Deworming programs should be based upon risk factors such as horse age, inherit resistance of each horse to parasites, management practices such as pasture or stall care, other horses pastured with the horse and weather conditions where the horse is kept.
For Colorado, it looks like your deworming schedule is sorta backwards. Your horse is more likely to pick up parasites during your summer when weather is moderate so you should focus more on parasite control during that season. The winter months when you are likley dealing with snow/ice and your horse is stalled and/or on hay rather than pasture is when you will have less risk of parasite reinfection. In really cold climates where horses are on hay and no pasture in winter, it may even be reasonable to discontinue deworming during the winter. You also appear to be deworming at 8 week intervals which isn't really an appropriate plan. You should be deworming based upon fecal egg counts or in situations where there is high risk of reinfection and a horse that is susceptible based upon egg reappearance periods for the drugs you are using to help minimize recontamination of the pasture. Only 1 drug has an 8 week egg reappearance period--ivermectin. There is no need to deworm for at least 12 weeks after using moxidectin (and using fenbendazole or pyrantel earlier than that really is likely to be ineffective as you will likely not have many adult parasites and that is what those drugs kill).
I would recommend that you talk to your vet about setting up a strategic deworming program based upon your horse and his situation. You will likely save money and help slow the developement of drug resistance in parasites while still keeping your horse's parasite load low.
Check out this webinar available through "The Horse Magazine" The Horse: Videos
(it takes a while to watch) or get a copy of their Jan. 2009 magazine (current issue) to learn more about why strategic deworming is so important.