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Wheat Pasture Poisoning. I. An Evaluation of Cereal Pastures as Related to Tetany in Beef Cows¹

University of Nevada, Reno 89557 and and US Department of Agriculture, El Reno, OK 73036, Bushland, TX 79012 and Ithaca, NY 14853

Abstract

Forage samples were obtained from three wheat and rye mixed pastures (replications) from December 1979, to April 1980 (140 d), while they were being grazed by mature cows at the Southwestern Livestock and Forage Research Station, El Reno, Oklahoma. These samples were taken three times during December, twice monthly in January and February and weekly in March and April. Samples were analyzed for dry matter, ash, N, K, Ca, Mg, P, ash alkalinity, aconitic, malic and citric acids, total lipids, NO₃-N, Na and total nonstructural carbohydrates (TNC). In vitro digestibility was also determined. The N:TNC ratio, estimated plasma Mg levels and K:(Ca + Mg) ratio (tetany ratio) were calculated. On d 105 (March 19), 16% (five of 32 head) of the cows developed tetany. At tetany, the forage K, protein, digestibility, ash alkalinity, aconitic acid and total lipids increased suddenly and markedly. Forage dry matter, TNC and ash decreased. Forage Ca and Mg were slightly below or equal to the animals' requirements and remained relatively constant during the period of tetany. Forage P increased about 2 wk before tetany and decreased after tetany occurred. The levels of forage NO₃-N and Na did not appear to be related to the incidence of tetany. The N:TNC ratios exceeded .4 immediately before tetany and two sampling periods thereafter. The estimated plasma Mg levels were lowest at tetany and shortly thereafter and tetany ratio exceeded 2.2 during most of the study, with peaks of 3.2 in December, at tetany and in early April.

¹ Contribution of the Nevada Agr. Exp. Sta., Journal Series No. 565 in cooperation with the USDA, ARS and Oklahoma Agr. Exp. Sta. This study was conducted while the senior author was on sabbatical leave in Oklahoma

² Uni. of Nevada, Reno.

³ USDA, ARS, Southwestern Livestock and Forage Res. Sta., El Reno, OK.

⁴ USDA, ARS, Conservation and Production Res. Lab., Bushland, TX.

⁵ USDA, ARS, U.S. Plant, Soil and Nutr. Lab., Ithaca, NY.

⁶ The authors wish to acknowledge the assistance of Christine S. Hayward in analyzing the plant samples for ash alkalinity, aconitic acid, citric acid and malic acid