Looking for the 3 way rotation chart. Had it saved to my favorites and it shows up for 1/2 a second then goes to this page. Can you repost this on your website? PLEASE bring this back. Or, is there someone out there who has it saved and can send it to me? Thanks, Ann
Actually, we specifically removed the rotation chart from our website and catalog because experts now agree that switching between classes of dewormers based on the calendar is neither the best way to protect our horses from parasites or to prevent resistance from developing in the worms themselves. While rotating between the three chemical classes of dewormers made sense when it was first introduced almost 40 years ago, for a number of reasons it is no longer the best defense against parasites and in fact, may be doing more harm than good.
First, each of the three chemical classes of dewormers (benzimidazole/pyrantel/ivermectin) has a different egg suppression period. That is, each class prevents eggs from being laid by adult female worms for a different amount of time. When benzimidazole is used, it only takes a month for strongyles to mature into egg-laying adults. It takes six weeks for this to happen after a pyrantel-containing product is given. For ivermectin, the egg-suppression period is the full eight weeks or two months that is commonly recommended in rotation charts. Moxidectin, the newest dewormer we have, suppresses egg laying for 84 days, or almost three months. So a parasite control program that rotates between these classes every eight weeks isn’t taking into account the differences in how long each dewormer “lasts.”
Second, experts have detected resistance to almost all of the chemical classes of dewormers. Throughout the country, there are pockets of parasite resistance where the benzimidazoles or the pyrantels are no longer effective against small strongyles in adult horses. Scientists have also found that, on some farms, ivermectin is ineffective in controlling ascarid (roundworm) infections in young horses. Therefore, it’s important to use a chemical class on your farm that you know works through fecal egg counts.
Third, parasites have life cycles that depend on external environmental cues like temperature and moisture. When it’s below about 40 degrees, eggs continue to be laid in manure (and onto your pasture) but they are no longer able to hatch into infective larvae. So there’s no need to deworm after the first hard frost in the fall in northern climates because horses aren’t picking up any new parasites. However, when temperatures begin to warm up in the spring, eggs that were lying dormant on your pasture all winter now hatch, which is why you should begin deworming again when temperatures remain consistently above freezing (it’s the opposite for southern climates). This is also why it’s a good idea to deworm your herd in the spring before they go out on pasture and deposit even more eggs that can then hatch and infect the horses.
As my parasitologist friend Craig Reinemeyer, PhD, says “you don’t get any points for going through the motions.” I encourage you to sit down with your veterinarian and develop a parasite control program that is appropriate for your horse, in your geographic area, that takes into account whether he is a low, medium or high egg shedder and incorporates other parasite control measures like manure management.
Read more about deworming methods here: 5 Things you need to know about dewormer