Conventional loose mineral with added red clover leaf (Trifolium pratense L.) reverses vasoconstriction associated with tall fescue toxicosis in steers

Endophyte-infected tall fescue (E + TF) can cause persistent vasoconstriction in ruminants and consequently fescue toxicosis. Legumes, including red clover (RC), contain isoflavones that are known hypotensive agents. Previous experiments have shown the benefits of overseeding RC or feeding other stored legumes on blood flow and growth performance of livestock challenged with E + TF; however, alternative supplementation strategies have not been explored. Therefore, the objective was to determine if low levels of RC in conventional loose mineral could mitigate fescue toxicosis symptoms. Mature, rumen fistulated steers (n = 12) received 1 of 4 mineral treatments (5 oz head d⁻¹ or 141.8 g head d⁻¹ dosed intraruminally; n = 3 steers per treatment): 1) 0% RC (w/w), 2) 10% RC, 3) 15% RC, or 4) 20% RC. All steers were first adapted to their basal diet (ad libitum corn silage + 2.27 kg dried distillers’ grains with solubles) for 14-days and then were subjected to a 14-day E + TF seed step-up challenge (day 0–7: 10 µg kg BW⁻¹ ergovaline + ergovalanine [ERV]; day 8–14: 15 µg kg BW⁻¹ ERV), followed by a 7-day withdrawal period. The caudal artery of each steer was imaged repeatedly over the course of the study using Doppler ultrasonography to monitor artery luminal areas (CAA). Jugular blood samples were serially collected to monitor serum prolactin concentrations. Data were analyzed by repeated measures using the mixed procedure of SAS with significance set at P < 0.05. A treatment by sample day interaction was detected for CAA (P = 0.0051) and prolactin (P = 0.0148). The E + TF seed challenge induced vasoconstriction (d1: −54.6%, d7: −26.8%, d14: −80.1%, relative to baseline) and prolactin depression (d7: −35.5%, d14: −83.3%, relative to baseline) in 0% RC steers (P < 0.05, in all cases). All three RC mineral treatments were effective at partially or fully alleviating vasoconstriction and prolactin depression with the 20% RC treatment being most effective on day 14 (CAA: 10% RC: +37.4%; 15% RC: +29.7%; 20% RC: +56.9%, prolactin: 10% RC: +125.3%; 15% RC: +132.1%; 20% RC: +149.2%, relative to 0% RC; P < 0.05, in all cases). All treatments fully recovered (above baseline) by the end of the withdrawal period (P < 0.05, in all cases). These results demonstrate that loose mineral amended with RC could be used to prevent the adverse effects of fescue toxicosis in E + TF grazing livestock. Additional experiments are required to determine the optimum isoflavone source and concentration required to mitigate fescue toxicosis.