Several things will influence your horse’s water intake during the winter months. Perhaps the most important consideration is the temperature of the water. In general, optimum temperature for drinking water should be 45 – 65° F. If you have extended periods of cold temperatures and are using water troughs that do not need a daily refill, then there is a good chance that your water temperature will drop below 45°.
Horses drink less water when the water temperature is outside the optimum 45 – 65° F range. Just like with humans, drinking a significant amount of water that is just above freezing, isn’t easy to do. During cold winters, horses will drink enough water to maintain body functions but may not be hydrated at normal levels. This subclinical dehydration is a cause for winter colic.
Other considerations for why your horses are drinking less water:
They are less active during winter months o They perhaps have less turn out time
Less pasture grazing
They have less routine work
A change in forage (such as replacing long stem hay with pellet hay)