The influence of wheat variety on the dose-response of annual ryegrass to diclofop-methyl (POST) was examined in the field in 1992 and 1993 in southern New South Wales, Australia. The aim was to determine if planting a strongly competitive variety of wheat improved control of annual ryegrass at reduced doses of diclofop-methyl. Suppression of ryegrass was dependent on herbicide dose, season, and wheat variety. In the absence of herbicide, dry matter (DM) production of annual ryegrass at 300 plants m−2 at anthesis was 500 g ha−1 with Dollarbird and Katunga compared to 1000 g ha−1with Rosella or Shrike in 1992. In 1993, DM was approximately 150 g ha−1 with Dollarbird or Katunga, and 350 g ha−1with Shrike or Rosella. Ryegrass DM was reduced by diclofop-methyl to a greater extent, relative to the weedy unsprayed controls, with less competitive varieties Rosella and Shrike than with the more competitive Dollarbird or Katunga. Diclofop-methyl at 0.28 kg a.i. ha−1 reduced DM of ryegrass growing with Katunga to less than 100 g m−2 in 1992, compared to more than 200 g m2 with the other varieties. In 1993, diclofop-methyl was more effective on ryegrass, and the same dose reduced ryegrass DM to almost zero in all varieties. Grain yields in unsprayed weedy controls of Dollarbird and Katunga were reduced approximately 20% by annual ryegrass compared with yields achieved with herbicides in both years. Yields of Rosella and Shrike in the unsprayed controls were reduced about 40% in 1992 and 60% in 1993. Only small increases in grain yields of all varieties occurred from diclofop-methyl doses above 0.13 kg a.i. ha−1. Poorly competitive varieties were dependent on herbicides to achieve grain yield potential and had a greater risk of weed survival when herbicide efficacy was reduced. In contrast, strongly competitive varieties, likely to retard build-up of weed seed in the soil, are less dependent on herbicides to achieve grain yield potential, and therefore result in reduced weed control cost.

The Environmental Protection Agency (EPA) is required by law to assure that the use of pesticides does not cause unreasonable risks to humans or the environment when risks are compared with benefits. Weed scientists conduct hundreds of comparative efficacy tests each year, but the results are often of little use to the Agency in benefit assessments because the tests are unpublished or otherwise unavailable to the Agency, the tests are conducted in a manner unusable for regulatory purposes, or there are inconsistencies between tests conducted year to year or at different sites. Despite the lack of high quality data, the Agency is compelled to make the best regulatory decision possible with the information at hand, and it may appear to some that decisions are based more on policy than science. EPA is looking for experimental methods that will improve the quality of benefits data available to the Agency.

Although water is crucial to the performance of preemergence herbicides, pesticide performance has rarely been related to irrigation management. This 2-yr study investigated the effect that amount of irrigation water applied had on activity of simazine. Three rates of simazine at 0, 1.12, and 2.24 kg/ha were applied to a 3-yr-old nectarine orchard that was irrigated with microsprinklers. The performance of simazine was compared between irrigation treatments initially targeted to provide water at 110 (efficient) and 175% (overwatered) of crop water requirements. Simazine effectiveness was based on the survival of oat and cucumber plants that were seeded at 0, 14, 28, 56, and 84 d after herbicide application. A longer time interval to 50% survival indicated prolonged herbicidal activity. Results were consistent between years in that simazine's performance was consistently greater in efficient irrigation treatments. The greatest increases were measured at the higher simazine application rate (2.24 kg/ha); overall averages for the length of time to reach 50% survival for cucumber were 50 and 23 d and for oats were 55 and 15 d for efficient and overwatered irrigation treatments, respectively. Use of an efficient irrigation management technique could have enhanced simazine's performance through a decreased leaching of residues from the weed root zone or less chemical or biological degradation (or both). Adoption of efficient irrigation management has been identified as a best management practice to mitigate leaching of pesticide residues to groundwater in coarse soils in California. This study indicates that efficient irrigation improves simazine performance and that both factors, pesticide application and irrigation management, should be considered when developing a weed management system.

Herbicide applications often do not reach their full potential because only a small amount of the active ingredients reaches the intended targets. Selecting the appropriate application parameters and equipment can allow for improved efficacy. The objective of this research was to evaluate the effect of droplet size on efficacy of six commonly used herbicides. Atrazine (1.12 kg ai ha−1), cloransulam-methyl (0.18 g ai ha−1), dicamba (0.14 kg ae ha−1), glufosinate (0.59 kg ai ha−1), saflufenacil (12.48 g ai ha−1), and 2,4-D (0.20 kg ae ha−1) were applied to seven plant species using an XR11003 nozzle at 138, 276, and 414 kPa and a AI11003 nozzle at 207, 345, and 483 kPa. Each herbicide, nozzle, and pressure combination was evaluated for droplet size spectra. Treatments were applied at 131 L ha−1 to common lambsquarters, common sunflower, shattercane, soybean, tomato, velvetleaf, and volunteer corn. Control from 2,4-D was observed to increase approximately 12% on average for all species except common lambsquarters as droplet size increased from medium to very coarse (Dv0.5 303 to 462 μm; Dv0.5 is droplet size such that 50% of spray volume is contained in droplets of equal or smaller size). Control with atrazine was near 95% for common lambsquarters, common sunflower, and soybean. Atrazine provided the greatest shattercane control using a medium (Dv0.5 325 μm) droplet, whereas the same droplet size provided the lowest tomato control. Control of common lambsquarters, shattercane, and tomato with cloransulam-methyl increased 79% when decreasing droplet size from extremely coarse to fine (Dv0.5 637 to 228 μm). Dicamba control of common lambsquarters increased 17% using a medium droplet compared with a fine droplet (Dv0.5 279 to 204 μm). Dry weight of common sunflower and soybean was reduced 21% using dicamba when using a very coarse spray compared with a fine spray classification (Dv0.5 491 to 204 μm). Common lambsquarters control using glufosinate increased 18% using a fine spray classification (Dv0.5 186 μm) compared with medium (Dv0.5 250 μm) and both very coarse droplet sizes (Dv0.5 470 and 516 μm). Conversely, tomato and velvetleaf control with glufosinate was maximized using a very coarse (Dv0.5 470 and 516 μm) or extremely coarse droplet (Dv0.5 628 μm) with increases of 11 and 25% compared with a fine spray (Dv0.5 186 μm). Saflufenacil control of volunteer corn was 38% greater using extremely coarse droplets (Dv0.5 622 μm) than fine, medium, and very coarse spray classifications (Dv0.5 257 to 514 μm). Overall, spray classifications for the herbicides evaluated play an important role in herbicide efficacy and should be tailored to the herbicide being used and the targeted weed species.