I have never wormed any of our chickens with a commercial wormer. Many experts recommend 'proactive' worming with a wormer twice a year, but I don't believe in administering any medications unless absolutely necessary.
Instead I rely on holistic preventatives. I have never had any trouble with worms in my flock, and actually have had fecal samples taken and no sign of worms have ever been found.
I use pumpkin seeds (fall), garlic (year round) and nasturtium (spring/summer) to combat worms year round because all are perfectly healthy and natural, with no withdrawal period during which you can't eat the eggs.
There are varying opinions on this subject and not much study has been done, but raw ground pumpkin seeds are thought to be a natural dewormer not only for chickens and ducks, but for sheep and goats as well. Twice a year, spring and fall, it is suggested that you feed your girls ground raw pumpkin seeds free-choice for a week.
The pumpkin seeds (as well as other squash in the cucurbitaceae family such as winter, butternut and crookneck squash) are coated with a substance called cucurbitacin that paralyzes the worms, and then the yogurt and molasses flush the worms out of the chickens' systems. Although there isn't much information or scientific evidence out there to prove or disprove the effectiveness of pumpkin seeds to combat intestinal worms, our chickens love the pumpkin, so I figure it can't hurt.
Pumpkins also contain antioxidants, as well as high levels of Vitamin A, both of which strengthen immune systems and help our flock better combat illness. The beta carotene in the pumpkins contributes to vibrant orange egg yolks as well as bright orange duck bills and feet. Another substance in the pumpkin, phytosterol, is thought to lower cholesterol, so I believe it probably makes our eggs lower in cholesterol as well.
Bottom line, the pumpkin and garlic, as well as the nasturtium, are nutritious, so I figure there's no harm....and besides, we've never had any problem with worms - so I'm a believer.
I also love to take it one step further and make our chickens Pumpkin Soup.
In the fall I save our Halloween pumpkins and grind up the seeds and pulp. In the spring I buy a bag of hulled plain pumpkin seeds and a can of pureed pumpkin and grind them up in my coffee grinder. Then I make soup.
Pumpkin Soup (serves 8-10 hens)
One entire bulb crushed fresh garlic
2 cups raw oatmeal
One shredded carrot
8 Tablespoons ground raw pumpkin seeds
1 Tablespoon blackstrap molasses
Pumpkin pulp in the fall from two small pumpkins (or one can of pumpkin puree in the spring)
Blend it all in the food processor and add enough plain unflavored yogurt to achieve a 'soupy' consistency.
Then pour some soup into each pumpkin half, garnish it with shredded carrot and serve it up ! Our chickens clearly love it! And when they are done with the soup they will eat the bowl!
References: For more information on using garlic and also nasturtium as well as the references for this information, please read my blog article Pumpkin Soup, Garlic & Nasturtium - Natural Wormers?