It is not possible for me to determine whether Barlett and Steele's evidence is convincing without more information about the specific evidence they present and the context in which they present it. In order to evaluate the persuasiveness of their argument, it would be necessary to examine the quality, relevance, and credibility of the evidence they present, as well as the logical coherence and clarity of their argument. Without this information, it is not possible to accurately assess the persuasiveness of their argument.
In general, it is important to consider a variety of types of evidence when evaluating an argument, and to be critical of any evidence that may be biased, incomplete, or otherwise unreliable. In the case of a claim about Monsanto's control over the world's agricultural seeds, it would be important to consider a range of evidence, including scientific studies, economic data, and other relevant sources. It would also be important to consider any alternative viewpoints or counterarguments that might challenge the conclusion being presented. Ultimately, the persuasiveness of an argument depends on the quality and relevance of the evidence presented and the logical coherence of the argument, as well as the ability of the argument to withstand scrutiny and critical analysis.