Influence of Chute Exit Velocity and Fescue Toxicosis on Steer Grazing and Finishing Growth, Immune Responses, and Carcass Characteristics

Abstract

Mixed breed steers (n=120) from 3 sources were weighed and evaluated for exit velocity. Steers were assigned an exit velocity treatment (high/low; based on relative ranking in measured exit velocities within source), and assigned to either toxic endophyte or novel endophyte tall fescue pastures (n=20 pastures; 6 animals/pasture) blocked by source with equal representation of exit velocity treatments in each pasture. Pasture groups were randomly assigned to a control or glucomannan supplement. Following grazing, steers were transitioned to drylot pens (n=40 pens) for finishing after 110d on pasture and harvested at approximately 681 kg. Drylot pen assignment was established by placing steers of the same exit velocity treatment within each pasture in the same pen, and steers were fed a corn-based finishing diet. Blood was collected on d110, 124, and 138 for evaluation of peripheral lymphocyte interferon-γ production, with additional blood collected on d138 to measure Leptospirosis pomona titer response. Supplementation did not affect responses (P>0.10). Finishing and carcass data were analyzed as a 2 × 2 factorial (endophyte and exit velocity treatments). Steers on toxic endophyte pasture had lower grazing average daily gain (P<0.01) and higher finishing average daily gain (P=0.07) and gain:feed ratios (P<0.01). Low exit velocity steers consumed more dry matter (P=0.06) but had lower gain: feed ratios (P=0.01) over the finishing period. Steers grazing toxic endophyte fescue had higher titer responses to Leptospirosis pomona (P=0.09), but peripheral lymphocyte interferonγ production was not different (P ≥ 0.25) between endophyte or exit velocity treatments. Analysis of carcass data detected higher kidney, pelvic, and heart fat for toxic endophyte steers (P=0.05), and low exit velocity steers had higher final yield grades (P=0.10). No other carcass differences were detected. These data indicate previous exposure to toxic endophyte tall fescue may lead to compensatory growth during finishing and that effects of exit velocity on intake, efficiency, and growth may be more complex than previously recognized.