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

Our results show that different mechanisms are probably at the origin of the high impacts of
PFCs on weed populations (see summary in Table 13). These mechanisms acted often
simultaneously and showed cumulative effects on the weed life cycle and several interactions.
First, the absence of soil tillage (and of herbicides) favoured the development of very
competitive crop canopies and root systems in the perennial crops suppressing weed growth,
which corresponds to a positive interaction between the factors. Second, hay cuttings might
temporarily alleviate the competition for light, which would correspond to a negative
interaction. But our results do not support this hypothesis; the weeds did not profit from the
temporally reduced competition for light. The competitive advantage of the forage crops
appeared in particular after the first cutting treatments (corresponding to a positive
interaction). The interaction between cuttings and competition depend thus strongly on the
regrowth and re-establishment abilities of the different species. In our case, both Dactylis and
Medicago crops had much better regrowth abilities than most of the weed species. The
regrowth abilities of different weed and crop species as well as the interactions between
cuttings and competition will be further analysed by specific experiments in controlled
conditions in the following chapter C.III.
Two results suggest that competition and cuttings
are both required to obtain a good weed control in PFCs. First, weed biomasses were rather
high before the first cutting treatment and decreased afterwards (Fig. 18). Second, some weed
species developed very high biomasses on small sub plots, where the perennial crop plants
were experimentally removed (data not shown).
C.II.4.4 Strength, limits, perspectives and preliminary recommendations
One strength of this experimental approach was to allow comparing different perennial crop
management practices at the same time, which was rarely done in previous studies. Two
crossing of important crop management factors (sowing season*crop species and cutting
frequency*crop species) and the comparison to the succession of annual crops with different
intercrop management practices permitted investigating some of the most important
mechanisms underlying the impacts of PFCs on weeds.
Results indicated that the perennial crop sowing date has the strongest impact on weed
communities among the three tested factors. However, such conclusions must be seed with
caution, as variations in the experimental treatments (other sowing dates, crop species and
cutting frequencies) might give different results. Futures studies might also integrate other
perennial crop management factors such as fertilisation rate (Fan and Harris, 1996), cutting
106