Nitrogen and weeds determine crop yields in arable organic crop rotations

By Jørgen E. Olesen, Ilse A. Rasmussen and Margrethe Askegaard, Danish Institute of Agricultural Sciences

A crop rotation experiment was initiated in 1996/97 at three sites in Denmark with the aim of investigating the possibilities of increasing the organic production of cereals. The sites are:

• Jyndevad (coarse sand) in Southern Jutland,
• Foulum (loamy sand) in Central Jutland
• Flakkebjerg (sandy loam) at Zealand
  

The crop rotations represent systems with different proportions of cereals and nitrogen fixing crops. However, here only results from the crop rotation with grass-clover prior to winter cereals are presented (Table 1). This crop rotation has been tested with four different combinations of catch crops (with and without) and manure (with and without).

Manure and weed control

All manure was applied as slurry in the spring at a rate corresponding to 40 pct. of the nitrogen demand in the cereal crops at rotation level, which implied that at all sites a rate of 50 kg ammonium-N/ha in slurry was applied to spring barley and winter cereals in the rotations shown in Table 1. All cereal and pulse crops were harvested at maturity. The grass-clover was used solely as a green manure crop, and the cuttings were left on the ground. All straw was also left in the field. The crops were irrigated at Jyndevad.

Harrowing was used to control annual weeds. However, a reduced effort was used in the rotations with catch crops as the catch crops were established by undersowing in spring. Perennial weeds were primarily controlled by stubble cultivation in autumn after cereal and pulse crops.

Grass-clover increases yield in winter wheat on loamy soils

The grain yields were lowest at Jyndevad, especially in the winter cereals (Figure 1). The highest yields in spring barley and pulses were achieved at Foulum. There was a tendency for lower yields in the pulses during the second course of the rotation, where lupin/barley was grown, compared with the first course of the rotation, where pea/barley was grown. This was partly caused by attack of grey mould in the lupins.

A high yield in winter wheat depends on a good nitrogen supply, which in these crop rotations mainly comes from the grass-clover that was incorporated prior to sowing the winter wheat. There was no grass-clover prior to the onset of the experiment in 1997, and this caused the lower wheat yields in 1997. The grass-clover was poorly established at Flakkebjerg in 1997, and this resulted in low wheat yields in 1998 at this site.

There was a good relationship between the clover proportion at the first cut in grass-clover and the yield of the following wheat at both Foulum and Flakkebjerg (Figure 2). A large proportion of the nitrogen fixation is lost by leaching at Jyndevad during winter, and the clover proportion therefore has little effect on yield at this site.

Catch crops increase yields in spring barley

Catch crops gave an average yield increase in spring barley of up to 0.78 t/ha during the first course of the rotation and 0.54 t/ha during the second course of the rotation (Table 2). The catch crop was dominated by ryegrass, and the greater yield increase at Jyndevad is associated with a higher risk for nitrate leaching at this site.

The yield increase from catch crops was considerably smaller at crop rotation level than in spring barley - on average of the first course of the rotation ca. 0.06 to 0.18 t/ha. The yield increase from catch crops was reduced from the first to the second course of the rotation, primarily as a result of a yield reduction from catch crops in the winter wheat at Foulum and Flakkebjerg. The yield reduction in the winter wheat occurred because the catch crop after the pulse crop had enhanced the growth of the spring barley crop, which reduced the growth of the undersown grass-clover and resulted in a smaller clover proportion in the grass-clover, resulting in a yield reduction in the winter wheat. This effect occurred three years after the establishment of the catch crop and could thus not be observed in the first course of the rotation. At crop rotation level, this effectively eliminated the yield benefits of catch crops at Flakkebjerg.

Weeds can offset the effect of manure

There was a consistent yield increase from manure application to the cereal crops at all sites. The average yield increase during the first course of the rotation was 27 kg grain per kg ammonium-N applied in slurry. There was a large variation between sites in the yield response during the second course of the rotation with lower yield responses on the loamy soils.

The amount of weeds, plant residues and catch crop biomass was recorded in all experimental plots. This made it possible to estimate the influence of the different factors on the grain yield of spring barley (Table 3). In the table, plant residues from the previous year has been joined with the effect of the catch crops. At Jyndevad and Flakkebjerg this effect was of similar magnitude as the effect of manure application, whereas the manure had a higher influence at Foulum. However, the weed pressure was so great at Foulum that it neutralized the effects of manure, catch crops and plant residues.

The large yield losses caused by the weeds were not just caused by the proportion of weeds, but also by very aggressive weed species, such as mayweed. At Jyndevad mechanical weed control was performed during the entire experimental period in spring barley before sowing the grass-clover in spring. This was not the case at Foulum and Flakkebjerg, where a late sowing of the grass-clover can reduce the emergence and lead to failure of establishing the grass-clover. However, as a consequence of the large weed pressure at Foulum, it as decided in 2001 also to apply weed harrowings in the spring barley at Foulum prior to sowing grass-clover.

Arable organic farming requires proper crop rotations

The current organic arable crop production in Denmark is characterised by a large import of both organic and conventionally produced animal manure. This means application of essential plant nutrients, in particular nitrogen. The results here show that this may not necessarily be a sufficient strategy to secure high and stable yields. If the manure is applied at a position in the crop rotation, where it may reduce the establishment of grass-clover then much of the manure effect may be subsequently lost. Also weeds may reduce much of the yield benefit obtained from manure application.

In organic farming there is thus a need for manure application to interact with a properly designed crop rotation (crops and catch crops) and an effective strategy for mechanical weed control.