Blood constituents in lactating Holstein cows influenced by hematocrit, sampling site, and diet protein and calcium.

Constituents were measured in jugular vein (days 0, 4, 14, 28, 43, 72, and 151 of lactation) and internal iliac artery and mammary vein (days 28, 72, and 151 of lactation) blood of 24 Holsteins. Six diets of grain:corn silage-urea contained percents of protein and calcium: 12, .6; 12, .9; 15, .6; 15, .9; 15, .6; 15, .9. Grain contained urea (diets 3, 4) or soybean meal (diets 5, 6). All cows were fed diet 3 the first 4 wk of lactation; then four cows were assigned to each of six diets. In jugular blood, calcium and phosphorus were lower on day 0. Hydroxyproline and ketone bodies peaked and magnesium was minimal on day 4. Hematocrit, urea nitrogen, and free fatty acids decreased with lactation. Jugular blood from younger cows had less ketone bodies, free fatty acids, and magnesium and more hematocrit, hydroxyproline, and phosphorus. Sampling site had arterial-venous differences for each constituent, with venous differences for hematocrit, free fatty acids, calcium, phosphorus, and magnesium. Hematocrit decreased with diets 1, 2, 3, 4, or .6% calcium. With a common hematocrit (36%) and change from baseline (day 28), urea nitrogen was less with 12% protein or .6% calcium. Phosphorus and magnesium decreased with 15% protein. More constituents were required from diet and tissues to maintain concentrations in blood as hematocrit decreased, indicating the importance of dietary calcium and protein concentration and quality.