Introduction One of the most important environmental factors affecting the dairy industry is the temperature changes that showed a negative impact on the industry over the past few years. Increasing temperatures have declined the production and reproductive performance of herds in the tropics. Huge losses are caused annually due to heat stress. Heat stress in dairy cows is caused by a combination of environmental factors (temperature, relative humidity, solar radiation and air movement). Continual genetic selection for greater performance results to increased sensitivity to heat stress. It was one of the reasons why lactation curve during summer has decreasing trend compared to spring in which lactation curve maintained within high levels. Dairy cows at the beginning of lactation have small chances to fight off a thermal stress, and thus it has the strongest effect on the production of milk in the first 60 days of lactation. A negative balance of energy in dairy cows at the beginning of lactation is even more increased by creating and emitting of higher quantity of thermal energy in the period when animals consume less food. For this reason, a high-yielding dairy cows are more sensitive to heat stress than cows having a lower genetic potential for milk production. Impacts of heat stress on reproductive efficiency have been well documented and reviewed. Heat stress has been shown to alter the duration of estrus, colostrum quality, conception rate, uterine function, endocrine status, follicular growth and development, luteolytic mechanisms, early embryonic development and fetal growth. Therefore, the purpose of this study was to investigate the effect of heat stress on yield of dairy cattle in different months of production and also to estimate genetic parameters of production and reproductive traits under heat stress.
Material and Methods In this study, 169655 records of 60322 dairy cows in different parity in Isfahan province of Iran were used. The studied traits included productive and reproductive traits. Milk test day and fat percent as productive traits and open days and days to first service were considered as reproductive traits. Climatic records of herds were collected from 7 stations less than 70 km away from herds and temperature-humidity index (THI) was calculated for each month in each herd. Dairy milk production records ranged from 5 kg to 60 kg for milk and milk fat percentage from 1 to 7%. Genetic and phenotypic trends were considered by regression of the estimated breeding values on year of the birth. The model used included the effect of herd-year of calving, calving month, parity and temperature-humidity index. An animal model was used to estimate genetic parameters of reproductive traits and random regression was used for production traits. SAS software was used to investigate the significance level of independent factors and DMU software was used to estimate genetic parameters.
Results and Discussion The results showed that the threshold of temperature-humidity index is 72 and more than it has adverse effects on performance. Average days open and difference to first service in different Parity 112 and 60 days respectively and was estimated the average heritability of days open and difference to first service 0.02 and 0.06, respectively. With increase in temperature- humidity index, mean of production traits decreased and this decrease for milk and fat yield traits occurred at temperature-humidity threshold of 72 and 65, respectively. The average of heritability was calculated as 0.32 for milk yield and 0.24 for milk fat percentage. The results indicated that with increasing temperature humidity index in 72 the genetic variance for both traits was increased. In on other words, the cows after heat stress (THI=72). Genetic differences were significantly increased. The results of this study indicate that the additive genetic variances were higher in early lactation (5 to 100 days of lactation), for both milk yield and fat percentage.
Conclusion Genetic variance increased with increasing temperature-humidity index (THI = 72) for the two traits of milk and fat production. Also breeding value of open day correction and difference to first calving under heat stress decreased. In other words, there is a significant genetic difference between animals exposed to heat stress after temperature-humidity index 72. Therefore, it may be possible to genetically identify animals more resistant to heat stress as parents of the next generation.