PHYSIOLOGICAL BASIS OF OVULATION IN CATTLE

Ovulation in cattle is a complex physiological process that relies on the coordinated action of the hypothalamic-pituitary-ovarian (HPO) axis [19]. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in a pulsatile manner, which is modulated not only by kisspeptin neurons, linking metabolic and environmental cues to GnRH secretion, but also by upstream neuroendocrine modulators such as neurokinin B and dynorphin, which fine-tune GnRH release [20].

The rise in follicle-stimulating hormone (FSH) levels triggers the recruitment and growth of clusters of ovarian follicles, but only one dominant follicle develops into the preovulatory phase [21]. This dominant follicle produces increasing amounts of estradiol, and when estradiol peaks, a positive feedback mechanism activates the hypothalamus and pituitary to induce the luteinizing hormone (LH) surge, a critical signal for ovulation [22]. Figure 1 summarizes the coordinated hormonal regulation governing normal ovulation in cattle.

Figure 1: Hormonal regulation and follicular dynamics underlying ovulation in cattle. The figure was prepared by the authors based on information synthesized from previous studies describing the hypothalamic-pituitary-ovarian axis, gonadotropin secretion, follicular wave dynamics, endocrine regulation, and ovulatory mechanisms [19–30]. The graphical illustration was generated with the assistance of ChatGPT (OpenAI, San Francisco, CA, USA) and designed using Canva (Canva Pty Ltd., Sydney, Australia). The final figure was reviewed, refined, and validated by the authors to ensure scientific accuracy and consistency with the cited literature.

Follicular development occurs in several waves during each estrous cycle, characterized by recruitment, selection, and dominance [23, 24]. Ovulation is restricted to the wave coinciding with luteolysis, when declining progesterone levels increase pituitary responsiveness to estrogen [25]. Under normal conditions, the LH surge triggers molecular changes in the follicle, including activation of proteolytic enzymes and remodeling of the follicular wall, culminating in oocyte release [26].

Breed and age influence HPO axis sensitivity; high-producing dairy breeds often display altered LH surge amplitude and timing compared with beef or dual-purpose breeds, while older cows exhibit reduced follicular responsiveness to gonadotropins, potentially prolonging the preovulatory phase [27, 28].

Delayed ovulation occurs when hormonal regulation or follicular dynamics are disrupted. Mechanisms include residual high progesterone from incomplete luteolysis, impaired estradiol synthesis, or decreased LH receptor sensitivity, all of which can extend the preovulatory period [29]. Management and environmental factors, such as stress, metabolic disorders, and nutritional imbalances, can disrupt pulsatile GnRH release, thereby delaying ovulation [30].