Mineral Utilization by Lambs and Guinea Pigs Fed Mg-Fertilized Grass and Legume Hays

Hypomagnesemic tetany, in both spring and winter forms, is a serious metabolic problem in beef cow herds in the northeast U. S. Control of the syndrome may be achieved by Mg supplementation of the diet, by selecting or breeding plants with the ability to accumulate Mg, or by increasing the concentration of Mg in forages by Mg fertilization or foliar application. The present study was designed to determine whether fertilization of forages with a moderate level of an available Mg fertilizer would effectively increase the concentration of Mg in the forage and improve the Mg status of ruminant animals. A number of grasses and legumes were grown in Pennsylvania and West Virginia, fertilized with kieserite (MgSO₄·H₂O) at the rate of 112 kg Mg/ha, and harvested as hays in 1973 and 1974. Magnesium concentration in the forages was determined by chemical analysis, and utilization of minerals evaluated with growing wether lambs in balance trials. The use of the guinea pig (Cavia porcellus) as an assay animal for mineral studies with ruminant species was examined in a limited number of trials. Magnesium fertilization increased the concentration of Mg in hays, from a mean value of 0.17 to 0.20% for all forages studied. Fertilization had no significant effect on dry matter digestibility (DMD) of the hays by lambs, and had a variable effect on apparent absorption of Mg. Apparent absorption of Mg was increased significantly, from 24.4 to 29.5%, by fertilization of the grass hays, with no effect (31.8 vs. 31.9% for non-fertilized and fertilized treatments) in the legume hays. Apparent absorption of Mg, Ca, and P was higher in legumes than in grasses, but there were no differences in absorption of K and S due either to type of hay or to fertilization. Magnesium fertilization resulted in a highly significant increase in Mg retention by lambs, 0.17 vs. 0.28 g/day for non-fertilized compared to fertilized treatments, for all forages. Retention of Mg, Ca, and P was markedly higher from the legume than from the grass hays. Fertilization had no effect on retention of Ca or P. Fertilization resulted in the most marked improvements in Mg retention from orchardgrass (Dactylis glomerata L.) when harvested (1974) at a vegetative stage in West Virginia and Pennsylvania. DMD values of forages were lower, and apparent absorption and retention of Mg much higher, in the guinea pig than in the lamb in comparative trials. Substantial species differences in the degree and nature of Mg utilization of alfalfa (Medicago sativa L.) hays fertilized with different levels of MgSO₄ suggest that the guinea pig would not be a suitable essay animal for mineral availability studies with the ruminant.