The effect on milk composition of feeding spring grass to cows

1. An experiment to investigate the seasonal increase in the solids-not-fat content of milk, which usually occurs in Britain when cows go from winter feeding to spring grazing, is described. 2. Two similar groups of cows were fed good quality winter rations at the same high level both before and after calving (about 115% of the Woodman standard after calving), until spring grass was available. One group of 7 cows was then fed for 6 weeks almost entirely on cut grass, returning to winter rations for a further 3 weeks. The control group of 6 cows continued to be fed throughout the experiment on winter rations and both groups remained indoors for the whole of the experiment. The estimated nutrient intake of both groups in all periods was kept substantially constant. 3. Changes in milk yield and composition were mainly those associated with advancing stage of lactation, and grass feeding had little specific effect. It is concluded that the increase in milk protein content which is normally observed soon after the start of spring grazing probably results from an improved plane of nutrition and not specifically from an increase in the intake of soluble carbohydrates. 4. Increases in the total nitrogen content of the milk as the experiment proceeded were contributed to uniformly by the casein, total albumin and β-lactoglobulin fractions.