Conversation so far:
Who was Norman Borlaug's most vocal critic?
mounted over the decades by environmentalists and some nutritionalists.
What was their biggest issue with him?
faced opposition by people who consider genetic crossbreeding to be unnatural or to have negative effects.
Did he prove them wrong?
CANNOTANSWER.
What did he think about his critics?
Borlaug dismissed most claims of critics, but did take certain concerns seriously.

Question: Which concerns did he take seriously?
Self-contained rewrite: Which concerns of his critics did Norman Borlaug take seriously?