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Heat Tolerance and Productivity of Dairy Cattle Under Tropical Climate in Suburb Area of Cambodia
Dissertation Abstract:
This study aimed to understand the factors affecting the dairy production in Cambodia to improve the dairy industry. The study investigated heat tolerance in dairy cows in a newly developed dairy farm in suburban Cambodia, using a physiological method that can contribute to the breeding of high performing dairy cows that can adapt to tropical climates. The study was conducted at the first experimental dairy farm established in Cambodia located at the Nagoya University Asian Satellite Campus in the Royal University of Agriculture (RUA) in the suburbs of Phnom Penh City. The effect of tropical temperature on the autonomic nervous activities in crossbred Cambodian dairy cattle was determined by analyzing heart rate variability (HRV). Holter-type electrocardiograms were recorded in five adult crossbred cows (Cambodian native x Holstein-Friesian) either in a sheltered area or under direct sunlight. Rectal temperatures and heart rates increased in all animals under direct sunlight compared to those in the shelter.
The power spectral analysis of HRV revealed that three out of the five cows studied showed a decrease in their parasympathetic nervous activity and a tendency to increase in sympathetic nervous activity under direct sunlight, or the likelihood to be more heat stressed. On the other hand, two cows did not show the tendency to increase in sympathetic nervous activity under sunlight exposure, hence, are likely to have higher heat tolerance. These results demonstrate that the power spectral analysis of the HRV is a useful tool to evaluate heat tolerance in individual dairy cows. The data from the HRV analysis would guide researchers and livestock farmers to breed high performing dairy cows that adapt to tropical climate.
In examining the difference of heat stress response between Cambodian local cattle and crossbred dairy cows based on the HRV analysis, results revealed that the frequency domain parameters of HRV in Cambodian local cows under direct sunlight showed uniform fluctuations. On the other hand, crossbred dairy cows showed varying responses to heat stress as shown by fluctuations of the frequency domain parameters of HRV. A number of dairy cattle seemed to respond similarly with Cambodian local cows when exposed to direct sunlight, indicating difference in response to heat stress between Cambodian local and crossbred dairy cows. There is a potential in establishing improved breeds of dairy cattle by selecting heat tolerant cows through the spectral analysis of the HRV.
In determining whether decrease in milk yield is associated with housing and feeding management, a comparison of the newly established systematic dairy farm in RUA and the local dairy farms located in the suburb area of Phnom Penh City was done. Four primiparous dairy cows fed at the RUA farm and four primiparous cows fed at three suburb local farms were used in the study. The average milk yields per day of RUA cows tended to be higher compared with those of local farm cows. Furthermore, milk yields in RUA cows with high milk production tended to decrease in the dry season when there is high ambient temperature. Moreover, difference in milk production between the RUA and local farms seemed to depend on feeding and management conditions. Higher dairy production can be achieved by improving feeding technique of dairy cows in Cambodia.
The importance of dairy management and the correlation between heat stress and HRV indices in domestic native crossbred dairy cattle in the tropical climate in Southeast Asia help to improve dairy cattle breeding in tropical and subtropical climate conditions. Moreover, the dairy cattle selection based on heat tolerance with HRV analysis could produce efficient and sustainable development of dairy industry in Cambodia as well as in other tropical countries in the region.
