I think it also depends on the duck breed, Indian runner ducks are apparently fine layers, not interesting for their meat I guess.
In terms of slaughter, ducks seem more difficult to me, they have such a nice personality that it makes it difficult to kill them (but I'm just too sensitive to kill animals at all

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I think the food depends on who you ask. As is often the case for humans and other animals. I have the idea that there is a trend mainly from America to feed everything a lot of protein (even people). I don't know enough about chicken feed to say anything meaningful about it myself, but I know that our chickens used to get mixed chicken grain, some shell grit and all kitchen scraps.
They all lived to be quite old, the oldest I can remember was 12, they were never sick (except just before they died). At the moment I take care of the chickens of someone from my village and she only gives wheat, for example, this does not seem optimal to me and I would not recommend it, but it is an older woman who has always done it this way. Apparently that works for her chickens.
I regularly read on this forum that you should give chickens/ducks special chicken or duck food because that is absolutely necessary. I have the impression that people forget that this processed food, before processing, consisted of whole food. The advantage of this specially formulated food is that the proportions are 'correct'. If you are going to compose yourself, it is important that you know a bit about which proportions you have to work with. I personally think it is even better to feed whole grains than the processed pellets, the nutrients are better preserved in whole grains. But only if you know what the right proportions are. I can't help you with that yet, I need to read up on that too

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I have recently started fermenting our duck food and they absolutely love it. Fermentation increases the nutritional value of feed, so that might be interesting to look at as well.