The effect of a low-protein ration on milk yield and plasma metabolites in Friesian heifers during early lactation.

1. Sixteen first-calf Friesian heifers were used in a continous treatment design experiment. For 2 weeks after calving they were given a 750 g concentrate, 250 g hay/kg ration with 169 g crude protein (nitrogen X 6.25; CP)/kg dry matter (DM). They were then divided into two groups of eight and given a high-protein (223 g CP/kg DM) or low-protein (107 g CP/kg DM) ration at the rate of 10.8 kg concentrate + 3.6 kg hay for 8 weeks. 2. Milk yield and composition, live weight and blood composition were monitored throughout. A digestibility trial was carried out with six animals on each treatment. 3. The low protein ration reduced DM, organic matter, energy and fibre digestibility significantly (P less than 0.001) so that intakes of digestible energy were not equal and the low-protein group lost more weight than the high-protein group. 4. Milk yield and the fat content of milk were lower in heifers given the low-protein ration (P less than 0.01). The lactose content of the milk was not affected and protein content only slightly reduced (P less than 0.01) by low-protein feeding. When the heifers were all changed onto an adequate protein (190 g CP/kg DM) ration in mid-lactation, those which had previously been under-fed protein appeared to recover in milk yield to the point they might have been expected to reach is given an adequate-protein ration throughout. 5. Concentrations of urea (P less than 0.001) and albumin (P less than 0.05) were reduced by underfeeding protein, but albumin concentration was affected less by diet than by stage of lactation. Blood concentrations of total protein, glucose, sodium, potassium, calcium, inorganic phosphate, iron, copper, haemoglobin and packed cell volume were unaffected by treatment. Blood magnesium concentration was slightly lower (P less than 0.01) with low-protein feeding.