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

Heggenstaller & Liebman (2006) compared a conventionally managed 2-year rotation (maize–
soybean), a 3-year rotation (maize–soybean–triticale undersown with red clover) and a 4-year
(maize–soybean–triticale undersown with lucerne–lucerne). The 3-year and 4-year rotations
were managed with 72% and 79% less herbicides than the 2-year rotation, respectively. Weed
populations profited from the herbicide reductions in the 3- and 4-year rotations but increases
of Abutilon theophrasti could be prevented due to low fecundity in triticale and low seedling
survival and fecundity in lucerne. In contrast, results for Setaria faberi were more
heterogeneous leading to population increases in some years.
Hiltbrunner et al. (2008) studied weed dynamics and diversities in 15-years field experiments
in Switzerland including winter wheat, maize, summer or winter barley, potatoes or oilseed
rape and temporary grassland in organic, integrated and conventional cropping systems. In the
organic systems, the diversification of crop rotations with temporary grasslands was an
important factor keeping weed pressure low, however, some species such as Taraxacum
officinale and Rumex obustifolius showed increasing tendencies in the perennial crops and
dominated the weed community in the following annual crops.
A.IV.1.4 Discussion and limits of the reviewed studies
Most of the reviewed studies indicated that PFCs have negative effects on some weed species
and positive effects on others (see also the species listed in Table 1 of Article 1). This
indicates that PFCs basically tend to change the weed community composition.
The generality of these findings may however be limited as most of the reviewed studies (i)
were based on field experiments conducted on one or few experimental sites, (ii) involved
rather short duration of forage crops (1-2 years) inserted in rather short experimental rotations
(2-4 years) and (iii) often focused on one or few locally important weed species. The only two
exceptions are the farmer interviews (Entz et al., 1995) and the weed surveys (Ominski et al.,
1999) which were both done in the same region in Canada (see above). These studies might be
closer to reality, where forage crops last often for more than 2 years on the fields and farmers
often do not apply fixed rotations as in the experiments but adjust their crop sequences and
forage crop duration depending on various economic and agronomic factors.
One mean for increasing the generality of experimental results is to understand the underlying
mechanisms. Unfortunately, most reviewed studies did not give many details on the
mechanisms causing the impacts on weeds. Authors observing reduced weed abundances after
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