عنوان مقاله [English]
Introduction Animal performance can be affected by the weather condition. Nowadays, study the effect of weather conditions on plants, animals and all living being is an interdisciplinary science, which has a great effect on the quality and quantity of their products. In order to survive and maintain their physiological activities, body temperature of warm-blooded animals should remain in a specific range. Therefore in warm days, in order to restrain their body temperature in an optimized range, their transpiration, breathing and heart beat raises, while the feeding rate decrease. Meanwhile, if the relative humidity was high, the cooling reaction does not response well and therefore animals’ body temperature raises, which puts animals under the heat stress. Subsequently, problems such as feeding rate and animal production reduction, reproductive problems, and increase in the rate of getting sick occur. In this regard, researches have shown that air temperature and environmental humidity are two key parameters in cows’ heat stress. To evaluate and assess the heat stress, most of the researchers use Temperature Humidity Index (THI). Numerous equations have been proposed for THI calculation, each having coefficients to be calibrated in different regions, which in return make their application confusing. The aim of this study is to determine the best THI equation for dairy industry in Mashhad region, and also find the reduction in milk production during the heat stress period.
Materials and Methods The meteorological data (daily wet and dry bulb and relative humidity records) were obtained from Iran meteorological organization. THI was calculated by 8 common equations for seven consecutive years (2007-2013). The daily milk production data was also obtained from improvement center of milk production in Karaj. Then the relationship between daily milk production and 8 obtained THI were calculated for both annual and seasonal time scale. At that point by using the correlation coefficients and P-value the best equation to determine THI was chosen. Finding the best THI equation for the study area, led to find a warning heat stress point. At the end, daily milk production during the heat stress was calculated and milk production reduction was estimated during the heat stress time.
Results and Discussion Results indicated that there was no significant correlation between annual milk production and obtained THI. Also the relationship between THI and milk production in seasonal scale (spring, fall and winter) was not significant. Nevertheless, only daily milk production in summer had a significant relationship with temperature-humidity index, which was negative. This means that daily milk production decreases as THI increases. Among 8 investigative equations, the forth equation (which calculate THI by using dry bulb and dew point temperature) had the best correlation coefficient and P-value with daily milk production. By definition the break point in summer daily THI and milk production, the heat stress warning point for Mashhad dairy industry was found. The results showed that when THI rises above 75, the milk production significantly decreases. To calculate the milk reduction corresponding to THI increase, the relationship between milk production and THI was estimated more than 75. Then by subtracting the calculated THI from average milk production in a non-stress day, the loss of milk production were calculated for each cow. Then by multiplying one cow milk lost, by the number of cows, the final milk production loss were calculated for each year. The results showed that the temperature humidity stress had a greatest effect on milk production, so that during a summer time, each cow averagely produce 138.8 liter milk less than usual. This amount is also equal to 1.5 liter milk per a day. During a 7 years period investigation, the total milk loss was calculated 401 ton.
Conclusion In conclusion, raising air temperature can cause a heat stress and a reduction in milk production. When the heat comes with high relative humidity, these effects highlighted and cows were under enormous stress therefore their body temperature transpiration, breathing and heart beat raise, while the feeding rate decrease, and as a result of that milk production reduced which can cause a great damage in diary industry.