Abstract
Ergot alkaloids in endophyte-infected tall fescue (E+TF) can negatively impact the welfare and growth of cattle by altering the physiology of the animal including systemic vasoconstriction resulting in decreased capacity for thermoregulation and increased heat stress susceptibility. The isoflavones in legumes, specifically red clover (RC), are known hypotensive agents and can be used to prevent vasoconstriction associated with E+TF. Based on the vasodilatory properties of RC, the objective of this study was to determine if low levels of RC could also prevent fescue toxicosis-related heat stress (HS). We hypothesized that cattle receiving E+TF seed with RC will have reduced rectal temperature (RT) and respiration rates (RR) in comparison to those that receive isoflavone-free orchardgrass (OG) when subjected to a HS challenge. Two blocks of 6 ruminally-fistulated beef-cross steers (Initial body weight (BW), block 1: 521.0±9.1kg and block 2: 360.8±11.6kg) were dosed intraruminally with E+TF seed (15 µg kg BW(-1) ergovaline + ergovalanine) daily, and stratified by BW into 2 treatments of either 25 g head d(-1) ground [5 mm] OG (E+OG) or RC leaf (E+RC). All steers were first adapted to their basal diet (ad libitum corn silage + dried distillers’ grains with solubles) for 14-d and then were subjected to treatments for 14-d including a 7-d thermoneutral period (TN: 21.0˚C, 64.0% humidity, Temperature-Humidity Index (THI)=67) followed by a 7-d HS challenge period (cyclical daily temperatures to mimic diurnal patterns; HS: 23.2˚C–33.6˚C, 39.5–70.0% humidity, THI=70.3–83.5). At 0630, 1230, and 1730-h each day, RR and RT were measured by counting flank movements for 15 s and using a rectal thermometer, respectively. Data were analyzed using the MIXED procedure with repeated measures of SAS (v.9.4) with significance observed at P ≤ 0.05. At the end of the adaptation period (no treatments), RR was 50±1.4 breaths·min(-1) and RT was 38.6±0.05 ˚C on average. By days 5 and 7 of the thermoneutral period E+OG steers had greater RT (+0.5˚C) and RR (+11 breaths·min(-1)) in comparison with E+RC steers, regardless of sampling time, respectively (P < 0.05). During the HS challenge, RT and RR continued to increase in E+OG steers from day 8 to day 12 (day 13 for RR 0630-h) peaking at 40.1˚C and 121 breaths·min(-1) at 0630, 40.3˚C and 143 breaths·min(-1) at 1230, and 40.6˚C and 161 breaths·min(-1) at 1730-h (P < 0.01). Rectal temperature and RR also increased in E+RC steers during HS period, peaking on days 12 and 13 (0630-h: 39.2˚C, 85 breaths·min(-1); 1230-h: 39.5˚C, 106 breaths·min(-1); 1730-h:39.4˚C, 109 breaths·min(-1)); however, on all days and sample times, RR and RT were lower in E+RC compared to E+OG steers (P < 0.05). Results of the current study indicate that a small amount of RC daily can partially attenuate fescue toxicosis-related heat stress.