US: EPA's official national breakdown places agriculture for US around 9% (this would not include any deforestation impacts at all, and transportation and building energy use associated with Ag supply chain would show up in those respective categories) https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions 

Globally, the EPA indicates a 24% emission contribution: https://www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data

The EU reports similar numbers (10% of EU emissions associated with AG): http://ec.europa.eu/eurostat/statistics-explained/index.php/Agriculture_-_greenhouse_gas_emission_statistics

If you are up for heavier reading - the IPCC is always a great place to go for background.  They work hard to report the science with as little hype as possible: https://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter8.pdf