Poor heat detection is the quiet profit-killer of most dairy herds. A cow you fail to spot in heat is a cow that doesn't get pregnant for another ~21 days — and every open day past target costs feed with no milk-cheque to show for it. The good news: heat detection is a skill you can sharpen for free.
The cycle you're working with Cows cycle on average every 21 days (range 18–24). Standing heat — the fertile window — lasts only about 12–18 hours, and high-yielding modern cows often show it for less. Miss the window and you wait three weeks for the next.
The one definitive sign: standing to be mounted A cow in true standing heat will stand still while other cows mount her. That's the gold-standard sign — everything else is a clue that points you to watch her, not proof she's ready.
Secondary signs (she's coming into or going out of heat) - Mounting other cows (but not standing to be mounted herself) - Clear, stringy mucus discharge from the vulva - A red, swollen vulva - Restlessness, bawling, walking the fence, reduced milk for a day - Roughed-up tail head / rubbed hair from being mounted - Chin-resting and sniffing other cows - A blood-tinged mucus discharge a day or two *after* heat (metestrus bleeding) — this means you've just missed a heat; mark her to catch the next one in ~18 days.
Watch at the right times Cows show heat most in the cool, quiet hours — early morning and late evening, away from feeding and milking distractions. Spend 20 focused minutes watching the group three times a day and you'll catch far more than glancing in passing. Most missed heats are missed simply because nobody was looking at the right time.
Aids that help Tail paint or chalk (rubbed off when she's mounted), heat-mount detectors, and — at the high end — activity/pedometer collars that flag the spike in steps a cow takes when in heat. Collars catch ~70% of heats automatically and shine in big herds where eyes can't cover everyone.
Then time the insemination right The a.m./p.m. rule: cow seen standing in the morning → breed that afternoon; seen standing in the evening → breed next morning. Aim to inseminate about 12 hours after the onset of standing heat, because ovulation follows ~24–32 hours after heat begins and the egg only lives 8–10 hours.
Close the loop in your records Log every heat you see — even unbred ones. Heats recorded let Vache predict the next one, flag cows overdue to cycle, and stop heifers and cows sitting open and unnoticed.
Sources University of Wisconsin Extension — Estrus Detection & Aids. Penn State Extension — Heat Detection and AI Timing. NADIS Animal Health — Heat Detection in Dairy Herds.