EFFECT OF THE LEVELS OF N-FERTILIZER, GRASS AND SUPPLEMENTARY FEEDS ON NITROGEN COMPOSITION AND RENNETING PROPERTIES OF MILK FROM COWS AT PASTURE

In a 2 x 2 x 3 factorial design grazing experiment we investigated the effect of fertilizer (none or 240 kg N/ha), amounts of clover grass a vailable (low or high) and type and level of daily supplementary feed for each cow (3.5 kg barley, 3.5 kg concentrate mixture rich in protei n and fat, or both, 7 kg) on the protein composition and renneting pro perties of their milk. The experiment was carried out in two successiv e grazing seasons (years) and included a total of 79 Danish Holstein c ows. The effect on milk protein composition was determined in both yea rs whereas the effect on renneting properties was determined only in t he second year. Fertilization of the clover grass significantly decrea sed total milk protein concentration (- 1.4 g/kg; P < 0.01) and tended also to decrease the relative proportion of whey protein N. Fertiliza tion had no effect on renneting properties. Increased availability of clover grass significantly increased milk protein concentration (1 g/k g; P < 0.05) and resulted in significantly poorer renneting properties , that is increased clotting time (P < 0.01) and decreased coagulum de velopment. These effects seemed to be mediated through an effect on th e pH of the milk (+ 0.05; P < 0.05) as the effect was markedly reduced when statistical correction was made for the actual pH. Use of the pr otein- and fat-rich concentrate mix (3.5 kg) significantly reduced the total protein content of the milk (P < 0.05) and increased the propor tion of non-protein N (NPN) in total N compared with use of the other supplementary feeds (P < 0.05). We found no effect on renneting proper ties of the different supplementary feeds. Throughout the grazing seas on and independent of the main treatments, the NPN proportion of milk N increased at the expense of casein N. At the same time, renneting pr operties became poorer, especially with high clover grass availability .