Postpartum acyclicity in suckled beef cows: a review

Area of Science:

• Reproductive endocrinology in livestock
• Animal science and veterinary medicine
• Beef cattle production systems

Background:

• Prolonged postpartum acyclicity in suckled beef cows causes significant economic losses.
• Factors influencing acyclicity include suckling, nutrition, calving season, and age.
• Uterine involution and ovarian follicular waves resume post-calving, but dominant follicles fail to ovulate.

Purpose of the Study:

• To elucidate the physiological mechanisms underlying postpartum acyclicity in beef cows.
• To explain the failure of dominant follicle maturation and ovulation.
• To present a model for postpartum acyclicity and the resumption of cyclicity.

Main Methods:

• Review and synthesis of existing research on postpartum reproductive physiology in beef cattle.
• Analysis of hormonal feedback loops involving GnRH, LH, estradiol-17beta, and opioid peptides.
• Examination of the influence of suckling and time post-calving on the GnRH pulse-generator.

Main Results:

• Failure of dominant follicle maturation is linked to insufficient Luteinizing Hormone (LH) pulses due to depleted pituitary stores and absent Gonadotropin-Releasing Hormone (GnRH) pulses caused by suckling.
• Post-calving, sensitivity of the GnRH pulse-generator to estradiol-17beta negative feedback persists, inhibiting GnRH pulses.
• Decreased sensitivity to negative feedback with increasing postpartum interval leads to GnRH/LH pulses, follicular maturation, ovulation, and cyclicity.
• The first postpartum ovulation may result in a short cycle due to premature luteolysis, potentially exacerbated by suckling-induced oxytocin release.

Conclusions:

• Postpartum acyclicity in beef cows is a complex endocrine-mediated process primarily driven by suckling and feedback mechanisms.
• Understanding these mechanisms is crucial for developing strategies to improve reproductive efficiency in beef herds.
• The presented model provides a framework for comprehending the transition from acyclicity to cyclicity post-calving.