The oestrous cycle, revisited: follicular waves and what actually decides conception

The oestrous cycle of the dairy cow is conventionally described as 21 days, but the real distribution in lactating herds runs from roughly 18 to 24 days, and the reason is structural rather than random. Ultrasound work beginning with Savio et al. (1988) and Ginther et al. (1989) established that the cycle is built from two or three successive waves of follicular growth, each with a recruitment, selection and dominance phase. Cows running two waves tend to have shorter cycles; three-wave cows tend to have longer ones.

Why wave number matters at the farm level

The practical consequence is follicle age. In a two-wave cycle, the ovulatory follicle has been dominant for longer before it ovulates, and an aged dominant follicle is associated with lower fertility than a freshly selected one. This is one of the clearest mechanistic explanations for why cows served on a long cycle, or after a prolonged period of luteal activity, sometimes conceive less readily even though the visible heat looks textbook.

The corpus luteum decides what happens next

Progesterone from the corpus luteum controls both cycle length and the uterine environment the embryo will meet. Sub-luteal progesterone concentrations during the luteal phase before service, and slow progesterone rise after service, have both been repeatedly associated with reduced conception and early embryo loss. Wiltbank et al. (2014) reviewed the mechanisms in detail and made the point that this is largely a metabolic phenomenon in high-producing cows rather than an ovarian defect.

High intake, high clearance

• High dry matter intake increases hepatic blood flow, which increases the rate at which oestradiol and progesterone are metabolised.
• The consequence is shorter, less intense expressed oestrus and lower circulating progesterone at the same rate of ovarian production.
• This is a normal consequence of high production physiology, not a disease — it is managed through detection technology and service timing, not by reducing intake.
• It is also the main reason visual heat detection performance has declined as herd yields have risen (Lopez et al., 2004; Roelofs et al., 2010).

What to take onto the farm

Three things follow directly. First, expect real variation in cycle length and build detection systems that tolerate it rather than assuming a fixed 21-day return. Second, treat energy balance and liver load as reproductive variables, because they change hormone concentrations directly. Third, when fertility drops in a well-managed herd, look at the timing and detection system before concluding the cows have a reproductive problem — the physiology above explains most of what is usually blamed on the cow.