Two 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 × Hereford ruminally cannulated steers (200 ± 6 kg BW) were blocked by BW and within block assigned to 1 of 4 treatments containing YCW at 0, 20, 40, or 60 g/d.  Tall fescue straw (579 ppb ergovaline) was provided at 120% the previous 5-d average intake with soybean meal 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 affected by YCW supplementation (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), but prolactin stores linearly increased as YCW increased (P = 0.05).  In Exp. 2, 60 Angus × Hereford cows (517 ± 5 kg BW; approximately 200 d of gestation) were stratified by BCS (5.0 ± 0.1) and randomly assigned to the same 4 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 control (CON).  Cows were provided ad libitum access to straw and were supplemented with soybean meal daily.  One cow was removed from the 40 g/d treatment due to clinical signs of fescue foot.  No differences (P > 0.20) were observed in pre- or post-calving BCS change or post-calving BW change.  Control cows gained more weight (P = 0.02) pre-calving compared to 0 g/d cows.  A linear increase (P = 0.04) in milk production at 60 d post-partum was observed as YCW increased.  Serum prolactin post-calving and change from initial to post-calving increased linearly (P = 0.02 and P = 0.06, respectively) with increasing YCW supplementation.  Also, post-calving serum prolactin was less for 0 g/d YCW compared with CON (P = 0.003) and 20 g/d (P = 0.04).  The YCW seems to alleviate the prolactin depression normally associated with fescue toxicosis and, therefore, has the potential to be used successfully with other management practices when feeding or grazing high-alkaloid tall fescue.


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