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Magnesium Utilization in Wethers Fed Diets with Varying Ratios of Nitrogen to Readily Fermentable Carbohydrate

U.S. Department of Agriculture, Ithaca 14853

Abstract

Two experiments were conducted to study magnesium utilization in wether lambs fed diets with varying ratios of nitrogen to readily fermentable carbohydrate (N:RFC). Semipurified diets fed to the lambs contained about 4.1% nitrogen (25.6% crude protein), .08% magnesium, .4% calcium, 2.4% potassium and varied amounts of readily fermentable carbohydrate provided mainly as either starch (experiment 1) or sucrose (experiment 2). Dietary ratios of N:RFC were .2, .4, .7 and .9 in experiment 1 (five lambs/treatment), and .2 and 1.6 in experiment 2 (nine lambs/treatment).

In experiment 1, ruminal fluid ammonia-nitrogen levels increased as starch intake decreased, but differences between treatments were not significant (P>.05). Apparent absorption, retention and plasma concentration of magnesium were not affected (P>.05) by decreased starch intake. In experiment 2, ruminal ammonia levels increased (P<.05) as sucrose intake decreased. Urinary excretion (P<.05) and apparent absorption (P<.1) of magnesium were higher in lambs fed supplementary sucrose than in those fed the diet without added sucrose; plasma magnesium levels were similar in both groups. Results of experiment 2 support the hypothesis that supplemental energy may enhance the availability of dietary magnesium to ruminants.

¹ U.S. Plant, Soil and Nutrition Laboratory, Agricultural Research Service, Ithaca, NY 14853.

² Snake River Conservation Research Center, Agricultural Research Service, Kimberly, ID 83341.

³ The authors gratefully acknowledge the advice and assistance of Dr. D. E. Hogue and Mr. C. L. Heffron, Department of Animal Science, Cornell University, in conducting the feeding trials, and the helpful suggestions of Dr. J. P. Fontenot, Department of Animal Science, Virginia Polytechnic Institute, in preparing the manuscript.