Hard stools occur when the horse does not consume enough water to keep the ingesta hydrated in the digestive tract. The large colon is a very thorough and efficient sponge which reclaims most of the water from the feed material as it passes through. If the horse does not drink enough to meet their basic needs, the body extracts too much water and as a result the manure is very dry and hard. If you can hear your horse poop from the other end of the barn because they are bouncy rock-hard little rabbit-sized balls, she is probably not drinking enough. This is especially problematic in the winter as the cold and reduced activity levels result in lowered water intake and dehydration. as well as lower availablilty of succulent forages such as grass. That is why we see more impaction colic in the cold weather.
Based on your description, this mare is not drinking enough water. The addition of a tablespoon of salt to her ration in the morning and evening will stimulate thirst, and you can use high moisture feeds such as well-soaked beet pulp, carrots, and hydroponic grass strips (great for ulcer horses too) to increase the water content of her diet. You can also actually add up to two cups of oil per day to her feed, but this is quite high in calories. As an alternative, you can use up to two cups of calorie-free mineral oil in her meals. You do not need to reduce the flax or supplement levels, go ahead and feed her however much is recommended to meet her nutritional needs. Another trick to encourage drinking in the winter is to offer warmed water, especially after she comes in from turnout, and to offer a separate pail of water with flavoured electrolytes in it. These strategies are usually enough to get picky drinkers to increase their intake. If you continue to have colic problems, I suggest that you have a thorough physical and dental exam performed to detect additional problems.
Melissa McKee DVMwww.mpequine.com