More than 80 different vine varieties are authorized for use in the Port vineyards of the Duoro; few of the growers will know exactly which vines they have growing at any one time. All but the most recently planted vineyards have a mixture of grapes, often with as many as 20 or 30 different varieties intermingled in the same plot. In recent years, some of the larger shippers, most notably Ramos-Pinto and Cockburn, have conducted their own research to identify the best vines for producing Port. Touriga Nacional, Tinta Barroca, Touriga Francesa, Tinta Roriz (Spain's Tempranillo) and Tinto Cão are the favored black-skinned varieties, although some growers prefer to use Sousao, Tinta Amarela and Mourisco Tinto. Gouveio (thought to be Verdelho), Malvasia Fina and Viosinho are generally considered to be among the best for producing white ports.