Thehorse.com - Full Article
Owners need to consider how they will meet their older horses' (or their younger, hard-keeping horses') nutrient requirements during the winter. Providing adequate energy is the prime concern, and how you will provide those extra calories depends on available feed and each horse's individual needs.
A good place to start is assessing your horse's body condition score (BCS). Horses with a BCS of greater than 5 will have some extra fat stores that can provide insulation and serve as a readily available source of energy when the daily ration falls short as the temperature drops.
In developing your feeding strategy, consider increasing your horse's hay intake to meet his energy needs. Hay is digested in the gastrointestinal tract by fermentation, which produces heat that the horse can use to maintain core body temperature. There is a limit as to how much hay he can consume daily. In most cases, he will consume 2.0-2.5% of his body weight per day. If he can't consume enough hay, then adding grain to the diet will also provide calories.
Temperatures well below freezing, or wet snow or freezing rain conditions, greatly increase a horse's energy requirements, especially if he's maintained outside. Rain and wind can cause the horse to lose the insulating capacity of his hair coat, and he'll use body reserves to maintain core body temperature, often resulting in weight loss...
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