Response of milking cows fed a high concentrate, low roughage diet plus sodium bicarbonate, magnesium oxide, or...

To determine the ability of mineral supplements to elevate depressed milk fat percent, we placed 42 Holstein cows in early to midlactation in seven blocks and assigned each to one of six treatments: control (25% corn silage and alfalfa haylage, 75% concentrate, mostly corn, dry matter); control plus magnesium oxide ground to pass a .425-mm sieve; control plus prilled magnesium oxide sieved to between 1.70 and .425 mm; control plus sodium bicarbonate; control plus reactive powdered magnesium oxide to pass a 45 microns sieve; and control plus powdered magnesium hydroxide. Sodium bicarbonate was 1% of diet as fed, magnesium oxide .5%, magnesium hydroxide .7% later reduced to .5%. Cows were fed control ration for 3 wk to induce milk fat depression, then were changed to treatment rations for 5 wk. Five cows (unblocked) were continued on each dietary treatment for a digestibility study after the feeding trial. All mineral supplements produced greater milk fat percent and yield of milk fat per day than control and all magnesium treatments produced greater milk fat percent than sodium bicarbonate. Magnesium oxide passing a .425-mm sieve produced the greatest increase of milk fat percent. Milk production was most for sodium bicarbonate treatment. Supplementation with magnesium increased ruminal magnesium concentration by factors of 1.26 to 3.75. Blood serum, urine, and fecal magnesium concentrations and fecal pH were more for cows fed magnesium than those fed sodium bicarbonate treatments. Kidney filtration ratios of element to creatinine increased for magnesium when diets were supplemented with magnesium and increased for phosphorus and sodium when diets were supplemented with sodium bicarbonate.