changes of milk fat, crude protein, true protein, npn and protein:fat ratio in holstein cows fed a high concentrate diet from early to late lactation

percentages and yields of fat, crude protein (cp), true protein (tp) and non-protein nitrogen (npn) wereassessed in milk of four groups of holstein cows, at early, mid and late lactation (days in milk 20-70, 71-110, 111-150 and 151-210), on a diet rich in concentrates during a hot summer. four milk samples were taken from individual cows at 10-day intervals. milk fat percentages were close to breed average and increased toward the end of lactation with a tendency (p = 0.055) for significant difference between the early and late lactation. percentages of cp (p = 0.14) and tp (p = 0.478), however, were low similarly among all groups and did not follow the normal pattern of the breed. percentage of npn was significantly higher in groups iii and iv (p = 0.005). yields of fat (p = 0.035), cp (p = 0.002) and tp (p = 0.001), but not npn (p = 0.589), decreased toward the end of lactation. there was a negative correlation (r = -0.354, p