Question about Egg Fertility in Ducks

Almost immediately. I have had them start producing fertile eggs within the first couple days after breeding. Of course, as you have already noted, fertility tends to drop in the summer then pick back up in the Fall somewhat, but still not quite on the level as in the Spring. They will continue to breed, but just produce fewer eggs (how many fewer depends on the breed and ranges from a lot fewer to slightly fewer) and have lower fertility. Our more prolific ducks will go into a Fall laying cycle that lasts up to I would say 6-10 weeks. For example, our Runners all but completely stopped for about 3 weeks and are now back in full swing (haven't checked fertility though). A lot of our bantams though haven't laid since the end of June/ first part of July.

 

I like to add that first time layers tent not to produce fertile eggs in the first 2 weeks. Or at least the fertility is very low. It will pick up after that. Breeders usually discard the first 2 weeks, because it is a waste of time to fill an incubator with less then good eggs.

 

Obviously it will not work without the mating.
I mean the first two week of laying. It's the time when their system has to settle in and as a result the eggs may not all be fertile. The same applies after the duck took a break for winter, but then it is only one week. This only applies to mallard derived domestic ducks. It's different in muscovies and other wild ducks.

 

Just wanted to add that if you've seen them breed twice, they've actually bred about two hundred times. They're pretty quick.

My girls are all still laying fertile eggs, even my new girls who just started laying a few weeks ago. There's an easy way to tell if a specific egg is fertile, but you have to crack it open. Still, if one is fertile there's a good chance the others will be too. Look here: https://www.backyardchickens.com/forum/viewtopic.php?id=16008.