Knowledge, science, and practice in ecological weed management: farmer–extensionist–scientist interactions - Ecological Management of Agricultural Weeds - page 110

while usually applied over the entire field, act against individual weeds. In
addition, fields across the agricultural landscape are different in their weed
patchiness and composition,due to founder effects,differing patterns of crop-
ping and weed control practices, soil and drainage, and location in the land-
scape. For example, Johnson et al . (1995) found that a particular weed species
will not necessarily have the same degree of clumping in different fields. How
to improve farmer planning and decision-making through simpler and more
accurate scouting of weed patches, estimation of damage, and extrapolation
of weed dynamics is a major challenge to farmers, extensionists, and weed
scientists.
In addition to being patchy, the presence of weeds in crop fields and across
the agricultural landscape is uncertain. For a specific field in a specific season,
when weeds will germinate, how fast they will grow in relation to the crop,
how much seed they will produce, and how effective crop growth and weed
control practices will be are difficult to predict (Ghersa & Holt, 1995).
First, the individuals of a weed species have a wider range of response to
weather conditions than the individuals of a crop planted in the same field.
Although each weed species has a relatively defined life cycle, individual
weeds within a species show a range of responses to moisture and temperature
cues (Chapter 9) (Dekker, 1997).
Second, unpredictable variations in weather during and between seasons
affect weed germination and growth, the relative development of the weeds
and the crop, and the effectiveness of weed control measures. In a single loca-
tion in Minnesota, for example, variable weather conditions from 1991 to
1994 included lingering snowpacks, a late cold snap, an exceptionally wet
spring, an exceptionally dry spring, and midseason droughts (Forcella et al .,
1996). These affected date of soil preparation and planting, date of weed ger-
mination, and herbicide effectiveness. Thus, although weed patches may
show some stability across years, actual weed density, weed phenology in rela-
tion to the crop, and weed seed production may be much more variable than
weed patch location and thus harder to predict.
Third,over several cropping seasons,nondirectional random shifts in weed
composition due to weather fluctuations and semipredictable directional
shifts in weed composition due to cropping patterns occur simultaneously.
This interaction contributes an additional dimension of uncertainty to weed
management. This may be further complicated by the occasional invasion of
new weed species.
Lastly, farmers manage weeds according to different criteria and con-
straints depending on the year. Small farmers are routinely affected by family
illnesses,economic crises,and low crop prices.Large-scale farmers often suffer