Please don't give your hens yoghurt, they are lactose intollerant. It just makes upset tummies worse! Use a proper probiotic designed for poultry. Net-tex do one, ACV works as one, and Antech do one too. Chickens aren't reared on milk, they are not mammals.
I'm afraid i don't agree with worming birds, if you have room to let them range they build up a natural immunity to worms. If kept in small perminant arks, then worm with a natural wormer such as ACV, this helps to build up a resistance without causing the wormers to become in affective. Far too many chemicals and drugs are used on birds who don't need it, and they build up a resiliance to them so they no longer work.
Good husbandry is the key to worm-free birds, keeping your ground clean and moving them to fresh ground whilst treating used ground with lime. Treating new bought in birds with a chemical wormer to rid them of any parasite burden is exceptable as others may not have such good husbandary as you, but after the initial treatment, a natural immunity is better for the birds. No one really knows if their birds have worms without sending poo samples off, or examining it under a microscope, so you may be giving them a host of chemicals and drugs that they don't need.
JubesX