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Wheat Pasture Poisoning. II. Tissue Composition of Cattle Grazing Cereal Forages and Related to Tetany¹

University of Nevada, Reno 89557 , US Department of Agriculture, El Reno, OK 73036 , Ames, IA 50010 and and Oklahoma State University, Stillwater 74078

Abstract

Thirty-two aged Hereford and Angus cows (avg 10.2 yr old) grazed mixed cereal (wheat and rye) pastures from December 6, 1979 to April 23, 1980 (138 d). On d 104 (March 19), five cows developed tetany. On the day of tetany, the mean plasma Ca of the cows with tetany was 4.1 mg/dl. Cows with tetany had plasma Mg concentrations of 1.7 mg/dl on the day of tetany, but 15 d before tetany and 6 d post tetany, levels were lower (1.2 mg/dl). Cows with tetany had large increases in plasma parathyroid hormone (PTH) as well as vitamin 1, 25 dihydroxy D₃ [l,25(OH)₂D], plasma glucose, lactic acid and ß-hydroxybutyric acid. Plasma K of these cows followed a similar trend as plant K, except it was not as pronounced. Plasma inorganic P of these same cows was exceedingly low, <3 mg/dl several times, although the forage P did not indicate a P deficiency. Levels of plasma hydroxyproline in these cows indicated that bone resorption may have been low during most of the grazing season, even though levels of plasma hydroxy proline were initially high and high subsequent to tetany. High levels of plasma glucose and lactic acid were associated with the time of tetany. Apparently, cows developed inappetence around the time of tetany, as suggested by the high levels of plasma ß-hydroxybutyric acid. Cows with wheat pasture poisoning appeared to respond to the severe hypocalcemia with large increases in plasma l,25(OH)₂D. Plasma l,25(OH)₂D changes were similar to plasma hydroxyproline changes (and possible bone resorption rates) and seemed inversely proportional to plasma Ca and directly proportional to plasma PTH concentrations. Generally, cerebrospinal fluid (CSF) composition was not affected as much as plasma composition, except for the low levels of CSF Ca. These data indicate that a metabolic deficiency of Ca (hypocalcemia) was the cause of tetany.

¹ Contribution of the Nevada Agr. Exp. Sta., Journal Series No. 566. In cooperation with SEA, USDA and Oklahoma State Univ. This study was conducted while the senior author was on sabbatical leave in Oklahoma.

² Anim. Sci. Division, Univ. of Nevada, Reno.

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

⁴ USDA, ARS, Natl. Anim. Disease Center, Ames, IA.

⁵ College of Vet. Med., Oklahoma State Univ., Stillwater.