Strong evidence for phytotoxicty limiting oat yields came from the Oat Cultivar Experiment. In that experiment, oat biomass was very supressed, and more so than weed biomass, where the oats were planted in years 1 and II of the rotation, but not where they were planted in fields in years III and IV of the rotation. In the rotation initiated in 1979 oats were being planted in year II, following fababeans (yr I). We attributed the low yields to incorporation of large amounts of residues following winter wheat and fababeans. These residues consumed oxygen in the soil causing formation of fermentation products which were phytotoxic to the oats early in the season. Weeds fared better because they germinate continuously and could respond when the residues were more decomposed and less phytotoxic.

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