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Burning Soybean Harvest Residue in Narrow Windrows to Kill Weed Seed

In a White Paper (updated Mar. 2021) posted on this website, guidelines for burning crop residues to destroy weed seeds following harvest as part of a Harvest Weed Seed Control (HWSC) strategy are presented. Results from research cited in that paper provide evidence that burning crop residues that are concentrated into narrow windrows by a specially designed chute at the rear of the combine can be an effective method for destroying weed seeds that are dispensed from the combine during crop harvest.

In a recent publication titled “Seed destruction of weeds in southern US crops using heat and narrow-windrow burning” [Weed Tech. 34:589-596 (https://doi.org/10.1017/wet.2020.36)], Dr. Jason Norsworthy and colleagues present results from research that was conducted in Arkansas to determine the temperature and duration needed to kill the seed of Palmer amaranth, barnyardgrass, johnsongrass, pitted morningglory, hemp sesbania, prickly sida, sicklepod, velvetleaf, and Italian ryegrass. These small- and large-seeded grass and broadleaf weeds frequently occur in southern US soybean fields. Procedures used in and results from the research cited in the publication follow.

•    The basis for conducting this research was 1) weeds that escape chemical control will continue to grow and produce seed , 2) seed that are retained by these weed escapes will enter the combine during harvest and will be redistributed in the field to replenish the soil weed seedbank, 3) the efficacy of narrow-windrow burning against weed seeds in soybean residue requires that seed of common problem weed species be evaluated for their response to the burning practice, and 4) the low cost of using this strategy makes it a viable HWSC option.

•    Viability of seed of targeted weed species was determined prior to placing into a high-fire kiln where these seed were subjected to 20 combinations of temperature (200, 300, 400, 500, and 600 deg. C; to convert deg. C to deg. F, multiply deg. C by 1.8 and add 32 to the answer) durations (20, 40, 60, and 80 seconds). Number of viable seed was determined after each treatment.

•    Heat index (HI) was calculated by summing the temperature above ambient (23.9 deg. C in these experiments) for each second duration of heat exposure. For example, a 400 deg. C temperature for 60 seconds would result in an HI of 22,566 [400 x 60 = 24,000 - (23.9 x 60 = 1,434) = 22,566] . Effective burn time (EBT) is defined as the number of seconds that a burn was above a specified temperature. For example, EBT 300 is the designation for the number of seconds that a burn was above 300 deg. C.

•    A field experiment was conducted in a production field of irrigated soybean to assess the efficacy of burning on the viability of seed of Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory. Viability of seed of these weed species was determined prior to their placement on the soil surface beneath the residue windrow.

•    Results from these experiments follow. 1) An HI of 22,600 was needed to kill all seeds of Palmer amaranth, barnyardgrass, and Italian ryegrass. 2) Lengthening the burn time–e.g. exposure time of weed seed to burning–reduced the temperature needed to achieve weed seed mortality. 3) Regardless of weed species and temperature duration, no seed kill was completely achieved at 200 deb. C. 4) Seed size had some impact on the mortality of seed–e.g., small-seeded species such as Palmer amaranth and barnyardgrass showed complete mortality at lower temperature/duration combinations than did seed of large-seeded species such as pitted morningglory and sicklepod. 5) As temperature increased for each species tested, viability decreased, thus showing that weed seed mortality can be achieved by heating. 6) The amount of soybean residue at the time of burning had an effect on results–i.e., the greater the amount of residue, the greater the weed seed kill. It is highly likely that residue resulting from an irrigated soybean crop will be greater than that resulting from a nonirrigated (NI) crop. Thus, the utility of burning residue to kill weed seed contained in residue from a harvested NI soybean crop is not known. 7) Wind speed had an effect on both HI and EBT–i.e., as wind speed increased, both HI and EBT decreased rapidly. 8) Regardless of the achieved HI and EBT, all seeds of Palmer amaranth, barnyardgrass, johnsongrass, and pitted morningglory were killed when the narrow windrows in the field experiment were burned. In fact, seeds of all but pitted morningglory were reduced to ash.. Even though seeds of pitted morningglory remained intact after burning, they were not viable.

The results from the above experiments showed that complete control of weed seeds expelled from the combine can be achieved by burning narrow windrows of soybean residue. The HI values that are needed for complete mortality of the nine weed species evaluated in the kiln study were easily achieved in the field burning experiments. Thus, weed seed that enter the combine during soybean harvest and that subsequently leave the combine with the soybean harvest residue are highly likely to be destroyed during narrow-windrow burning. This then will significantly reduce the amount of seed that will be available to replenish the soil weed seedbank. Of course, this process will be most effective against those weed species that retain a higher percentage of their seed at soybean harvest time so that they enter the combine and are expelled in the soybean residue that is burned in these narrow windrows.

Further research is needed to determine 1) how this HWSC practice might affect the carbon and nutrient recycling benefits derived from soybean production, 2) how this removal of soybean residue from sloped fields might affect erosion potential of these sites, 3) the impact of this practice on soil microbial activity, and 4) how this practice might affect soil fertility management in fields with these burned strips..

It is doubtful that this tool for weed management/control has been/is being used on a wide-scale basis in the Midsouth. However, with the increase in problematic herbicide-resistant (HR) weeds in soybean, the steadily decreasing chemical weed control options, and the continuing evolvement of HR weeds, it is reasonable to assume that alternative methods such as residue burning will necessarily become more commonplace to complement herbicide weed control. Using narrow-windrow burning as an HWSC tool should result in a diminishing soil weed seedbank, and this, coupled with an effective herbicide weed management program, should help to improve weed control in soybeans over the long-term. Certainly, such alternative weed control measures should at least be implemented in soybean fields that have significant weed escapes that have produced/will produce seed at soybean harvest time.

Composed by Larry G. Heatherly, Mar. 2021, larryheatherly@bellsouth.net