Josh Copes, Donnie Miller, Daniel Stephenson, Syam Dodla, Owen Clark and Melanie Netterville

In Louisiana, winter annual weeds can be difficult to control in spring prior to planting because of their advanced size and growth stage. Additionally, glyphosate-resistant Italian ryegrass is widespread in Louisiana and control options are limited. LSU AgCenter researchers have demonstrated the effectiveness of applying a soil residual herbicide in late October to early November to manage these winter weed populations. Winter cover crops are an investment in crop production systems and are implemented to address an identified soil health or production management concern.

Realizing the effectiveness of applying soil residual herbicides in controlling winter weeds and the benefits offered by a winter cover crop, research was conducted in 2018 and 2019 at the Northeast Research Station in St. Joseph, Louisiana, and in 2018 at the Dean Lee Research and Extension Center in Alexandria, Louisiana, evaluating the use of the herbicide Zidua (pyroxasulfone) in selected cover crops. Zidua is a very effective grass and small-seeded broadleaf soil residual herbicide used in soybean, corn and winter wheat. The study evaluated two rates of Zidua along with a nontreated comparison. It was applied in the fall either one or three weeks after emergence of drill-seeded cereal rye, crimson clover, tillage radish and Austrian winter pea. Crop injury and weed control ratings were collected approximately every two weeks after treatments were applied. Cover crop and weed biomass were collected from each plot prior to termination near the end of January each year.

Because the extent of cover crop injury varied between year and location, data is shown separately for each year in Table 1. The most sensitive cover crop to Zidua based on visual injury and regardless of rate or application timing was tillage radish. It was followed by crimson clover and cereal rye. Winter pea had the greatest tolerance to Zidua. Cover crop injury ratings averaged across application timing, cover crop and location for 2018 showed that injury was similar between the 2 and 4 ounces per acre rates of Zidua (Table 2). Regardless of cover crop or rate applied, injury was greatest when Zidua was applied at the earlier timing when averaged across location in 2018 (Table 2).

In 2018, henbit and annual bluegrass control were similar between the 2 and 4 ounces per acre rate of Zidua regardless of application timing at 28 days after treatment. Henbit was controlled 53.7% and 61.5%, and annual bluegrass was controlled 82.5% and 89.5% at the 2-ounce-per-acre rate and 4-ounce-per-acre rate, respectively. These winter weeds were controlled greatest when Zidua was applied one week after emergence regardless of rate applied. At 28 days after treatment, henbit and bluegrass control were similar across all cover crops.

When averaged across application timing and location in 2018, cover crop biomass was not affected by Zidua rate. In 2019, however, Zidua injury to tillage radish resulted in a reduction in biomass. In each year, cereal rye provided the greatest reduction in weed biomass followed by tillage radish. When averaged across application timing, cover crop and location, weed biomass in 2018 was greatest in the nontreated plots compared to the biomass observed with both the 2 and 4 ounces per acre Zidua rates.

The data indicate that Zidua applied in the fall to select cover crops at one or three weeks after emergence could be a viable practice to help control winter annual weeds. Although henbit control averaged only 58% across Zidua rates, the cover crop would have a competitive advantage to outgrow the injured and not optimally growing henbit. Most cover crops exhibited good to excellent tolerance to Zidua at both application timings. Cover crop injury was greater in 2019 than in 2018, with injury again greatest for tillage radish (Table 1). Tillage radish exhibited fluctuations in injury across locations and years. This was attributed to above average rainfall received, which enhanced injury in certain cases and lowered biomass produced. Therefore, Zidua should not be included in tillage radish cover crop systems. When compared to the nontreated, biomass of the cereal rye, crimson clover and winter pea were not affected by Zidua treatments. Applying Zidua in the fall in conjunction with a cover crop can provide effective winter annual weed management. Cover crops evaluated proved competitive with weeds. Addition of a soil residual herbicide with a cover crop should provide better weed management than either alone. Further research is needed to determine additional cover crop species’ tolerance to Zidua. Based on this research, tolerant cover crops should be drill-seeded into a clean seedbed and Zidua applied within one to three weeks after emergence.

Josh Copes is an assistant professor of agronomy, and Donnie Miller is a professor of weed science, both at the Northeast Research Station, St. Joseph, Louisiana. Daniel Stephenson is a professor of weed science and specialist at the Dean Lee Research and Extension Center, Alexandria, Louisiana. Syam Dodla is an associate professor of soil fertility and irrigation at the Red River Research Station, Bossier City, Louisiana. The research associates working with this project were Owen Clark and Melanie Netterville, both at the Northeast Station.

(This article appears in the fall 2020 issue of Louisiana Agriculture.)

The data indicate that Zidua applied in the fall to select cover crops at one or three weeks after emergence could be a viable practice to help control winter annual weeds. This field shows cover crop growth three weeks after emergence when Zidua was applied at 2 ounces per acre. Photo by Josh Copes