Oats cv. Cashel and Drummond were sown at Lincoln, New Zealand on 16 September, 2 November or 21 December 1994; and on 7 February, 3 April or 16 May 1995. Biomass and leaf area were measured at one- to four-week intervals from the end of tillering. Crops grew for periods ranging from just over 100 days to nearly 300 days. The crop sown in February died before maturity, and the one sown in April was affected by frost. Biomass production was much higher for the April sowing (over 20 t/ha) than the 7.5-15 t/ha for the other sowings, and was higher for Drummond than for Cashel at all sowing dates. Leaf area indices did not exceed 4.5 except for the February and April sowings of Drummond. There was a strong linear relationship between cumulative biomass production and intercepted radiation for all sowing dates, with the same rate of increase in biomass with radiation increase for both cultivars. Grain yields estimated from quadrat samples were higher than header yields for all sowings, but especially for the April sowing in which grain weights and harvest indices were lower than for the other sowings. Fifty-seven percent of grain yield came from post anthesis photosynthesis and 43% from remobilization from other plant parts. Oat yield was dependent on the amount of radiation intercepted by the crop and harvest index. Changes in conversion of radiation to biomass were not significantly affected by the range of conditions experienced by the crops.