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

of respondents) years after forages. Forage crops mostly lasted between 3 and 9 years on the
fields. This duration mainly depended on forage yield, only 12% of the farmers adjusted it to
maximize rotational benefits (Entz et al., 1995).
A.IV.1.2 Regional weed survey
Ominski et al. (1994; , 1999) compared the weed communities in i) 63 cereal fields following
3-6 year old alfalfa stands and ii) 54 cereal fields following at least 5 years of annual cereal
grain crops. Cereals after alfalfa were characterized by lower densities of Avena fatua,
Brassica kaber, Cirsium arvense, and Galium aparine, higher densities of Taraxacum
officinale and Thlaspi arvense while Amaranthus retroflexus, Chenopodium album,
Polygonum convolvulus, and Setaria viridis had no consistent or no significant differences.
A.IV.1.3 Field experiments
The largest experimental study was done by Andersson & Milberg (1996; , 1998) on 3 sites in
southern Sweden. They compared 4 nitrogen application rates and three 6-year rotations
comprising either (i) a 2-yr grass ley, (ii) a 2-yr legume-grass ley, or (iii) spring wheat
followed by a repeatedly harrowed fallow applied since 26-30 years. These 2years phases
were always followed by winter turnip rape, winter wheat, oats and barley, which was
undersown in the two ley rotations. The weed communities differed strongly between the sites
and the crops (highest in turnip rape) but did not differ consistently between the fertilisation
and rotation treatments and none of the three rotations developed any major weed problems.
Norris & Ayres (1991) observed that yellow foxtail [Setaria glauca (L.) Beauv.] invasion was
lowest when alfalfa was cut with an 37-days interval, intermediate for a 31-day interval and
highest for a 21-day interval. In two out of three years, delaying the irrigation (14 days instead
of 7 days after cutting) further reduced S. glauca density. While yields increased with the
cutting interval, economic return was best for the intermediate 31-day cutting interval due to
lower forage quality with the 37-day interval.
Gill & Holmes (1997) reported that some farmers in southern Australia include a 2-3 years
pasture phase into crop rotations to manage herbicide resistant ryegrass (Lolium rigidum) and
Avena fatua. A review of several small field experiments in southern Australia indicated that a
combination of grazing by sheep, cutting and other IWM techniques can successfully deplete
the seed bank of problematic Lolium weeds.
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