The thermo-neutral zone is the range of temperatures between the lower and upper critical temperatures to which an animal may become acclimatized. In new born calves and up to four weeks of age this is between 10⁰C and 25⁰C. Calves at one month it is between 0 degree C and 23 degree C.

Outside these temperatures i.e. if it is too cold calves must burn energy just to maintain core body temperature.
A calf requires 2% more energy for every 1 degree C below the thermo-neutral zone, therefore at freezing point (0 degree C) a calf requires 20% more milk/feed just for maintenance

Dairy breed calves do not have enough body fat to maintain their own body temperature in cold weather. Feed is often limited by number of feeds per day and the amount given per feed. Keeping the calf warm therefore is the most cost effective way of maximising the feed given.

General rule for the use of calf jackets on young or sick calves is that: – if you feel the need of a coat then so does a calf!