Bronco Wines is to challenge a law that says California vintners using the word "Napa" on their labels will be required to make their wine primarily from grapes grown in Napa County.A bill signed by California Governor Gray Davis closes a loophole ina federal law that says if a wine label contains a geographic term,at least 75% of grapes used to make the wine must have beengrown in that area.The loophole was that brands named before 1986 were not subject to the 75% ruling.The bill, which takes effect on 1 January 2001, was sought by the NapaValley Vintners Association, which represents 177 wineries.The group was prompted by Napa-based Beringer Wine Estates' January sale of its Napa Ridge brand to Bronco Wine of Ceres, California, for $40 million. Napa Ridge was subject to the loophole.The bill was opposed not only by Bronco, but also by Central Valleyfarmers who have signed contracts to provide grapes to Bronco.Bronco issued a statement on Thursday saying the new law "willhave an enormous and lasting negative impact on the California wineindustry".The company said it would challenge the new law in court.The bill, the statement said, "is economic protectionist,special-interest legislation that sanctions an unfair competitiveadvantage for just one of the 45 counties that produce wine andgrapes, Napa County."