Conclusion
Little conclusive evidence was found to support the use of onland
rather than in-furrow wheeling during ploughing, nor for the
use of light tractors rather than heavier ones, in terms of their effect
on crop yields and indirect effects on weeds, provided ploughing
is performed under favourable moisture conditions. For perennial
weed species with shallow creeping rhizomes or roots (E. repens
and S. arvensis), both weed numbers and the total above-ground
weed biomass were mostly 50% lower with deep ploughing than
with shallow ploughing. The greatest advantage of deep ploughing
appeared in the control of C. arvense, which in some cases was
reduced by more than 90% compared to shallow ploughing. Further
studies are required to establish whether the greater effect of deep
ploughing in spring on C. arvense, found in this study, compared to
its effect on species with shallower root or rhizome systems, applies
to all deep rooted creeping perennials. Depth of ploughing affected
both perennial weed infestation and yield levels consistently. With
regard to ploughing depth in organic cropping systems dominated
by cereals, the need to combat perennial weeds by deep ploughing
weighs probably at least as much as any possible beneficial effect
of shallow ploughing on stimulating nutrient turnover.