Protect and Enhance the Nation's Resource Base and Environment
Weed Control With Less Herbicide in Arizona Tree Crops
Impact Nugget
A weed-detecting automatic spot sprayer system reduced the amount of
herbicide by 67 percent in an orchard irrigated with microsprinklers
(i.e, only part of the orchard floor is irrigated), and about 50 percent
in flood-irrigated lemon and pecan orchards. If the automatic spot spray
system were adopted on 50 percent of the lemon and pecan acreage in
Arizona, with a 50 percent reduction in the amount of herbicide applied
(currently about 2 gallons of glyphosate per acre annually at a cost
of $14 per gallon), the annual savings would be about $14 per acre at
the cost of current weed control programs, resulting in a statewide
savings of about $252,000 per year.
Issue
Weeds covering an orchard floor compete with the trees for water and
nutrients, reducing fruit or nut yield at harvest. Growers at different
times have tried mowing, disking and cover crops for weed control. Mowing
causes a weed shift to grass and nutsedge species that can be difficult
to control and disking can prune roots and damage tree branches and
trunks. Although leguminous cover crops suppress weeds they also compete
with trees for water and may reduce yields. Weed populations occur in
patches across fields and are usually controlled with chemical herbicides.
Most growers currently broadcast spray post-emergence herbicides on
the entire orchard floor, regardless of where weed patches are located.
A few growers spot-spray weed patches where they exist and thereby reduce
the amount of chemical used to control weeds.
Broadcast spraying the orchard floor wastes chemical and increases
the herbicide load in the environment. Spot spraying reduces the amount
of herbicide wasted but is labor-intensive, slow and costs more than
broadcast spraying herbicides despite reducing the amount of herbicide
sprayed. University of Arizona researchers are investigating how growers
can reduce herbicide use and save money by automatically spot spraying
herbicides only where weeds exist in Arizona tree crops.
What has been done?
A comprehensive series of weed management studies in lemons have been
conducted by faculty in the University of Arizona College of Agriculture
and Life Sciences at multiple sites since 1993. These studies have proven
that clean culture, where the orchard floor is kept completely free
of weeds using herbicides, still provides the best environment for lemon
growth and maximizes yield, both for the first harvest and for the cumulative
yield of all harvests in a season. Similar studies in pecans were conducted
in two locations in southern Arizona where the orchard floor along the
tree row is free of vegetation but a grass strip is grown in the middle
between the rows.
An optical chlorophyll-detecting spray system currently on the market
detects weeds on the orchard floor and automatically triggers spot applications
of herbicide directly to them without spraying bare ground. Called the
NTech WeedSeeker, this boom-mounted system is used in some commercial
orchards in California, where it was developed, but it has not caught
on in Arizona. The barriers to the adoption of this technology include
its high cost and its untested performance under Arizona conditions.
UA researchers have conducted tests of the automatic spot sprayer technology
in commercial lemon orchards in Yuma and Hyder, Arizona and in pecan
orchards in Sahuarita and Bowie, Arizona.
Impact
Study results show that conventional broadcast sprayers and the weed-detecting
automatic spot sprayer system result in comparable weed control in lemon
and pecan orchards with both types of systems providing good or commercially
acceptable weed control. The type of irrigation used in an orchard makes
a difference in the distribution of weeds and the amount of herbicide
used. The weed-detecting automatic spot sprayer system reduced the amount
of herbicide by 67 percent in an orchard irrigated with microsprinklers
(i.e, only part of the orchard floor is irrigated), and about 50 percent
in flood-irrigated lemon and pecan orchards.
Potential impact: The combined acreage for both crops in Arizona is
about 36,000 acres. If the automatic spot spray system were adopted
on 50 percent of the lemon and pecan acreage in Arizona, with a 50 percent
reduction in the amount of herbicide applied (currently about 2 gallons
of glyphosate per acre annually at a cost of $14 per gallon), the annual
savings would be about $14 per acre at the cost of current weed control
programs, resulting in a statewide savings of about $252,000 per year.
Costs savings increase as the cost of the herbicide program used for
weed management increases.
In addition, there would be additional savings due to reductions in
the amount of time workers spend mixing and loading herbicide sprayers,
and health benefits for workers who have less exposure to herbicides.
Environmental benefits would accrue as well, with 50 percent less herbicide
released into the environment compared to conventional spray methods.
Funding
Arizona Citrus Research Council
Yuma Mesa Pest Abatement District
Arizona Department of Agriculture–specialty crop grant program
Hatch Act
USDA regional pecan grant
Contact
William B. McCloskey, associate specialist
Department of Plant Sciences
The University of Arizona
PO Box 210036
Tucson, AZ 85721-0036
(520) 621-7613 office
(520) 621-7186
Email: wmcclosk@ag.arizona.edu
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