We normally don’t recommend that you worm your chickens on a regular basis until you have confirmed that they have worms. Chickens are susceptible to several different types of worms, and the treatment varies for those different types.
Also, using wormer can interfere with your chicken’s ability to develop a natural resistance toward worms, as we discuss below.
If it seems like your chickens have worms, you can ask your local veterinarian to do a fecal test. This should be inexpensive and will tell you both if your chickens have a problem and if so, what type of worms they have. Then based on that information, you could treat them with the appropriate wormer.
To a large extent, you can prevent or reduce worm problems by practicing good flock management. If you house your chickens in chicken tractors, then move them regularly to fresh ground. This breaks the life cycle of the worms and reduces the likelihood of the chickens re-injesting the parasites or their eggs.
If you raise them in a stationary coop, keeping the bedding fresh will reduce problems with worms. Also, be sure to wash out waterers regularly and feeders, and make sure your chickens have a good supply of fresh, clean drinking water at all times.
Normally, your chickens will start to develop a resistance against parasitic worms, as long as they’re not being stressed by other problems. You can also have your vet do a fecal test about 4 times a year to monitor worm levels in various seasons. Based on your findings with the fecal test and based on your vet’s recommendations, you may come up with a yearly worming schedule that, together with proper sanitation and management, keeps worm problems under control and, to a large extent, prevents them.
If you’d like to read more about this, Gail Damerow’s book Storey’s Guide to Raising Chickens is a good source of information, along with her Chicken Health Handbook.