Interaction between level of barley supplement, season and stage of lactation on performance of dairy cows at pasture

The purpose of the study was to investigate the general effect on animal pr oduction of different levels of supplementation with a starch-based concent rate to pasture, and, more specifically, to look at its interaction with st age of lactation and time of season. Three different levels of rolled barle y were introduced as supplements to pasture in two herds grazing in a conti nuous grazing system. Intake of barley in the three treatments was as follo ws: 0.6 (L), 2.3 (M) and 3.6 (H) kg dry matter daily, and the total level o f supplement was 4.0 kg dry matter above that. Production was analysed in t wo ways: (i) cows that lactated during the entire grazing period and (ii) a ll cows in the herd. The second analysis was made in order to calculate the effect of season and stage of lactation. The analysis comphending cows tha t lactated in the entire grazing period included 110 cows which were on ave rage 15 weeks post-partum and yielding 24.8 kg milk at the start of the exp eriment. The milk yields in the three treatments were 22.2 (L), 23.6 (M) an d 23.4 (H) kg. The fat content decreased significantly with increased level s of concentrate to 4.02 (L), 3.90 (M) and 3.75 (H) g kg(-1) milk, while pr otein content was not significantly affected. The analysis comphending all cows in the herd included 206 cows which were on average 18 weeks post-part um and yielding 23.7 kg milk during the grazing period. There was no signif icant interaction between stage of lactation and level of concentrate on th e production response, but the marginal response in milk to the level of co ncentrate increased significantly during the season.