Graduate Thesis Or Dissertation

An appraisal of risks and benefits in the provision of selenium for beef cattle

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  • Three experimental grazing trials were conducted to explore the hypothesis that in the Klamath Basin, a higher level of selenium supplementation is necessary than those proven adequate elsewhere. Two forage species were investigated: Festuca arundinaeca and Agropyron repens, in 0.61 hectare plots, both grown in a selenium deficient soil area. Selenium treatments were offered free choice in a salt-mineral mixture, to growing beef heifers. In Experiments I and II the animals were rotationally grazed between the grasses, while in Experiment III they were grazed by forage type. Average total weight gain was used to measure performance, and selenium status was monitored by whole blood analysis. Selenium was supplemented at levels of 0, 25, 50, and 100 ppm added to the salt-mineral mixture in Experiment I; 0, 25, 50, 100, 200, and 300 ppm in Experiment II; and 0, 200, and 400 ppm in Experiment III. In Experiment I selenium supplementation increased (P<.05) weight gains of grazing heifers. Linear regression indicated that a higher level of selenium supplementation might result in increased (P<.02) weight gains. Only the highest level of supplementation (300 ppm) resulted in an increase (P<.05) in weight gain in Experiment II. Experiment III results indicated a increase (P<.01) in weight gain from supplementation of selenium at both the 200 and 400 ppm levels on wheatgrass, but no added increase above the 200 ppm level. There was no increase in weight gain on tall fescue. Whole blood selenium levels closely paralleled the level of selenium supplementation. No toxic or detrimental effects were observed from selenium supplementation at any level. The results substantiate the concept that adequate selenium supplementation depends on a number of factors, including soil and plant levels, and forage type and demonstrate problems inherent in using a single level to cover variable conditions. In the Klamath situation, the approved level of 20 ppm selenium in a salt-mineral mix proved insufficient for maximum performance.
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