1. An investigation to compare winter and spring defoliation of S.147 oats, and incorporating an application of nitrogen, was carried out during the seasons 1950/1, 1951/2 and 1952/3.

2. Forage available in the spring was considerably greater than that available in the winter, but it had a lower crude-protein content.

3. Compared with the undefoliated oats, both spring and winter defoliation reduced the number of productive tillers by significant amounts, but nitrogen had no effect on tiller numbers under defoliated and undefoliated conditions.

4. Spring and winter defoliation caused significant reductions in grain yields; with winter defoliation the decline in yields was substantial. On the undefoliated and spring-defoliated oats nitrogen increased yields by significant amounts.

5. Spring and winter defoliation reduced straw yields, but nitrogen on both treatments and on the undefoliated oats gave significant yield increases.