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