Monsanto, the giant of agricultural biotechnology, has been buffeted by setbacks this year that have prompted analysts to question whether its winning streak of creating ever more expensive genetically engineered crops is coming to an end. The latest blow came last week, when early returns from this year's harvest showed that Monsanto's newest product, SmartStax corn, which contains eight inserted genes, was providing yields no higher than the company's less expensive corn, which contains only three foreign genes. Monsanto has already been forced to sharply cut prices on SmartStax and on its newest soybean seeds, called Roundup Ready 2 Yield, as sales fell below projections. Sales of Monsanto's Roundup, the widely used herbicide, has collapsed this year under an onslaught of low-priced generics made in China. Weeds are growing resistant to Roundup, dimming the future of the entire Roundup Ready crop franchise. And the Justice Department is investigating Monsanto for possible antitrust violations. Until now, Monsanto's main challenge has come from opponents of genetically modified crops, who have slowed their adoption in Europe and some other regions. Now, however, the skeptics also include farmers and investors who were once in Monsanto's camp.