It seems like there must be a very big temptation for your dog. In order to solve the problem, you must find out where he/she is going. My guess would be to meet other dogs, to catch rabbits or perhaps terrorize farm animals. Either way, it is irresponsible to allow your dog to do this. Firstly, make sure that your garden is secure! Make sure that the fencing is complete, with no holes underneath it, and that your gates self-close by spring action. Secondly, do some detective work. See if any of your neighbors have seen where your dog goes and ask them to phone you if they see your dog.

One thing to do is to reward your dog greatly upon his return, perhaps with a lot of attention and a big biscuit. These suggestions should work, but in the longer term you may have to seek advice regarding spaying. This will make all-round behavior much better.