An analysis of a herd-based lactation curve reveals the seasonal effect from dairy cows fed under high ambient temperatures
The present investigation was carried out to demonstrate the effect of high ambient temperature (HTa) on lactation
performance with herd-based measurement. One large dairy farm in Thailand with more than 300 crossbred Holstein
cows milking each month was selected. The meteorological data was recorded once a week during the period of
observation. Milking cows (n=90)were observed for a 10-month lactation curve based on their calving seasons: Summer,
rainy and winter. The lactation curve was fitted by nonlinear regression using the reduced lactation persistency model. The
climatic conditions revealed a high degree of temperature and humidity index (THI) . The average THI throughout the
year was 84.8 ± 0.5. The THI from summer (86.9 ± 1.0) was significantly higher than that from winter (82.7 ± 0.5, P<0.05).
An analysis of the average lactation curve revealed that the total 305-day lactation yield (5407 kg) from this farm was
higher than the average phenotypic 305-day lactation yield from dairy cows fed in Thailand (4698 kg). Lactation curves
from different seasons revealed the potential effect of HTa on mammary gland function. Specifically, the area on the
curve of the summer lactation curve was significantly lower than that of the winter curve by16% (P<0.05). This effect
was apparently from the shortest lactation persistency. In conclusion, dairy cows fed in Thailand were under a high
degree of HTa and had compromised lactation performance. Lactation persistency appears to be one of the main
phenotypic traits for breed selection for dairy cows fed in the tropical country.