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

crop treatments, weed emergence was even more reduced in the Dactylis crops even if this
crop showed higher initial weed densities and higher seed production. In this crop, the soil
surface was generally harder and often intertwined with fine and dense grass roots, while the
soil surface in the Medicago crops was looser and often wetter, and comprised less superficial
roots (Medicago roots are known to grow deeper than Dactylis grass roots), which might have
favoured weed germinations.
C.II.4.3.2 Competition (B)
Competition for common growth resources (light, water, nutrients) and space is probably the
most important direct interaction between crops and weeds. An established and dense plant
canopy can affect several important stages of the weed life cycle (see Fig. 20 and Table 13 for
an overview). Germination rates may be reduced due to a lack of light stimuli and a modified
light quality under the canopy (Huarte and Arnold, 2003). Initial seedlings growth may be
affected due to a reduced availability of water and nutrients or allelopathic interactions (Xuan
and Tsuzuki, 2002). After seedling emergence, competition (for light) is probably the most
important factor determining weed growth, biomass and seed production in arable fields of
temperate cropping systems. There are several indications that competitive interactions played
an important role for weed (and crop) growth in our experiment:
• When pooling all crop treatments, crop and weed biomass showed strong negative
correlations for all measurement dates during the first year of the experiment suggesting a
strong impact of competition (Fig. 19). In particular, the initial weed biomass was high in
all treatments and plots, where the initial crop biomass and competitive ability was low.
This was the case in the cocksfoot crops, which generally showed a slower initial
establishment than alfalfa and winter wheat (Fig. 18). Moreover, the autumn-sown
perennial crops showed a better establishment than the spring-sown perennial crops. When
these two ‘unfavourable factors’ were combined (spring-sown cocksfoot crops), initial
weed biomass was highest (Fig. 18) and in turn reduced crop growth due to competition
during the first month. However, the initial competitive disadvantages of spring sowing
and of Dactylis crops disappeared already during the first year ( Fig. 18).
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