, fallscalving beef cows and their calves. Treatments were endophyte infection levels of 20% (low; SEM = 3.5), 51%, (medium; SEM = 1.25), and 89% (high; SEM = 2.4; 4 replications/treatment). Five cow-calf pairs grazed in each replicate (n = 60 cow-calf pairs/yr) for 84 d (phase 1) starting on December 2, 2004 (yr 1), and December 1, 2005 (yr 2). After 84 d of grazing each treatment, the cattle were commingled and fed as a single group (phase 2) until weaning in April of each year. Phase 2 allowed measurement of residual effects from grazing stockpiled tall fescue with varying levels of endophyte infection. Pregrazing and postgrazing forage DM yield, forage nutritive value, and total ergot alkaloid concentrations of forage were collected every 21 d during phase 1. Animal performance data included cow BW, ADG, and BCS, as well as calf BW and ADG. Animal performance was monitored during both phases. Endophyte infection did not affect (P = 0.52) apparent intake (pregrazing minus postgrazing forage DM yield) of stockpiled tall fescue, because each cow-calf pair consumed 16 +/- 1.7 kg/d regardless of treatment. Cow ADG during phase 1 was -0.47 +/-0.43 kg for the low treatment, which was greater (P < 0.01) than either the medium (-0.64 +/-0.43 kg) or high (-0.74 +/- 0.43 kg) treatments. However, cows that had grazed the high or medium treatments in phase 1 lost -0.43 and -0.57 (+/-0.24) kg/d, respectively, which was less (P < 0.01) BW loss than the cows in the low (-0.78 +/- 0.24 kg/d) treatment during phase 2. By the end of phase 2, cow BW did not differ (528 +/-27 kg; P = 0.15). Body condition score for cows in the low treatment was greater (P = 0.02) than that of the medium and high treatments at the end of phase 1. Body condition scores did not change appreciably by the end of phase 2, and differences among treatments remained the same as at the end of phase 1 (P = 0.02). In contrast to cow performance, calf ADG was unaffected (P = 0.10) by endophyte level and averaged 0.73 +/- 0.07 kg during phase 1 and 0.44 +/- 0.04 kg during phase 2. Our data suggest that fall-calving herds can utilize highly-infected tall fescue when stockpiled for winter grazing, with little impact on cow performance and no impact on calf gain.