Evaluation of Milestone Herbicide for Early Post-emergence Weed Control in Citrus on the Yuma Mesa
David L. Kerns, Assistant Specialist, Entomology, Yuma Agricultural Center
Tony Tellez, Research Specialist, Yuma Agricultural Center
When applied early-postemergence, Milestone at 0.75 and 1.0 lbs-ai/A provided excellent control of
summer annual grasses and broadleaf weed for as long as 4 months, equivalent to Karmex. Princep and Krovar
also provided good weed control but appeared to offer slightly shorter residual against some broadleaf
species. Solicam, although it provided good weed control, particularly of the grasses, was the weakest
material evaluated in this trial. Although Karmex is notorious for causing crop injury on the sandy
soils of the Yuma Mesa, we did not detect any injury from this herbicide or any others in this trial.
Weed control in citrus grown in Arizona is an important consideration for every citrus grower.
Not only do weeds rob trees of water and nutrients, but many also harbor serious insect pests such as
citrus thrips and eye gnats. It is not a question as to whether weeds in citrus should be managed, but how.
Currently in Yuma most citrus growers control weed through mechanical cultivation and Round-up. Mechanical
cultivation can be an effective tool for weed control, but presents several problems. Frequent cultivation
is necessary to maintain weed densities at acceptable levels. Also, damaging the trees by striking the
trunk is of concern, particularly on small trees. Cultivation can also damage trees by destroying shallow
roots. Additionally, disking weeds into the soil provides a breeding ground eye gnats. Presently,
to avoid eye gnat problems, Yuma County citrus growers are offered a financial incentive not to disk
under weeds. Round-up is by far the most popular alternative to cultivation, however many growers are
concerned with chronic phytotoxicity problems from drift of Round-up on the lower leaves of the trees.
The only other alternatives to cultivation and Round-up for weed control in citrus are the pre-emergence
herbicides. Although several pre-emergence herbicides have proved effective in Yuma citrus, many growers
have chosen to avoid these materials. Most citrus in Yuma County is grown on extremely sandy soil, which
increases the probability of crop injury from herbicide. In the past, growers have reported crop injury
when using Karmex, Krovar or Princep. Concerns of crop injury with Karmex have resulted in this product
being made unavailable for use in Yuma County. Although other products such as Solicam and Surflan, have
not been associated with extensive crop injury in Yuma County, many growers choose not to use these products
due to the cost of the material, short residual on sand, or lack of fit due to weed spectrum. There is great
potential for a long lasting citrus herbicide that has a broad spectrum of activity on sand, and a minimal
risk of crop injury.
Milestone (azafenidin) is a new herbicide being developed by DuPont. Milestone is a unique chemistry
that acts by inhibiting the enzyme protoporphyrinogen oxidase which is involved in the biosynthesis of
chlorophyll. Milestone has primarily been tested pre-emergence, is used at low rates, and has proven to
offer 4 to 10 months control of a variety of annual broadleaf and grass weeds with little risk of crop
injury. The purpose of this study was to investigate the efficacy and crop safety of Milestone when
used early post-emergence on flood irrigated lemons grown on sandy soil on the Yuma Mesa.
Materials and Methods
This test was conducted on three-year old 'Limoneira 8A Lisbon' lemons on C. volkameriana rootstock grown
at the University of Arizona, Yuma Mesa Agricultural Center in Yuma, AZ. Artificially seeded and naturally
occurring weeds were evaluated. Artificially seed weeds included two annual broadleaf weeds: redroot
pigweed (AMARE) and common purslane (POROL), and several annual grasses: green foxtail (SETVI), green
sprangletop (LEFDU) and field sandburr (CCHIN). Naturally occurring weeds included broadleaf weeds:
hemp sesbania (SEBEX), horse purslane (TRTPO) and hyssop spurge (EPHHS), and sixweeks grama grass (BOBBA).
The test site was cultivated clean of standing weed 2 days before seeding with weeds. Artificially
seeded weeds were broadcast over the test area on 04/23/97 and lightly incorporated to 0.5 inches using
a rotor-tiller. Following seeding, this trial was irrigated every other week with about 3 inches of water
This trial was a randomized complete block design with 4 replicates. Treatments included an untreated
check, Milestone at 0.5, 0.75 and 1.0 lbs-ai/A, Karmex DF at 3.0 lbs-ai/A, Princep 80W at 3.00 lbs-ai/A and
Solicam at 4.00 lbs-ai/A. All treatments include Kinetic non-ionic surfactant at 0.25% v/v. On 05/05/97
when the herbicide applications were made the weed were 0.25-4.0 inches in height. Treatments were applied
using a 7 ft. wide CO2 powered hand-boom sprayer with flat-fan nozzles calibrated to deliver 20.00 gallons
per acre. Plots were 10 ft wide X 30 ft in length. Each plot was bordered on one side by a lemon tree and
on the other side by a 3 ft running check. Evaluations were made by rating the % weed control and % crop
phytotoxicity at 7, 14, 30, 60, 120, 240 and 360 days after treatment.
Results and Discussion
Seven days following application, Milestone at all three rates, Karmex and Krovar exhibited excellent
activity on all broadleaf and grass weeds present
(Table 1). Princep appeared to have slightly slower
activity on CCHIN, and Solicam was weaker across all weed species. Although many weeds in the Solicam
treated plots were alive, most showed signs of poisoning.
By 14 DAT, all products appeared equally effective towards the grasses, but Solicam appeared slightly
weaker towards the broadleaf weeds except AMARE ((Table 2).
Lower values in the Solicam treated plot may
be slightly misleading, because one plot appeared to be subject to washing out during irrigation.
Although weed control was evident in this plot, it was never equal to the other three Solicam-treated plots.
By 30 DAT, efficacy among herbicides was similar to the 14 DAT readings, all products except Solicam were
providing greater than 90% control across all weed species
(Table 3). At 60 DAT, all herbicides tested
were still providing excellent control of the grasses
(Table 4). Among the broadleaf weeds there was
much more variability in control than at previous evaluations. The Solicam plot that was subject to
wash, continued to lose soil and thus weed control in this plot continued to worsen. It was evident
the Milestone and Karmex were still providing slightly superior weed control than the other products
Late in the summer, weeds in the untreated plots and in the running checks and alleys had gotten too
tall and dense. To preserve the integrity of the health of the grove, weeds in the test area were mowed
to a height of approximately 4 inches on 08/23/97. Additionally, weed densities in the check area had
dropped considerably due to shading and crowding.
At 124 DAT, or 4 months after treatment, weed control in the Solicam-treated plots was beginning to slip,
especially among the naturally occurring weeds
((Table 5), probably because of the more extensive seed bank
and non-uniform germination relative to the artificially seeded weeds. Additionally, control of TRTPO began
to slip in the Krovar-treated plots, and Solicam, Krovar, Princep and the Milestone at 0.5 lbs-ai/A all
appeared slightly weaker on SEBEX. AMARE control with the low rate of Milestone also appeared to begin
Initially we had intended to collect data at 8 and 12 months after treatment, but the summer annual we
were evaluating early in the trial had cycled out by October. Although some winter annuals had emerged,
they were not plentiful and differences among plots were not detectable.
At no time in this trial did we detect any evidence of crop injury by any of the products tested,
although Karmex has a particularly bad reputation for crop injury on the Yuma Mesa. Overall, Milestone
at 0.75 or 1.0 lbs-ai/A and Karmex appeared to offer the best weed control for as long as 4 months.
Princep and Krovar also provided good weed control, but began to slip by 4 months. Although weed control
with Solicam was not bad, it appeared to be the weakest product evaluated.
This is a part of publication AZ1051: "1998 Citrus and Deciduous Fruit and Nut Research Report," College of
Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1051/az10518.html
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