Effects of isoflavone supplementation, via red clover hay, on the growth and postgraze physiological recovery of beef steers grazing endophyte-infected tall fescue pastures.

Livestock grazing endophyte-infected (E+) tall fescue can exhibit persistent systemic vasoconstriction and fescue toxicosis. Isoflavones in legumes, most notably red clover (RC), are known hypotensive agents. The objective of the experiment was to evaluate the effect of isoflavone supplementation via RC hay, every day or every other day (QOD), on average daily gain (ADG) of steers grazing E+ tall fescue pastures and their physiological recovery after grazing when managed on a non-toxic diet (28 d). In Spring 2018 (62 d) and 2019 (65 d), 36 Angus× steers were blocked by initial body weight (BW) into pastures (n = 6 pastures year-1) assigned to one of three supplement treatments: Dried distillers' grain with solubles (DDGS) only (CON; pasture + DDGS), 7.5% RC hay daily (based on 300-kg steer consuming 2.5%-BW; 7.5%-RC; pasture + DDGS + RC hay), or 7.5% RC hay QOD (7.5%-RC QOD; daily rotation of either: CON or 7.5%-RC). All supplements were isonitrogenous, and shrunk BW was collected at the start and end of the grazing period for determination of ADG. Each year, immediately after the grazing period, three steers were randomly selected from each pasture and moved to outdoor partially covered pens where they were fed a non-toxic diet for a recovery period of 28 d. The caudal artery of each steer was imaged immediately after grazing and then every 7 d over the recovery period using Doppler ultrasonography to monitor artery luminal areas (CAA). Jugular blood samples were collected immediately after grazing and on day 28 of recovery for determination of serum prolactin. Data were analyzed using the Mixed procedure of SAS with repeated measures, and significance was set at P ≤ 0.05. Steer ADG was greater with 7.5%-RC daily or QOD (7.5%-RC: 0.77 kg d-1; 7.5%-RC QOD: 0.83 kg d-1), compared to CON (0.55 kg d-1; P < 0.01). Isoflavone supplementation via RC hay during the grazing period resulted in 2.5× larger CAA and higher serum prolactin concentrations immediately after grazing and over the recovery period, respectively, compared with CON steers (P < 0.05). Furthermore, steers supplemented with 7.5%-RC during the grazing period were never vasoconstricted over the recovery period, but CON steers did not exhibit CAA recovery until day 21 (P < 0.05). Supplementation of isoflavones via RC hay daily or QOD is an effective management strategy to improve ADG in cattle grazing E+ tall fescue pastures and reducing the time required for physiological recovery post-grazing.