Feeding behaviour is the first vital sign
A thriving calf in its first weeks is an enthusiast: it comes to its dam or the bucket eagerly, drinks strongly, and is up and moving between feeds. The calf who hangs back, starts slowly, breaks off early or has to be encouraged to drink is showing you the earliest and most useful warning there is. Note, too, the calf who nurses constantly but never seems satisfied — she may be getting less than she appears to be. Watch at least one full feed a day rather than glancing from the gate, because appetite changes are subtle before they are obvious, and the day a calf goes off milk is the day the clock starts.
What the body shows before anything is named
Stand back and read the calf's outline. A thriving calf looks filled out along the ribs with a rounded belly after feeding; a calf who is not thriving looks tucked up and hollow in the flanks, loses that bloom within days, and grows a dull, staring coat that stands up instead of lying sleek. Sunken eyes and skin that stays tented when you gently pinch it on the eyelid or neck point to dehydration, which in a scouring young calf can deepen quickly. Check the manure as you go: firm-to-soft is unremarkable in a milk-fed calf, but watery scours that soil the tail and hocks drain fluid and energy fast. Weigh or tape the calf weekly if you can — a flat line confirms what the eye suspects.
The navel, the breathing and the company kept
Two checks catch problems owners miss. Run a hand over the navel: in the first weeks it should be dry, cool and shrinking, so a navel that is swollen, warm, painful or damp-smelling needs attention rather than waiting. Then watch the breathing at rest — a young calf should breathe easily and quietly, so persistent coughing, rapid or laboured breathing, or discharge from the nose or eyes belongs on your list. Behaviour completes the picture: a calf who isolates from its dam or the group, lies down far more than its penmates, or stops joining in the short bursts of play normal for the age is telling you something is wrong.
When watching becomes a vet call
Some combinations should not wait for tomorrow. A calf off its milk with watery scours, signs of dehydration such as sunken eyes or a dry mouth, a hot swollen navel, laboured breathing, or a calf who is weak, cold-eared and slow to rise needs a vet the same day — young calves have small reserves and decline faster than older cattle. Call sooner still for a newborn who never got going strongly. Keep your dated notes of feeds, manure and behaviour to hand for the call, because the pattern over days tells the vet far more than a single impression, and follow the vet's guidance rather than reaching for leftover treatments.
Those dated feeding and behaviour notes are exactly what a Pet Passport in KinsLe is for — the calf's records kept together in one packet, so the vet hears the whole first-weeks story, not just today's chapter.