Use of an Experimental High‐Magnesium Tall Fescue to Reduce Grass Tetany in Cattle

Grass tetany (hypomagnesemia) continues to be a problem for the livestock industry. An experimental cultivar of tall fescue (Festuca arundinacea Shreb.) selected for high Mg concentration, HiMag, was compared against four other cultivars for its ability to increase serum‐Mg and reduce the risk of grass tetany. Forage from HiMag contained 22% more Mg, 18.5% more Ca, and 9% more P than other cultivars; K levels were not different. Regardless of cultivar, concentrations of Ca and Mg were lower while K and P were higher during the spring than the autumn. The tetany ratio [K/(Ca + Mg) on Mc basis] was lower for HiMag than for other cultivars (1.34 vs. 1.65); all cultivars had a higher ratio during spring than autumn (1.91 vs. 1.31). In cattle (Bos taurus) grazing trials, blood serum from steers grazing HiMag contained 8% more Mg in autumn 1993 but was not different in spring 1994. Blood serum Mg was not different for cows (either dry or nursing calves) during autumn grazing, but approached significance (P = 0.09) during the spring 1995 calving season (2.32 vs. 2.07 mg/dL for HiMag vs.other cultivars). Except for differences between calves on HiMag compared with Kentucky‐31 in autumn 1995 (58 vs. 43 lb, respectively), animal weight change was not affected by cultivar. Normal forage Mg concentrations (>0.20%) and tetany ratios below 2.2 for all cultivars in every season except spring 1995 may explain the lack of consistent animal response. No clinical symptoms of grass tetany were observed at any time during these studies. However, the higher Mg concentration and lower tetany ratio suggest that HiMag could provide a means of reducing the incidence of grass tetany in livestock during periods when risk of the disorder is high. The Problem Grass tetany (hypomagnesemia) causes economic losses from death or reduced performance of livestock throughout the world. Attempts to reduce the incidence of grass tetany through application of fertilizer Mg have not generally been successful. Dietary Mg supplementation adds to the annual labor and cost of production and does not always ensure all animals receive adequate amounts of the mineral. Selective breeding of forages for higher levels of Mg and lower tetany ratio may offer a long‐term solution to the grass tetany problem. Objectives of this study were to evaluate an experimental selection of tall fescue, HiMag, for its ability to reduce the risk of grass tetany through higher Mg concentration and lower tetany ratio of forage, and higher blood serum Mg concentrations of beef cattle under grazing conditions. Literature Summary Grass tetany is characterized by low blood Mg in livestock resulting from low Mg content of feed or reduced absorption of Mg, usually when grazing early, lush cool‐season forages. Forages containing less than 0.20% Mg and a “tetany ratio” [K/(Ca + Mg) as moles of charge (Mc)] greater that 2.2 have higher risk of inducing grass tetany. Progress has been made in breeding cool‐season forages with higher Mg concentration and lower tetany ratio. Grazing and feeding studies with high‐Mg Italian and perennial ryegrass cultivars in the United Kingdom have resulted in mixed responses with sheep. Initial results with HiMag have shown a favorable mineral concentration and tetany ratio, but no animal responses to grazing HiMag have been reported. Study Description Twenty, 1‐acre pastures on a predominantly Creldon silt loam soil at the Southwest Research Center near Mt. Vernon, MO, were planted to four endophyte‐free tall fescue cultivars, AU Triumph, Kentucky‐31, Martin, and Mozark, and the experimental cultivar, HiMag. The study consisted of five individual grazing trials using cross‐bred beef steers, dry cows, or cow/calf pairs during three autumn and two spring seasons between 1993 and 1995. Forages were sampled each season and analyzed for Mg, Ca, K, and P concentration, and the tetany ratio was calculated. Cattle were weighed at the beginning, middle, and end of each trial to determine average change in weight. Blood serum was collected at each weighing for determination of Mg, Ca, K, and P status of the cattle. Applied Questions Can cool‐season forage grasses be selectively bred to reduce the risk of grass tetany? Forage from HiMag tall fescue consistently had higher Mg (22%), Ca (18%), and P (9%) concentration than other tall fescue cultivars. Potassium levels were not different among cultivars. This resulted in a lower tetany ratio for HiMag compared with the other cultivars. Higher Mg concentration (>0.20%) and lower tetany ratio (<2.2) suggest that HiMag tall fescue can reduce risk of occurrence of grass tetany. Do cattle grazing HiMag tall fescue show improved Mg status and reduced incidence of grass tetany? Results of this study were not conclusive. Cattle in two of the five trials did show higher blood serum Mg concentrations on HiMag,. However, with the exception of the spring 1995 trial, all cultivars had acceptable Mg content (>0.20% in forage) and tetany ratios (<2.2). Cattle grazing all cultivars had Mg levels above the critical value of 1.7 mg/dL in blood serum. Thus, animals were at low risk for grass tetany and, because blood Mg concentrations were a1ready normal, did not always respond to the higher Mg content of HiMag. No signs of grass tetany were observed in any of the trials. Recommendations Based on mineral concentration of the forage, HiMag appears to have a lowered risk for grass telany. Additional grazing research under conditions that maximize the risk for grass tetany are needed to evaluate animal responses to HiMag.