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.

The variable incidence of grass tetany in cattle within the spring tetany season suggests that considerable day‐to‐day change may occur in forage cation concentrations. An awareness for these fluctuations would seem to be important as research scientists attempt to understand the tetany syndrome. The objectives of this study were to observe changes in forage K, Ca, and Mg concentrations and the K/(Ca + Mg) milliequivalent ratios of smooth bromegrass (Bromus inermis Leyss.) at 2‐day intervals during the spring tetany season. An established stand of grass which had received N rate (0, 125, and 500 kg N/ha in 1970 and 0,250, and 500 kg N/ha in 1971 and one‐half the latter amounts in early 1972) and K source (0 and 125 kg K/ha annually as KCl, K2SO4,, and K2SO4,‐MgSO4) treatments was used for the spring of 1972 sampling. Fluctuations in forage cation concentrations were related to N and K fertilizer treatments, existing differences in soil exchangeable bases and pH due to previous fertilization and cropping, precipitation patterns, and changes in mean daily air temperature. Acidic soils resulting from previous N fertilization were associated with a 20 to 48% increase in bromegrass Mg concentrations. Mean soil Mg/K ratios of 2.8, 6.1, 7.6, and 7.8 for the 0–2.5, 2.5–5.0, 5.0–7.5, and 7.5–15 cm soil depths did not seem to result in higher forage Mg and lower K as the growing season progressed and root activity presumably increased at deeper soil depths. Potassium fertilizer significantly increased forage K concentration and decreased forage Ca and Mg. Forage Ca was decreased more by K2SO4,‐ MgSO4 than by KCl or K2SO4 fertilization. Significant but relatively small increases in forage Mg resulted for K2SO4,‐MgSO4, fertilization compared with other K sources. Increases in K/(Ca + Mg) cation ratios suggest a greater grass tetany risk for the intermediate level of N fertilization, followed closely by the high level of N. Nitrogen fertilization in the absence of K generally produced less risk compared with N check with or without K fertilization. Fertilization with K produced a relatively large increase in cation ratios at intermediate and high N levels, with values near 2.2 during late April and exceeding 2.2 during early to mid‐May. These data illustrate a simultaneous shift in two indices of grass tetany risk; that is, an increase in cation ratio above 2.2 and a decrease in forage Mg concentration below 0.20%. Both indices of risk became more serious during rainy periods and following a temperature rise above 14 C. Also, both are more serious with NK fertilization, whereas, N fertilization without K reduced the apparent risk.

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