In the year 2000 we dewormed every 3 months and rotated the deworming chemical used.  Around 2004 we increased it to every 2 months based on recommendations of horse showing professionals, vets, magazine articles, and deworming manufacturers.  A few years later the recommendations where changing to worm every month during the growing season for horses that live with others in a group setting and drop it to every 3 or 4 months during the winter time.

Vets are now emphasizing doing fecal exams throughout the year so that you can fight the parasites with a specific deworming chemical based off your horses results.  The parasites have been getting more and more resistant to the chemicals being used with 2 out of the 3 drugs of choice progressively becoming less and less effective; the 3rd is starting to show signs of the same thing too which is why vets are recommending fighting the parasites only when the numbers get excessive.  More information can be read at http://www.veterinarypracticenews.com/vet-dept/equine-large-animal-dept/if-one-dewormer-works-try-another.aspx and http://www.thehorse.com/ViewArticle.aspx?ID=15854

Another route that is being taken by some which isn't publicized as much is to not deworm but instead increase the overall health of the horse and build up their immune system.  The thinking is that one can not eradicate all parasites just like we can't get rid of all germs. Instead if one has a strong immune system and has great health then the parasites don't really have an affect on the horse.  Also it is is believed that we can not constantly fight the parasites with chemicals without it also taxing the horse's body too.  One article on this thought can be read at http://www.rainbowacrestables.com/horsehealtharticles/parasitecontrolvsdeworming.html  We are implementing this route by increasing the nutritional value of our feeding program (see Supplements) and or perhaps randomly taking fecal exams of 10% of the herd and addressing the deworming on a as needed basis.