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Item Effects of increasing tetany risk ratio and magnesium supplementation on mineral balance and feeding behavior by ruminants(Montana State University - Bozeman, College of Agriculture, 2011) Norvell, Tess Marie; Chairperson, Graduate Committee: John Paterson.Three experiments were conducted to evaluate the effects of increasing the tetany risk ratio on blood serum Mg levels, nutrient digestion, mineral balance, and Mg supplementation source and feeding behavior. Experiments 1 and 2 were randomized complete block designs, with 24 wethers were maintained in individual metabolism crates and assigned to four treatments (6 wethers/treatment). Experiment 1 compared tetany risk ratios (TRR) of 1.6, 2.3, 2.9, or 3.5. Dry matter, NDF, and N digestibilities were not different between treatments. Nitrogen retention decreased (P < 0.05) as the tetany risk ratio increased. No treatment differences were recorded in Mg, Ca, K, or P balance. The TRR 2.3 treatment had the greatest decrease (P < 0.05) in serum Mg after 15d. Experiment 2 compared tetany risk ratios of 1.5, 2.6, 1.5 plus MgO, or 1.5 plus MgCl. The TRR 2.6 treatment showed increased (P < 0.05) DM, NDF, and N digestibilities when compared to all other treatments. Nitrogen digestibility decreased (P < 0.05) with the addition of supplemental Mg. No treatment differences in Mg, Ca, K, or P balance were recorded. The TRR 2.6 treatment had the greatest decrease (P < 0.05) in serum Mg after 5 d. Experiment 3 was a cafeteria study using a switchback design, 23 Angus heifers were weighed and randomly assigned to one of two locations (11 heifers in drylot, and 12 heifers on pasture) containing two mineral supplements (0.0% Mg and 10.0% Mg). The groups were rotated between locations after 15 d for 30 d of measurements. Individual mineral consumption (grams/d), feeder attendance (trips/d), and feeding duration (seconds/d) were measured using a GrowSafe® individual feeding system. Heifers consumed 119% more (P < 0.01) 0.0% Mg supplement each day than the 10.0% Mg. Heifers made almost twice as many (P < 0.01) trips to the feeder, and spent an additional 91.3 s consuming the 0.0% Mg than the 10.0% Mg. Total mineral intakes were 87.3 % greater (P < 0.01) when supplemented on pasture compared to drylot. During the first 15 d, total mineral intakes were higher (P < 0.01) than for the second 15 d.