Evaluating Winter Pasture Nitrogen and Weight Gain of Cattle Fed Ionophore.

A study was initiated during two consecutive winter grazing seasons (2005-2007), using two annual ryegrass (RG) (Lolium multiflorum Lam.) cv. Jumbo grazing systems to determine if adding ionophore to replacement heifer grazing supplement improves animal performance and nutrient absorption efficiency, therefore reducing nutrients excreted in urine and manure. The two RG, grazing systems [tilled seedbed (TS) vs. over-seeding (OS) into dormant bahiagrass (Paspalum notatum Flugge)] were compared for possible interaction with the dietary treatments. Eight 1.33 ha pastures were utilized. Pastures were seeded in the late fall. Six crossbred yearling beef heifers were allocated to each pasture. All heifers were fed a pelleted concentrate (ground corn) either with or without ionophore (monensin sodium). Forage and soils were periodically sampled from a high congregation area (area where feed, water, shade and mineral were provided) and a low congregation area (back of pasture). Monensin concentrations in heifer excretia ranged from below detection (< 0.05 mg kg^-1) to over 1 mg kg^-1. Forage crude protein (CP) was not affected by grazing system but it peaked with the March clipping at 18% CP. Forage dry mass was 9.7 and 3.9 Mg ha^-1 yr^-1 for the TS and OS treatments, respectively, in year 1 and only 6.5 and 3.4 Mg ha^-1 yr^-1, respectively, in year 2. Although both years were droughty, year 2 received only 70% the rainfall of year 1, which affected TS pasture yield more than OS pasture yield. Initial soil water content in the upper 10 cm of the OS pasture treatment was approximately 50 % of the TS pasture but lower soil depth moisture tended to be greater in the OS treatment, throughout the season. Soil moisture and bulk density tended to be greater in the congregation zones. Nitrogen interactions will be presented.