What crops out there do farmers grow in abundance that are useful for the bee keeper and bees ,I know some people move hives to fields of osr and beans is there anything else?
We all know of Oil Seed Rape (OSR) / Canola, but I too have heard that the more modern strains have less nectar, Finman often refers to this type of crop.
Then there is Clover, usually planted as a seed mix with grass, as it aids the grass to grow, unfortunately it can be cut early so not left to flower properly.
After that the only other mainstream crops with flowers that may have nectar, that I am aware of are peas and potatoes, I've heard that peas do give some nectar, I know nothing about potatoes, except they have pretty purple flowers.
As has been mentioned, there are some niche crops such as Borage or Lavender, but in this country (Britain & Ireland) I know of no other crops of worth that farmers plant.
On the continent you have a greater range, such as sunflowers, and so on.
In case your wondering my understanding is that Maize produces no nectar but large amounts of pollen, unfortunately the bees do not like it.
As for trees
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I've read that Black Locust produce the most nectar, but they can take several years to get up to full size to get the figures quoted and it only really flowers for up to 10 days, which is ok if it's just a few trees and a strong hive.