According to the EPA, as of 2015 agriculture is responsible for 9% of American GHG emissions and transportation is 27%.

https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...

Maybe you're thinking of the global level, where agriculture is comparable to transportation and the IPCC grouping "agriculture, forestry, and other land use" (AFOLU) is greater:

https://www.ipcc.ch/pdf/assessment-report/ar5/wg3/ipcc_wg3_a...

I would venture that the reason that latter figure doesn't come up much in American policy discussions is because American policies can do a lot more about American transportation emissions than American policies can do about foreign agriculture or deforestation.