Knowledge, science, and practice in ecological weed management: farmer–extensionist–scientist interactions - Ecological Management of Agricultural Weeds - page 115

yields from specific localities in a field were below average one season and
above average in others.This complicates the fine-tuning of input levels,since
weather may be difficult to predict.
Weed observations can also be incorporated into the data system to locate
problem areas for additional observation from one crop to the next. Variable-
rate applicators permit fine-tuning herbicide applications to soil type or
patches of particular weed species. Sprayer prototypes controlled by weed
sensors have also been developed to apply post-emergent herbicides only
where weeds are present (Thompson, Stafford & Miller, 1991). Patch spraying
has been calculated to save from 9% to 97% in herbicide use,compared to field-
wide application for the control of the perennial weed Elymus (Elytrigia) repens
in cereal grains in England (Rew et al .,1996).Little saving occurred when weed
patches were extensive, a wide buffer was sprayed at the patch edges, and the
areas below threshold were sprayed with a lower herbicide dose.High savings
resulted when weed patches were few and concentrated,no buffer at the patch
edges was sprayed, and the areas below threshold were not sprayed.
The second approach, also in mechanized agriculture, builds on the multi-
ple interactions among diverse living organisms and the physical environ-
ment in a crop sequence to minimize the impact of weed variability and
uncertainty. Weeds are managed by manipulating a diversity of factors that
negatively affect weed population dynamics and favor the crop over the weed
in crop-weed interactions. Non-noxious weed complexes are maintained at
below-threshold levels through crop rotation, cover crops, timely tillage and
cultivation, crop residue management, choice of crop varieties, and other
tactics. The ecological approach focuses on the design of multiple-year crop-
ping systems that suppress weeds rather than directed, short-term control of
weed patches. This approach is well illustrated by the study of potato-based
rotations under three contrasting weed management treatments (conven-
tional,reduced input,and mechanical) and two soil managements (Gallandt et
al ., 1998) described in Chapter 5. In the southern Brazilian state of Santa
Catarina, thousands of farmers routinely use a diversity of green manures
either intercropped with main crops or as covers during fallow periods to
prevent soil erosion,suppress weeds,reduce weeding costs,and build soil tilth
(Bunch,1993).Green manure management is mechanized with animal-drawn
implements, which flatten standing cover crops, conserving the cut biomass
on the soil surface, and clear a narrow furrow for planting. Since 1987 the
combination of green manures, animal manures, and soil and moisture
conservation has produced yield increases of over 65% for maize and soybeans.
Labor costs for weeding and plowing have also declined. As both the potato-
based system and the minimum-till green manure system show, the