330. Soy Essential Amino Acids and Isoflavones Mitigate Ergot Alkaloid-induced Serotonin Suppression in Beef Cattle.

Abstract Soybean meal supplementation has been shown to mitigate ergot alkaloid-induced vasoconstriction and serotonin suppression in beef cattle. However, the mechanisms underlying these responses have not yet been fully established. The objective of this study was to evaluate whether supplementation with soy essential amino acids (AA) or isoflavones mitigates ergot alkaloid-induced vasoconstriction and serotonin suppression in beef cattle. Six ruminally-cannulated Angus × Holstein steers [body weight (BW) = 381 ± 12.5 kg] were randomly assigned to a 6 × 6 Latin square design. Treatment periods lasted 7 d and were separated by 14 d washout periods. Treatments consisted of: 1) non-toxic novel endophyte-infected tall fescue seed (NTE); 2) toxic endophyte-infected tall fescue seed (TE); 3) TE supplemented with soybean meal (TE+SBM); 4) TE supplemented with soy essential AA (TE+AA); 5) TE supplemented with soy isoflavones (TE+ISO); and 6) TE supplemented with both soy isoflavones and essential AA (TE+ISO+AA). Soybean meal (for TE+SBM) or corn (for all other treatments) was supplemented at 2.75 g/kg BW on a dry matter (DM) basis. Purified soy isoflavones (genistein and daidzein) and crystalline essential AA were supplemented to match the amounts of isoflavones and essential AA provided by the TE+SBM treatment. Steers were pair-fed a corn silage-based diet daily according to the lowest DM intake recorded on the previous day. Caudal artery cross-sectional area (CAA) and hemodynamics were assessed using color Doppler ultrasonography at 3, 5, and 7 h after morning feeding on day -1 (last day of the washout period) and day 7 of each treatment period. Blood samples were collected from the jugular vein at 0 h (before feeding) and at 1, 2, 4, 6, 8, 10, and 12 h after feeding on d 7. Serum concentrations of tryptophan, kynurenine, 5-hydroxytryptophan, serotonin, and 5-hydroxyindoleacetic acid were quantified using HPLC. No treatment × time interactions (P ≥ 0.27) were observed for serum serotonin or hemodynamic variables. Serum serotonin concentrations were decreased (P < 0.05) by 46% in steers receiving TE compared with NTE. Supplementation with SBM increased serum serotonin concentration by 55.6% relative to TE. Similarly, serum serotonin concentrations were increased by 44.2%, 40.4%, and 75% in steers receiving TE+AA, TE+ISO, and TE+AA+ISO, respectively, compared with TE. Compared with NTE, all TE-containing treatments reduced (P ≤ 0.01) CAA, blood flow, and stroke volume by 45.6%, 40.5%, and 38.2%, respectively. These results demonstrate that soybean meal and purified combinations of amino acids, isoflavones, or both attenuate ergot alkaloid-induced serotonin suppression that occurs during fescue toxicosis. These findings suggest that the increased serum serotonin seen with SBM supplementation is likely mediated by both essential AA and isoflavones. However, none of the supplementation strategies alleviated ergot alkaloid-induced vasoconstriction.