Another agricultural group said it has pulled its convention out of San Francisco in retaliation for the city’s support of a table-grape boycott. The Produce Marketing Assn. is the sixth group to cancel a convention in the city since the Board of Supervisors voted in December to endorse a boycott of non-union table grapes sponsored by the United Farm Workers. The California Farm Bureau estimates that the city has lost more than $25 million in convention business because of its support of the national boycott. But city officials say that the bureau’s figures are inflated and that the true cost to the city is much less. UFW leader Cesar Chavez started the boycott five years ago to protest the use of pesticides that he says are dangerous to farm workers and consumers.