Repeated clover subcropping as a strategy for commercial organic grain production

Summary

There is a shortage of organic grains, and a need to develop organic production systems which ensure high and stable grain yields. Commercial organic grain production is mainly a task for farms with few or no livestock, and the development of well functioning strategies for nutrient supply and weed control in commercial organic cereal production under such conditions, is a considerable scientific challenge. The nutrient losses should be minimized and grain yields maintained at economically satisfactory levels over time. A promising strategy involves undersowing of clover in grains where yields can be taken and nitrogen (N) be supplied simultaneously. The main objective of this project was to evaluate if repeated undersowing of clover is a well functioning green manuring strategy to obtain high and stable cereal yield on stockless organic farms. Further goals were to quantify effects of repeated undersowing of clover on grain yields and: 1) Field N balance, 2) Mobilization of soil mineral phosphorus (P) and potassium (K) and 3) Weed infestation. Two systems for N supply to organic spring grains were compared at Bioforsk Arable Crop Division, Apelsvoll and Kise in central south east Norway. Both systems consisted of cover crops which are grown for the benefit of the soil and/or other plants. Repeated undersowing of clover in the grains in four consecutive growing seasons (2002 –2005) was compared to a treatment with whole-season green manure in the second year. The residual effect of all treatments was measured in 2006.
The use of legumes, undersown in grains as well as used as a whole-season green manure, significantly increased the subsequent grain yield in an organic production system with no other fertilizers applied. The average grain yield over four years was larger with continuous undersowing than with a whole season green manure in the second year of the period. Hence, in organic stockless grain production systems it seems to be a better option to harvest cereals each year with undersown legumes, than to use a whole growing season each fourth year to produce nutrients for subsequent grains. However, when legumes are included in most years of the crop rotation, there is a risk that plant health problems may arise, such as nematodes or fungi. No beneficial effects of repeated clover undersowing was recorded with regard to control of annual weeds. However, the expected competitive effect of the undersowing on the weeds may have been obscured by an increased N supply from the undersown clover. Not only the grain plants, but also the weeds responded with increased growth when clover was grown in the previous season. In an experiment addressing perennial weeds it was shown that the undersown crops may retard the growth of sowthistle. With respect to soil tillage, shallow ploughing and rotary tilling in the autumn will retard the growth of perennial weeds, especially Canada thistle and coach grass.