US nuclear-war planners in the 1970s or 1980s estimated that there was enough food in silos in the US to feed the survivors for about 3 years. I think they assumed that 50% or 60% of Americans would survive. Usually, most of the grains and beans in those silos would go to feeding livestock, but in an emergency they could be used to keep people alive instead.
So, unless the livestock-feed supply chain has tightened up significantly since the 1970s or 1980s, the US would have about 3 years to get mechanized agriculture back up and running.
Also, ISTR that they estimated that most cars and trucks will survive the attack, but since the electrical systems of automobiles have changed drastically since the 1980s, maybe the US's current inventory of automobiles is a lot more sensitive to electromagnetic pulses. (Cars and trucks are relevant because the US would need some way to transport the grain and beans in silos, most of which are on farms or near farms, to where the people are.)