Effects of potassium fertilization of fescue pastures and oral administration of potassium chloride on metabolism of magnesium, calcium and potassium in lactating beef cows

The purpose of this study was to compare the effects of potas-sium (K) fertilization of fescue pasture and oral administration of potassium chloride (KCl) on plasma Mg, Ca and K in lactating beef cows. Four pasture trials were conducted in February and March of 1982 and 1983. Pasture I (control pasture) was fertilized at a rate of 112 kg of N/ha and 169 kg of P/ha. Pasture II (K pasture) was fertilized at the same rate as pasture I plus 224 kg of K/ha. One half of the animals on each pasture were given 280 grams of KCl via gelatin bolus. Milk, urine and fecal samples were collected twice daily. The Cr₂0₃ acid detergent lignin (internal-external) technique was used to estimate fecal dry matter output. The weigh-suckle-weigh method was used to estimate daily milk production. Creatinine ratios were used to determine urine volume. Plasma Mg was depressed by pasture fertilization in both years. Potassium chloride bolusing had no effect on plasma Mg. Sixteen of 24 animals on the K fertilized pasture became hypomagnesemic. Plasma Ca was variable and inconsistent. Plasma K was not affected by treat-ment. However, plasma K of animals in trial II in both years had sig-nificantly higher plasma K. This may be a reflection of the higher forage K in trial II. From these results it seems the incidence of hypomagnesemic tetany may be increased by heavy K fertilization of fescue pastures and not by KCl intake.