MILK-PRODUCTION FROM GRASS-SILAGE DIETS - EFFECTS OF HIGH-PROTEIN CONCENTRATES FOR LACTATING HEIFERS AND COWS ON INTAKE, MILK-PRODUCTION AND MILK NITROGEN FRACTIONS

To examine the effects of increasing the crude protein (CP) content of concentrates at either equal concentrate intake (increasing CP intake ) or at reducing concentrate intake (equal CP intake), 44 multiparous Holstein-Friesian cows were allocated to one of five treatments for we eks 4 to 18 of lactation. The treatments were 6 kg dry matter (DM) per day of concentrates containing nominally 200, 300 or 400 g CP per kg DM or 9 or 3 kg DM per day of concentrates containing 200 or 600 g CP per kg DM respectively. In addition 23 first-calf cows (heifers) were offered 5 kg DM per day of concentrates containing 200, 300 or 400 g C P per kg DM. All the animals were offered first-cut perennial ryegrass silage ad libitum. Increasing the concentrate CP content at equal int ake caused only small and non-significant increases in silage intake b y both heifers and cows. It significantly increased yields of milk and milk protein for both groups but whereas the cows responded to both i ncrements of CP, the heifers only responded to the first. The concentr ations of total protein, true protein, casein and non-protein nitrogen (NPN) in milk were significantly increased but the concentration of w hey protein remained unchanged. Increasing CP concentration while the concentrate ration was reduced enhanced silage intake but had no signi ficant effect on yields of milk or milk solids. The concentrations of total protein, true protein (non-significant), casein and NPN all incr eased but whey protein was unchanged. Overall the concentrations of ca sein and NPN were linearly related to the dietary CP concentration. Wi th both concentrate strategies the proportion of true protein in total milk protein decreased and the proportion of NPN increased with incre asing dietary CP concentration but the changes, though highly signific ant, were relatively small. It is concluded that the strategy of reduc ing the concentrate ration while maintaining concentrate CP intake cau ses only small reductions in milk solids production but it is dependen t for success on supplies of high-quality grass silage to substitute f or the concentrates.