Revisiting the Timing of Insemination at Spontaneous Estrus in Dairy Cattle

SIMPLE SUMMARY: The possibility of artificial insemination (AI), mainly in dairy cattle, revolutionized animal and human reproduction procedures over the 20th century. In effect, AI in dairy cattle is considered the most important technological advancement in the field of animal breeding. Cows show estrous behavior over an 8-to-20 h period yet they are refractory to the bull some 10–12 h before ovulation. As spermatozoa need to persist in the female reproductive tract for 6–8 h before they are capable of fertilization, in the past 80 years the time interval for AI in cattle with fresh or frozen-thawed semen has been 24–6 h before ovulation. Recent evidence suggests this interval should be reduced. ABSTRACT: Cows show spontaneous estrus over 8–20 h but become refractory to the bull about 10–12 h before ovulation. This indicates that ovulation occurs 10–12 h after the end of estrus behavior, yet spermatozoa from the bull ejaculate need to undergo maturation and capacitation for 6 to 8 h in the female reproductive tract before they are capable of fertilization. Traditionally, the onset of estrus has been considered the best timing for artificial insemination (AI) in cattle, that is, 6 to 24 h from the first signs of estrus. However, recent findings suggest this interval should be reduced to 16 to 6 h before ovulation, bringing it closer to the end of estrus. In this review, the end of estrus rather than its onset is proposed as the best guide for AI timing in dairy cattle, and physiological indicators of late estrus are discussed such as relaxation of the intravaginal part of the uterus, a lower cervical mucus viscosity and a softer pre-ovulatory follicular consistency as simple cues indicating a cow is ready for service.