VN November 2021

Vetnuus | November 2021 36 Regulars I Zoetis Livestock Column Oestrous synchronisation of the oestrous cycle of the cow, paired with artificial insemination (AI), are effective breeding management tools available to the modern beef or dairy producer. Artificial insemination allows producers toutilise valuable genetics, albeit local or international. 3 Oestrous synchronisation involves manipulating the oestrous cycle using exogenous hormones to get as many cows on heat or inseminated at the same time as possible. 4 Using these technologies, producers can improve reproductive efficiency and make maximum use of their time and other resources. Management is made more accessible due to a shortened breeding season and more uniform calf crop. 3 However, for any synchronisation programme to be successful, there must be a thorough understanding of various concepts involved. Firstly, the oestrous cycle of the animal and the hormonal interplay that leads to successful ovulation. Secondly, knowledge of the available exogenous hormones, how they function and why they are administered at specific times. Follicular waves Follicular wave dynamics are an essential consideration in an oestrous synchronisation program. Groups of Gonadotropin (FSH) dependent follicles grow in wave-like patterns. 1 These waves are always present, even before puberty and during pregnancy, but not every wave ends in an ovulation. 1 The duration of an individual animal’s oestrous cycle will depend on the number of follicular waves that occur. 1 Most cows exhibit either a two or three-wave cycle. If the day of ovulation is regarded as day 0, the second wave will emerge on days 9 and 10 for two wave cycles. 8 For three-wave cycles, the second wave will emerge on day 8 or 9 and the third wave on day 15 or 16. 8 This will result in an oestrous cycle duration of 20 days for two wave cycle cows and 23 days for three-wave cycle cows, on average. 1,8 A surge in FSH can be detected approximately 2 – 4 days (peaking at 1 – 2 days) before the emergence of a group of recruited follicles that are ~4 – 5 mm in diameter. 1,8 From the group (also known as a cohort) of follicles, one follicle is selected as the dominant follicle (diameter approximately 8 mm) about three days after wave emergence, and FSH levels start to decline. 1,8 The estradiol and inhibin released by the dominant follicle further suppress the FSH to levels below that required by the smaller follicles to grow. 1,8 The dominant follicle then continues to grow for another ~6 days while the rest of the follicles undergo atresia (selection). 1,8 At this stage, the dominant follicle becomes less dependent on FSH and more responsive to LH. 1,8 The stage of the cycle and the hormones that predominate at the time of maturation of the dominant follicle will determine whether the follicle will ovulate or undergo atresia. 1 In other words, the fate of the Corpus Luteum (CL) will determine whether ovulation will occur. At high progesterone concentrations (no luteolysis), the LH pulses are insufficient to lead to ovulation. 1,8 In such instances, the dominant follicle will begin to undergo atresia after about 2-3 days, estrogen and inhibin levels will fall, FSH levels will rise, and a new follicular wave will emerge. 1,8 The second (in two-wave cycles) and third (in three-wave cycles) follicular waves usually end in ovulation if it coincides with luteolysis. 1 With luteolysis, progesterone levels start to fall, which leads to an increase in the frequency of the LH pulses. 1 The growing follicle releases higher concentrations of oestrogen, which leads to positive feedback via the hypothalamic-pituitary axis and an LH surge triggering a biochemical cascade that results in ovulation. 1,8 At this time, the follicles will be approximately 15 – 20 mm in diameter. 1 Available hormones and types of synchronisation programmes While it is important to know the different hormones available, it is also important to understand why the hormones are given at specific times and/or concentrations. Most synchronisation programmes aim to manipulate the luteal phase, synchronise the follicular wave emergence, LH surge and the subsequent ovulation. 2 There are several synchronisation programmes available, but in general, they are either Prostaglandin F2 α (PGF2 α) -, Gonadotropin-Releasing Hormone (GnRH)-, or Oestradiol-based. 7,8 PGF2 α is used to induce luteolysis of a functional CL which will lead to a drop in progesterone levels. A functional CL means that the animal needs to be cycling and have a CL of at least > 5 days of age since it will be refractory to the effect of PGF2 α before this time. 7 PGF2 α -based Zoetis Animal HealthPages FOR ANIMALS. FOR HEALTH. FOR YOU. Understanding oestrous synchronisation in cattle Dr Chantelle Erwee, Zoetis South Africa (Pty) Ltd Technical Manager: Ruminants