A horse’s nutrition requirements change throughout the year depending on his exercise and the ambient temperature of his environment.

As the weather gets colder outside, he needs to produce more heat to maintain his body temperature. Rain and wind will also affect his ability to stay warm.

Horses are foraging animals. They digest and eat hay. The act of digesting causes heat production, so it is important that they have access to hay continually. Additionally, there is a threshold of temperature below which your horse cannot stay warm without added forage calories to his diet.
Healthy horses with thick coats can maintain warmth at lower temperatures than horses with thin coats. Older horses often have thinner coats and need more calories sooner in the winter.

See this article from Clinton Anderson for details about “Lower Critical Temperature (LCT)”, which is the temperature below which metabolic heat production must be increased to maintain body temperature.

Whether to blanket or not blanket we will leave for another time.