farmer jeff jorgensen is busy harvesting soybeans. he also grows corn on about 2,000 acres in the southwest corner of the state. farming is big business in iowa. the state is the biggest producer of corn in the country, and it's second only to illinois in the production of soybeans. for jorgensen, whose family's been farming for four generations, it's all about keeping his yields as high as he can. >> this is the yield monitor here. >> reporter: and one of the biggest battles he fights is against weeds. >> a weed in the field's going to take moisture, going to take sunlight, nutrients away from the plants surrounding it. and that's why we have to keep clean fields. >> reporter: but jorgensen says keeping "clean" fields has been getting harder and harder. like many farmers, he relied for years mainly on an herbicide called roundup that's manufactured by monsanto. roundup use exploded in the mid '90s with the introduction of new, genetically modified crops that dominate the market today. the crops were engineered to withstand roundup, s