Effects of different methods, sources and levels of selenium supplementation and fertilization on beef cattle and forage tissue levels

Several experiments were conducted in Northwest and Northcentral Florida to determine effects of different methods and sources of selenium supplementation and fertilization of forages on the performance and tissue levels in beef cattle and on tissue selenium levels of two forage species. Organic selenium (Se-yeast) proved to be more effective in stimulating weight gains and liver selenium concentrations than an inorganic source (sodium selenite) for Angus steers and heifers. Plasma selenium was also higher with the organic source, although none of the sources achieved adequate levels in the plasma. However, in Angus steers and heifers neither the organic nor the inorganic selenium sources were able to alter plasma concentrations of the thyroid hormones T 3 and T4 after administration of thyrotropin-releasing-hormone. In adult Angus cows, a selenite (Mu-Se) treatment was not effective in raising plasma selenium concentrations while barium selenate (Deposel) maintained a constant level in plasma. Neither the Mu-Se or Deposel treatments attained plasma selenium adequacy (0.07 mg/L) but were above the critical level (0.03 mg/L). The organic (Se-yeast) mineral mixture treatments did increase plasma selenium concentrations and attained adequacy. The control and Mu-Se treatments did not achieve adequate selenium concentration in liver at any time, but the Deposel did so at 24 mo. The salt mineral mixtures (Se-yeast) were adequate at 6, 12, and 24 mo. Concentrations of selenium in milk were not adequate and declined with time as did the colostrum selenium concentrations. Neither dam injected Mu-Se or Deposel was effective in raising or maintaining calf plasma selenium concentrations and the trend was to decrease with time. However, the free-choice mineral mixtures (Se-yeast) were effective in attaining and maintaining adequate plasma selenium concentrations in calves. Spraying of Selcote-Ultra, a selenium fertilizer, on bahiagrass was not very effective in raising forage selenium to adequate levels (>0.1 mg/kg) beyond 8 wks at the 5 g/ha rate while the 10, 15, and 20 g/ha rates were still adequate at 12 wks after application. Topdressing of Selcote-Ultra on fescue is an adequate method to increase forage selenium as the levels were more than adequate 22 wks after application. Organic selenium (Se-yeast) was more effective in raising and maintaining adequate selenium concentrations in tissues of beef cattle while the fertilizer worked better when topdressed on forage than when sprayed.