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As dogs get older it is possible for their heat cycles to become irregular and even stop all together. HOwever, 6 years old is not that old really. Dogs can also have silent heat cycles. With a silent heat cycle the dog is going into heat but the outward signs are not evident. I've found many females in a silent heat are still more affectionate than normal and an experienced stud still can sense they are in heat.
Now dogs can have a variety of heat issues including prolonged heat, a shortened heat or a split heat. Read about these here:
And there are medical causes for a heat cycle to not occur such as hypothyroidism. Often a thyroid condition will affect the heat cycle. So it would be a good idea to have your vet take a look at her. In addition, be sure your vet gives the ok to breed. If she has never breed before, 6 years old may be a little late to start. She might have a more difficult time at delivery than a younger dog.
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