Do you have any idea about whether your soil is maybe chemically extreme = very acidic or very alkaline?
Beans are also in general quite sensitive to drought but some fodder pea cultivars are quite tolerant. Eso for example - we had Eso this year and there were basically 4 months of drought. The pea kernels were not exactly plentiful and are small but they plump up nicely when soaked and are given a warm welcome by the ducks.
Soybeans (cooked as they should be) in contrast are not that interesting to them, they eat some but meh.
I would love to try sweet lupines but haven't yet done anything about it.
Here's what Perplexity says and based on this I'm getting the impression that lupines would not be a "ooooh cocaine" situation that one gets when feeding peas; and also that it might be necessary to shell the seed which adds cost or creates a problem if there is no practical way to do it at scale:
https://www.perplexity.ai/search/can-ducks-eat-sweet-lupines-an-Grfsyz.uRPu.GTcw1YbxOA#0