posted 8 years ago
This perhaps doesn't address the question as asked, but a couple thoughts:
Ducks seem much less inclined to eat during hot weather than chickens, guineas, or turkeys. Eating produces body heat, which is perhaps disincentive enough. Couple that with the ducks' proclivity to lay on insulating fat, and it's no wonder they don't want to eat much.
Even raising them to an older age may not help. We raise Pekins for our local market. The window for getting a cleanly plucked carcass with Pekins is 8-9 weeks, and again at 16-17 weeks. We had one batch that wasn't big enough at 9 weeks (they were rather skittish and didn't eat enough), so we raised them on to 16-17 weeks, just finishing them last week. They were bigger then, but not by a lot. Even though they were considerably older, they still didn't eat enough because of the heat.
I've become convinced that the best times to finish ducks (at least here in zone 6) are April, when it's still cool enough to allow for ready fat deposits, and not again until cooler fall weather (mostly October). This probably doesn't help if you're raising ducks for a fair competition, but there you go.