Get your calves into their comfort zone
Comfort is a big factor in healthy thrive for calves.
I always try and look through the eyes of a calf. It pays to get down to calf level and assess their environment. Remember a calf will spend up to 20 hours a day lying down.
A lot of dairy calves naturally have poor fat covers so they can get cold quite quickly in low air temperatures or with cold winds.
Cold calves require more feed or energy to produce heat in the absence of a heat source or warm nesting area.
My own opinion is you can’t beat deep straw bedding which allows calves nest and stay warm.
A lie back area or shelter in calf pens can often provide this.
This is also the principle behind calf igloos which has made them so successful. It allows the calf a warm shelter in times of cold. This is also why calf jackets work so well by providing calves with their own warm micro-climate.