Preliminary Study of Cotton Weed Control Strategies Using Over-The-Top Herbicides
William B. McCloskey, Plant Sciences Department
A study was conducted at the Maricopa Agricultural Center to compare various cotton weed control programs that combined the use of the over-the-top herbicides Buctril, Roundup Ultra and Staple with reduced or eliminated preemergence herbicide applications in dry planted cotton. Roundup Utra provide superior large seedling Palmer amaranth control compared to Buctril or Staple. The yield and end of the season weed control data showed that BXN-Buctril and Staple weed control strategies require the application of a dinitroaniline herbicide prior to planting. The data also suggest that a band application of Prowl in combination with timely postemergence Buctril or Staple applications would be just as effective as preplant incorporated application of Prowl. In contrast to the Buctril and Staple herbicide treatments, the end of the season weed control ratings and seed cotton yield were the same for all of the Roundup Ready cotton treatments suggesting that Roundup Ready cotton weed control strategies do not necessarily require a dinitroaniline herbicide (Prowl or Treflan) application prior to planting.
The development of Staple® herbicide and transgenic cotton varieties resistant to Buctril® (BXN® cotton) and Roundup Ultra® (Roundup Ready® cotton) provide cotton growers with new tools for cotton weed control. Staple and Buctril control broadleaf weeds while Roundup Ultra controls or suppresses a broad spectrum of weed species. Until recently, cotton weed control programs lacked selective postemergence herbicides that effectively controlled broadleaf weeds or purple and yellow nutsedge (Cyperus rotundus and Cyperus esculentus, respectively) in small cotton. Control of weeds in small cotton (i.e., the time interval between cotton seedling emergence and the one foot tall growth stage of cotton) is especially important because weed competition during this period can severely reduce yield. The development of Fusilade DX, Poast Plus, and Select 2EC provided cotton growers with selective, postemergence herbicides for the control of annual and perennial grasses in small cotton. Thus, cotton growers now have effective postemergence herbicides that control almost all of the weeds found in Arizona cotton fields. This array of postemergence herbicides make it possible to reduce or eliminate the use of preplant incorporated (i.e., preemergence herbicides) before or at planting. Thus, the objective of this study was to compare various cotton weed control programs that combined the use of over-the-top herbicides with reduced or eliminated preemergence herbicide applications.
Materials and Methods
Two field studies were conducted at the University of Arizona, Maricopa Agricultural Center (MAC) in 1997. The experimental design used for both studies was a randomized complete block design with six blocks in which individual plots were four, 40 inch rows by 70 feet long. The soil at both field study sites was a sandy clay loam that contained 1.06 % organic matter, 54 % sand, 22 % silt, and 24 % clay. Prowl (pendimethalin) preplant incorporated herbicide treatments (see data tables for rates and application methods) were applied to flat ground and incorporated on April 22, 1997 with a field cultivator consisting of three ranks of spring steel teeth that each had a small sweep on the end. The field was then listed and beds were formed. The Prowl band applications were made using a Dickey Machine Works Incorpovator. This implement sprayed a 20 inch band of Prowl on top of the beds and contained a pair of offset rolling cultivators for each row that immediately incorporated the herbicide. At the rear of the Incorpovator was a rubber-tired roller that flattened and firmed the soil on the bed top of each row following herbicide incorporation. Cotton seed, either BXN 47, DP5690RR, or DP5415, was planted in the dry plant field about 0.5 inches deep in dry soil on May 2, 1997 and irrigated on May 3rd to initiate germination. The wet plant experiment was initiated on April 16, 1997 when the preplant incorporated Prowl was applied and incorporated as in the dry plant experiment. The wet plant field was listed and pre-irrigated on April 17, 1997. The field was roto-mulched on April 30th, planted with cotton seed, either BXN 47, DP5690RR, or DP5415, on May 2, 1997 and the soil mulch cap removed on May 6, 1997. Unfortunately, the seed was not planted deep enough and emergence was very poor. The field was irrigated on May 13, 1997 to obtain a good stand of cotton. Since the treatment list for the wet plant experiment was very similar to the dry plant experiment, the need to irrigate in order to obtain an adequate plant population essentially made the experiments identical. Thus, only the results of the dry plant experiment are discussed further in this paper.
The dry plant field was cultivated on May 27, 1997 leaving an 8 inch uncultivated area in the center of the bed that was uncultivated. The postemergence herbicide treatments were applied on May 28, 1997 using a tractor mounted, PTO driven roller pump sprayer traveling at 4.4 mph. Two nozzles per crop row were used to apply the herbicides at the 4 true leaf stage of cotton. Buctril 4EC, Staple 85SP, and Roundup Ultra were applied in a 20 inch band using two TeeJet 80015EVS nozzles per crop row at 22 psi in a carrier volume of 22 gpa. No adjuvant was added to the Buctril or Roundup Ultra spray solutions but 0.5% X-77 (non-ionic surfactant) was added to the Staple spray solution. See data tables for herbicide treatment rates (lb a.i./A), cotton growth stage at the time of application, and for the dates weed control efficacy data was collected. Standard University of Arizona cotton production and insect control recommendations were followed to produce the cotton crop. The experiment was defoliated on September 19, 1997 by spraying 10 oz. of Ginstar per acre. A second application of Ginstar at 8 oz./A on October 16th was required to obtain complete defoliation. The center two rows of the four row plots were machine harvested on October 15, 1997. Analysis of variance and mean separation using Tukey's Honestly Significant Difference test were used to elucidate differences between treatments.
Results and Discussion
In the dry plant experiment, there were three different preplant herbicide treatments combined with each of the three varieties. When the first weed control data were collected on 5/27/97, treatments 5, 10, and 15 had not yet received any herbicide applications and the weed populations in these plots were a measure of the weed population densities at the study site. The major weeds early in the season were Palmer amaranth (Amaranthus Palmeri) and Wright groundcherry (Phylaris wrighti); there was very little annual morningglory (Ipomoea hederacea) at this site. The weed control ratings for Palmer amaranth, about 50% control, reflex the fact that the weed population densities were much higher in the wet plant experiment. Comparison of treatments 1, 10, 15 with all of the rest of the treatments shows that a Prowl application, either banded on the bed top or broadcast on flat ground and incorporated with a disk, provided superior control of Palmer amaranth compared to untreated plots. The same comparisons with respect to groundcherry control illustrated that Prowl applied broadcast and incorporated on the flat resulted in better groundcherry control than no herbicide application. The band application of Prowl provided an intermediate level of control that was not significantly different from the other two types of applications although the data suggest that banding on the bed top was inferior to the broadcast applications. The difference in weed control between the two species was probably due to the fact that Palmer amaranth is a weed species easily controlled by dinitroaniline herbicides such as Prowl and Treflan compared to Wright groundcherry, especially in alkaline soils such as the soil at the study site.
After the efficacy of the Prowl treatments was evaluated, the plots were cultivated leaving about an 8 inch uncultivated band in the center of the beds, and then the postemergence treatments of Buctril, Roundup Ultra, and Staple listed in Table 2a, Table 2b, and Table 2c were applied. Application of Buctril to large seedling pigweed (i.e., Palmer amaranth, 4 to 8 leaf, 4 inches tall) burned the leaves off of the seedlings but many shoots survived and regrew. Treatments 1 and 2 (Prowl PPI) had almost no pigweed at the time of the postemergence applications and thus control was not improved (Table 2a). Similarly, the DP5415 plots treated with Prowl PPI, treatments 11 and 12, had almost no pigweed when sprayed with Staple so that control was not improved (Table 2c). Pigweed control was not improved in treatments 13, 14, and 15 because Staple did not adequately control large seedling Palmer amaranth although the plants were stunted for a couple of weeks. Roundup Ready cotton plots treated with Prowl PPI (treatments 6 and 7, Table 2b) had no pigweed and control was not improved by spraying a postemergence herbicide. However, in contrast to Buctril treatments 3 to 5 and Staple treatments 13 to 15, 1 quart/A of Roundup Ultra was very effective in killing all of the pigweed in treatments 8, 9, and 10 substantially improving the percent control of Palmer amaranth in these treatments (Table 2b). Wright groundcherry was much more sensitive to all three herbicides than pigweed so that groundcherry remained essentially only in treatment 16 which did not receive an early postemergence herbicide spray.
The data indicate that the Buctril and Staple postemergence herbicide applications at the 4 leaf growth stage of cotton were made at a subotimal timing that resulted in poor or no control but Roundup Ultra was effective at the same timing on large seedlings of Palmer amaranth and Wright groundcherry. Previous studies have shown that Buctril and Staple herbicide applications in dry planted cotton need to be made at the 1 to 2 true leaf growth stage of cotton so that the weeds are also small (i.e., 1 to 2 true leaf). Previous studies have also shown that, if annual morningglory is present, Roundup applications must also be timed to treat 1 to 2 leaf morningglory which is freqently at about the time that cotton reaches the 1 to 2 true leaf stage.
The end of the season weed control ratings (Table 3a, Table 3b, and Table 3c) were very similar to the control ratings after the postemergence applications of Buctril, Roundup Ultra, and Staple (6/16/97) because most of the weeds present at the end of the season were Palmer amaranth plants that escaped cultivation, the postemergence herbicide sprays and were too large to be controlled by the layby herbicide applications. Comparing treatment pairs that did or did not received a layby application of Caparol (e.g., 1 versus 2, 3 versus 4, 6 versus 7, 8 versus 9, and etc.) indicated that there was little weed control or seed cotton yield benefit from the layby herbicide application (Table 3a, Table 3b, and Table 3c). This result is in agreement with previous work that suggests there is little benefit from making layby herbicide applications if annual morningglory is not present in the field and if a good stand of competitive cotton is present.
The seed cotton yield and end of the season weed control data suggest that BXN-Buctril and Staple weed control strategies require the application of a dinitroaniline herbicide prior to planting. For example, treatment 5 (without Prowl PPI) yielded significantly less and had poorer weed control than treatment 1 or 3 which did include Prowl PPI. (Table 3a). Similarly, treatment 15 (without Prowl PPI) yielded significantly less and had poorer weed control than treatment 11 which did include Prowl PPI. (Table 3c). The data also suggest that a band application of Prowl in combination with timely postemergence Buctril or Staple applications would be just as effective as preplant incorporated applications of Prowl. In contrast to the Buctril and Staple herbicide treatments, the end of the season weed control ratings and seed cotton yield were the same for all of the Roundup Ready cotton treatments (Table 3b) suggesting that Roundup Ready cotton weed control strategies do not necessarily require a dinitroaniline herbicide application prior to planting.
The author wishes to thank the Arizona Cotton Research and Protection Council for funding the MAC studies and for funding the demonstrations of over-the-top herbicide technology in farmer's fields at various locations in Arizona in 1997.
This is a part of publication
AZ1006: "Cotton: A College
of Agriculture Report," 1998, College of
Agriculture, The University of Arizona, Tucson, Arizona,
85721. Any products, services, or organizations that are
mentioned, shown, or indirectly implied in this
publication do not imply endorsement by The University of
Arizona. The University is an Equal
Opportunity/Affirmative Action Employer.