1Department of Animal Health Management, School of Veterinary Medicine, University of Shiraz, Shiraz, Iran
2Department of Food Hygiene and Public Health, School of Veterinary Medicine, University of Shiraz, Shiraz, Iran
3Graduated from School of Veterinary Medicine, University of Shiraz, Shiraz, Iran
Percentages and yields of fat, crude protein (CP), true protein (TP) and non-protein nitrogen (NPN) were assessed 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<0.001) between the percentages of NPN and TP. However, no correlation was observed between the percentages of NPN and CP. Despite a very strong correlation between yields of CP and TP (r = 0.983, P<0.001), the correlation between the yield of CP and that of NPN was much weaker (r = 0.473, P<0.001). It is concluded that percents and yields of milk fat, close to breed average are attainable at any stage of lactation on high concentrate diets in hot weather. However, regarding milk protein, cows may not perform well. TP may be more affected than CP.