SUPPLEMENTAL PROTEIN, MAGNESIUM AND SELENIUM PLUS VITAMIN E FOR BEEF COWS FED STRAW DIETS IN WINTER

Sixty-four cows were fed diets containing 94% barley straw for 83 days in the winter to evaluate the effects of supplemental protein, magnesium, and selenium plus vitamin E on cow health, feed intake, and winter weight change. Overall, the cows consumed 47 MJ of digestible energy daily which was only 70% of their calculated requirements. Eleven cows did not complete the test; eight of these cows died. Abomasal impactions were found in three of the cows. One animal fed the diet with supplemental protein but without supplemental magnesum exhibited symptoms of acute hypomagnesemia. When the protein level of the total diet was increased from 5.2 to 6.2% the cows gained more (P < 0.05) weight and increased (P < 0.10) feed consumption by 17%. Plasma urea nitrogen concentrations were increased (P < 0.01) from 3.7 to 4.6 mg/100 mL. Cows that received the low protein diet gained more weight when supplemental magneisum was provided than those that were fed a similar diet without magnesium. They also tended (P < 0.10) to eat more feed. The mean magnesium concentration in blood plasma was only 1.4 mg/100 mL, which confirmed the existence of a magnesium deficiency in cows fed large amounts of straw. Plasma magnesium concentrations were increased (P < 0.10) by between 0.04 and 0.08 mg/100 mL for each gram of increased magnesium intake. Plasma calcium was decreased (P < 0.05) from 8.9 to 8.4 mg/100 mL when magnesium oxide was included in the concentrate mixture. Supplemental selenium plus vitamin E had no major effect on animal performance, even though dietary intakes of selenium on the control diets were only 43% of the listed requirements. It was concluded that more than 6% concentrate is required in straw-concentrate diets for beef cows and that it may be necessary to supply supplemental protein, magnesium, selenium and manganese with such diets.