https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis... The EPA site shows transportation and electricity as the main contributions to carbon.
The grains are not the issue, the subsidies in the US are so strong that they are not a real factor. There is so much excess food produced they export it to other countries for instance. https://modernfarmer.com/2019/01/congress-finally-passed-a-n... https://abcnews.go.com/US/dairy-farmers-dumping-milk-amid-co... https://edition.cnn.com/2020/04/15/business/milk-dumping-cor... Fresh food is often wasted, and the dumping of milk is to prevent the prices from going too low. This was happening before corona as well, this paper is a pretty good summary from a marxist perspective on overproduction. https://www.e-ir.info/2016/07/07/agricultural-overproduction...