Cud chewing is the cheapest rumen sensor

A resting dairy cow that is not heat-stressed should spend a large share of her day ruminating. Each bolus is re-chewed, mixed with saliva, and sent back. That loop is mechanical (smaller particles) and chemical (bicarbonate buffer).

Count cows lying or standing quietly after a meal. Many herds use a simple barn walk: a clear majority of resting cows should be chewing. Empty bunks, slug meals, heat, pain, or a ration that cannot hold a fiber mat all cut that number.

This is not a lab test. It is a daily check you can do without a pH bolus. If chewing falls and manure loosens in the same week, open the starch-and-SARA universe next — the same cow, a tighter question.