Palmer amaranth arrived about five years ago in western Nebraska and it has become more common in the region. Palmer pigweed interference has been shown to significantly reduce the yield of major row crops.
For example, interference from Palmer’s amaranth throughout the season reduced soybean yield by 68% at a density of 10 plants per row metre, corn yield by 91% at a density of eight plants per meter row and groundnut yield by 68% at a density of five plants per meter row (Burke et al. 2007, Klingaman and Oliver 1994, Massinga et al. 2001).
However, Palmer’s amaranth interference has not been studied previously in dry beans. To provide more information to dry bean growers on the potential impact of uncontrolled Palmer pigweed, studies were conducted in 2020 and 2021 at the Panhandle Research and Extension Center, Scottsbluff, Neb., to quantify the loss of yield and potential seed production of Palmer amaranth in competition. with dried beans throughout the season.
Palmer amaranth densities assessed were: 0, 0.2, 0.3, 0.5, one and two plants per meter row (Table 1). These studies were conducted on erect pinto beans with a population of 85,000 plants per acre and a row spacing of 22 inches. Emerging Palmer amaranth plants were tagged with zip ties to identify which plants should be kept to maintain the correct density, and all other weeds were removed several times a week by hand weeding. At the end of the season, dry bean yield, yield components and seed production were measured.
Palmer’s pigweed was able to reduce dry bean yield by 77% at the highest density tested (two plants per meter row) (Figure 1). A 5% yield loss, a level where a dry bean grower will begin to notice lower yields, has been estimated at a single Palmer amaranth plant per 50 meters or four Palmer amaranth plants per 1,000 feet. .2. Dry bean plants are more susceptible to Palmer amaranth interference than corn, soybeans, or peanuts because higher densities of Palmer amaranth were required to significantly reduce the yield of corn, soybeans, and peanuts. groundnut compared to dry beans.
Yield components of dry beans were negatively affected by Palmer pigweed interference throughout the season (Figure 2). The number of pods per plant was reduced by 69% when dry bean plants competed with Palmer’s pigweed at a density of two plants per meter row. The number of seeds per pod was reduced by 16% as the density of Palmer amaranth increased to two plants per meter of row. Hundred-seed weight was reduced by 18% as Palmer amaranth density increased to two plants per meter row.
The reduction in dry bean yield was attributed to a reduction in the number of pods per plant as palmer pigweed density increased.
The highest Palmer amaranth seed production per plant was found at the lowest assessed density – 0.2 plants per meter row – and this seed production began to decline as the amaranth density increased. Palmer was increasing (Figure 3). Regardless of the density level of Palmer amaranth, seed production was exceedingly high – a single female Palmer amaranth plant was capable of producing 91,000 to 376,000 seeds.
If only 1% of Palmer amaranth seeds germinate, the next growing season approximately 910 to 3,760 plants will emerge for each uncontrolled Palmer amaranth plant.
Implications of weed management
Since there are limited POST herbicide options in dry bean in Nebraska, effective weed management approaches must be applied to manage Palmer pigweed and avoid yield loss and seed set in floor. Outlook’s PRE fb POST sequential applications have proven effective in controlling Palmer’s Pigweed, which also allows growers to switch to corn the following year. Rotation with corn is key to effective weed management, as taller crops are more competitive against Palmer pigweed and there are more herbicide options to control Palmer pigweed biotypes resistant to herbicides (Mahoney et al. 2021). A “zero tolerance” approach to weed management is encouraged, as a single Palmer amaranth plant can significantly increase seeds in the soil seed bank.
Planning is underway for the 2022 research season. New herbicides for use in dry beans, including pre-harvest emergence, post-harvest emergence, and holding options will be tested, along with on-farm control trials of Palmer amaranth. If you are a grower interested in conducting on-farm weed control trials in dry beans, contact Nevin Lawrence for more information.