Supermarket contracts, opportunity cost and trade-offs, and farm household welfare: Panel data evidence from Kenya

•Panel data models of the effects of supermarket contracting on farmers’ welfare.•Supermarket suppliers have higher income and dietary diversity than traditional channel suppliers.•Supermarket channel stayers sustain higher income gains than dropouts or newcomers.•Positive spill-over effects of supplying supermarkets on other sources of income.•Wider supermarket participation can improve household nutrition.