There are instances when calf has no appetite for milk or supplements and this calls for close attention. I have always found this in newborn calves and the grown ones.
Here under are causes and how I deal with them:--
-For newborn calves after 24hrs of their birth, i check if lack of appetite is a result of atresia ani (internal secretion organ was not formed, how it is checked? Insert your finger into its anus, you will feel blockage, finger will come with slippery mucus like or a bit of blood) if so i euthanize it.
-Some calves that scour loose appetite, so give the calf ringer lactate and sodium bicarbonate in it to balance Ph, then after stimulate its rumen by giving it electrolytes or rumenos.
-Calves with acidosis: these are becoming acidic, excrete black dung and don't want to drink and eat. Give ringer lactate together with sodium bicarbonate, inject vitamin for 3 to 4 days and also warm water ie 2litres for 2 to 3 days.
-Intestinal blockage; this maybe due to hairball or eating soil,from my experience çhances of such calf to survive are minimal however you can give it parafin oil in warm water and also ringer lactate for some good days. Note some survive if detected early.
-Dehorning: some of dehorned calves may drink very little or not drink at all on that very or next day due to stress generated during dehorning.You can feed them electrolytes using fingers or tube and inject them vitamins.