پژوهشهای تولیدات دامی (Oct 2024)
The Effect of Heat Stress on the Performance and Genetic Parameters of Milk Production, Reproduction, and Somatic Cell Scores in Iranian Holstein Cows
Abstract
Extended Abstract Background: Improving the ability of animals to deal with adverse environmental conditions is a challenge in the livestock industry. Adverse environmental conditions, especially heat stress, are important economic issues in animal husbandry because they negatively affect the production of dairy cows. Although animals can adapt to hot climates, their adaptation mechanisms may be detrimental to their productive performance. The genetic potential of an animal can play an important role in controlling reductions in milk production, reproductive performance, and herd udder health in stressful weather conditions. Therefore, this study aimed to investigate the effect of heat stress on milk production traits, reproduction, and somatic cell score (SCS) and to identify the best temperature-humidity index suitable for studying heat stress in Iranian Holstein dairy cows. Moreover, the superior genetic fathers for heat tolerance were identified and their genetic value in functional traits was investigated under the genetic evaluation for heat stress. Methods: The estimated genetic parameters related to heat stress tolerance were milk production traits, udder health (SCS), and reproduction (open days and intervals from calving to first insemination). The data included 145,731 test day records of Holstein cows from the first lactation in 323 herds of the Iranian Breeding Center between 2008 and 2018. Test day records were merged with daily temperature-humidity index (THI) values based on weather records of weather stations. Each record included minimum, maximum, and average daily temperature, average daily relative humidity, and average daily rainfall. Genetic parameters were estimated using an animal model and random regression under the Bayesian method using GIBSF90 software. Variance components were estimated using multi-trait repeatable test day models with random regression on THI values. The models included herd test day classes and DIM as fixed effects, and normal weather, heat tolerance, and permanent environment as random effects. Results: The production and performance traits with increased THI (THI>72) showed a phenotypic and genetic decrease while the level of SCS increased with increasing THI. The average milk production in the THI index range of < 72 underwent a relatively stable trend with an average of 35.72 kg, but the average milk production decreased to 31.25 kg with the increase of the index from the threshold of 72. Thus, it can be claimed that the optimal THI point for milk production was between 72 and 70 THI (9-13 °C). The average open days was 114.47 ± 28.64 days, and the calving interval to the first insemination was estimated at 62.38 ± 16.42 days. The highest and the lowest lengths of open days and the interval from calving to the first insemination were respectively observed during summer (135.41 and 69.71, respectively) and in winter (100.07 and 54.64, respectively). In addition, the milk production and reproduction traits showed the highest genetic slope ratio, which indicates that they are more affected by heat stress at high levels of THI. Since this trait is known to reflect the mobilization of body reserves, using its changes in hot conditions can be a very cost-effective biomarker for heat stress in dairy cows that balances consumption and motility in hot conditions and highlights the need to include the additive genetic effect in the heat resistance evaluation model. Furthermore, the lactation stage in which cows experience heat stress intensifies the effect of heat stress on milk production. Mid-lactation and early-lactation cows are the most and, least affected, respectively, while late-lactation cows are moderately affected. High-producing cows were more affected by heat stress than low-producing ones. On the other hand, a positive genetic correlation was detected for SCS between neutral and heat stress, hence continued selection for lower SCS will result in a favorable selection response for SCS under heat stress. Conclusion: Heat stress is one of the important factors that negatively affects the milk production, reproduction, and health of Holstein cows. The effect of heat stress can be reduced by modifying the environment (feeding and cooling) or by the genetic selection of animals that are less affected by heat stress. The results of this research show that productivity, reproductive performance, and SCS are significantly affected by the THI-. Since the genetic potential of animals is different under the influence of heat stress and considering the global temperature changes, it is recommended to consider the effect of the animal's resistance to heat stress as a factor in the evaluation indices of superior bulls and dams to produce and breed resistant animals in herds, especially in tropical areas. The optimal THI for Iranian Holstein cows was below 72 for productive, reproductive, and health traits. Disturbances in various reproductive functions due to heat stress can lead to low conception rates in the first insemination, thereby increasing the calving interval and open days, especially during the warm season.
