Research was conducted in North Carolina from 2007 through 2010 at the Central Crops Research Station located near Clayton and the Upper Coastal Plain Research Station located near Rocky Mount. Soils were a Johns sandy loam (fine-loamy over sandy, siliceous, semiactive, thermic Aquic Hapludults) at Clayton and a Goldsboro fine sandy loam (fine-loamy, siliceous, subactive, thermic Aquic Paleudults) at Rocky Mount. Efficacy of graminicides was evaluated in separate experiments in non-crop areas. Plot size was 2.4 by 5.0 m.

Treatments included all possible combinations of clethodim (Select 2EC herbicide, Valent U.S.A. Corporation, Walnut Creek, CA 94596) or sethoxydim (Poast herbicide, BASF Corporation Agricultural Products, Research Triangle Park, NC 27709) with no chloroacetamide herbicide, dimethenamid-P (Outlook, BASF Corporation, Research Triangle Park, NC 27709); or S-metolachlor (Dual Magnum, Syngenta Crop Protection, Inc., Greensboro, NC 27419); with or without 2,4-DB (Butyrac 200, Albaugh Inc., Ankeny, IA 50021); and with or without pyraclostrobin (Headline, BASF Corporation, Research Triangle Park, NC 27709). Clethodim, dimethenamid-P, pyraclostrobin, sethoxydim, S-metolachlor, and 2,4-DB were applied at 0.14, 0.84, 0.17, 0.21, 1.1, and 0.28 kg ai ha⁻¹, respectively. Crop oil concentrate (Agri-Dex nonionic spray adjuvant, Helena Chemical Company, Collierville, TN 38137) at 1.0% (v/v) was included in all treatments. Treatments were applied in 140 L ha⁻¹ aqueous solution from a municipal water source using CO₂-pressurized backpack sprayer with flat-fan nozzles (TeeJet TP8002 flat-fan spray nozzles, Spraying Systems Co., Wheaton, IL 60189) at 275 kPa.

Large crabgrass control was evaluated during 2007, 2008, and 2009 while goosegrass control was evaluated during 2009 and 2010. Goosegrass and large crabgrass were 15 to 20 cm in height at the time of application. Visual estimates of percent weed control were recorded 21 d after treatment using a scale of 0 to 100% where 0  =  no weed control and 100  =  complete weed control (Frans et al., 1986). Foliar chlorosis, necrosis, and plant stunting were considered when making the visual estimates.

Laboratory experiments were conducted to compare physicochemical compatibility of the graminicide combinations at rates and spray volumes used in field experiments. Deionized water at pH 6.3 was used in the laboratory experiments. Agrochemicals were mixed in the following order: emulsifiable concentrates (clethodim, pyraclostrobin, sethoxydim, S-metolachlor, and 2,4-DB), and soluble liquids (crop oil concentrate and dimethenamid-P). Graminicides solutions were prepared in a final volume of 80 ml in sterilized plastic specimen cups (Specimen cup120ml-53 ST ORG CAP, Fisher Scientific, NJ 07410) of 120 ml capacity. After mixing different agrochemicals, solution was vortexed (Vortex Genie 2, Fisher Scientific, Fairlawn, NJ 07410) immediately and examined for precipitates followed by determining pH using a portable pH meter (Oakton portable pH meter, Fisher Scientific, Fairlawn, NJ 07410). Solutions were allowed to settle for 6 h after the time of mixing, examined for precipitates, vortexed, and re-examined for precipitates followed by pH determination. Similarly, mixtures were allowed to settle for 24 h after the initial solution preparation using the same procedure. Presence or absence of precipitates were determined visually and described as Yes or No, respectively. Any visual depositions on the bottom of the specimen cup or the presence of suspended solids in the solution were considered as precipitates. Two types of precipitates, temporary and permanent, were noticed when herbicides were mixed with other agrochemicals. Temporary precipitates went back into solution upon vortexing while permanent precipitates did not go into solution after vortexing.

The experimental design in the field was a randomized complete block and treatments were replicated four times. In the laboratory, the experimental design was a completely randomized design and the experiment was repeated with two replicates in each experiment. Data for visual estimates of percent grass control and solution pH were subjected to ANOVA using the PROC MIXED procedure of SAS (Statistical Analysis Systems version 9.2, SAS Institute Inc., Cary, NC 27513) with year and sampling intervals as random variables for field and laboratory studies, respectively. Data for visual estimates of percent weed control were transformed to the arcsine square root prior to analysis. However, non-transformed means are presented because transformation did not affect data interpretation. Means of significant main effects and interactions were separated using Tukey's pair-wise comparison tests at P  =  0.05.

Interactions of graminicide by 2,4-DB by pyraclostrobin and graminicide by chloroacetamide herbicide by pyraclostrobin were significant for goosegrass control. Large crabgrass control was affected by the interaction of graminicide by pyraclostrobin by 2,4-DB. Significant interaction of chloroacetamide herbicide by 2,4-DB was noted for both goosegrass and large crabgrass.

When pooled over years and chloroacetamide herbicide treatments, goosegrass control with clethodim was greater than control with sethoxydim when pyraclostrobin or 2,4-DB was not included in the mixture (Table 1). Clethodim plus pyraclostrobin controlled goosegrass less than clethodim alone when 2,4-DB was not included in the mixture. Regardless of pyraclostrobin, control by clethodim applied with 2,4-DB was similar to that by clethodim alone. Goosegrass control with sethoxydim was not negatively affected by pyraclostrobin but only when applied with 2,4-DB; in absence of 2,4-DB, pyraclostrobin did not affect goosegrass control with sethoxydim (Table 1).

When pooled over years and 2,4-DB treatment, clethodim plus pyraclostrobin controlled goosegrass less than clethodim alone when dimethenamid-P or S-metolachlor was not included in this mixture (Table 2). When compared with clethodim alone, chloroacetamide herbicides did not affect goosegrass control irrespective of pyraclostrobin treatment. In the absence of chloroacetamide herbicides, control of goosegrass with sethoxydim plus pyraclostrobin was similar to sethoxydim alone. When dimethenamid-P or S-metolachlor was applied with sethoxydim, goosegrass control was similar regardless of pyraclostrobin treatment when comparing within a chloroacetamide herbicide or with sethoxydim alone.

Combinations of graminicides with dimethenamid-P or S-metolachlor controlled goosegrass more effectively than graminicides alone irrespective of 2,4-DB (Table 3). On the other hand, addition of 2,4-DB to graminicides plus S-metolachlor provided similar control of large crabgrass as compared to graminicides alone. However, the control of large crabgrass with graminicides plus chloroacetamide herbicides was greater than graminicides alone.

Results for large crabgrass control by clethodim and sethoxydim were surprising in that pyraclostrobin reduced control by clethodim but not sethoxydim when 2,4-DB was not included (Table 4). Control of large crabgrass was greater with clethodim than with sethoxydim. In contrast, applying 2,4-DB with graminicides alone or in mixture with pyraclostrobin resulted in similar control.

Previous research indicates that clethodim was more effective in controlling broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash] and large crabgrass than sethoxydim (Jordan et al., 1993; Lancaster et al., 2005a). As noted for goosegrass control by clethodim, co-application of clethodim with pyraclostrobin reduced large crabgrass control compared with clethodim alone. Lancaster et al. (2005a) reported that large crabgrass control was reduced in three of four experiments when sethoxydim was applied with pyraclostrobin.

Results suggesting that chloroacetamide herbicides can mitigate adverse interactions of graminicides with other pesticides or increase control over the graminicide alone were surprising. Data in the peer-reviewed literature are limited with respect to defining interactions of graminicides and chloroacetamide herbicides. The inconsistent impact of 2,4-DB on interactions of graminicides with pyraclostrobin also was surprising. In previous research (Chahal et al., 2012d), pyraclostrobin and 2,4-DB influenced efficacy of graminicides independently.

Although interactions were noted in these experiments among pesticide combinations, the magnitude of negative impact did not exceed 19% when comparing within a graminicide (Tables 1, 2, and 4). Also, the improved control in presence of chloroacetamide herbicides did not exceed 14% (Table 3). The biological significance of these interactions is difficult to quantify. None-the-less, these data do indicate that in some instances control by clethodim and sethoxydim can be negatively or positively impacted by pesticide combinations included in this study but at a relatively low level.

The interaction of graminicide by chloroacetamide herbicide by 2,4-DB by pyraclostrobin was significant for solution pH determined for graminicide combinations. Solution with clethodim or sethoxydim alone had pH 4.24 or 4.11, respectively, over the sampling intervals (Table 5). In the absence of 2,4-DB and pyraclostrobin, addition of dimethenamid-P and S-metolachlor to clethodim or sethoxydim solutions slightly decreased solution pH compared with solution containing clethodim or sethoxydim alone. Regardless of chloroacetamide herbicides, fungicide, and graminicides, 2,4-DB added to graminicide solution increased pH greater than 6.0. Regardless of graminicides, fungicide, and 2,4-DB, solutions containing dimethenamid-P had pH lower than solutions with S-metolachlor (Table 5).

In a few instances, solution with graminicides combinations formed temporary precipitates at 0, 6, and 24 h sampling times (Table 6). For instance, a combination of clethodim plus crop oil concentrate plus pyraclostrobin with or without S-metolachlor formed temporary precipitates across sampling times but solution was reestablished after vortexing. In contrast, combination of sethoxydim plus crop oil concentrate plus pyraclostrobin did not form any precipitates regardless of sampling time.

The application of results from the pH component of this research is unknown. Although pH can impact herbicide efficacy, the impact of mixtures was relatively minor, either positively or negatively on graminicide performance. Results from the precipitate observations suggest that these pesticides can be mixed with minimal concern of permanent precipitates forming which could cause major issues with application and sprayer cleanout.

Previous research (Chahal et al., 2012a 2012b 2012c; Lancaster et al., 2005a 2005b) with three or more components in tank mixtures often used in peanut have been evaluated but did not include chloroacetamide herbicides. Results from the current research suggest that the chloroacetamide herbicides dimethenamid-P and S-metolachlor will not adversely affect control by graminicides or the previously defined interactions of graminicides with 2,4-DB or pyraclostrobin. These experiments demonstrated that pyraclostrobin consistently reduced goosegrass and large crabgrass control by clethodim. When applied alone, clethodim controlled goosegrass and large crabgrass more effectively than sethoxydim. Lack of a consistent reduction of sethoxydim activity by pyraclostrobin may have been a reflection of relatively poor control by sethoxydim regardless of co-applied pesticides. Of the herbicides used in these experiments, 2,4-DB had the least effect on graminicides. Results of physicochemical compatibility indicated that addition of clethodim, sethoxydim, dimethenamid-P, S-metolachlor and 2,4-DB changed the carrier pH dramatically. Although several clethodim and sethoxydim combinations with chloroacetamide herbicides, 2,4-DB, and fungicide formed temporary precipitates at different time intervals, pesticides and crop oil concentrate could be brought back into solution after agitation. The impact of changes in pH on efficacy was not determined in this experiment. Future research measuring efficacy and pH affects with solutions used in the field would be more informative than results reported here. Lack of permanent precipitate formation from these mixtures is important in eliminating one possible negative impact of co-application of these pesticide combinations.