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Over the last 60 years, grass tetany has been recognised as a significant lethal condition in sheep and cattle.Outcomes from this study include documentation of the likely precursors to grass tetany, ways to recognise these precursors, and long term practices that will enable producers to minimise livestock deaths. The benefit of this research to beef producers is that the environmental circumstances thought to be associated with outbreaks of grass tetany have been identified, along with remedial action that can be taken to prevent deaths occurring.Recommendations to industry on best practice to be adopted by leading producers to minimise outbreaks of grass tetany are made.This study provides an alternate strategy for the management of grass tetany in beef cattle, to the more clinical approaches previously recommended. It is suggested that losses from this economically important metabolic disease can be minimised if management practices of beef cattle producers in eastern Australia can incorporate a more holistic approach to farm management, which takes account of the soil/plant/animal/climate inter-relationships. / Master of Science (Hons)

Thesis (M. Sc.) (Hons.) -- University of Western Sydney, Hawkesbury, 2000. / Includes bibliographical references (leaves 136-152).

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Three experiments were conducted to study the effects of dietary potassium intake on magnesium metabolism. In experiment 1, 24 wether lambs were used in two metabolism trials to determine the minimum level of potassium that depresses magnesium absorption. Two magnesium levels (.1 and .2%) were fed with four levels of potassium (.6, 1.2, 2.4 and 4.8%) in a 2 x 4 factorial arrangement of treatments. Each trial consisted of a 10-day preliminary period followed by a 10-day collection period. Greater amounts (P<.05) of magnesium were absorbed and retained when animals were fed the high level of magnesium (.2%). Increasing the level of potassium in the diet increased (P<.05) fecal magnesium excretion. Magnesium absorption decreased linearly with increasing levels of potassium. The greatest depression in magnesium absorption occurred when the potassium level was increased from 1.2 to 2.4%. A further large decrease in absorption occurred when the potassium level was increased from 2.4 to 4.8%. Serum magnesium was depressed when higher levels of potassium were fed. Potassium absorption was not affected by level of dietary magnesium, but increased (P<.05) with increasing levels of dietary potassium. In experiment 2, two metabolism trials were conducted with 18 wether lambs, equipped with abomasal and ileal cannulae, in a 2 x 3 factorial arrangement. Two levels of magnesium (.1 and .2%) and three levels of potassium (.6, 2.4, and 4.8%) were used. Chromic oxide was used as an indicator to determine digesta flow. Each trial consisted of a 10-day preliminary period, a 7-day collection of feed, feces and urine, and a 6-day sampling of feed, abomasal and ileal fluid and feces. Magnesium was absorbed mainly prior to reaching the small intestine, followed by a low net absorption in the large intestine. Increasinq potassium level resulted in a linear decrease in magnesium absorption. Preintestinal magnesium absorption decreased (P<.05) with each increase in dietary potassium. Large amounts of magnesium flowed into the small and large intestines and was excreted in the feces when higher levels of potassium were fed. Lambs fed the high level of magnesium (.2%) absorbed larger amounts of magnesium from the stomach. Magnesium absorbed, expressed as a percent of intake, was not affected by level of magnesium. Experiment 3 consisted of three metabolism trials with six Angus x Simmental steer calves, per trial, equipped with abomasal and ileal cannulae. Steers were fed .1% magnesium and three levels of potassium, .6, 2.4 and 4.8%. Procedures were similar to those used in Experiment 2. Magnesium absorption decreased linearly (P<.05) when high levels of potassium were fed. The primary site of magnesium absorption was the preintestinal region, followed by a net secretion into the small and large intestines. Preintestinal magnesium absorption was decreased approximately 50% when steers were fed 4.8% potassium. Small amounts of magnesium tended to be secreted into the small intestine when the high level of potassium was fed. As observed in experiment 1, serum magnesium levels were depressed when increasing amounts of potassium were fed. Potassium absorption increased with increasing levels of dietary potassium. The effect of high dietary potassium on magnesium absorption was similar to those reported in experiments 1 and 2 with sheep. / Ph. D.

Five crossbred heifers were slaughtered at different times and rumen and omasum tissues were removed: The epithelium was separated from the muscle layer and mounted in parabiotic chambers. The tissues were incubated for 2 h in buffer at 39 C and aerated continuously in 95% O₂ 5% CO₂ . The buffers were similar to those which have been used previously in mineral transport studies, but were modified by varying the levels of Mg and K while keeping the osmolarity constant. Potassium was included in the buffers on the mucosal side in appropriate ratios to Mg to correspond to the K: Mg in diets fed to ruminants which would contain .1% Mg and .6, 2.4 and 4.8% K, (low, medium and high, respectively). A fourth buffer which contained physiological concentrations of Kand Mg, was placed on the serosal side of the parabiotic chamber. Flow of Mg was calculated by: F = C₁V₁ - C₀V₀ / AΔt where C₁ = final concentration, C₀ = original concentration, V₁ = final volume, V₀ = original volume, Δt = incubation time, A = area of the tissue exposed to the buffer, and F = flow of Mg (mg/cm²/h). Blanks were included which contained physiological levels of Mg on both side of the chamber to adjust for tissue effects. Magnesium transport tended to be 10 times greater through the rumen than the omasum. This indicates that the rumen is the primary site of Mg absorption in cattle. Potassium tended to depress Mg transport across both tissues. This technique has only limited application in mineral research. / M.S.