Estrus

What is Estrus?

Estrus, commonly known as "heat," is the phase of the reproductive cycle when a cow is sexually receptive and will stand to be mounted by a bull or accept insemination. It is triggered by rising estrogen levels as a dominant follicle matures on the ovary.

A typical estrus period lasts 12–18 hours in cattle, though it can range from 8 to 30 hours depending on the individual cow, breed, and environmental conditions. During estrus, the cow displays characteristic behaviors: standing to be mounted, restlessness, bellowing, mucous discharge from the vulva, and reduced feed intake.

The estrus cycle — the interval between one estrus and the next — averages 21 days in cattle (range: 18–24 days). After estrus ends, ovulation occurs approximately 10–12 hours later. The released egg travels down the oviduct where it can be fertilized by sperm from natural service or artificial insemination.

Detecting estrus accurately is one of the most important — and challenging — aspects of dairy and beef cattle management. Missed heats are the leading cause of extended calving intervals and reduced reproductive efficiency.

Signs of Estrus to Watch For

The gold-standard sign is "standing heat" — the cow stands still when another cow mounts her. But not all cows show clear standing behavior. Watch for these secondary signs: (1) Restlessness — walking the fence line, searching for other cows, frequent position changes. (2) Mucous discharge — clear, stringy mucus hanging from the vulva, sometimes visible on the tail or hindquarters. (3) Mounting activity — mounting other cows (not just being mounted), chin-resting on other cows' backs, or sniffing the vulva of other cows. (4) Reduced feed intake — cows in heat typically eat 10–15% less than their pen mates. (5) Bellowing — increased vocalization, especially at night. (6) Swollen, reddened vulva — visible in some cows, particularly heifers.

The AM-PM Rule and Breeding Timing

A widely used guideline is the "AM-PM rule": if standing heat is observed in the morning, breed in the evening; if observed in the evening, breed the following morning. This targets insemination approximately 12 hours after the start of standing heat, which aligns with optimal sperm transport and egg viability. However, this rule is a simplification. With timed AI protocols (using prostaglandin and GnRH to synchronize ovulation), breeding is scheduled regardless of observed heat, which can improve pregnancy rates in herds with poor heat detection.

Economic Impact of Missed Heats

Every missed heat cycle costs 21 days of extended calving interval. At $6–$10/day in lost production (reduced milk during late lactation, additional non-productive days, and delayed genetic progress), a single missed heat costs $126–$210 per cow. For a 200-cow herd with a 90% detection rate (10% missed heats), the annual cost of missed heats is approximately $25,000–$42,000. Improving detection from 90% to 95% saves $12,500–$21,000/year — making heat detection technology (activity monitors, tail paint, mounting detectors) one of the highest-ROI investments in dairy management.