When Vache flags a cow whose milk has dropped, treat it like a smoke alarm: something changed, and the yield is just the smoke. The faster you find the fire, the less production you lose. Work the list from most-common and cheapest-to-check downward.
1. Is it the cow, or is it the whole herd? If several cows dropped together, look at management, not medicine: did the feed change, run out, or spoil? Water supply interrupted? Weather turned hot (heat stress cuts intake and yield within a day)? A new milker or a changed routine? Herd-wide drops are almost always feed, water, heat, or routine.
2. Mastitis — strip the quarters. A hot, hard quarter or abnormal milk explains a drop instantly and is the single most common single-cow cause. Always rule this out first.
3. Where is she in lactation? A natural decline of ~8–10% per month after peak is normal — that's the lactation curve, not a problem (see "Understanding the dairy lactation curve"). A *sudden* drop off the expected curve is what matters.
4. Fresh cow (under 6 weeks)? Think metabolic. - Ketosis — fresh cows pulled into negative energy balance stop eating and milk falls; breath/urine may smell sweet. (See "Signs of ketosis in fresh dairy cows.") - Milk fever / sub-clinical hypocalcaemia — around calving, a cow short on calcium drops milk and goes wobbly or down. - Displaced abomasum, retained placenta, metritis — all hit fresh cows and all cut intake and yield.
5. Is she sick anywhere? Take her temperature. A fever points you to infection (mastitis, metritis, pneumonia, tick-borne disease). Lameness cuts yield because a sore cow lies down less and eats less — check her feet and her locomotion.
6. Is she in heat? Cows on heat commonly drop milk for a day and bounce back — log the heat, it's useful breeding data, and don't chase a phantom illness.
7. Did the milking itself change? Incomplete milking-out, a unit falling off, a liner or vacuum fault, or a stressed/rushed parlour all lower yield without the cow being ill.
The fast field routine Temperature → strip quarters → days-in-milk and heat status → feet and rumen fill → then feed/water/heat if it's herd-wide. Most drops are explained inside five minutes with no drugs.
Why logging the cause pays off "Milk dropped" tells you nothing next month. "Milk dropped — ketosis, day 12" builds a pattern that tells you your transition-cow management needs work — which is a fixable, farm-level win, not a cow-by-cow firefight.
Sources AHDB Dairy — Fresh Cow Management. Merck Veterinary Manual — Ketosis, Hypocalcaemia, Mastitis. University of Minnesota Extension — Troubleshooting Drops in Milk Production.