The Use of Norflurazon (Zorial 5G)
Two experiments were conducted during 1997-98 to study the use of norflurazon (Zorial 5G) for purple nutsedge control in alfalfa. In experiment 1, Zorial 5G was applied in spring 1996 and 1997 at application rates of 1.0, 1.5, 2.0, and 3.0 lb a.i./A. Split applications were made each summer in four of eight treatments for total annual Zorial 5G rates of 2.0, 3.0, and 4.0 lb a.i./A-year. In the fall of 1997, each plot was divided into two subplots. One set of subplots was disced twice and replanted with alfalfa. Alfalfa planted into these subplots was not affected by the residual soil concentrations of norflurazon. In the other set of undisturbed subplots, the residual soil concentrations of norflurazon continued to provide substantial suppression of purple nutsedge in 1998. However, by August 1998, the 3 lb a.i./A-year rate only provided fair nutsedge suppression (51%), while the 4 lb a.i./A-year rate still provided satisfactory suppression (75%). At rates of 2 lb a.i./A-year or less, the residual effects of norflurazon declined to low levels in 1998 and did not keep purple nutsedge population levels from rebounding to pretreatment levels. In experiment 2, the efficacy of single spring (April 30) applications of Zorial 5G (norflurazon) at rates of 1.0, 1.5, 2.0, 2.5, and 3.0 lb a.i./A; of Treflan TR10 (trifluralin) at a rate of 2 lb a.i./A, and of Visor 5G (thiazopyr) at rates of 0.25 and 0.50 lb a.i./A were evaluated. As of August 4, 1998, purple nutsedge was the predominate weed species in this experiment. Zorial 5G applications at rates ranging from 1.5 to 3.0 lb a.i./A provided moderate (57-68%) purple nutsedge suppression. Rates above 1.5 lb a.i./A did not significantly improve purple nutsedge control suggesting that in new alfalfa stands, 1.5 lb a.i./A may be an appropriate rate for the first part of a split application. To date, single applications of Visor 5G at 0.25 and 0.50 lb a.i./A and Treflan TR-10 at 2 lb a.i./A have provided poor (17-33%) purple nutsedge suppression.
Purple nutsedge is difficult to control in established alfalfa stands. EPTC granules (Eptam 20G) applied just before irrigation can suppress purple nutsedge in established alfalfa when treatment is initiated prior to emergence of nutsedge shoots in the spring and if the field is retreated four to five times during the season (Tickes, 1990). Although nutsedge foliage in the hay was reduced by multiple Eptam applications, Tickes (1991) found that the number of nutsedge shoots per unit area continued to increase. Thus, Eptam treatments must be continued for more than one season and probably for the life of the stand to maintain suppression of purple nutsedge. The necessity for repeated EPTC treatments and the lack of long-term reductions in purple nutsedge (Cyperus rotundus) populations in alfalfa fields has prompted the investigation of alternative control methods.
Zorial 5G is a granular formulation of the preemergence herbicide norflurazon that Novartis Crop Protection, Inc. has recently labeled for use in alfalfa for the control of many annual broadleaves and grasses and for suppression of nutsedges. Small quantities were commercially available during 1998 and increased quantities are expected to be available in 1999. Norflurazon is absorbed by soil colloids, is not subject to leaching and has an average half life of 45-180 days in soil which provides weed control for 6 to 12 months at rates used in alfalfa. Susceptible weed seedlings must absorb norflurazon through the roots and then translocate the herbicide to shoots. Norflurazon inhibits the biosynthesis of carotenoid pigments which protect chlorophyll from photo-degradation. Thus in the presence of norflurazon, weed seedlings emerge with chlorotic leaves that turn white in sunlight due to the loss of chlorophyll. Susceptible weed seedlings die following the destruction of leaf tissue and exhaustion of food reserves. The current label restricts annual use rates to 2 lb active ingredient (a.i.) per acre (A) per year. Norflurazon may be applied to healthy stands of established alfalfa no earlier than five months following emergence and, because of its long soil residual, treated acreage may only be rotated to cotton, soybeans, peanuts, or asparagus for 16 months following the last application of Zorial 5G.
Knowles et. al. (1997, 1998) conducted field experiments to determine the optimum use and effectiveness of norflurazon for purple nutsedge (Cyperus rotundus) control in established alfalfa. Good nutsedge control (80-95%) was obtained in established alfalfa following a two year program during 1996-97 that involved split spring and summer applications of Zorial 5G made immediately after hay harvest and bale removal but prior to an irrigation. Annual Zorial 5G application rates of 1.5 to 2.0 lb a.i./A provided good nutsedge suppression under light to moderate infestations (30 to 50% cover). However, dense nutsedge infestations (65 to 100% cover) required annual Zorial 5G application rates of 3.0 to 4.0 lb a.i./A for 90% or better purple nutsedge control. Zorial Rapid 80 provided poor purple nutsedge control in established alfalfa because it was difficult to get the liquid spray past the foliage of dense stands of nutsedge and onto the soil where irrigation could incorporate and activate the herbicide. The research reported here was conducted to determine the optimum use of norflurazon in newly planted alfalfa fields and to determine the effect of residual norflurazon on the establishment of alfalfa in the fall.
Materials and Methods
A field experiment was established during 1996 in Parker Valley (located in southwestern La Paz County) to determine the effect of norflurazon herbicide formulations on actively growing purple nutsedge in two year old alfalfa. The experiment was conducted on a Gilman loam soil with 1.8, 44, 40, and 16% organic matter, sand, silt, and clay, respectively. The treatments and rates of norflurazon are listed in the data tables and consisted of treatments that included various annual amounts of norflurazon applied either in a single spring application or in two applications (i.e., split applications). In 1996, the first applications were on April 30, 1996 and the second set of applications were applied on August 7, 1996. In 1997, treatments were reapplied on March 6, 1997 and the split application treatments were applied on June 17, 1997. The granule formulation of the herbicide was applied with a Valmar small plot granule applicator (model 1255 PT). The treatments were replicated four times and arranged in a randomized complete block design. Individual plots were 20 ft wide by 60 feet long.
On October 31, 1997, each plot was split in half to create two 20 by 30 ft. subplots. One 30 ft long subplot was disced twice and replanted to ‘CUF 101' alfalfa as described by Knowles et al. (1998) to determine the residual effects of the 1996-97 Zorial 5G applications on alfalfa planted as a rotational crop 3-6 months after the herbicide applications. The other 30 foot long subplot remained intact to determine how long the residual norflurazon would suppress purple nutsedge in the year following the last herbicide applications. Winter weed control was evaluated in the newly planted subplots on March 5, 1998 (125 days after planting). Stand counts and visual phytotoxicity observations were taken on April 2, 1998 in the newly planted subplots. Nutsedge control ratings following each hay harvest were initiated on May 30, 1998 in the intact established subplots.
This experiment was initiated on April 30, 1998 to examine the optimum use of norflurazon (Zorial 5G), thiazopyr (Visor 5G), and trifluralin (Treflan TR10) for weed control in a new stand of alfalfa. The treatments are listed in Table 3 and included single applications of Zorial 5G and Visor 5G and split applications of Zorial 5G and Treflan TR10. The field was prepared and planted as described in a study of the effect of norflurazon on alfalfa stand establishment (Knowles et al., 1998). Grower collected seed from ‘CUF 101' alfalfa was planted on October 29, 1997 in a sandy loam soil similar to that described above. The herbicide treatments were applied on April 30, 1998 using a Valmar granule applicator (model 1255 PT) following a cutting and hay removal and were incorporated by flood irrigation on May 5,1998. There were no emerged purple nutsedge shoots or summer annual grasses present at the time of application. Weed control was evaluated following each hay cutting starting on May 27, 1998.
Results and Discussion
A study involving a two year Zorial 5G application program was initiated in the spring of 1996 in two year old ‘CUF 101' alfalfa that had about 61 to 74% purple nutsedge groundcover within the alfalfa stand (Knowles et al., 1997). Two annual Zorial granule applications during April and August of 1996, and March and June of 1997 at total rates of 3 to 4 lb a.i./A-year provided good purple nutsedge control in established alfalfa in 1997 (Knowles et al., 1997). The residual soil concentrations of norflurazon from these treatments continued to provide substantial suppression of purple nutsedge in 1998 (Table 1). However, by August 1998, the 3 lb a.i./A-year rate only provided fair nutsedge suppression (51%), while the 4 lb a.i./A-year rate still provided satisfactory suppression (75%). At rates of 2 lb a.i./A or less, the residual effects of norflurazon declined to low levels in 1998 and did not keep purple nutsedge population levels from rebounding to pretreatment levels. Shepardspurse (Casella bursa-pastoris), a winter annual species, was also significantly controlled by the residual concentrations of norflurazon with very good control at total annual rates of 2 lb a.i./A and greater (Table 2). No phytotoxic effects were observed in the established alfalfa at the application rates used in this study (data not shown).
Alfalfa planted into the existing treatment plots in fall 1998 following extensive field preparation was not affected by the residual soil concentrations of norflurazon. The ground in this experiment was ripped, disced twice, cultipacked, and land planed prior to planting. Stand counts conducted in April 1998 indicated that establishment was not significantly affected and the seedlings did not show residual norflurazon phytotoxic effects (Table 2). In contrast, alfalfa seedlings in the split plots with established alfalfa showed substantial norflurazon injury symptoms. These seedlings all died as a result of norflurazon injury and autotoxicity affects.
Since only the first application of the split
applications have been applied, the preliminary data from
this experiment showed the affects of single spring
applications of Zorial 5G (norflurazon) at rates of 1.0,
1.5, 2.0, 2.5, and 3.0 lb a.i./A; of Treflan TR10 at a
rate of 2 lb a.i./A, and of Visor 5G (thiazopyr) at rates
of 0.25 and 0.50 lb a.i./A. These initial herbicide
treatments were applied 5 months (22 weeks) after
planting before purple nutsedge and summer annual grasses
emerged. As of August 4, 1998, purple nutsedge was the
predominate weed in this experiment. Single spring Zorial
5G applications at rates ranging from 1.5 to 3.0 lb
a.i./A provided moderate (57-68%) purple nutsedge
suppression (Table 3). Previous experiments have shown
that adequate nutsedge control requires a second of
application of Zorial 5G in the summer (Knowles et al.,
1997). Rates above 1.5 lb a.i./A did not significantly
improve purple nutsedge control suggesting that in new
alfalfa stands, 1.5 lb a.i./A may be an appropriate rate
for the first of two split applications (Table 3). A
companion study showed that this rate of Zorial 5G as
well as rates up to 4 lb a.i./A applied 6.4 weeks after
planting did not affect stand establishment or the yield
of a new alfalfa stand (Knowles et al., 1998). To date,
single applications of Visor 5G at 0.25 and 0.50 lb
a.i./A and TR-10 at 2 lb a.i./A have provided poor (17 to
33%) purple nutsedge control. However, these treatments
were applied primarily to determine their effects on
summer annual grass weeds (e.g., sprangletop, junglerice,
and barnyardgrass). Summer annual grasses were only
beginning to emerge at the last purple nutsedge
evaluation date so the efficacy of the herbicides on
grasses has not yet been evaluated.
Under the extreme nutsedge pressure where 65 to 100% of the ground is covered by nutsedge such as in the field where experiment 1 was conducted, greater than labeled Zorial 5G rates (2 lb a.i./A-year) of 3 to 4 lb a.i./A-year of applied as a split application were required to provide satisfactory purple nutsedge control. Thus, a label change incorporating an increased annual application rate will be necessary for satisfactory purple nutsedge control in some fields. The data from these studies reinforces our previous conclusion that annual Zorial 5G application rates of 2.0 lb a.i./A-year may provide satisfactory nutsedge control in light to moderate infestations (30 to 50% nutsedge ground cover).
The current Zorial 5G label states:
Rotational Crop Restrictions
The replanting of alfalfa in the fall of a year in which split applications of 3.0 to 4.0 lb a.i./A-year were used appears to be possible in some soils such as the loam soil (44% sand) present in this study if the soil profile is substantially mixed prior to planting. However, preliminary data from a study conducted in a soil with about 65% sand at the University of Arizona, Maricopa Agricultural Center indicates that substantial seedling alfalfa injury can occur in a fall planting following high rate split Zorial 5G applications even with soil mixing. From a crop rotation perspective, the safest approach is to terminate Zorial 5G applications in the year in which alfalfa or other fall crops will be planted. Further research is necessary to determine if eliminating the summer portion of a split application (i.e., making only the spring application of a split treatment) will be sufficient to allow safe fall replantings of alfalfa. Such a strategy would probably still provide satisfactory purple nutsedge control in the last spring and summer of the alfalfa stand. It should also be noted that in the low desert regions of Arizona, summer alfalfa production usually requires two irrigations per cutting. In areas or situations where less irrigation is used or less rainfall is received, the potential for injury to rotational crops from residual norflurazon would be expected to be greater.
This is a part of publication
AZ1059: "1998 Forage and
Grain Agriculture Report," College of Agriculture,
The University of Arizona, Tucson, Arizona, 85721.