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

However, some differences in density and diversity of the emerged weed vegetation were
detected. During the establishment phase, cocksfoot crops showed higher weed densities and
biomasses than alfalfa, which might be due to reduced competition. In contrast, cocksfoot
crops had even lower weed densities and diversities than alfalfa during the second half of the
experiment, which might be due to reduced weed emergence (see chapter D.II ‘Underlying
mechanisms’ below).
C.II.4.2.3 Cutting frequency
Hay cuttings are often the only mechanical disturbances in PFCs and might have strong direct
and indirect impacts on weeds (destroying weed shoots and modifying the competitive
environment). It might thus be surprising that the difference between the high and the low
cutting frequency (5 vs. 3 cuts per year) had only rather low impacts in the field experiment.
These low differences are in contrast to the high differences between annual and perennial
crops that also differed by the cutting frequency (annual cereal crops were harvested only once
per year).
The lower cutting frequency led to slightly increased final weed species diversities. This may
indicate that the lower cutting frequency allowed a higher number of different species to
successfully terminate their life cycles. In contrast, no significant effects could be detected on
final plant densities, although plots with the lower cutting frequency showed higher weed
biomass at some measurement dates. Two antagonistic processes might have caused this
balanced result: the lower cutting frequency might have reduced the direct destruction of weed
shoots but also lead to longer periods where the soil is covered by the crop canopy increasing
competition for light. The latter mechanism was visible in cocksfoot, where the lower cutting
frequency led even to higher cumulated crop biomass production (Fig. 17). This was also
observed in other studies. Bell et al. (1989) showed that yield and competitive ability of
Bromus willdenowii-grasslands in New Zealand increased when cutting frequency was
reduced (40-50 days instead of 10, 20, or 30 days). In a similar way, Hoveland et al. (1996)
reported that Medicago sativa grasslands in Georgia, USA, showed better regrowth with a 4 or
6 weeks interval compared to 2 weeks, where yields were reduced by 50% and weed invasion
favoured.
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