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

growth of any (crop or weed) plant leading to temporarily extended vegetation cover and
competition against weeds. However, older perennial crop stands may show higher spatial
heterogeneities and ‘gaps’ that may be occupied by weeds. The vigour of the perennial
crops may decrease with time due to plant senescence and mortality, which is often the
reason to terminate the perennial crop stand (Entz et al., 1995).
4) Herbicide use is often lower in PFCs as compared to annual crops, or even completely
absent as in organic systems. In most cases, herbicides are only occasionally used during
the establishment phase and sometimes for stand termination. Herbicide use reductions
may be possible as the weeds are suppressed due to the other mechanisms listed here or by
alternative non-chemical weed control techniques adapted to perennial crops (Summers,
1998). Several weed species may also be tolerated in forage crops, as they may have good
forage values, while other weed species such as Rumex crispus L., and Conyza canadensis
(L.) Cronc. may be rejected by livestock or may even be toxic such as Senecio vulgaris L.
(Summers, 1998). Reduced herbicide use in the perennial crops may especially benefit all
plant species with high herbicide sensitivities.
5) Fertilization and irrigation schemes in PFCs may also differ from annual crops. Nitrogen
fertilization is often reduced or absent thanks to nitrogen-fixing legume crop species.
Irrigation may be less necessary than for annual crops due to the deep roots of many
perennial crops. Both modifications may reduce weed growth and seed production.
Table 4: Characteristics of annual crops and PFCs with possible impacts on weeds.
(Further indirect effects are marked in the main text).
1)
Belowground
disturbances:
2)
Aboveground
disturbances:
3)
Crop
competition:
Annual grain crops Perennial forage crops
Potential effects on weeds
Soil tillage and sowing No soil disturbance throughout Reduced seed germination
operations at least once per the whole duration of the crop [lacking tillage stimulus,
year, often more frequently, (2-6 years), accumulation of (Huarte and Arnold, 2003),
sometimes additional plant debris on the soil surface mulch], increased plant
cultivation for mechanical
(mulch)
survivorship (no physical
weed control
damage)
Mostly one cutting per year Frequent cuttings (2-5 per year) Reduced plant survival, reduced
for crop harvest
(forage harvest) seed production (damage of
plant canopies) (Gill and
Holmes, 1997)
High only during some High during the whole Reduced germination, growth
seasons of the year (weak vegetation period due to deep and reproduction (limiting
after sowing, at crop and dense rooting systems and growth resources) (Schoofs and
senescence and absent after intense canopy closures, except Entz, 2000)
crop harvest) during the establishment phase
and directly after cuttings
28