Rotational deworming has the same inherent problems that 'just in case' antibiotic use has had for humans. It causes the parasites to gain resistance to the deworming medications. So, the latest advice from experts is to switch from a rotational schedule to FEC testing.
FEC stands for fecal egg count.
FEC Testing is your guide
A fecal egg count measures the number of parasite eggs in your horse's manure. You can take the sample to your vet or send it off to an independent laboratory, and they'll send you a number back such as '50 EPG' or '500 EPG.'
The EPG stands for eggs per gram.
If the number is low (such as less that 250 EPG,) then good news, your horse probably has a good natural immunity to parasites and may not need to be dewormed as frequently.
However, if the test comes back with a high count (such as more than 250 EPG), then your horse is probably carrying a lot of adult worms that are spreading them all over your pastures to other horses.