For Argentina:
https://ipad.fas.usda.gov/rssiws/al/ar_crop_prod_prov.htm?commodity=Corn&arg2=Production
Here's some better ones for Brazil:
https://ipad.fas.usda.gov/rssiws/al/br_cropprod_s.htm?commodity=Soybean&country=Brazil
https://www.usda.gov/oce/weather/pubs/Other/MWCACP/Graphs/Brazil/BrzSoyProd_0509.pdf