Factors affecting calcium, phosphorus, and magnesium status of dairy cattle on the Oregon coast.

Blood plasma from 126 milking cows in 21 herds was analyzed for calcium, phosphorus, and magnesium in October, 1974, and in the following April. For all three minerals, time of sampling, herds, and time of sampling by herd differed. Mean values in October and April were calcium, 10.1 and 9.0; phosphorus, 5.96 and 4.88; magnesium, 1.98 and 1.85 mg/100 ml. Calcium also differed between stages of lactation. Mean calcium for cows milking less than 100 days, 101 to 200 days, and greater than 200 days and dry was: 9.41, 9.70, and 9.80 mg/100 ml. Differences in winter feeding practices among herds were responsible for time of sampling, herd, and time of sampling-by-herd differences.