Factors of farm management, metabolic factors and reproductive characteristics on fertility of heat stress cows

Abstract

Heat stress is a major problem of economic loss in the dairy industry worldwide, including milk production, cattle's health, and reproduction problem. Thailand locates in the Tropical zone and is presented with heat stress problems all year round. This thesis' primary purpose is to study and understand the effects of heat stress on fertility in dairy cattle related to farm factors, cattle factors, reproduction, and ova factors. The studies in this thesis were conducted using in vivo and in vitro techniques to help farmers in smalllolder dairy farms improve their cattle's fertility. The heat stress factors on reproductive performances was observed. Microenvironments, barn characteristics, and reproductive performances data were collected and analyzed to find the correlation, using Cox's proportional hazard model. The results showed that days open and days to first insemination were related to the calving season, barn characteristics, and farm managements. All were related to heat stress. The problems of heat stress on reproductive performances, particularly in tie-stall barn type located in several areas in Thailand, are restricted to the primary signs of estrus behaviors. Therefore, the combination of secondary signs of estrus behavior observed in tie-stall barns were considered. Chapter 2 studied the optimal time for artificial insemination (AI) after detecting the secondary signs of estrus, and factors related to reproductive characteristics and the environment in farms, which were associated with increasing conception rate. The results showed that the optimal time for AI is 24 hours (h) after detecting the secondary signs of estrus behavior. Besides the secondary signs of estrus and their combinations, the appearance of cervical mucus discharge during insemination and season were related to the conception in dairy cattle inseminated 24 h after estrus detection. Seasonal effects on dairy cattle's fertility due to heat stress induces the cow's body temperature elevation, directly affecting the follicular pool development, COCs quality, and the quality of oocyte maturation and its capability to fertilize. These deleterious effects can affect cattle's fertility for a few weeks after exposure to the heat. Chapter 3 studied the reproduction problems, the seasonal effect on oocytes, by collecting COCs between different seasons to classify their quality into three classes, following the cumulus cell layers, and in vitro maturation. This study found that the highest Class I COCs had more than three layers of cumulus cells in the summer or hot season. The maturation competence of Class I and Class II was not related to the seasonal difference in the Tropical zone. A study of seasonal effects and environmental climates on the day of Class I COCs collection of the genes related to immature oocyte's quality was determined in Chapter 4. Genes were classified into three groups: 1) genes related to heat stress, 2) genes related to steroidogenic, and 3) genes related to apoptotic. This study showed, from the genetic expression, that the genes related to heat stress decreased in rainy season, in high- temperature and humidity (HT-HH) climates, whereas the genes related to steroidogenesis increased. Moreover, these expressions in HT-HH were similar to those found in low temperature and humidity (LT-LH) climates. However, the season was not affected by the expression of genes related to steroidogenesis and apoptotic. To understand the effects of heat stress on the development of oocyte into blastocyst, chapter 5 studied the oocyte maturation in vitro at different temperatures. being 37.0, 38.5, and 40.0 °C for cold, standard and heat stress conditions, respectively, with maturing period between 17 and 23 h. This study included in vitro fertilization on blastocyst developmental competence in different co-incubation periods of 4, 6, 8, and 20 h. Moreover, the oocyte maturation competence was assessed by the nuclear maturation at the metaphase II stage and cortical granules distribution in ova's cytoplasm at different temperatures and periods of in vitro maturation. The genetic expression on cumulus cells and matured oocytes related to their quality were measured in this chapter. This study showed that the 17 h heat-stressed ova produced the highest percentage of blastocyst compared to the other temperatures during 4 h fertilization period. Oocyte maturation at 40.0 °C for 17 h showed no difference in the percentage of blastocyst from the one matured at 38.5 °C for 23 h with 20 h fertilization period. Nuclear maturation of matured oocyte cultured at 40.0 °C for 17 h had developed to metaphase II stage faster than the one cultured at 38.5 °C for 17 h. There was no different to the matured oocyte cultured in the standard conditions at 38.5 °C for 23 h. The study of genes found that the expression of apoptotic genes on cumulus cells increased significantly in oocytes matured for 17 h and 23 h at 40 °C In conclusion, heat stress affected cattle's fertility in the Tropical zone on factors of barn, microenvironment, and season. The secondary signs of estrus detection and reproductive characteristics on reproductive management in tie-stall type at the day of insemination were related to conception risk at 24 h after estrus expression. The effects of seasonal and environmental-climate were associated with COCs' quality and quantity, and genetic expression. Besides, heat stress accelerated oocyte maturation, resulting in early fertilization.