Parasite control fall and winter recipe for our area: We usually recommend a September/October deworming (only one month or the other, and preferably Sept for future reference) with Quest or Quest-Plus (you will have to use Quest as the Plus option will probably not be available until mid 2011). Then 3 months later (Dec-Jan) use the ivermectin-praziquantel combination to get the last of the bots and also the tapeworms. Zimectrin Gold or Equimax are examples of this combo... familiarize yourself with the active ingredients so as to get the best deal. Yes, there are new deworming strategies, but these two dewormings are somewhat universal, regardless of population densities or fecal egg count results.
If by chance you were able to use Quest-Plus during the Sept/Oct deworming, then it would be okay to use just plain ivermectin at the Decem/Jan deworming... again, shop around for the active ingredient as generic ivermectins are fairly inexpensive.
Your young horses such as foals/weanlings upwards to age 2 seem to end up with round worm problems. Acquired immunity seems to take care of them in the older horses. We highly recommend your weanlings get a 5 day, double-dose safeguard (the oneies-twoies are good to slip some extra strongid in the rotation). All the buzz concerning resistance of worms to dewormers, fecal egg counts, rotation of dewormers,etc. generally refers to small strongyles in adult horses. Roundworms have different resistance patterns (strongid and safeguard usually work well with them whereas ivermectin and quest don't) and require more frequent dosing intervals, so it gets confusing when you read about deworming strategies and the sort because they usually refer to adult horse worm problems. Remember the youngsters get adult worms as well, so you have to direct your therapy to include them as well.
Please check with us if there are any questions regarding what to do.