Alan G. Dexter, John L. Luecke and Richard K. Zollinger
Extension Sugarbeet Specialist, Sugarbeet Research Specialist, North
Dakota
State University and the University of Minnesota, and Extension
Weed Control Specialist, North Dakota State University, Fargo.
Poast first received a federal label for grass herbicide registered for sugarbeet until 1995. In 1996, other postemergence grass herbicide registered for sugarbeet. Prism received a federal label late in the 1995 growing season and Poast Plus received a 24c Special Local Need label for use on sugarbeet only in North Dakota in 1995. Poast Plus will continue to be labeled in North Dakota in 1996 unless EPA revokes the registration. Assure II may receive a label in time for the 1996 growing season but do not plan on using Assure II until a label has been approved. Ultima 160 along with Poast will be available for sugarbeet in the 1996 season. Ultima 160, Poast and Poast Plus all have sethoxydim as the active ingredient but Ultima 160 has 1.3 lb/gal of sethoxydim while Poast has 1.5 lb/gal and Poast Plus had 1.0 lb/gal. For the first time, sugarbeet growers will be able to make choices in 1996 regarding what to use for postemergence grass control. The objective of this paper is to present selected data collected over the past few years which illustrates similarities and differences among grass control herbicides.
An experiment or two have been conducted each year since the early 1980s where grasses are seeded with a grain drill and the grass control herbicides are applied across the strips of grass with a bicycle wheel type sprayer delivering 8.5 gpa through 8001 nozzles at 40 psi. The grasses are intentionally treated when they are large to enhance separation among treatments. Grasses are typically 4 to 10 inches tall when they are treated. Natural infestations of green foxtail, yellow foxtail and wild oats also are treated and evaluated when present.
In 1995, barley, oats, white proso millet, Siberian foxtail millet and yellow foxtail were evaluated. In 1994, barley, oats, wild oats, yellow foxtail and green foxtail were evaluated.
Poast, Ultima 160, Assure II and Prism all gave excellent grass control when applied with Scoil to 6 to 7-inch tall barley, oats, white proso millet and Siberian foxtail millet and 2 to 5-inch tall yellow foxtail (Table 1). The addition of Betamix, Betanex, Betamix Progress or UpBeet to the grass herbicides resulted in reduced control of one or more species of grass. Prism generally was antagonized less by the broadleaf herbicides than Poast, Ultima 160 or Assure. Stinger had no effect on grass control from Poast in other experiments (data not shown). Poast and Ultima 160 were antagonized by broadleaf herbicides more on barley and oats than on yellow foxtail and millets while Assure II was antagonized more on yellow foxtail and millets than on barley and oats. Poast was antagonized similarly by Betamix or UpBeet + Betamix so the efficacy of UpBeet on grasses did not help overcome the observed antagonism. However, UpBeet alone antagonized Assure II less than UpBeet + Betamix. Antagonism was nearly eliminated by application of the broadleaf herbicide first, skipping three days, and then applying the grass herbicide. Results from other experiments (data not shown) demonstrated that only one day delay between grass herbicide and broadleaf herbicide will eliminate antagonism if the grass herbicide is applied first.
Grass herbicides plus an oil adjuvant, like Scoil, will provide grass control superior to the grass herbicides alone or grass herbicides plus a surfactant (data not shown). Also, Betamix + oil often causes more sugarbeet injury than Betamix alone so eliminating the oil in combination would be desirable to reduce the risk of sugarbeet injury. In this experiment (Table 1) Poast + Betamix and Ultima 160 + Betamix gave less grass control than Poast + Betamix + Scoil or Ultima 160 + Betamix + Scoil. Increasing the Poast or Ultima 160 rate by 50% without Scoil gave grass control similar to Poast or Ultima 160 + Betamix + Scoil at the lower rate. So, increasing the grass herbicide rate by 50% will substitute for the oil in combination with Betamix but the level of control will still be less than the grass herbicide plus oil with no broadleaf herbicide. Grass control from Ultima 160 + Betamix Progress was better than from Ultima 160 + Betamix when Scoil was not included but control was similar when Scoil was included.
The first half of the split treatments were applied to 4 to 9-inch tall wild oats, barley and oats and to 0.5 to 4-inch tall green foxtail and yellow foxtail (Table 2). The second half of the split treatments and the single treatments were applied five days later to 6 to 10-inch tall wild oats, barley and oats and to 2 to 5-inch tall green foxtail and yellow foxtail. The average level of grass control was generally less in 1994 (Table 2) than in 1995 (Table 1) indicating that environment has an impact on grass control from these herbicides. Poast + Dash HC, Prism + Sun-It II and Assure II + Scoil gave similar control of grasses in 1994. Betamix + UpBeet antagonized grass control from all three grass herbicides with a single application. Two applications of Poast at 1 pt/A + Betamix + UpBeet (no Scoil) gave similar control of foxtail spp. but less control of the large-seeded grasses compared to one application of Poast at 1 pt/A + Dash but two applications of Poast at 1 p/A + Betamix + UpBeet + Scoil gave grass control as good as one application of Poast + Dash HC.
Antagonism of grass control from Prism by Betamix + UpBeet was not overcome by doubling the Prism rate in 1994 (Table 2) when no oil was added to the combination. A half-rate of Prism applied twice in combination with Betamix + UpBeet + Scoil gave less foxtail spp. control and similar large-seeded grass control compared to Prism + Sun-it 11 applied once at a full rate. Assure IIat 0.5 pt/A + Betamix + UpBeet applied twice gave less control of foxtail spp. but similar control of large-seeded grasses compared to one application of Assure II at 0.5 pt/A + Scoil. The oil additive had little effect on grass control from Assure II + Betamix + UpBeet, unlike Prism and Poast. The results in Table 2 suggest that antagonism of grass control from Poast by Betamix + UpBeet can be overcome by two applications of Poast + Betamix + UpBeet + oil additive with a full rate of Poast each time. Two applications of Prism at a half-rate did not totally overcome antagonism so perhaps a full rate twice may also be needed with Prism. Antagonism of control of large-seeded grasses was overcome by two full-rate applications of Assure II + Betamix + UpBeet with or without oil but this did not overcome the antagonism of control of the foxtail spp.
Treatments were applied to 6 to 9-inch tall oats and 3 to 6-inch tall foxtail millet in 1992. The level of grass control was greater and the amount of antagonism by Betanex was less in 1992 (Table 3) than in 1994 (Table 2) or 1995 (Table 1). In general, control will be greater and antagonism will be less when soil moisture is good and the grasses are actively growing. Also, small gasses are easier to control than large grasses. In 1992 (Table 3), Poast + Scoil, Prism + Scoil and Assure II + Scoil gave total control of oats and foxtail millet. Poast and Betamix and Assure II + Betamix with no Scoil gave less control of oats and foxtail millet than Poast + Scoil and Assure II + Scoil but foxtail millet control was still quite good, 90 to 93%, even with the broadleaf herbicide and without the oil. Prism + Betanex without oil gave 98 to 100% control of oats and foxtail millet. The results in 1992 suggest that a combination of a grass herbicide plus a broadleaf herbicide with no oil additive can give good grass control when the environment favors good herbicide efficacy. Good soil moisture, small grasses and target grasses that are very susceptible to the selected grass herbicide all favor good herbicide efficacy. Prism was antagonized less than Assure II or Poast by broadleaf herbicides in 1992 and 1995 suggesting that Prism may be a good choice for mixing with a broadleaf herbicide. However, all grass herbicides including Prism were antagonized severely in 1994.
All treatments were applied with an oil adjuvant, AT plus 411F at 1 qt/A. The single treatments and the first. half of the split treatments were applied May 27, 1983 to a native population of 3 to 9-inch tall quackgrass and the second half of the split treatments were applied 3 weeks later to 12 to 18-inch tall quackgrass (Table 4). The Assure used in this experiment was the old formulation of Assure but the rates have been adjusted to Assure II equivalent rates. Quackgrass control from split application generally was better than from a single application of Poast or Assure. Assure generally gave better quackgrass control than Poast.
Grass herbicides plus an oil additive provided more consistent grass control than grass herbicides plus a broadleaf herbicide plus an oil additive. Leaving the oil additive out of the combination will reduce the risk of sugarbeet injury from the broadleaf herbicide but also may further reduce grass control. Prism was antagonized less by broadleaf herbicides in 1992 and 1995 than Poast or Assure II, so Prism may be a good choice for mixing with broadleaf herbicides. However, Prism was antagonized severely in 1994. A grass herbicide plus a broadleaf herbicide without oil may give adequate grass control if good soil moisture, actively growing grasses, small grass size and highly susceptible grass species favor good efficacy from the grass herbicides. Even Betamix or Betamix Progress alone will give good control of small foxtail spp. in some environments. A grass herbicide at a full rate plus a broadleaf herbicide plus an oil additive applied twice generally gave grass control equal to a grass herbicide applied once at a full rate plus an oil additive even in conditions unfavorable for good efficacy from the grass herbicides. Increasing the rate of grass herbicide by 50% did not always overcome the antagonism from the broadleaf herbicides with a single application.
1995 Sugarbeet Research and Extension Reports. Volume 26, pages 83-87.
