Graduation date: 2007Presentation date: 2007-02-05Two experiments were conducted to evaluate the influence of a yeast-derived cell wall preparation (YCW) on forage intake and digestibility, ruminal fermentation characteristics, serum prolactin and prolactin stores, and milk production in beef cattle consuming high-alkaloid tall fescue straw. In Exp. 1, 16 Angus x Hereford ruminally cannulated steers (200 ± 6 kg BW) were blocked by BW and within block assigned to one of four treatments containing YCW at: 1) 0, 2) 20, 3) 40, or 4) 60 g hd-1 d-1. Tall fescue straw (579 ppb ergovaline) was provided at 120% the previous 5-d average intake with soybean meal (0.068% BW; CP basis) used as a CP supplement. In the 29-d digestion study, total DM, OM, and NDF intake, and DM, OM, and NDF digestibilities were not different (P > 0.13). Linear decreases in ruminal indigestible ADF outflow (P = 0.10) and liquid dilution rate (P = 0.03) were noted as YCW increased. Weekly serum prolactin was not affected by treatment (P > 0.50); however, prolactin stores linearly increased as YCW increased (P = 0.05). In Exp. 2, 60 Angus x Hereford cows (517 ± 5 kg BW; approximately 200 d gestation) were stratified by BCS (5.0 ± 0.1) and randomly assigned to the same four YCW treatments as Exp. 1 (447 ppb ergovaline high-alkaloid straw) with the addition of a low-alkaloid straw (149 ppb ergovaline; no YCW supplementation) as a positive control (CON). Cows were provided ad libitum access to straw and were supplemented with soybean meal (0.1% BW; CP basis) daily. One cow was removed from the 40 g hd-1 d-1 treatment due to clinical signs of fescue toxicosis. No differences (P > 0.20) were observed in pre- or post-calving BCS change or change in post-calving BW. Control cows gained more weight (P = 0.02) pre-calving compared to 0 g hd-1 d-1 cows. A linear increase (P = 0.04) in milk production was observed as YCW increased at 60 d post-partum. Serum prolactin post-calving and change from initial to post-calving increased linearly (P = 0.02 and P = 0.05, respectively) with increasing YCW supplementation. Also, post-calving serum prolactin was higher (P = 0.002) in CON compared to 0 g hd-1 d-1 cows. The YCW seems to alleviate some symptoms of the fescue toxicosis and, therefore, has the potential to be used successfully with other management practices when feeding or grazing high-alkaloid tall fescue.