Thanks a lot guys!
I'll start a new thread about threshing, because I have constraints to specify, and also to explain the difficulties I've had until now.
As for the initial points of this discussion, I'd like to share my impression and my remaining question.
1) Apparently solved: bending stems
By now it seems pretty clear that the yield is ok, and the bending was not a problem in itself.
Two other elements make me pretty convinced that it was not due to sunlight but to overfertilization.
The first element is that sunlight was unequal on the field. Some parts got more sunlight during winter, some parts more during the spring and early summer, and some were in between, but I don't see any difference across the field. I am thinking that if sunlight was the issue, I would have seen a difference, at least at the end of spring.
The second element is that I read that one should not amend the soil with compost more than half a kilogram per square meter, because otherwise the soil is too light and the roots cannot grab hold, which makes the plants bend over. I didn't add compost but sheep manure, but still, I think I added way more than half a kilo per square meter. I think I emptied a wheel barrel every 4 square meters.
So I am under the impression that this is the explanation for bending plants.
2. Remaining question: heterogeneity
I thus feel confident that I understand why plants were bending, but I still have no idea why the field was so heterogenous. Some plants were a lot taller than others, some dried way earlier than others, etc.
Do you have any idea what could have caused this?
Thanks a lot again very much for the help!