Waterhemp (Amaranthus tuberculatus [Moq.] Sauer) escapes are common in midwestern U.S. soybean [Glycine max (L.) Merr.] fields due to the continued rise in herbicide-resistant (HR) populations. In a conventional harvesting system, weed seeds are harvested with the crop grain and spread back on to the field. Harvest weed seed control methods such as chaff lining concentrate weed seed-bearing crop and weed chaff into a narrow row (chaff line). These chaff lines (30- to 50-cm wide) are undisturbed the following growing seasons, under the assumption that the chaff line creates an environment less favorable for weed seed germination and survival. Field experiments were conducted in a soybean–corn (Zea mays L.) rotation in 2020 and 2021 in Ames, IA, and Roland, IA, to quantify the effectiveness of chaff lining for managing A. tuberculatus seeds. About 70% of the A. tuberculatus seeds were retained on the mother plant at soybean harvest in 2020. The chaff lining system concentrated more than 99% of the A. tuberculatus seeds exiting the combine into the chaff line. Although A. tuberculatus population density in 2021 was 76% higher inside the chaff line than outside the chaff line, A. tuberculatus aboveground biomass was 63% lower inside the chaff line than outside the chaff line at 12 wk after corn planting. Similarly, A. tuberculatus inside the chaff line had delayed emergence compared with A. tuberculatus outside the chaff line. Application of preemergence herbicides in corn inside the chaff line delayed A. tuberculatus emergence by more than 2 wk compared with A. tuberculatus outside the chaff line. Additionally, a follow-up postemergence herbicide application in corn was needed only inside the chaff line to manage A. tuberculatus, suggesting the possibility of lower overall herbicide use. These results support implementing chaff lining in soybean-based crop systems of the U.S. Midwest to help manage HR A. tuberculatus seedbanks.

Greenhouse and field experiments were conducted with a roller applicator at Lincoln, Nebraska, during 1979 and 1980. Glyphosate [N-(phosphonomethyl)glycine] concentrations of 5, 10, and 20% and carpet saturations of 50 and 75% controlled shattercane [Sorghum bicolor (L.) Moench] when applied to the top 30 cm of the plant in greenhouse research. In the field, glyphosate concentrations of 5 to 20% with a carpet saturation of 50% controlled shattercane acceptably in soybeans [Glycine max (L.) Merr.], but a concentration of 2.5% with 25% carpet saturation did not. Weed control was comparable whether speed of application was 3.2, 6.4, or 9.6 km/h. Shattercane control in grain sorghum [Sorghum bicolor (L.) Moench.] was excellent at glyphosate concentrations of 5, 10, and 20% and at carpet saturations of 50 and 75%, and sorghum injury was minimal at 25 and 50% carpet saturations. The roller applicator was compared to a ropewick applicator for shattercane control in sorghum. Excellent weed control (90% or greater) with minimal crop injury was obtained with the roller applicator at glyphosate concentrations of 10 and 20% at application speeds of 3.2 and 6.4 km/h and with the ropewick applicator with glyphosate concentrations of 35 and 50% applied at 3.2, 6.4, and 9.6 km/h.

Other research has shown that a roller applicator can selectively control weeds that protrude above the crop, conserve over 80% of the herbicide, eliminate spray drift and splash, and obviate recirculation of the spray solution. The carpet on the roller applicator must be moist enough to transfer ample herbicide to the weeds it contacts but dry enough to prevent dripping of herbicide solution from the carpet. An electronic moisture sensor and solenoid valve system was designed to permit operation of the roller applicator through various weed densities while automatically maintaining a proper level of herbicide solution in the carpet of the roller.

An electrical discharge system (EDS) was evaluated in field studies conducted in 1977 through 1979 in western Nebraska for its ability to control weed escapes in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′). Nine weeks after sugarbeets were planted, kochia [Kochia scoparia (L.) Schrad.] had attained a height above sugarbeets sufficient for EDS treatment. Redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters (Chenopodium album L.) generally attained sufficient height above sugarbeets 11 and 13 weeks after sugarbeet planting. Sugarbeet root yields were reduced 40, 20, and 10% from competition by kochia, common lambsquarters, and redroot pigweed, respectively. Treatment of kochia, redroot pigweed, and common lambsquarters with EDS in some cases resulted in a reduction in weed height. The EDS treatments reduced the stand of all weeds 32, 39, and 47% for 1977, 1978, and 1979, respectively. Although the EDS treatments failed to kill many weeds, it did suppress the competitive ability of the three weeds to the extent that sugarbeet yields were higher in areas receiving EDS treatments than areas receiving no EDS treatment.

Late-season weed escapes are often ignored because they rarely cause crop yield penalty. Traditional weed management recommendations are based on the economic threshold (ET) approach, wherein management is required if the predicted current-season yield loss is greater than the cost of control interventions. While ET-based weed management can reduce current-season production costs and promote farmland biodiversity, it does not consider the long-term biological and economic consequences associated with late-season weed seed production. An important concern is that late-season weed seed production will replenish the soil seedbank, ensuring future weed problems. In the context of herbicide resistance evolution, allowing late-season weed seed production can be problematic because the probabilities of occurrence of resistant mutants rise with increases in seed production. A key component of herbicide resistance mitigation and management is preventing seed production and buildup of the soil seedbank. Late-season weed management efforts constitute additional expenses to growers, which cannot be recouped in that growing season, but any such investment must be weighed against the perceived long-term benefits. It appears that management of late-season weed escapes is valuable in a number of situations, and the degree to which management interventions should be employed can be case-specific. Adoption of economic optimum thresholds (EOTs), which can be established using bio-economic models, will be useful for making management decisions for late-season weed escapes. In systems vulnerable to herbicide resistance evolution, bio-economic resistance thresholds (BERTs) will be appropriate and bio-economic resistance models (BERMs) will be helpful for establishing such thresholds for specific production scenarios. Management considerations for late-season weed escapes are discussed, and knowledge gaps for future research are identified.