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

D.I EVIDENCE OF THE IMPACTS OF PFCS ON WEEDS
D.I.1 Differences in species composition between current crops
Comparisons of the weed community composition found in various crops showed that
differences between annual and perennial crops were even higher than the well-known
differences between autumn and spring-sown annual crops (Fig. 1 and Table 3 of Article 1).
Indicator Species Analysis showed that most of the frequent weed species either preferred
perennial lucernes (including Taraxacum officinale, Veronica persica, Crepis sp., Poa
trivialis, Silene latifolia, Capsella bursa-pastoris, Picris sp.) or annual crops (including
Mercurialis annua, Galium aparine, Fallopia convolvulus, Chenopodium album, and Cirsium
arvense) (Table 4 of Article 1). The results thus indicate that perennial lucernes suppressed
many weeds that are widespread (and sometimes problematic) in annual crops while favouring
other species. Stellaria media and Alopecurus myosuroides were the only frequent weed
species (present in more than 5% of the fields) that showed about equal frequencies in annual
and perennial crops (Fig. 2 of Article 1). Such large-scale comparisons of weed communities
between annual and perennial crops have probably not been published previously, in contrast
to the comparisons between different annual crops (e.g., Andreasen et al., 1996; Murphy et
al., 2006; Fried et al., 2008).
The main advantages of this large-scale study are linked to the random distribution of the
fields and the use of data covering three years, which increased the spatial and temporal
generality compared to the previously published studies, mainly based on small-scale field
experiments (cf. literature review A.IV.1). However, this first approach has also three limits:
1) The large-scale weed surveys are only based on one survey per year and may thus miss
weed species growing during other seasons.
2) As in other previous studies, weed communities in spring- and autumn-sown annual crops
were not evaluated at the same dates, which might increase the differences in species
composition. In contrast, the differences between perennial crops and autumn-sown annual
crops are not concerned by this limit, as they were surveyed at the same period.
3) Finally, this approach does not say anything about the reproductive success of the recorded
species, which will be important for long-term effects.
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