We normally think of salt blocks as a warm weather requirement. Horses do require about 1-2 ounces of salt per day to provide help meet their requirement for sodium and chloride. This requirement can increase to 4-6 ounces of salt per day in hot climates or under exercise where losses in sweat increase greatly. Inadequate salt in the diet can result in abnormal eating behavior such as licking or chewing objects which have salt on them (fork handles etc.) or licking/eating dirt. Water intake may also decrease, increasing the risk of impaction colic. In more extreme cases, horses will stop eating and may experience muscle incoordination.
If horses are salt starved, it may be a good idea to limit the amount of salt put out for them until they have adjusted their intake. It is essential that fresh clean water at an appropriate temperature be available at all times as well.
Commercial feeds normally contain 0.5-1.0% salt, so horses on this type of feed will typically consume less free choice salt than horses not receiving salt in their feed. They may still benefit from having salt available free choice.
Providing salt free choice is a good management tool that can help your horse eat and drink well all year long!
Source: The Feed Room, Roy J.