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 densities 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.
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