Impact of Mineral Supplementation on Intake Behavior and Blood Flow in Growing Beef Cattle Exposed to Fescue-Derived Alkaloids

Consumption of endophyte-infected tall fescue by cattle has repeatedly been shown to negatively impact dry matter intake (DMI), growth rate, and circulating prolactin concentrations. The objective of the current study was to determine if a mineral supplementation would mitigate the negative effects of fescue-derived alkaloid consumption on feed intake, circulating prolactin, and the vascular system. Twelve Angus crossbred steers were used in a triplicated Latin square design consisting of three 28 d experimental periods. Treatments consisted of three top-dressed mineral supplements (142 g/head/d): a non-medicated control (CON), commercially available Fescue EMT® Mineral Defense (EMT), and a test prototype (EMT 2). Each period was composed of an adaptation/washout subperiod (d 1-14), a step-up sub-period (d 15-21), and a sampling subperiod (d 22-28). During the adaptation/washout sub-period, all steers received CON and were fed a basal diet, composed primarily of alfalfa haylage and corn silage, 1.5 x NEm. During both the step-up and sampling subperiods, steers received their appropriate supplemental treatment and were fed the basal diet containing 15% KY-31 endophyte-infected seed on an as-fed basis. The amount of feed offered was increased incrementally during the step-up period to allow for ad-libitum intake by d 19 and throughout the sampling period. Steers were housed indoors in individual pens and the room temperature was cycled above thermoneutral (~29.4°C) 16 h from 0600-2200 and thermoneutral (~21.1°C) 8 h from 2200 to 0600 to mimic summer conditions. A light:dark cycle was set corresponding to the room temperature cycle. Steers were fed each d at 0830 and the amount offered was recorded. DMI was determined daily based on the DM of feed delivered and the corresponding ort. Meal feeding behavior (i.e., meal size, meal number, and meal duration) was monitored using feed bunks suspended from load cells with communications to a data handler. Daily water consumption was determined using in-line meters. Rectal temperature and respiration rate were recorded daily at 1200h during the treatment sub-period. The arterial luminal cross-sectional area of the caudal artery was measured using a Doppler ultrasound on d 14, d 21, and d 28 of each period. Blood samples were collected on d 14 and d 28 of each period for the determination of circulating prolactin. The EMT treatments reduced (P ≤ 0.04) DMI during the last 7 days of the treatment sub-period. The EMT treatments did not affect water intake (P = 0.67) during the treatment sub-period. The EMT treatments tended (P = 0.07) to decrease meal frequency but did not affect (P ≥ 0.37) meal size or duration. Prior to treatment on d 14, the arterial luminal cross-sectional area of the caudal artery was smaller for steers (P ≤ 0.008) assigned to EMT treatments compared with those assigned to CON. On d 21, the arterial luminal cross-sectional area was greater for EMT (P = 0.007) and EMT 2 (P = 0.10) compared with control. On d 28, the arterial luminal cross-sectional area tended (P = 0.06) to be greater for EMT than control. EMT (P = 0.0004) and EMT 2 (P = 0.02) reduced endophyte-induced vasoconstriction on d 21. Endophyte-induced vasoconstriction on d 21 tended (P = 0.09) to be less for EMT compared with EMT 2. On d 28, endophyte-induced vasoconstriction was lower for EMT compared with CON (P = 0.002) and EMT 2 (P = 0.07), with no difference (P = 0.11) between CON and EMT 2. Prolactin concentration decreased following the addition of endophyte-infected seed, but concentration was unaffected (P ≥ 0.50) by EMT treatments. Rectal temperature and respiration rate were unaffected (P = 0.12) by EMT treatments. Overall, induction of fescue toxicosis was achieved throughout the experiment. The EMT 2 formulation was not as effective in reducing vascular constriction and did not benefit the EMT formulation. This data suggests EMT formulation is effective in partially alleviating the negative impacts of fescue-derived alkaloids when consumed by cattle