Undersowing in a northern climate: effects on spring cereal yield and risk of nitrate leaching

The disadvantages associated with invariable cereal cropping, concern about nutrient leaching and the price of nitrogen (N) fertilizer have increasingly become the subjects of discussion during recent decades. An undersown crop, which grows together with a main crop and post-harvest, could address such disadvantages. The aim of this study was to gain knowledge needed to develop undersowing for Finnish conditions in the interests of optimizing cereal production and meeting environmental goals. Studies were made on the effects of undersown species and management practices on biomass and N yield of undersown crops, grain yield of spring cereals and soil nitrate levels. In total, 17 plant species were undersown in spring cereals during the field experiments carried out between 1991 and 1999 at four sites in south and central Finland. After analysis of preliminary results, eight species were studied more thoroughly. Two legumes, one grass species and one grass-legume mixture were included in long-term trials in order to study annually repeated undersowing. Simultaneous broadcasting of seeds instead of specific undersowing was also studied. Moreover, seeding rates of undersown crops and N fertilization application rates during annually repeated undersowing were researched. Italian ryegrass (Lolium multiflorum Lam., IR) absorbed soil nitrate N (NO3-N) most efficiently in autumn and timothy (Phleum pratense L.) in spring. The capacity of other grass species to take-up N was low, or it was insufficient considering its negative effect on grain yield. Red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) were well suited to annually repeated undersowing, supplying fixed N for cereals without markedly increasing the risk of N leaching. Autumn oriented growth rhythms of the studied legumes were optimal for undersowing, whereas those of grasses were less well suited despite variation in the trait among species. All species were compared according several critical features required of an undersowing system. A model of an adaptive undersowing system was outlined in order to emphasize allocation of measures according to needs. After defining the goal of undersowing, many decisions need to be made. When the primary consideration is reduction in N leaching, a mixture of IR and timothy is advantageous. Clovers represent suitable replacements for N fertilization because their positive residual effect is greater than the negative effect of increased competition. A mixture of legume and nonlegume is a good choice when increased plant diversity is the main target. Seeding rate is an efficient means for adjusting the levels of competition and N effects. Broadcasting with soil covering equipment can be used to establish an undersown crop. In addition, timing and method of cover crop termination play an important role in the outcome. Continuous observation of the system is needed as, for instance, ambient conditions significantly affect theplant diversity is the main target. Seeding rate is an efficient means for adjusting the levels of competition and N effects. Broadcasting with soil covering equipment can be used to establish an undersown crop. In addition, timing and method of cover crop termination play an important role in the outcome. Continuous observation of the system is needed as, for instance, ambient conditions significantly affect the growth of an undersown crop and N release from crop residues can increase over the long term.