A preliminary study done at A&M showed no effect from using DE for horses.
Since up to 50% of adult horses in a herd will control GI parasites on their own and pasture management practices will also greatly reduce parasite reinfection rates, you are better off to focus on pasture management and strategic deworming to minimize parasite loads in your horses. Lots of horses only need deworming twice yearly----6 times a year is overkill in any situation.
Assess your horse's natural resistance to parasites by running fecal egg counts 3-4 months after their last deworming and during the season when parasite reinfection rates are highest for your area which is going to be the summer months. Once you know which of your horses have a good natural resistance to parasites and which tend to carry higher loads, you can set up a deworming program with between 2-4 dewormings per year during the spring-fall to control parasite recontamination of your pastures. You shouldn't need to deworm during the winter months just due to the weather conditions.
Take the time to watch the webinar that was just aired recently on TheHorse.com. It will help you see how to control parasites with as little chemical intervention as possible. The Horse: Videos