Effect of selenium concentration on feed preferences by cattle and sheep

Selenium-accumulator plants are reputed to be unpalatable to livestock. The objective of this study was to determine if sheep and cattle could discriminate between forages and feeds with different concentrations of Se. In the first study, cattle and sheep preferences for intermediate wheatgrass (Thinopyrum intermedium), alfalfa (Medicago sativa), and western aster (Symphyotrichum ascendens) of varying Se concentrations were assessed. The Se concentrations ranged from 0.8 to 50 mg/kg (DM) in grass, 1.4 to 275 mg/kg in alfalfa, and 4 to 4,455 mg/kg in aster. Selenium concentration had no influence (P > 0.05) on the initial or subsequent preferences of sheep or cattle for grass or alfalfa. Cattle developed an aversion to aster after consuming 95% of the plant material during the first brief exposure and subsequently refused to eat any aster. Sheep consumption of aster was variable, but their preference was not driven by Se concentration. In the next study, cattle and sheep were offered pellets at 1.5% of BW (as fed) that contained increasing concentrations of Se from aster (control and 5, 25, 45, and 110 mg/kg Se). In trial 1, all pellets were offered. In Trials 2 and 3, all pellets were offered with the exception of the 5 mg/kg Se pellet and the 5 and 25 mg/kg Se pellets, respectively. In trial 1, consumption of the control pellet by cattle was greater on all days compared with other Se pellets (P < 0.001). Cattle ate more (P < 0.001) of the 5 mg/kg Se pellet than the higher Se pellets on d 3, 4, and 5. Sheep ate greater amounts of the control and 5 and 110 mg/kg Se pellets compared with the 25 and 45 mg/kg Se pellets (P < 0.0001) on d 1, and sheep consumed primarily the control and 5 mg/kg Se pellets thereafter. In trial 2, cattle and sheep consumed more (P < 0.0001) of the control Se pellet than the 25, 45, and 110 mg/kg Se pellets. In trial 3, cattle consumption of the control and 45 and 110 mg/kg Se pellets differed on d 2 and 3 (P < 0.001), except there was no difference (P > 0.95) in cattle consumption of the control and 45 mg/kg Se pellets on d 1. Sheep consumed primarily the control and 45 mg/kg Se pellets. We conclude that high Se concentrations in fresh forages had no effect on initial consumption by cattle or sheep. When given Se pellets, initial responses were variable, but the results indicate that cattle and sheep adjusted their intake over time to avoid excessive intake of Se.