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.