Fenbendazole is the only wormer I have ever heard of to do a five day course with.
It treats specifically encysted small strongyles (in the five day course, but is effective against roundworms (It is also used in cats and dogs)
In Europe we give it April and October (and I also worm with Praziquantel for Tapes at these times)So my Spring routine will be
Fenbendazole (Panacur/Safeguard or generic cattle drench) five day course at recommended dose for area.
Ten days later Ivermectin based wormer.
Ten days later Praziquantel (or you can dose with a combo at the first ten day mark, I juts prefer to do it separately)
This just about clears out everything and allows them to go into Spring free of worms.
At this time I also move the mares onto the Spring pasture, which has rested all winter, and bring in the Show horses.
I realise not everyone can do this, but a good harrowing at this time, plus a very good sweeping or pick up of poo, will help no end.
I worm my foals with Fenbendazole at ten days- but I do not worm the mare on day of foaling, I worm routinely and do not break the cycle.
The first two wormings of foals I will use Fenbendazole, then I switch to Ivermectin if the foal is big and strong.