Site-specific weed control matches
site-specific conditions (such as soil properties and weed infestation
densities) with the proper herbicide and application rate. Spatially
variable herbicide-rate applications can achieve the most effective
application, because each part of the field receives a precise amount
of herbicide based on its need. The benefits of this technology include
a reduction in spray volume and consequently lower herbicide costs,
time savings because of fewer stops to refill, and less nontarget
spraying, which reduces potential environmental risks.
Reductions in herbicide use achieved with site-specific applications
depend on the level of weeds in the field, but can be as high as 40%
to 50%. In an evaluation of site-specific, postemergence weed control
of broadleaf and grass weeds in corn, showed a 51% reduction in rimsulfuron
and an 11.5% reduction in bromoxynil plus terbuthylazine use, compared
with conventional herbicide spraying in spring barley, a nonsignificant
yield increase was observed when weeds were controlled in patches,
but 41% less herbicide was used compared with whole-field spraying.
Scientist at the University of California tested the hypothesis that
weed patches present in specific locations of a field before the previous
year’s harvest indicate where weeds will be present during the
following growing season. Mapping these weed patches indicates where
herbicide should be applied, and conversely, the absence of weeds
indicates where little or no herbicide is required. Although sampling
is often performed on a larger grid than the grid used for pesticide
application, geostatistics allows the estimation of weed populations
between sample points, and thus the application map can be made to
correspond with the width of the spayer. Our objective was to evaluate
site-specific herbicide applications of a pre-emergent herbicide using
two types of weed maps developed from weed counts made the previous
year, and to calculate the herbicide savings.
We conducted a variable-rate experiment on an 11-acre portion of a
79-acre field located in Yolo County. The crops were processing tomato
in 1999 and sunflower in 2000. We developed weed maps from the tomato
crop and used them to develop variable-rate applications the following
year to sunflower. In sunflower, a pre-emergent herbicide is appled
either before planting and mechanically incorporated, or after planting
but before crop or weed emergence and incorporated mechanically or
by irrigation. We studied the effectiveness of variable-rate application
of a pre-emergent herbicide, although this technology can be used
for post emergent herbicides as well.
Processing-tomato seeds were planted from May 4 to 8, 1999. A pre-emergent
herbicide, napropaminde (Devrinol), was applied in an 8-inch band,
centered on the crop row before tomato planting. The field was hand
weeded on May 26 and cultivated on June 3. A lay by postemergent herbicide,
trifluralin (Treflan), was applied on the sides of the bed and in
furrows on June 20. Another hand-weeding followed on June 27. Furrows
and sides of beds were again cultivated on July 26. The crop was harvested
from Sept. 10 to 14, 1999.
Using weed maps developed from a tomato crop, we developed variable-rate
application maps for the following year. Ethalfluralin (Sonalan) was
applied postplant, pre-emergent and followed by two cultivations.
Weed distribution was mapped in the tomato crop. The density of the
weed population was assessed in two ways: (1) by cumulative weed-seedling
counts throughout the crop season or (2) by mature-weed counts ar
the time of crop harvests. Weed densities were estimated using a grid
165 feet wide (across beds) and 185 feet long (along the direction
of beds). The measurement unit was a 20-inch-by 20-inch quadrat for
seedling counts, and a 15-feet-by-17-feet grid cell for mature-plant
counts. All data points were assigned north and east coordinates (georeferencing)
to allow the weed maps to be spatially analyzed in a geographic information
system (GIS).
Weed population densities estimated by the different methods were
used to create continuous weed-density maps, utilizing an interpolation
method to estimate weed densities between the sampled locations. The
interpolated weed-density maps were used to create treatment maps
based on weed infestation levels. The field map was divided in to
a matrix of cells, and the average weed infestation level was estimated
for each cell.
Infestation levels were defined as weed-free (less than 10 seedlings
per square yard or less than one mature plant per square yard), medium
(11 to 30 seedlings per square yard or one to three mature plants
per square yard) or high (more than 30 seedlings per square yard or
more than 3 mature plants per square yard). Levels were arbitrarily
set to cover the range of observed densities. Herbicide treatment
maps were created by assigning varying herbicide rates to each location
according to infestation levels, and dividing the field into zones
receiving the same herbicide rate. The GIS map information is downloaded
directly into the sprayer controller.
In this study, variable rate herbicide applications based on weed
infestation maps developed just before the previous year’s harvest
provided effective weed control. The results showed that when information
about the spatial distribution of the previous year’s weed seedlings
or mature weeds was used, weed conrol was comparable to unifrom, one-rate,
herbicide applications, while the total amount of herbicide applied
decreased. Herbicide use was reduced an estimated 39% for the seedling
map and 24% for the mature map approach. However, incorporating the
weed-seed redistribution from harvest to application time into treatment
maps could further improve weed control.
