ANIMAL-RELATED FACTORS

Factors directly related to the individual animal play a significant role in regulating reproductive function and significantly influence the occurrence of delayed ovulation in cows [74]. Variations in age, parity, breed type, and postpartum physiological condition influence hormonal responses, follicle development, and reproductive recovery rates [75].

Parity is a major factor influencing ovarian function [76]. Cows can be broadly categorized as heifers (young primiparous cows, first parity, ≤3 years of age) and multiparous cows (second parity and above, >3 years of age). Primiparous cows show a higher incidence of delayed ovulation, primarily due to competition between somatic growth and milk production for energy resources [77]. In young cows, the metabolic capacity to support ovarian function is not yet fully optimized, making NEB more likely, resulting in decreased GnRH and LH pulsations [78]. Multiparous cows generally exhibit more consistent follicular responses to gonadotropins and a more favorable metabolic hormone profile, thereby reducing the risk of delayed ovulation [1].

Age-related reproductive changes also influence ovarian responsiveness. As cows age beyond 6–7 years, there is evidence of reduced follicular reserve, lower granulosa cell proliferation, and decreased sensitivity to gonadotropins, all of which can prolong the preovulatory phase and increase the likelihood of ovulatory delay [74].

Differences in cattle breeds also influence reproductive sensitivity to environmental and nutritional conditions [46]. High-yielding dairy cattle, such as Holsteins (Bos taurus), are more susceptible to delayed ovulation than dual-purpose or local breeds, due to their high metabolic demands, which affect energy status and metabolic hormones such as insulin and IGF-1 [79]. Conversely, beef and indigenous breeds, including Zebu-derived cattle (Bos indicus), typically exhibit better reproductive resilience and tolerance to nutritional and thermal stress [80]. These differences are partly related to genetic variation in reproductive and metabolic pathways, including gonadotropin signaling (LHCGR and FSHR), steroidogenesis (CYP19A1 and Steroidogenic acute regulatory protein [STAR]), and energy metabolism (Leptin and IGF1); however, current genetic markers remain limited in predicting ovulation outcomes, and caution is required to avoid overinterpretation [81]. Comparative studies have shown that Bos indicus cattle often maintain more stable LH pulsatility and follicular growth under heat stress compared with Bos taurus breeds, reducing the risk of ovulatory delay.

The postpartum period is a critical phase that significantly influences the accuracy of ovulation [82]. During this period, cows experience significant metabolic changes due to the onset of milk production, accompanied by decreased reproductive hormone levels and increased energy requirements [83]. NEB in early lactation suppresses LH pulse frequency, inhibiting preovulatory follicle maturation and prolonging the interval to first postpartum ovulation [8]. Furthermore, incomplete uterine involution can trigger subclinical inflammation and increase prostaglandin production, which disrupts follicle development and CL function [36]. Subclinical endometritis activates inflammatory cytokines (e.g., tumor necrosis factor-alpha [TNF-α] and Interleukin-1 beta [IL-1β]), which suppress GnRH secretion via neuroendocrine pathways, increasing delayed ovulation risk [84].