 Complete the following phrase:  Crops included wheat, rye, barley and oats. Peas, beans, and vetches became common from the 13th century onward as a fodder crop for animals and also for their nitrogen-fixation fertilizing properties. Crop yields peaked in the 13th century, and stayed more or less steady until the 18th century. Though the limitations of medieval farming were once thought to have provided a ceiling for the population growth in the Middle Ages, recent studies have shown that the technology of medieval agriculture was always sufficient for the needs of the people under normal circumstances, and that it was only during exceptionally harsh times, such as the 