Abstract
Data were compiled and statistically analyzed on the lactation of 50% and 75% Thai native-Friesian crossbred and purebred Friesian cows that were fed at a national institute in Chiang Mai, Thailand. More than 30% higher milk production was obtained in the 75% crossbred compared with that in the 50%, but this amount of milk production in the upgraded breed was still about half that of purebred Friesians; 2,138 kg, least squares means during an average lactation period of 279 days in the 50% crossbred, 2,847 kg during 277 days in the 75% crossbred and 5,585 kg during 308 days in the purebred. Environmental stress due to tropical climate was alleviated by the use of electric fans and water sprinklers in the feeding house during the hot season, and improved diet seemed to enable purebred Friesians to keep their ability to produce a milk quantity of more than 6,500 kg per year. This special care was not given to crossbreds and significantly negative correlations were found between daily minimum temperature and humidity during the initial 100 days of lactation and total milk production and average daily milk yield in the 75% crossbreds. However, these correlations were not found in the 50% crossbreds.