The production of tops and roots in green manure crops of Westerwolds, Italian and perennial ryegrass was measured at 4 sites in 1968-70. When sown on the same date in late summer the yield of Westerwolds ryegrass was highest and that of perennial ryegrass lowest. Under unfavourable conditions the yield of Westerwolds ryegrass sown into cultivated stubble could be lower than that of undersown crops of the other 2 grasses. Cultivation of the stubble was liable to render the field inaccessible in wet weather, therefore undersowing of the green manure crop was advisable. Although perennial ryegrass had the lowest yields of tops its root production was equal to that of Italian ryegrass. High rates of N were needed for good production, but on soils liable to capping and in which good root penetration was desirable the application of 30-50 kg N/ha was adequate.