Studies On Heat-Tolerance Of Holstein Cows

The environmental management of dairy cows in high temperature was based on thermoregulation for heat tolerance. The practical index system according to the traits for heat tolerance could supply standardizaiton for the breeding of thermostability stock, the institution of nutrition standardization and the swarming management of dairy cows. This study presented heat tolerance index model, determined undetermined coefficients , and evaluated their feasibility.Part one: The physiology and productivity indexes of dairy cows were determined respectively at different stages: short-term heat stress, lasting heat stress and decreased heat stress. Based on original heat tolerance coefficient modle HTC=100-18 (BT-38.3) , the models of heat tolerance index HTI_(Tr) = 100(MY/mMY) - a (Tr-mTr) +b | Tr-Tr'| including the information of milk yield, rectal temperature and body temperature circadian rhythm and the modle HTI_(Tb)=100(MY/mMY) -a (Tb-mTb) +b | Tr-Tr' | which replaced rectal temperature with mean body temperature were presented. Undetermined coefficients were determined. The results showed that the traits of undetermined coefficient for mathematic distribution were obvious at the lasting heat stress. Undetermined coefficient a and b were 20 and 10 respectively in the model including rectal temperature information. And in the model containing mean body temperature information, a and b were respectively 18 and 10.Part two: The difference on heat- tolerating types of Holstein cows was studied in high temperature (short-term heat stress, lasting heat stress and decreased heat stress). The milk yield, rectal temperature, mean body temperature, rectal temperature difference and respiratory rate for different heat- tolerating Hostein cows were compared and analyzed. The results indicated that the milk yield from high heat-tolerating cows was higher than that from low heat- tolerating cows and the high heat-tolerating cows had lower rectal temperature than the low heat- tolerating cows at three stages. The change of mean body temperature for different heat- tolerating types of Holstein cows was analogous to that of rectal temperature at three stages. And at the lasting heat stress stage, the high heat- tolerating cows rised their rectal temperature quite highly in order to transmit heat quickly to accommodate environment, and their recovery capabilities were also high.Part three: Based physiology and productivity indexes of Chinese Holstein cows were determined in high temperature and the heat tolerance of cows were evaluated with the established heat tolerance index HTI_(Tr) and HTI_(Tb). Then the physiological nature on different types of heat- tolerating Holstein cows were obtained to verify that whether the established heat tolerance index were valid and feasible. The results showed that the thermoregulation on the high and low heat-tolerating Holstein cows was coherent to that of cows at lasting heat stress stage in trial 2. Both the heat tolerance index HTI_(Tr) and HTI_(Tb) for different types of heat-tolerating cows presented the same physiological characteristics, which were that the high heat- tolerating Holstein cows had higher mean skin temperature and milk yield, but lower mean body temperature and respiratory rate than the low heat- tolerating cows at the lasting heat stress. So establishment on heat tolerance indexes were valid and feasible.