The diffuse leaching of plant nutrients from agricultural soils is part of the problem of the eutrophication of fresh water systems and coastal sea waters. Among the measures taken to reduce the leaching is keeping the soil with plant cover during autumn and winter. In areas with a predominance of annual crops this can be achieved by the undersowing of catch crops. In the current work it was investigated how the time of undersowing affected the barley (Hordeum distichon L.) crop and the catch crop biomass production. As catch crops Italian ryegrass (Lolium multiflorum L.), perennial ryegrass (Lolium perenne Lam.) and red clover (Trifolium pratense L.) were used. The biomass of the catch crop was markedly reduced with delayed undersowing, but net biomass production in late autumn was generally not affected. On light soil the barley yield was only to a small extent affected by the undersown catch crop, and with delayed undersowing the yield was even less affected. On heavier soil the barley yield was only marginally affected regardless of time of catch crop undersowing. An alternative to the undersowing of catch crops would be relay cropping of two cereal crops, one spring crop and one winter crop. As their initial competitive capacities are similar there is a risk that the winter crop will affect the spring crop yield adversely. To be able to set the competitive relationship between the crops favourably for the relay cropping system to be effective, a means to delay the winter crop germination is needed. Delayed undersowing has the risk of damaging the spring crop by the drilling procedur itself, and this was also noticed in the catch crop field experiments. In a couple of experiments the technique of coating seeds with polymers to achieve delayed germination was explored. The materials used were cellulose lacquer (in one experiment with the addition of lanolin) or an acrylic plastic, a so called primer. Wheat (Triticum aestivum L.) and oil seed rape (Brassica napus L.) were coated in house with several coating levels and germinated under controlled conditions in Petri dishes. In one experiment the coated seeds were germinated under three different temperature levels and three different moisture levels. It was found that delays could be achieved and that with increasing coating level the delays increased. It was also found that at low germination temperature (5.5 °C) for plastic coated seeds there was almost no difference in temperature sum needed from sowing to germination regardless of coating level. At high germination temperature (13.9 °C) there was a marked increase in temperature sum needed to germination and it increased with increasing coating level in comparison with uncoated seed. The resulting germination pattern was hypothetically explained by the dependence of water and oxygen uptake on temperature, in relation to the permeability of the coating materials. The permeability of the materials was probably rather constant over the temperature range. To gain the necessary control of the desired germination delay further investigations are needed, in collaboration with other scientific disciplines
