During the last several centuries, the high granite mesas in eastern Portugal south of the Douro river have been a major rye growing region. About half the rye produced was carried away to feed urban populations. The loss from the village agro-ecosystem of the nitrogen in the rye grain was replaced because farmers used giesta (Cytisus, Fabaceae), a nitrogen fixer, to make the compost with which they fertilized their crops. Farmers used to keep about 20 sheep for every hectare they cultivated. These sheep produced very few lambs, could be milked only briefly, produced low quality wool, and were not eaten. Farmers kept so many sheep to mix their excrement with giesta. Two experimental rye plots in the county of Trancoso, Beira Alta Portugal were grown with traditional technology except one had giesta plus sheep manure, and the other had only giesta, but enough extra to add the same amount of nitrogen. By the end of the growing season, rye in the plot with giesta plus manure was approximately twice as productive, and much of the giesta added without manure had not decomposed. The carbon/nitrogen ratio of mature giesta is near 30, which is too high for giesta alone to rot readily in the soil. Manure lowers the C/N ratio to near 20, which enables it to rot and release nutrients at the rate rye needs them.