HORMONAL THERAPIES

Hormonal therapy is the most commonly used method to treat delayed ovulation in cattle, particularly in postpartum dairy cows, which are susceptible to folliculogenesis and ovulatory response disorders [165]. Exogenous hormone administration aims to normalize endocrine function, stimulate follicle maturation, optimize the LH surge, and ensure ovulation occurs at the physiological time, thus supporting increased reproductive efficiency [166]. However, the choice of hormonal agent must consider follicular status, metabolic condition, and potential risks associated with repeated hormonal exposure. Effectiveness may vary significantly between herds depending on management, nutrition, and overall health status.

Strategy / Approach | Mechanism / Action | Application / Description | References
Hormonal therapies | Stimulate ovulation through exogenous hormones | GnRH induces artificial LH surge in dominant follicles; hCG acts as LH agonist and promotes luteinization. Synchronization protocols (Ovsynch, G6G, Double Ovsynch) coordinate ovulation, improve follicle uniformity, and optimize AI timing. Choice depends on follicular status, metabolic condition, and herd management. | [165–175]
Nutritional management | Optimize energy status and micronutrient supply | Maintain ideal BCS 3.0–3.5; prevent or minimize NEB; supplement selenium, zinc, copper, manganese, vitamin E, and vitamin A. Supports follicle maturation, LH sensitivity, steroidogenesis, and overall ovarian function. | [176–186]
Environmental and stress management | Reduce environmental and stress effects on reproduction | Manage heat stress through improved ventilation, cooling systems, and shading; optimize stocking density, barn comfort, and lighting. Helps maintain hormonal balance, estrus expression, and ovulatory competence. | [187–195]
Postpartum management | Support postpartum recovery for timely ovulation | Implement uterine health programs to prevent endometritis/metritis; optimize transition cow management with BCS monitoring, balanced nutrition, and energy balance control. Enables early differentiation of physiological vs. pathological delayed ovulation and accelerates reproductive recovery. | [196–201]
Assisted reproduction technologies | Synchronize ovulation and improve AI timing | Fixed-time artificial insemination (FTAI) with hormonal protocols (GnRH + prostaglandins) standardizes ovulation timing and reduces sperm–oocyte asynchrony. Integrate with nutritional, environmental, and hormonal management; optimize semen type (conventional vs. sex-sorted). | [202–208]

The administration of GnRH is the primary strategy for stimulating LH release from the pituitary gland [20]. In cows with a dominant follicle that is already in the preovulatory phase but has failed to rupture, GnRH injection can trigger an artificial LH surge and induce ovulation within 24–30 h [167]. The efficacy of GnRH is strongly influenced by the degree of follicle maturation; follicles that have not reached optimal size or physiological competence tend to have a lower response, limiting the effectiveness of GnRH in some individuals [168]. Therefore, the timing of GnRH administration is crucial in the management of delayed ovulation [169]. Reported ovulation or conception success rates following appropriately timed GnRH treatment range from approximately 50–70%, but these rates are context-dependent and can vary with parity, energy balance, postpartum interval, and herd management practices.

Despite its widespread use, GnRH administration has limitations. Repeated or poorly timed injections may result in suboptimal follicular responses, unnecessary treatment costs, and reduced overall protocol efficiency [20]. GnRH is generally preferred when a mature dominant follicle is present and pituitary responsiveness is intact, and is less effective in cows with severe endocrine disruption, systemic disease, or poor metabolic status [169].

Recent studies in North Romania demonstrated that strategic GnRH administration at the time of artificial insemination in cows raised under extensive systems can improve conception rates while being cost-effective, reducing economic losses associated with repeat inseminations [170].

Other therapeutic strategies, such as prostaglandin-based synchronization protocols or combined GnRH-prostaglandin treatments, have also been explored to enhance fertility outcomes in similar extensive farming systems, with variable success and economic implications depending on herd management, drug costs, and labor availability [167].

Human chorionic gonadotropin (hCG) is used as an alternative or complement to GnRH because it acts as an LH agonist with high affinity for LH receptors in the ovaries [170]. Administration of hCG directly stimulates granulosa cell luteinization and facilitates ovulation, with a longer-lasting effect than GnRH [171]. However, repeated hCG use carries specific risks, including antibody formation against hCG, potential ovarian overstimulation, and abnormal luteal development, which limit its routine application. Therefore, hCG is often reserved for cows that fail to respond adequately to GnRH or exhibit clear signs of pituitary insufficiency [164].

Synchronization protocols such as Ovsynch, G6G, and Double Ovsynch offer a more structured therapy strategy [172]. Ovsynch begins with GnRH administration to regulate follicle growth, followed by prostaglandin F2α to lyse the CL, and a second GnRH to trigger coordinated ovulation [173]. Meanwhile, the G6G and Double Ovsynch protocols add a pre-synchronization phase to optimize hormonal responses and standardize follicle status prior to insemination [174]. These protocols generally achieve conception rates of 35–55% per insemination, but success varies with herd health, metabolic status, and management efficiency [175].