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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 M c 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 Ib, 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. |
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