Soybean meal mitigates vasoconstriction and serotonin suppression during fescue toxicosis in beef cattle

Legume supplementation has been shown to mitigate fescue toxicosis in beef cattle. However, no studies have directly compared the use of soybean-based feeds to alleviate symptoms of fescue toxicosis under controlled environmental heat and equalized feed intake. The objectives were to evaluate the effects of soybean hulls, whole soybeans, or soybean meal (SBM) supplementation on the mitigation of vasoconstriction, heat stress, and inflammatory responses in beef cattle during fescue toxicosis. Ten ruminally-cannulated Angus × Holstein steers were used in a 5 × 10 Latin rectangle design. Five treatments were evaluated: 1) non-toxic endophyte-infected tall fescue seed (NTE), 2) toxic endophyte-infected tall fescue seed (TE), 3) TE + soybean hulls (SBH), 4) TE + whole soybean (WSB), or 5) TE + SBM. Treatment periods lasted 7 d, followed by a 14-d washout. Heat stress was induced by maintaining room temperature at 32 °C during the light phase and 21 °C during the dark phase. Feed intake and feeding behavior were recorded daily. Caudal artery cross-sectional area (CAA) and hemodynamics were assessed before feeding (0 h), 4 h, and 8 h after the morning feeding on days -1 (baseline) and 7. Skin and rectal temperatures, respiration rate, blood pressure, heart rate were collected at 0 h, 4 h, and 8 h on days 1 and 7. Blood was collected at 0 h and 4-h on days 1 and 7. Treatments did not influence (P > 0.84) total dry matter (DM) intake. Steers receiving TE or TE + WSB consumed a greater (P = 0.02) number of meals compared with NTE and TE + SBH steers. Relative CAA decreased (P = 0.02) by 15.1 percentage units for TE compared with NTE and increased by 19 percentage units for TE + SBM compared with TE. Systolic blood pressure tended to increase (P = 0.06) in steers fed TE compared with steers receiving NTE or TE + SBM. Respiration rate, rectal temperature, and neck skin temperature were not influenced by soybean feed supplementation (P ≥ 0.24). Rump skin temperature decreased (P = 0.02) in steers receiving TE compared with NTE and TE + SBM. SBM supplementation increased (P = 0.01) serum serotonin concentration by up to 34% compared with other TE-containing treatments. Dietary supplementation of soybean-based feeds has the potential to alleviate symptoms of fescue toxicosis in cattle. Among soybean-based feeds evaluated, SBM was the most effective because of the mitigation of ergot alkaloid-induced vasoconstriction, promotion of vasodilation, and restoration of serum serotonin and normal feeding behavior in beef cattle challenged with fescue toxicosis.