*3.1.1 Oestrus expression*

The time interval and potency of oestrus are also abridged by heat stress. Heat stress reduced follicular estradiol, which may have lowered the level of oestrus. The physical inactivity brought in by heat stress may also be a factor in decreased oestrus speech. During oestrus, cattle's become less active hence, less likely be carried out to other cattle's. As a result, dairy cattle's in the summer had about half as many mounts per oestrus as dairy cattle's in the winter. It also diminished the oestrus activity as a result, the Cattles decreased motor activity, which is an attempt to minimise endogenous heat production. In mammalian oestrus also serves as a behavioural predictor, indicating whether or not the female is bred close to ovulation due to a climatic variation shows 80% impact on estruses which are failed to be identified in heat-stressed cattle's [9]. The long period of time towards high temperature decline the rate of pregnancy by shortening the oestrus signs and lowering their severity of pregnancy [9]. Furthermore, during the summer, hot weather triggers ovulation without any symptoms of oestrus [9]. The key cause of imbalanced heat detection represents lower intensity of E2 in blood which alter the steroid genic mechanism of heat stress which is disturbed by granulose cell (GCs) [14]. The increased in rectal temperature seemed to have less effective towards binding protein such as insulin like growth factor, level of progesterone in dominant follicle and in E2 [14]. These various responses must be taken into account in terms of exposure period, oestrus cycle, nutritional quality [14] and other environmental factors including wind and humidity. Since lactation in cattle's produce of high heat can result in milk productivity and ovarian function in lactating cattle's differs from dry cattle's and heifers [14].

*Adverse Impact of Heat Stress on Bovine Development: Causes and Strategies for Mitigation DOI: http://dx.doi.org/10.5772/intechopen.99307*

## *3.1.2 Follicular development and oocyte quality*

The phase of ovarian folliculogenesis takes about 180 days. At birth, the ovary contains primordial follicles containing an oocyte [5]. The accumulations of primordial follicles in the ovarian pool result in rising of follicles enhance the follicular dynamics and at last increase in the pre-ovulatory follicle. The early changes in inhibit, estradiol, and progesterone have been shown to stifle the growth of primordial follicles. The heat stress imbalance the development of intermediate-size (6-9 m) follicles as result in earlier emergence and sometimes decrease or delayed in dairy cattle's [5]. The possibilities of folliculogenesis are reduced due to which considerable amount of plasma inhibin secreted by small size and intermediate follicles [4]. The earlier development of the pre-ovulatory follicle and a rise in the period of dominance [4] are also correlated with a lower rate of conception [4]. However, follicles produce oestrogen; a hormone that causes cattle's to become overheated. Since smaller follicles contain less oestrogen than larger ones, oestrus activity will be reduced. The oocytes and somatic cells that synthesise estradiol are originate from ovarian follicles whereas estradiol has a number of functions, including inducing of oestrus and cause the LH surge. Heat stress disturbs the follicle range and lengthens follicular waves, lowering oocyte output. It also allows for the development of multiple dominant follicles, which explains why cattle's conceiving in summers has more twins. Heat stress also damages the somatic cells (theca and granulosa cells) within the follicles. The variations in folliculogenesis patterns are likely to accompany changes in oocyte quality caused by heat stress. In cattle's exposed to heat stress, follicular dominance is reduced, ensuing in a boost large number of large follicles on the ovary. Over a longer period of time of ovulatory follicle dominance can enhance the higher secretion follicle stimulating hormone (FSH) and decrease the secretion of estradiol-17 hormone and inhibin hormone [13]. The heat stress in dairy cattle's reduced estradiol production and granulosa cell viability, as well as androstenedione production by thecal cells [15]. The conditions of follicles beneath heat stress is forced by the metabolic markers as result disturbs the level of glucose in blood and also imbalances the levels of non-esterified fatty acid (NEFA). In the cool season, the level of glucose in bovine follicular fluid is around 85% of the level of plasma glucose, and in the hot seasons, the follicular glucose level falls substantially with a corresponding drop in blood glucose level [15]. On the other hand, heat stress does not show any impact on balance of Non-esterified fatty acid despite a substantial increase in plasma levels [15]. The studies suggested that at the time of summer, the conditions of follicles is effected by level of nutrition in blood and level of components of biochemical in body of cattle. However, the concentration of oxygen in fluid of follicular does not get disturbed by heat and non-stressed conditions [15].

### *3.1.3 Corpus luteum*

The progesterone, which is required for embryonic growth, is secreted by the Corpus luteum. The lack of luteal deficiency describes the condition of corpus Luteum that does not secrete sufficient amount of progesterone to maintain the pregnancy as result decrease in the fertility of cattle's. When cattle's are subjected to long-term, persistent, seasonal heat stress, their progesterone levels normally drop significantly [16]. These are may be due to a disturbance in the Corpus luteum formation process, low synthesis of under hyperthermia, or imbalanced preovulatory follicles that shape a Corpus luteum with suboptimal purpose [16]. The lack of progesterone has been observed in at the of summer in luteinized granulose cell and theca cell compared to winter. However, a less significant corpus luteum

and lesser progesterone plasma concentration in the later dioestrus result from a smaller diameter and less steroid concentration in pre-ovulatory follicles, which can compromise embryo implantation and development [13].

However, future studies may require a large number of animals to concluded the comparatively weak consequence of acute heat stress during the delayed follicular processes.
