186. Red Clover Leaf Supplementation Improves Thermoregulation in Cattle.

Abstract Previous research has demonstrated that flavonoid supplementation via legumes, including low levels of red clover (RC) leaf, can partially alleviate fescue toxicosis-associated vasoconstriction. However, no research has evaluated how this vasoactivity might relate to thermoregulatory capacity. Therefore, the objective was to determine if low levels of RC could improve thermoregulation with or without a fescue toxicosis challenge. Twenty-four ruminally cannulated Angus × Holstein steers (body weight [BW], block 1: n = 12; 521.0±9.1-kg and block 2: n = 12, 360.8±11.6-kg) were stratified by BW and randomly assigned to one of four treatments: 1. endophyte-free tall fescue (E-TF) seed with orchard grass (OG) control (E-OG; ground 5-mm), 2. endophyte-infected tall fescue (E+TF) seed (15 µg/kg BW ergovaline+ergovalanine) with OG control (E+OG; 25 g/head/d), E+TF seed with ground RC leaf (E+RC; 25 g/head/d RC leaf), or E-TF seed with ground RC leaf (E-RC). All steers were first adapted to their basal diet (corn silage-based diet, ad libitum) for 14-d and then were subjected to treatments for 14-d including a 7-d thermoneutral (TN) period (d-1 to 7; Temperature-Humidity Index [THI]:67) followed by a 7-d heat stress (HS) period (d-8 to 14; THI:70.3–83.5). Treatments and seed were dosed via the rumen cannula daily prior to feeding (0730-h). On the last day of the adaptation period (baseline) and daily thereafter, caudal artery hemodynamics using color Doppler ultrasonography, rectal temperature (RT), and respiration rate (RR) were collected at 1230-h. Data were analyzed using the MIXED procedure with repeated measures of SAS (v.9.4). At baseline, RR was 50 breaths/min and RT was 38.5 ˚C. By d-4 of the TN period and for the remainder of the study, caudal artery luminal areas were greater and blood flow rate were faster in both RC treatments than E+OG, with E-OG being intermediate (P < 0.01). The E+OG steers had greater RT (+0.3˚C) and RR (+13 to 19 breaths/min) in comparison with all other treatments, by d-5 and 6 of the TN period, respectively (P < 0.05). During the HS period, RT and RR increased in all treatments, peaking on d-13 (P < 0.05). On d-10 to 14 of the HS period, RT was highest in E+OG (39.8 to 40.3 ˚C), lowest in both RC treatments (-0.7 to -1.2 ˚C, relative to E+OG), and intermediate in E-OG (-0.4 to -0.7 ˚C, relative to E+OG) (P < 0.05). Similarly, RR was highest in E+OG (83 to 143 breaths/min), lowest in E-RC and E+RC (-35 to -45%, relative to E+OG), and intermediate in E-OG (-18 to -31%, relative to E+OG), over the entire HS period (P < 0.05). In conclusion, supplementation of 25-g of red clover leaf daily altered arterial hemodynamics and improved thermoregulation in steers with or without fescue toxicosis challenge.