Categoría del consumidor protegido:

Adjustments of the rotations and management

The management of the rotations described above is the result of a continuing proces with many discussions between the project participants. From the start of the project in 1997 until now we have made some adjustments in both the design of the rotations and in the management. Our policy is, that as long as we do not interfere with the three key factors (see Olesen et al., 1999) improvements to the rotations and management are permitted. After 1997 rotation 1 in Jyndevad was changed from barley, 1st _{year grass-clover, 2}nd _{year grass-clover, winter wheat to the one presented in Table 1. The reason for}

this was that crop rotations with a high level of grass-clover already had proved their sustainability (Askegaard et al., 1999). The change made it possible to compare spring wheat in rotation 1 to winter wheat in rotation 2, both following grass-clover and also to compare the pulse crops, lupins in rotation 1 with pea/barley in rotation 2. In 1998 we introduced more clover in the rotations in order to increase both the N2-fixation and the diversity. Red clover was added to the grass-clover fields of the heavier

soils and four clover species (described above) were mixed with the ryegrass-catch crop and used in selected crops. From 1999 slurry was applied to winter cereals in mid April instead of, as earlier, at the start of May. From the colour and growth rate of the winter wheat in the spring it was clear that even following a grass-clover crop, the wheat suffered from N-deficiency at the beginning of the growing season. The row distance was increased at Jyndevad and Foulum in selected crops without catch crops in order to improve the weed control by mechanical hoeing. A successful establishment of the white clover is a prerequisite for the success of rotation 4 (with catch crop). Oat undersown with pure white clover showed a vigorous growth at Foulum in 1997 and 1998. The plant density was therefore reduced in 1998 (from 400 to 300 germinating grains m-2_{) and in 1999 we increased the row distance of the oat}

in order to increase the penetration of light to the clover. In order to increase yields triticale was introduced as the second winter cereal in rotation 4 in 1999 instead of winter wheat.

Dilemmas

In the planning of the crop rotations and their management we often came across situations where the factors we wanted to optimise were counteracting each other. The crop-weed-nitrogen triangle was often the reason for this. Figure 1 illustrates the triangle and the management options.

We have increasing problems with couch grass at Jyndevad. To control it we may perform repeated harrowing after harvest of the crops even in the rotations with catch crop. The dilemma is that nutrient leaching increases when the soil is bare. If leaching losses increase, the crop growth may be reduced the following year and the weed propagation may increase. In order to postpone the problems we have decided to carry out more frequent cuts in the grass-clover fields and in this way reduce the build-up of couch grass.

Another example of a dilemma is the way we establish the catch crops. The catch crops are typically sown at the same time as the cover crop. In this way we maximise the chances of having a well- established catch crop in the autumn and winter, which is able to reduce leaching losses. If the spring establishment fails we can have another try after harvest. Prioritising the catch crop in this way may, however, reduce yield and there is a risk of weed propagation because harrowing is not possible.

Figure 1 Interactions between crop, weeds and N-level and the management measures.

Decisions about whether a weed harrowing should be carried out after emergence of the crop or not may lead us into dilemmas because a weed harrowing can damage the crop, especially if the plants are weak or sparsely distributed in the rows. A choice often has to be made between the short-term protection of the crop against mechanical damage and the weed regulation, which may have long-term consequences. In the rotations with no manure application we have considered reducing the plant density in order to increase grain weight. This may however increase the risk of crop damage from the harrowing, because a sparse or weak crop is easily covered with soil when harrowing is performed. Research needs sometimes counteracts with the best cropping practice. As an example we use pure cereal varieties instead of a variety mixture, because it makes it easier to register attacks of diseases. This procedure, however, may reduce the yield results from e.g. Flakkebjerg and Holeby relative to Jyndevad and Foulum because of a climatically induced higher infection risk at Flakkebjerg and Holeby. Questions which need to be answered

• Which weed species can be tolerated in the different systems? What are the thresholds for weed harrowing? The knowledge must be coupled to different N-levels and different crop competitive abilities.

• In which way do we obtain the best utilisation of the grass-clover for green manure? This concerns both the treatments in the growing season (number and intensity of cuts) and the N-release to the succeeding crop.

• How often can the clover species be grown considering the risk of propagation of diseases in clover, and do different clover species share the same diseases?

W EED S N -LEV EL C RO P C A TC H C RO P M echanical w eed control Seedbed V ariety Species

Sow ing tim e

Row distance

Plant density

M anure application m ethod

M anure application tim e

LO C A TIO N : C lim ate

• The pea fraction of the harvested pea/barley mixture is lower at Holeby compared with Jyndevad and Foulum. This may be caused by a higher N-level in the soil and possibly restriction of root growth in the soil at Holeby. How can we increase the production of pulses at Holeby?

• Can the protein content of the cereals be increased?

• Which strategy can control couch grass? This question includes the combined effect on crop yield and N-level.

• What are the effects of the different systems on soil compaction? How will the plant production be influenced in the long-term in systems with different degrees of compaction?

• The inter-cropping of winter cereal and white clover in rotation 4 needs to be improved. The main problem is probably N-deficiency and partly take-all. What can be done in order to improve the N- release to the winter cereal?

• The policy on the rotation experiment is that nutrients other than N must not be yield- limiting. When a limitation is detected the actual nutrient shall be applied. Which criteria and methods can be used to determine yield limitation? The standard soil test methods for P and K are probably incapable of predicting a yield-limiting deficiency.

• How do we increase the success of establishment of cover crops, especially legume cover crops?

• How should the manure be distributed between different crops in the rotation?

References

Anonymous (1997). Vejledning og skemaer, grønne marker, sædskifte- og gødningsplan, gødningsregnskab, 1996/97. Plantedirektoratet, Landbrugs- og Fiskeriministeriet.

Askegaard, M., Eriksen, J., Søegaard, K. & Holm, S. (1999) Næringsstofhusholdning og planteproduktion i fire økologiske kvægbrugssystemer. DJF-rapport 12, 112 pp.

Olesen, J.E., Rasmussen, I.A., Askegaard, M. & Kristensen, K. (1999). Design of the Danish crop rotation experiment. In: Designing and Testing Crop Rotations for Organic Farming (Eds. J.E. Olesen, R. Eltun, M.J. Gooding, E.S. Jensen & U. Köpke). DARCOF Report no. 1. Denmark: Research Centre Foulum.

Rasmussen, I.A., Askegaard, M. & Olesen, J.E. (1999). Plant protection in organic crop rotations for grain production. In: Designing and Testing Crop Rotations for Organic Farming (Eds. J.E. Olesen, R. Eltun, M.J. Gooding, E.S. Jensen & U. Köpke). DARCOF Report no. 1. Denmark: Research Centre Foulum.

Ten years experience of all-arable