Outcomes of developmental yield formation processes in oats, namely number of panicles/m2, number of grains/panicle, mean grain weight and incidences of aborted and tertiary grains, were measured in a series of experiments in Northern Ireland and the Republic of Ireland in 1997/98 and 1998/99. Seed rate (200 and 300 seeds/m2), nitrogen (0–200 kg/ha) and plant growth regulator (chlormequat chloride) treatments were applied to the spring oat varieties Aberglen and Barra in one spring- and two autumn-sown experiments, and to the winter varieties Gerald and Image in one autumn-sown experiment. Large variation in number of panicles/m2 and grains/panicle was observed between the experiments and varieties and in response to seed rate, with number of grains/panicle generally being inversely related to number of panicles/m2. At the higher rates of nitrogen rate both number of panicles/m2 and grains per panicle increased. Mean grain weights were relatively constant and were largely determined by variety. Chlormequat chloride had relatively little effect on the yield components, the most consistent being small reductions in mean grain weight.

Tertiary grains occurred rarely in the varieties and agronomic treatments used in the programme but were more frequent at higher rates of nitrogen in most of the experiments. Numbers of aborted grains were usually higher where grain numbers were higher although the effects of variety, seed rate and nitrogen on aborted grains were not consistent. Structure of the grain population, i.e. the relative proportions of primary and secondary grain, was stable despite the large differences in number of panicles/m2, spikelet numbers and mean grain weight.

The greater yield and growth enhancing effects of nitrogen compared with seed rate and plant growth regulator were apparent in responses by developmental processes active later in the life cycle, namely production of tertiary grains and grain filling.

Wide variation in number of panicles/m2 (c. 200–450) and number of grains/panicle (c. 55–145) produced in the crops grown under a very full expression of agro-ecological conditions in this programme was accompanied by small variation in mean grain weight (c. 38–47 mg/primary grain and c. 22–29 mg/secondary grain) and in numbers of tertiary (<2/panicle) and aborted grains (<10/panicle). Plasticity of development throughout the life cycle, manifested both as the established yield components and in numbers of tertiary and aborted grains, was largely effective in ensuring stability of mean grain weight and quality in oats.