This is a good time of year to do this as pastures are low & slow growing so a higher incidence of sand intake & worm recycling. Be mindful that Magnesium Sulfate is Epsom Salts & has a dehydrating affect on the horses body so ensure adequate water is available & be a good idea to lighten the workload so not too much sweating if being fed this. Ideally this is good to do a week or so before worming. This is to ensure the gut walls are as clean as possible so your worm treatment can do its job properly.
Read more at The Horse: http://www.thehorse.com/articles/35183/psyllium-magnesium-sulfates-sand-clearing-ability-studied