I think cooler weather is just more comfortable for them, with a standing body temp at 100 degrees plus (101.3 iirc) Much movement at the body temp rises (why I'm a believer in gutting, or quarter ASAP). When it is real cold, I think they balance extra movement to raise their temps, but forage for high energy with minimal effort diets. Ever notice young deer on a cool am, or pm run and play? cool weather and they feel "frisky".
This is the main benefit to whole corn (good quality with no cracks). IIRC from a class I took years ago at UoG, each whole un-cracked, quality kernel of corn, has the equivalent of 1 BTU of energy, if the shell is breached, that energy is released. Corn is also harder on a deer's gut and it takes a little time for their system to adjust to it and is why they recommended feeding it in small amounts year round.
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