hi Buffy, I can't answer your question about how old you should keep a buck, they can live to over 12 years. I believe there might be some standard set at several rabbitrys, in order to maximise production. However in a smaller home rabbitry used to make extra money, you can keep older bucks that have specific traits you really want, but you must keep young bucks that wont go sterile with the heat, too. This way you can breed as soon as the doe will allow, after the heat lets up, and will not be too much of a problem later. Yet you can still get litters with the old buck too. The way you described the kindling the problem may have been with the does not being fully recovered from the heat themselves, before being bred. I've had the same problem from time to time over the years, usually after a rather long heat spell with a nice break in which I bred then the heat came back. bad timing on my part. The only good answer is to be wealthy enough to have large climate controlled buildings, in other words air conditioned barns. Wouldnt that be nice? Ray