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Effect of Magnesium and Energy Supplementation on Blood Constituents of Fall-Calving Beef Cows

University of Kentucky^1,2,, Lexington 40506

Abstract

Forty-eight fall-calving Angus cows were utilized to study the effects of supplemental energy and magnesium on serum metabolites and body weight changes. The three groups of cows (control, MgO and energy) grazed similar pasture that had been fertilized with 112 kg ammonium nitrate/ha on July 23. All groups of cows were fed equal quantities of hay when forage was inadequate. The magnesium oxide (MgO) group was fed 56.8 g MgO and the energy-supplemented group 2.3 kg of corn per head daily. All groups of cows were rotated among pastures weekly and blood samples taken bi-weekly. Cows in the control, MgO and energy-supplemented groups lost 132, 112 and 86 kg, respectively, during the study. Tall fescue forage sampled from 9/5 to 1/23 averaged .18% Mg. Serum Mg was highest in the MgO group of cows. Energy supplementation tended to result in higher serum Mg, but was dependent on time in the trial. Serum calcium and potassium were influenced to a lesser degree by treatment than was serum Mg. Serum free fatty acids (FFA) were reflective of losses in body weight. Energy supplementation delayed the elevation of serum FFA until the latter portion of the trial, whereas they were elevated early in the control and MgO-supplemented cows. Serum glucose levels differed significantly among groups at most of the sampling times, with the MgO supplemented cows tending to have the lowest levels. Blood urea-N was lowest in the energy-supplemented cows and highest in those fed the MgO supplement. These data suggest that Mg supplementation influences metabolism not only by resulting in elevated blood levels but also through its influence on energy and protein metabolism. Energy supplementation resulted in slightly higher serum Mg and influenced FFA and serum urea-N concentrations.

¹ Department of Animal Sciences.

² This paper (78-5-85) is published with the approval of the Director of the Kentucky Agricultural Experiment Station.