Diversifying crop rotations with temporary grasslands - Université de ...

• Weed biomasses decreased in summer (June-July 2007) both in the cut and the uncut
treatments (Fig. 18), which was probably due to the high Dactylis and Medicago crop
biomass.
• The reappearance of some weeds in Medicago crops in 2008 might be due to the reduced
competition of this crop immediately after the cutting operations in contrast to Dactylis,
where regrowth started immediately after cutting.
• The high impact of competition was also visible through the fact that big weed plants
developed mainly in gaps of the crop canopy or when the perennial crop was
experimentally removed on small sub-plots (data not shown).
• While the crop growth dynamics were similar for winter wheat and the autumn sown
alfalfa in 2006/2007, all perennial crops had strong competitive advantages since
2007/2008 compared to the annual spring barley. Although spring barley was sown quite
early in the year (February 2008), the development of a competitive vegetation cover was
much quicker in the established perennial crops (Fig. 18). This was probably an important
factor leading to very low absolute and relative weed biomasses in the perennial crops
since the second year. In contrast, some weed species including G. aparine, A.
myosuroides, Polygonum spp, C. album and Sinapis arvensis developed very high
biomasses (per plant and per m 2
) in the annual crop ( Fig. 18),
even causing crop yield
reductions (data not shown).
Competition was thus probably an important factor for the decreasing weed biomasses in the
perennial crops. This ‘success’ was the reason for the initial negative relation between crop
and weed biomasses became weaker with time and disappeared in 2008 (Fig. 19).
C.II.4.3.3 Hay cuttings (C)
Hay cuttings present generally the only mechanical disturbances in PFCs. It destroys all plant
organs above about 5 cm from the soil surface and may therefore considerably reduce biomass
and seed production of tall weeds (Table 13). The impacts of cuttings on arable weeds are not
frequently studied, probably as this kind of crop management is not very important in annual
crops, which are mostly harvested only once per year with cutting heights that are often much
higher than mowing of PFCs. In annual crops, cutting for crop harvest has thus probably
limited impacts on weeds in contrast to the soil tillage operations applied after harvest.
Therefore, the impacts of cuttings on weeds are mostly studied in the context of permanent
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