Effects of altering the energy and protein supply to dairy cows during thedry period. 2. Metabolic and hormonal responses

We offered 48 multiparous Holstein-Friesian cows one of three different dry -period diets for 6 wk before predicted calving: A) a grass silage and barl ey straw mix (60:40) on a dry-matter basis), with a low protein content; B) grass silage only (medium protein); or C) grass silage plus 0.5 kg of high protein corn gluten meal thigh protein)/d. A standard grass silage-based d iet was offered to all cows following calving. All animals were blood sampl ed at weekly intervals during the dry period, and at wk 1, 3, 5, 7, 13, 17, and 21 of lactation. Nitrogen balance was measured in a subset of 12 cows (four per treatment) at 3 wk before calving, and at wk 8 and 18 of lactatio n. Nitrogen balance in the dry period increased significantly as dietary pr otein supply increased [9, 34, and 50 g of N/d (SED 8.8) for the three diet s, respectively]. We measured no residual effects of dry-period treatment o n N balance during lactation. Plasma concentrations of growth hormone becam e significantly higher in the late dry period in cows offered the silage an d straw diet, although treatment differences disappeared after calving. Ins ulin concentrations were significantly affected by treatment throughout the dry period and indicated inadequate nutrient supplies from the low protein diet, leading to losses in body reserves of energy and protein. The result s indicate that animals offered the low protein diet accumulated less N per day than is required for fetal development, suggesting that protein must h ave been repartitioned from maternal tissues.