Q: When I separate my hens after the breeding season some of them continue to lay eggs like chickens. Is this harmful to them? Is there any reasonable way to reduce their tendency to lay eggs?
Breeding hens laying in the off season is definitely not a desirable thing. It really depends on your loft setup -- in other words, how you are housing the hens, as to whether you can stop them from laying or whether you should try to capitalize on the extra eggs. I have raised some very good breeders from hens laying in the off season.
I assume you have taken the proper precautions by not housing your hens in a loft with nest boxes or box perches. Have them sitting on vee perches or something similar instead. In other words, don't provide a place where your hens can get cozy. However, it often is the case with older hens (2 years or older), that two of them will end up in a corner of the loft and lay eggs. Under no circumstances should you ever throw those eggs away. Place the eggs into a nest bowl and let the hens sit as long as they will. In this way they will overbrood. Eventually they will leave the eggs and may very well start a nesting cycle again. However, by leaving the eggs alone, the hens will lay new eggs only every 30 days or so. If you were to throw the eggs away immediately, the hens would have new eggs in 8 to 10 days again and this cycle would just keep repeating, being very detrimental to the hens. They are not Leghorn chickens, but pigeons, and we want to conserve all their resources.
When you see two hens starting to huddle in the corner and one happens to be an exceptional hen, take her to your best cock when she is about 5 days into mating. Let them be together for maybe 30 seconds. The hen will readily stoop for him within a few seconds and the cock will usually top her very quickly. If the mating process does not take place within the first minute, chances are the cock will start to peck the hen and reject her. You need to remove the hen and try again the next day. You are not trying to bond a pair together. You are only trying to set up a one night stand! (Or 30 seconds, as the case may be.) If you do not have any special loft in which you can put the birds together, you can simply use your young bird loft when the youngsters are being trained or exercised. Finally, after the birds mate, take the hen back to your hen loft and let her proceed with her maternal instincts.
Once you see which eggs are fertile, throw out the ones that her mate (hen) has laid. If you have another pair of hens that is also down on eggs, you can give that pair one of these fertile eggs. You now have 2 pairs of hens occupied raising one healthy youngster per nest.
Mating a hen in this way gives you some really fun possibilities. You can now use an excellent hen with a different breeding cock than normal -- or perhaps use her with one of your promising flying cocks -- or perhaps try that AU champion that is coming along on the race team. You are letting this matey hen raise a special set of youngsters in the off season. Many times this turns out to be a unique mating that you would not be able to accomplish during the regular breeding season. Like I said, I have bred some really exceptional birds in this way.
Now back to your original question as to how to stop hens from mating and laying eggs. Many people try to curtail the laying process by feeding barley or by cutting the energy intake in general. I consider this to be a very bad idea in the middle of the summer when the hens are going into a heavy molt. Instead of following this lean diet that many fanciers use, I would suggest that some newly harvested wheat and vetch should be included in the birds' diet so they will grow good feathers for the next season. Wheat is harvested in July about the time when your birds are separated and getting ready to start their main molt.
The ideal way to house your breeding hens would be to give them access to a fly pen all day. In other words, lock them out into a fly pen which gives them a lot of sunshine and fresh air. Let them into the loft only in the evening to be fed and to sleep.
You can go one step further and place a young rabbit into the flypen. A rabbit will always sit in the corners of your flypen and basically will run from one corner to the next. This stops hens from getting comfortable in any of the corners. The only negative to this is that a rabbit will end up urinating in one of the corners and that can be a mess. Also, a rabbit will want to chew on anything made out of wood when housed in this way. So, no wooden feeders or 2 x 4's that the rabbit can chew on. A flypen is by far the best and healthiest environment for your breeding hens. The other option you may use is to put a half-grown chicken in with those breeding hens. For some strange reason, the chicken will take it upon itself to patrol the floor of your flypen and will constantly chase your hens up. So you have your choice at the end of the summer whether to have a gourmet meal of rabbit or chicken.