Weed management: a need for ecological approaches - Ecological Management of Agricultural Weeds - page 26

countries who are not yet fully reliant on herbicides, transitions toward eco-
logical weed management systems may involve substantial agroecosystem
redesign but lack intermediate stages of improved herbicide efficiency and
input substitution. None the less, system redesign in developing countries is
still likely to emphasize the improved use of multiple tactics, local biological
resources,on-farm labor and knowledge,and skills for timely monitoring and
decision-making.
Increased efficiency in the use of conventional inputs
Because weed populations are often distributed patchily throughout
fields with many areas having low densities (Mortensen, Johnson & Young,
1993; Cardina,Johnson & Sparrow,1997),increased efficiency in herbicide use
can be achieved by treating weed populations only where and when their den-
sities warrant it (Mortensen,Dieleman & Johnson,1998).Johnson,Mortensen
and Martin (1995) mapped weeds in commercial Nebraska maize fields in
which pre-emergence herbicides were applied only in a band over crop rows
and found that, on average, 71% of the intra-row area was free of broadleaf
weeds and 94% free of grass weeds; 30% of the area between crop rows was free
of broadleaf species and 72% free of grasses.They concluded that if herbicides
were applied only where weeds were present or exceeded a threshold density,
large reductions in herbicide use would be possible.
Recent advances in real-time sensing technologies may soon allow spraying
weeds with post-emergence herbicides on the scale of individual plants;
remote sensing and geographic information systems already allow herbicide
applications to be made on the scale of small sections of fields (Hanson,Robert
& Bauer,1995; Mortensen et al .,1995; Mortensen,Dieleman & Johnson,1998).
Backpack sprayers and wick applicators are also suitable for locally targeted
use of herbicides. Accurate predictions of the timing and location of weed
emergence based on better knowledge of seed bank dynamics may also allow
farmers to avoid unneeded herbicide applications. If the costs of monitoring
weeds and applying herbicides at specific locations are lower than broadcast,
prophylactic applications, then direct cost savings and greater returns will be
possible at the farm level.
Substitution of benign inputs
The input substitution approach can involve replacement of conven-
tional herbicides with new synthetic materials (Zoschke, 1994) or microbial
products (see Chapter 8) that have shorter residual periods, less mobility, and
lower toxicity to humans and other nontarget organisms. Input substitution
can also involve partial or complete replacement of herbicides with mechani-