Breeding2 min read
Cow heat signs and the best time for artificial insemination (AM–PM rule)
Missing one heat costs about 21 days of milk. Learn the signs of heat, the AM–PM rule for AI timing, and how to stop missing silent heats.
A cow comes into heat about every 21 days (anywhere from 18 to 24). Each missed heat pushes her next calving back by three weeks and means three weeks less milk over her life. Good heat detection is the cheapest way to make a dairy farm more profitable.
Signs of heat
The most reliable sign is standing heat: the cow stands still when another cow mounts her. Other signs:
- Clear, stringy mucus from the vulva (like egg white)
- Swollen, red and moist vulva
- Restless, bellows more, walks more than usual
- Mounts other cows, sniffs and licks them
- Milk drops a little for a day; she eats less
- Rubbed hair and dirt on the tail head
Mounting other cows is a sign of heat, but standing to be mounted is the sign to time insemination by.
When to watch
Heat often starts at night and standing heat lasts only about 12–18 hours (shorter in hot weather and in high-yielding cows). Watch the herd at least 3 times a day for 20–30 minutes: early morning, midday and late evening, when the animals are calm and not eating.
The AM–PM rule
The egg is released about 10–14 hours after standing heat ends, so insemination works best in the second half of heat.
| First seen in standing heat | Inseminate |
|---|---|
| Morning | The same evening |
| Afternoon or evening | The next morning |
If you can only call the AI technician once a day, inseminate once about 12 hours after you first saw standing heat. Inseminating too early or too late is one of the main reasons cows "repeat".
Silent heat
Some cows ovulate with few or no signs, especially thin cows, cows in the first 2 months after calving, and cows in summer heat. To catch them:
- Use the 21-day calendar: watch extra carefully 18–24 days after the last heat
- Feed enough energy and minerals; thin cows often show weak heat
- Ask your vet to examine cows that have shown no heat 60 days after calving
After insemination
- Record the date, time, bull or semen code and the technician's name.
- Watch for return to heat 18–24 days later. If she comes back, she is not pregnant.
- Have a pregnancy check done about 45–60 days after insemination.
- Expected calving is about 283 days after the successful insemination.
Let the app count the days
In FarmFlows, record heats and inseminations under Breeding. The app works out the next expected heat, the date to check for return to heat, the pregnancy check date and the expected calving date, and shows reminders on your dashboard. Your vet can also see them if you invite them to your farm.
Quick checklist
- Watch 3 times a day, 20–30 minutes each
- Standing to be mounted = true heat
- Seen in the morning → inseminate in the evening; seen in the evening → next morning
- Watch again 18–24 days later
- Pregnancy check 45–60 days after AI