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

perennial crop treatments showed rather high weed biomasses (Fig. 18). For both spring-sown
perennial crops and for the autumn sown cocksfoot crops, weed biomasses exceeded crop
biomasses during the first months after crop sowing, while they were lower in both autumn
sown alfalfa crops, which showed a quicker and more homogeneous initial crop establishment.
After the first hay cutting, the proportion of weed biomass decreased strongly in nearly all
perennial crop treatments. After 2-3 cutting operations, weeds accounted for no more than 1-
20% of total biomass and decreased further to 0% in most treatments and replicate plots (Fig.
18). These decreases tended to be quicker (a) for all plots with the higher cutting frequency (5
cuts/year) and (b) in the three cocksfoot crop treatments, where weed biomass decreased often
to 0% already during the first year (2007) and stayed that low until the end of the experiment.
In contrast, substantial weed biomasses re-appeared in some alfalfa plots during the second
year (2008), especially in those with the low cutting frequency (3 cuts/year).
In general, all weed species showed a higher mortality and a much weaker regrowth capacity
after cuttings than both perennial crop species. However, the biomass of broad-leafed weeds
often decreased more quickly than the biomass of grasses. Until the first cut in 2008, grasses
(mainly B. sterilis and A. myosuroides)
accounted for 20 to 60% of the biomass in alfalfa with
a low cutting frequency. Most of these grass plants germinated already in autumn 2007,
survived the winter but disappeared at the first cutting in 2008. This ‘problem’ did not appear
in any cocksfoot crop treatment nor in the spring and autumn sown alfalfa with the higher
cutting frequency, where weed densities accounted for only 0-15% of the total biomass in
2008 and decreased further with time ( Fig. 18).
While all perennial crops showed decreasing weed biomass during the experiment, the
opposite was true for annual crops, where broad-leaved weed species including Galium
aparine, Polygonum sp., and Sinapis arvensis as well as grasses such as A. myosuroides
showed increasing biomass during the two years ( Fig. 17,
Fig. 18). In the first year (2007),
most of the winter wheat plots showed much lower weed biomasses than the perennial crops,
weeds accounted for 0-20% of total biomass. This proportion was much higher in 2008, where
mainly dicotyledonous weeds accounted for up to 40% of the total aboveground biomass (Fig.
17, Fig. 18).
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