Ontario, Canada-based Grober Nutrition offers the following advice on most effectively using calf blankets:
Why should you use a calf blanket?
Calf blankets are a great tool to help a young calf regulate body temperature. When a calf is born, they are not capable of regulating body temperature. Calves are born with very little brown fat (1-3% on average for a 100 lb. calf) – this is what can be used as energy to the young calf … not very much! When a calf is comfortable they will turn feed into gain more efficiently. In other words, a calf that isn"t comfortable will need more milk replacer to grow.
When is it best to use a blanket?
Use blankets in conjunction with adequate bedding (straw is best for warmth and comfort). Using a blanket without adequate bedding will not achieve an optimal environment for a calf – it will be like using a blanket to stay warm with no clothes on.
Use a blanket for newborn calves or calves that are struggling with illness. Blankets will not make up for cold drafts – properly ventilated barns are still a necessary aspect to raising strong calves.
Use a blanket if the ambient temperature is below 40'F (if calves are well bedded with straw, this will raise their ambient temperature). Calves older than three weeks should not require blankets if housing is adequate, bedding is dry, fresh and plentiful and the area is free of drafts.
How do you use a blanket?
Put on a blanket (which should cover their torso) and be sure to check them periodically. If a calf is sweating then the calf will become wet and this can increase their chances of illness and will definitely increase their level of discomfort. Check areas where there are straps/secures to ensure they are not too tight – it is best if you can fit your flattened hand inside the blanket.
Source: Calf GroFacts by Grober Nutrition