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Agricultural Research & Education Center
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Bradenton AREC Research Report BRA1982-18 October, 1982
RELATIVE EFFICACY OF PARAQUAT
AND DIQUAT FOR WEED CONTROL1
J. P. Gilreath2
Use of paraquat as a directed spray to control emerged weeds in row
middles of plastic mulched crops is a common practice in Florida. Suggested
use rate ranges from 0.5 to 1.0 Ib. active ingredient (a.i.)/acre (1). Some
growers have found certain weed species, such as nightshade (Solanum spp.),
to be difficult to control at the suggested rate. In addition, paraquat is
not labeled for use in all crops. Discussions with industry representatives
about future use registrations have suggested that in the future more emphasis
may be placed on registration of diquat for directed spray applications in
many crops. This is based in part on the establishment of acceptable residue
levels for diquat in irrigation water and the potential for extrapolation of
some of this registration data to the registrant's petition for establishing
tolerance levels in food crops.
Research was conducted to determine the optimum use rate of paraquat
and diquat for control of four common weed species.
Materials and Methods
Treatments consisted of a weedy check and 0.25, 0.5 and 1.0 lb. a.i./acre
of both paraquat and diquat. Surfactant, X-77, was included with each
treatment at a rate of 8 oz/100 gal of spray preparation. Experimental plots
4.5 feet by 10 feet were established in a randomized complete block design
with 4 replications. Soil type was Myakka fine sand with a pH of 6.3. Treat-
ments were applied at 9 AM on November 18, 1981 with a CO2 back pack plot
sprayer equipped with two 11004 UniJet flat fan nozzles and operated at 27 psi
pressure and 3 mph for a total output of 26.6 gal/acre. The weather was clear
at the time of application with air and soil temperature of 660F, 50% relative
humidity and wind less than 2 mph. Soil moisture was at field capacity and 0.13
inches of rain was recorded three days after application. Weed species
evaluated and their size at time of application are listed in Table 1. Weed
control was evaluated December 1, 1981 using a visual 0 to 10 rating scale
where 0=no control and 10=100% complete control. Since the composition of the
populations of both grass and broadleaf weeds was equally divided between each
of fwo species, evaluations were made for grass control and broadleaf control
rather than for each individual species. Data were analyzed by analysis of
variance with treatment means ranked by Duncan's new multiple-range test, P=0.05.
1Mention of a specific herbicide does not constitute or imply a recommendation
or endorsement by the author or the University of Florida.
2Assistant Professor of Horticulture (Weed Science).
Results and Discussion
All of the herbicide treatments provided some degree of weed control
(Table 2). Grass control effected by 0.25, 0.5 and 1.0 lb. a.i./acre paraquat
was not significantly different from that provided by 1.0 lb. a.1./acre diquat,
but all of these were superior to the grass control obtained with 0.25 and
0.5 lb. a.i./acre diquat. There was no significant difference in grass control
between 0.25 lb. a.i./acre and 0.5 lb. a.i./acre diquat. Paraquat at 0.5 and
1.0 lb. a.i./acre provided significantly better broadleaf weed control than
0.25 lb. a.i./acre paraquat and 0.25 and 0.5 lb. a.i./acre diquat, but was
not significantly different for broadleaf weed control from the 1.0 lb. a.i./
acre rate of diquat. A definite rate response was exhibited by diquat for
broadleaf control. The 1.0 lb. a.i./acre rate of diquat gave significantly
better broadleaf control than the 0.5 lb. rate which, in turn, was superior
to 0.25 lb. a.i./acre. Paraquat applied at 0.25 lb. a.i./acre was equivalent
to 0.5 lb. a.i./acre diquat and was significantly better than 0.25 lb. a.i./
acre diquat for broadleaf weed control.
Based on these results, weed control will vary depending upon compound,
application rate and weed species to be controlled. In general, the recommended
use rates of 0.5 to 1.0 lb. a.i./acre for paraquat provide adequate grass and
broadleaf weed control, but comparable levels of control would require the
1.0 lb. a.i./acre rate of diquat.
1. William, R. D. 1976. Weed control guide for commercial vegetable production
in Florida. Fla. Coop. Ext. Ser. Circ. 196G.
Table 1. Weed species and size at time of treatment, fall 1981.
Category Species Common name Size
Grass Digitaria ciliaris Crabgrass 6 leaves
Eleusine indica Goosegrass 8 leaves
Broadleaf Amaranthus viridis Smooth pigweed 5 inches
Chenopodium album Lambsquarters 6 inches
Table 2. Relative efficacy of diquat and paraquat for weed control. Fall 1981.
Treatment Rate (b. a.i./acre) Control rating
Paraquaty 0.25 8. -p 6.1b
Paraquat 0.50 9.5a 9.5a
Paraquat 1.00 9.7a 9.9a
Diquaty 0.25 5.6b 3.5c
Diquat 0.50 5.5b 5.5b
Diquat 1.00 8.la 9.2a
Weedy check 0.5c O.Od
ZWeed control was visually evaluated December 1, 1981 on a 0 to 10 scale where
0=no control and 10=100% control.
YAll applications of paraquat and diquat were made with 8 oz of X-77 per 100 gal
of spray preparation.
XMeans within a column followed by the same letter are not significantly different
at the 5% level, as determined by Duncan's new multiple range test.