in normal circumstances coz large urban population at the moment through urban gardening state could help and organize many even mids the concrete jungles, the cuban example is most famous such one  but are “they” willing to push such survival preparedness or wait with open arms for this kind of opportunity so would roll out full blown transhimanist technocracy for which dreamed for so long as euroatlantic elites when “they” could not be anyhow threaten by anyone except own elitist factions!?
anyway one good substitute for Corn when Chickens are in question is Kenaf  so get some seed reserves in time, it will make Your pains way lesser around feeding and expenditures, simply good protein improvisation eg. in comparison to casava leafs  and with the high yield ideal even for big improvised farms also useful for beading 
Hunger and poverty as we know it can be solved by introducing kenaf as a human and animal
feed. Kenaf is a fast growing plant that grows up to 20 feet tall. It is in the hibiscus family and
is currently grown mainly for its fiber to make burlap, paper and cloth. However, when the plant
is less than 4 feet tall it is correct to think of kenaf as a vegetable. It has extremely high protein
content – up to 34%. Enviro-inventor Bill Loftus has researched using kenaf as a chicken and
rabbit feed and as a human consumption food – used in salads made from the leaves or boiling it
to make greens.
This high yielding crop can be eaten raw or cooked. Feed kenaf pellets to rabbits and chickens
to create a self-sustaining eco-community. The animal feed can also be sold for income
The process: When kenaf is grown and harvested correctly you get several yields. The first 2 or
3 cuttings are for animal feed or human consumption. As it grows up to 20 feet tall, the leaves
are continuously harvested as animal feed for chickens and rabbits. When it is fully grown, the
farmer gets to sell the kenaf stalk for firewood
Animal Feed: Leaves can be dried and turned into different sized pellets as high protein
feed for rabbits, fish, chickens, and goats.