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