Growth Performance and Serum Mineral Concentrations of Stocker Calves Grazing Wheat Pastures and Fed Different Sources of Magnesium1,2

Magnesium supplementation has been shown to benefit feedlot cattle and is typically added at high concentrations to mineral supplements for calves grazing wheat pasture. Two experiments were conducted to determine the effects of Mg-mica supplementation on performance and serum mineral concentrations of stocker calves grazing wheat pasture. Supplemental Mg was provided at 6.1 g/d of supplemental Mg/ animal, Monday through Friday. In Exp. 1, 36 mixed-breed stocker calves (243 ± 3.9 kg BW) were stratified by BW and gender and allocated randomly to one of nine groups of four animals for a 112-d grazing study. Weight gain, shrink percentage, and serum Mg, Ca, K, Cu, and Zn concentrations did not differ (P>0.10) among calves fed either weathered Mg-mica (WMM), unweathered Mg-mica (UMM), or MgO. All serum mineral concentrations were within normal physiological ranges. In Exp. 2, 64 mixed-breed stocker steers (275 ± 1.7 kg BW) were stratified by BW and allocated randomly to one of 16 groups of four animals. Weight gain during a 50-d grazing study and subsequent feedlot period did not differ (P>0.10) among calves fed either no added Mg source or WMM, UMM, or MgO. Serum Cu was higher (P<0.05), and serum Mg was lower (P<0.05), from steers fed MgO, but all serum mineral concentrations were within normal physiological ranges. Although Mg-mica is relatively high in Fe (4%), using it as a supplemental Mg source appeared to have no negative impact on growth performance or serum mineral concentrations of calves grazing wheat pasture.