Heat Detection Without Technology
Mucus discharge, mounting behavior, bellowing — master the 18-hour window using your eyes and a simple log.
The 18-Hour Window
A cow's fertile window lasts approximately 18 hours, but the optimal insemination time is narrower — mid to late standing heat. Ovulation occurs 10-14 hours after the end of standing heat, meaning cows detected in the morning should be inseminated that same day, and those detected in the evening should be inseminated the following morning. This is the AM/PM rule, and it remains one of the most reliable guidelines in dairy breeding.
Primary Signs of Heat
Standing to be mounted is the only definitive sign of heat. Secondary signs include: mounting other cows, clear mucus discharge from the vulva, swollen and reddened vulva, restlessness and increased walking, bellowing, decreased feed intake, a drop in milk yield, and tail raising/fluffing. A cow in standing heat will stand immobile when another cow attempts to mount — this is the signal you're waiting for.
Using a Simple Heat Detection Log
A notebook, whiteboard, or basic spreadsheet is often more reliable than expensive technology — if used consistently. Record every observation morning and evening. Mark each cow's cycle day (cows cycle every 18-24 days). Flag any cow that hasn't been seen in heat by 40 days post-calving for veterinary examination. The simple act of writing down observations trains your eye to notice subtler signs over time.
Common Mistakes That Miss Heats
Observing cows only once daily catches less than 50% of heats. Peak heat activity typically occurs between midnight and 8 AM. Checking cows during feeding or milking misses most behavioural signs. Overcrowding makes heat expression difficult. Cows on slippery concrete rarely express strong heat. And silent heats (ovulation without visible signs) are more common in high-producing cows — don't assume they're cycling just because you haven't seen anything.
The AM/PM rule saves pregnancies. Watch for standing heat — it's the only reliable sign. Observe twice daily, keep a log, and investigate any cow not seen in heat by 40 days in milk.