Linseed and Flaxseed are the same thing. Many people feed flax (either whole, ground, or in oil form) to add omega 3 to the horses diet. Flax has a 4:1 ratio of omega 3 to omega 6 supplying approximately 6.4 or so grams of omega 3 per ounce. With many horses having limited to no access to pasture grass and also with some being on a diet high in omega 6's (from grain) omega 3 supplementation is important. It also has anti-inflammatory properties, especially helpful for arthritic horses. A lot of vitamin/mineral supplements use flax as their base.
Yeast culture is supposed to aid in hindgut function. It probably has other benefits, but that is the only one I know of without looking into it further.
Here are a few interesting links on yeast culture- Yeast Cultures http://www.ker.com/library/equinews/v12n1/v12n114.pdf