In the URL below, yeras appears in a list of crops grown in the 19th century. It is along with hortalizas = garden vegetables, frutas=fruits, listed after barley, wheat, etc. The usual word for herb is yerba OR hierba, the spelling is not totally fixed. My Spanish dictionary has the 2 conventional spellings, but yera is not listed with any meaning. Because of the context and known spelling variations, and because it is not anything common enough for my dictionary to list and yet is common enough to be a major crop, I think yera is a variation of yerba. It means herb, with fragrant oils, as required for culinary or medicinal purposes. Yerba can also be ordinary grass, even a weed. The crop would be the useful kind!