Reports of hypomagnesaemia, a Mg deficiency in ruminants, occur most frequently when livestock graze the spring growth of grass pastures. To minimize these animal losses it is important to know whether differences exist among forage grasses for potential risk of Mg deficiency in ruminant livestock. The objectives of this study were 1) to determine if several cool-season forage grasses accumulate K, Ca, and Mg in the same proportions during the spring period of growth and 2) to observe the effect of temperature fluctuations on changes in K, Ca, and Mg concentrations of these grasses.
Nine cool-season forage grasses were harvested on a daily basis during mid- to late-spring. Cation concentrations in the forage were determined, and the K to (Ca + Mg) cation ratios were calculated.
This study provides for the classification of reed canarygrass, (Phalaris arundinaceae L.), orchardgrass (Dactylis glomerata L.), tall oatgrass [Arrhenatherum elatius L. (Mert. and Koch)], and Canada wildrye (Elymus canadensis L.) as forage grasses with K to (Ca + Mg) cation ratios exceeding 2.2, and thus may be classified as those more likely to cause grass tetany in ruminants. On the other hand, smooth bromegrass (Bromus inermis Leyss.), Kentucky bluegrass (Poa pratensis L.), crested wheatgrass [Agropyron cristatum L. (Gaerta)], tall wheatgrass [Agropyron elongatum Host. (Beauv.)], and meadow foxtail (Alopecurus pratensis L.) generally have K to (Ca + Mg) cation ratios of less than 2.2 and appear to be less likely to cause grass tetany in ruminants.
For all grasses except meadow foxtail, a sudden increase in mean daily air temperature above 14 C was associated with a corresponding rise in cation ratio, with values reaching a peak approximately 5 days later. This suggests that the risk of hypomagnesaemia in grazing ruminants increases during these periods of temperature fluctuation. However, those grasses that initially had K to (Ca + Mg) cation ratios exceeding 2.2, presumably would continue to offer a greater risk compared to grasses initially found to have ratios less than 2.2.
Potassium concentration was the primary factor responsible for grass species having either a high or low cation ratio throughout the spring season, whereas fluctuations in Ca and Mg concentrations were responsible for the short-term variation in ratios as influenced by temperature. With the exception of Canada wildrye and meadow foxtail, forage grasses having higher cation ratios also contained higher absolute concentration of Mg.