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

grasslands (Magda et al., 2004; Hald, 2007), sometimes for set-aside fields (Dalbies-Dulout
and Dore, 2001), but only rarely for temporary grasslands (Norris and Ayres, 1991; Hoveland
et al., 1996; Graglia et al., 2006) or as (in-row) mowing treatments in annual crops used for
weed control (Donald, 2007).
In our experiment, several indications show that hay cutting had strong impacts on weed
population dynamics:
• The first 1-2 cutting operations in perennial crops after crop (and weed) sowing had strong
negative effects on weed biomass (Fig. 18) suggesting that cutting enhanced the
competitive advantage of perennial crops, who showed a quicker regrowth than most
weeds.
• In reducing weed biomass and seed production, the cutting operations were probably an
important factor causing the low weed plant densities in perennial crops compared to
annual crops, who were cut only once per year (Fig. 12, Fig. 13).
• In contrast, modifying the cutting frequency in the perennial crops (3 vs. 5 cuts per year)
caused relatively small effects (see above). When pooling all weed species, differences
between the high and the low cutting frequency had no effects on the final weed densities.
When looking at all measurement dates during the 2.5-years experiment, differences
between the low and the high cutting frequency were only visible in alfalfa, where the
lower frequency led to lower weed plant mortality rates and thus higher weed plant
densities at the end of 2007 and increased weed biomass at the beginning of 2008.
However, these weeds disappeared at the first hay cutting in May (Fig. 18). In cocksfoot,
variations of the cutting frequency had no impacts, as weed densities were always very low
(probably due to the dense superficial roots or other mechanisms suppressing weed
emergence).
• Besides the differences between annual and perennial crops, cutting might have also
caused some of the differences in weed plant and seed bank composition between spring
and autumn sown crops. While autumn-sown crops were cut already twice in spring 2007
(27/4 and 7/6), the spring-sown crops were cut for the first time in summer (13/7) and
some spring emerging species might have successfully produced seeds before.
C.II.4.3.4 Interactions between the three factors
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