From Imperial Spain: 1469-1716, by J. H. Elliott (Penguin, 2002), 2nd ed., Kindle Loc. 2057-2076:
In spite of the increasingly grave problem of the national food supply, Ferdinand and Isabella adopted no vigorous measures to stimulate corn production. On the contrary, it was in their reign that the long-continuing struggle between sheep and corn was decisively resolved in favour of the sheep. The great expansion of the mediaeval wool trade had revitalized the economic life of Castile, but there inevitably came a point at which further encouragement of Castilian wool production could only be given at the expense of sacrificing agriculture. This point was reached in the reign of the Catholic Kings. The importance of the wool trade to the Castilian economy, and the value to the royal treasury of the servicio y montazgo, the tax paid the Crown by the sheep-farmers, naturally prompted Ferdinand and Isabella to pursue the policies of their predecessors and to take the Mesta under their special protection. As a result, a whole series of ordinances conferred upon it wide privileges and enormous favours, culminating in the famous law of 1501 by which all land on which the migrant flocks had even once been pastured was reserved in perpetuity for pasturage, and could not be put to any other uses by its owner. This meant that great tracts of land in Andalusia and Estremadura were deprived of all chance of agricultural development and subjected to the whim of the sheepowners. The aims of this policy were obvious enough. The wool trade was easily subjected to monopolistic control, and, as a result, it constituted a fruitful source of revenue to a Crown which, since 1484, had found itself in increasing financial difficulties, exacerbated by the flight of Jewish capital. An alliance between Crown and sheepowners was thus mutually beneficial for both: the Mesta, with its 2½ to 3 million sheep, basked in the warm sunshine of royal favours while the Crown, whose control of the Military Orders gave it some of the best pasturing lands in Spain, could draw a regular income from it, and turn to it for special contributions in emergencies.
There were no doubt certain unintended advantages to Castile, in the intense royal encouragement of the wool industry. Sheep-farming requires less labour than arable farming, and the vast extent of the pasture-lands helped to produce a surplus of manpower which made it easier for Castile to raise armies and to colonize the New World. But on the whole the favouring of sheep-farming at the expense of tillage can only appear as a wilful sacrifice of Castile’s long-term requirements to considerations of immediate convenience. It was in the reign of Ferdinand and Isabella that agriculture was confirmed in its unhappy position as the Cinderella of the Castilian economy, and the price which was eventually to be paid for this was frighteningly high.