The effect of concentrate-to-forage ratio on the milk-yield response to supplementary protein

Abstract

Twenty-four British Friesian dairy cattle in their first lactation were used in a change-over design experiment with four periods, each of 4 weeks duration. The aim of the experiment was to examine the effects of varying both concentrate crude-protein concentration and the proportions of concentrates and grass silage in the diet on milk yield and composition. Twelve treatments were examined in a 3 × 4 factorial arrangement. Concentrates containing four levels of crude protein (CP), ranging from 120 to 206 g/kg fresh weight were offered in diets containing either 400, 500 or 600 g concentrates per kg dry-matter (DM) intake. The grass silage offered as the basal forage had a DM concentration of 214 g/kg and a digestible organic matter concentration of 682 g/kg DM. Milk yields (kg/day) for cows offered diets containing 400, 500 and 600 g concentrates per kg DM were 17·2, 18·1 and 18·3 respectively; and 17·4, 17·6, 18·0 and 18·4 for cows offered diets with concentrates containing 120, 147, 176 and 206 g CP per kg fresh weight respectively (pooled s.e. 0·13). Increasing either the proportion of concentrates in the diet or the CP concentration of the concentrates did not significantly affect milk fat concentration, but did increase milk protein concentration. There was a trend towards a greater milk-yield response to protein supplementation with low- rather than high-concentrate diets, although it appears that this trend may only be apparent in situations where total food intake is restricted.