Vegetation Management in Avocados
Department of Botany and Plant
Sciences,
Studies were conducted (at several different
locations) to determine if the rates of herbicides can be decreased by applying
them in combinations and with adjuvants.
Combinations of glyphosate
(Roundup), paraquat (Gramoxone),
diuron (Karmex), fluazifop (Fusilade 2000), sethoxydim (Poast), prodiamine, pendimethalin
(Prowl), oxyfluorfen (Goal), lactofen
(Cobra), linuron (Lorox), norflurazon (Solicam), napropamide (Devrinal), oryzalin (Surflan), simazine (Princep), and new sulfonylureas with each other and with four spray additives
were tested.
Low rates of some herbicides in combination are
effective. More labeled uses of low-rate combinations are needed. Glyphosate mixes well with most herbicides, but its
activity may be slowed by photosynthetic inhibitors such as simazine
and diuron. Paraquat and diuron work well together on some weeds, but not on others;
the mixtures cannot be used in avocados until diuron
is registered. A simazine and norflurazon
mixture controls a large number of annual weeds, but the high cost of the
latter will reduce its usefulness. Some herbicide combinations are antagonistic
and should not be used.
New research with sulfonylureas
is of particular importance. They can be used at less than one ounce per acre.
Some have low potential for causing environmental problems. There are
sulfonylurea herbicides with soil and/or foliar activity. However, for best use
they will be added with or in rotation with other herbicides to provide broad
spectrum weed control and to prevent buildup of resistant weed populations. The
sulfonylureas provide the potential as replacement
for the long residual and leachable herbicides now
used. However, they will be developed for avocados only if the industry
aggressively seeks their registration. The avocado industry in
Research on application of herbicides through the
sprinkler systems (herbigation) shows promise for
controlling weeds in the wetted area around sprinklers. This research will be
performed with norflurazon, trifluralin,
oryzalin, and oxyfluorfen.
Both the Environmental Protection Agency and the California Department of Food
and Agriculture will monitor such uses very closely; and eventually, only
short-residual, nonleaching herbicides will be
allowed to be used through herbigation. These include
trifluralin, oryzalin, and oxyfluorfen.
The transition of avocado orchards out of nontillage with herbicides being studied to determine if
herbicide applications can be either skipped or stopped after soil residual
herbicide treatments have been made for several years. Within two to three
years after discontinuance of long-term soil residual herbicide treatments,
over 30 weed species were identified that reinfest
orchards. Early invaders include weed species that were established in the area
before herbicide treatment began. Those commonly observed orchards removed from
soil residual treatment included several spurges, chickweed, bermuda grass, malva,
pigweed, annual bluegrass, annual sowthistle, sedge,
oxalis, scarlet pimpernel, rescue grass, wild mustard, common groundsel,
Johnson grass, flax leaf Fleabone, and little lovegrass. The shift in the vegetation complex should be
monitored with different management practices. It is obvious that managed
vegetation will require greater and more intense management and greater overall
energy input than herbicide-based nontillage weed
control. Middle management with low rates of Roundup sprayed once or twice a
year and a wick application of Roundup to control escaped tall weeds appears to
be promising in areas accessible to the equipment. The success of such a
program will depend upon knowledge of weed species, herbicide selection, and
timing of treatments.
Formulation of sprays to reduce leaching of
herbicides is being investigated. Sodium polyacrylate
added to simazine spray before application acts as a
spray adjuvant/thickener and an adhesive to bind the herbicide to the soil. It
is not yet known how well the polymers will perform under diverse field
conditions in preventing ground water pollution while allowing adequate weed
control. It is also probable that the best polymer for each leachable
soil-applied herbicide will be different. The effect of long-term use of synthetic
polymers on the soil will have to be determined.
Practical Applications and Summary
The project will insure that economical, effective,
and environmentally safe weed control methods remain available to