Hi Yoav,
I found this thread while researching Chevrotin, and found your question about breeding goats at different times to keep milk supply going...
*Most* breeds of goats are seasonally polyestrus, short day breeders... so generally breed October through March, with appx. 153 day gestation. So there is a 6 month window for breeding, give or take months or weeks depending on where you're located etc.
Some dairies use artificial insemination, some will keep bucks on the farm, some will bring in a buck/bucks to breed their does, etc., so there are various reasons for either breeding them all at same time, or being able to selectively breed particular does at different times. Also, some might breed at a time the does will be at maximum production for show season, as well.
The LaMancha dairy goat is known for extended lactation without having to be re-bred. Some have gone 4 years producing usable milk quantities without stopping. I'm building my herd of LaManchas and will select for does that show extended lactation traits :-)
~Laurie
Doesn't it make sense to breed some of the goats now and others later so that you always have at least some milk? This is a question from a guy who has no idea about owning dairy animals so please forgive me...