A lawsuit claiming Monsanto Co.’s popular weed killer Roundup causes cancer was dealt a blow by a judge’s conclusions that the opinions of the experts testifying against it are “shaky,” a potentially devastating development for the case getting to trial.  U.S. District Judge Vince Chhabria is the first judge to weigh in on the toxicity of the world’s most popular herbicide...Any key witnesses who are cut from the lineup may profoundly shape the outcome of more than 300 lawsuits collected before the judge — all the cases in federal courts that seek to hold Monsanto liable for its failure to warn about the risks of using Roundup.  The San Francisco judge heard from about a dozen witnesses including toxicologists, statisticians and an oncologist...It remains to be seen which witnesses will be allowed to testify at trial on behalf of more than 700 farmers, landscapers and gardeners claiming that exposure to glyphosate — through skin contact or inhalation — caused their non-Hodgkin lymphoma...At this stage of the case the judge is acting as a gatekeeper to exclude evidence not backed by scientific rigor, and evaluating whether the witnesses are qualified as experts to present their conclusions to a jury...All the plaintiffs’ experts except Ritz relied on a 2015 determination that glyphosate is a probable human carcinogen by the International Agency for Research on Cancer...