02-12-2008, 06:50 PM
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Cold weather is going to lower the risk of reinfection so you can often deworm less during the winter. However if the goal is to minimize pasture contamination then all horses in the pasture should be on a deworming program that is based upon egg reappearance periods for a year.
Generally, drop fenbendazole from your deworming rotation. It's got wide-spread resistance issues with strongyles.
If you feel the need to rotate dewormers, use ivermectin, moxidectin and pyrantel (though this isn't a true rotation as ivermectin and moxidectin are both avermectins, but they are also the only products with no known resistance issues in strongyles). You will need to deworm again 4 weeks after using pyrantel, 8 weeks after using ivermectin and 12 weeks after using moxidectin. This type of scheduling will greatly reduce the egg shedding into the pasture. Spring and fall you need to use and ivermectin/praziquantel or a moxidectin/praziquantel product to treat for bots and tapeworms.