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