Alternative strategies are needed for management of glyphosate-resistant (GR) horseweed in soybean. Integrating a cereal rye cover crop with soybean planted in narrow rows may improve control and reduce herbicide selection pressure for herbicide-resistant horseweed biotypes. Four site-years of experiments were conducted in Michigan to determine if fall-planted cereal rye terminated with glyphosate 1 wk prior to (early termination) or 1 wk after (planting green) planting in combination with narrow row soybean improved GR horseweed management. At postemergence (POST) herbicide application, horseweed biomass was reduced 71 to 90% when soybean was planted into cereal rye, regardless of termination time, compared with no cover across all row widths. Planting green or narrow row soybean suppressed horseweed through soybean harvest. When glyphosate was applied POST (non-effective), horseweed biomass was 36 to 46% lower when planting green compared with early terminated cereal rye and no cover. Similarly, planting soybean in 19- and 38-cm rows reduced horseweed biomass 48 and 28%, respectively, compared with 76 cm rows. Cereal rye did not affect soybean yield pooled over 3 of 4 site-years; however, narrow row soybean yielded 11 to 18% higher than 76 cm rows. Soybean yield was 11% higher when an effective POST herbicide was applied. In conclusion, fall-seeded cereal rye or narrow row soybean suppressed horseweed compared with no cover and 76 cm rows; however, the effects of early termination did not last throughout the growing season in most cases. Delaying cover crop termination by planting green reduced horseweed biomass and density through soybean harvest, but reduced yield in 1 site-year due to an increased incidence of white mold. These cultural practices have a positive influence on suppressing horseweed that should be part of an overall horseweed management strategy; however, the use of an effective POST herbicide is still needed for complete season-long horseweed management.

Glyphosate-resistant (GR) horseweed is a problematic weed for Michigan soybean growers. Additionally, rosette- and upright- horseweed growth types have been observed co-emerging during mid- to late-summer in several Michigan fields. In the greenhouse, shade levels from 35 to 92% reduced rosette and upright horseweed biomass 31 to 99% compared with the upright growth type grown under 0% shade. Greater reductions in biomass occurred under 69 and 92% shade. Thus, increased shading by planting in narrow rows and/or planting green into cereal rye may improve horseweed suppression. A field experiment conducted over three site-years compared the effect of fall-planted cereal rye terminated with glyphosate 1 wk after planting (planting green) with a preemergence (PRE) residual herbicide program (glyphosate + 2,4-D + flumioxazin + metribuzin) on horseweed control in soybean planted in three row widths (19-, 38-, and 76-cm). Planting green or applying a residual herbicide program across all row widths reduced horseweed biomass 86 to 91% and 95 to 99%, respectively, compared with soybean planted with no cover in 76 cm rows, 4 to 6 wk after planting (WAP). At soybean harvest, when a noneffective postemergence (POST) herbicide (glyphosate) was applied horseweed biomass was 42 and 81% lower by planting green or applying a residual herbicide program compared with no cover, respectively. Similarly, planting soybean in 19 cm rows reduced horseweed biomass compared with 38- and 76-cm rows. When an effective POST program was applied, similar horseweed biomass reductions were observed by planting green or applying a residual herbicide across all row widths. Additionally, soybean yield and economic returns were similar between planting green and applying a residual herbicide in 1 of 2 site-years. Integrating planting green and an effective POST herbicide program offers an alternative horseweed management strategy to applying a residual preemergence herbicide program.

Horseweed [Conyza candensis (L.) Cronquist] grows in one of two distinct growth phenotypes, so called “rosette” and “upright” growth types, and they have recently been observed co-occurring in Michigan fields. Previous research found that “upright” plants from two glyphosate-resistant populations were 3- and 4-fold less sensitive to glyphosate than their rosette siblings. Further experiments were conducted to investigate whether differential glyphosate sensitivity of the growth types was due to glyphosate retention, absorption, or translocation. The total amount of glyphosate retained on C. canadensis leaf surface was similar for both growth types; however, on a per weight and area basis the upright growth type retained 21 and 18% less glyphosate, respectively. Glyphosate absorption was up to 85%, 168 HAT, and was not different between the rosette and upright growth types or between the susceptible (S) and resistant (R) biotypes. Additionally, there was no difference in translocation between the two growth types within each biotype at any time point. Interestingly, at 168 HAT 14C-glyphosate translocation was higher in the S rosette compared with the two growth types from the R biotype; however, the S upright-type was similar to both R growth types. Thus, glyphosate resistance in the R biotype may be due to an alternative mechanism instead of impaired translocation which has been cited as the primary mechanism of glyphosate resistance in C. canadensis. These results suggest that reduced glyphosate retention on a per weight and area basis of the upright growth type may contribute to increased glyphosate tolerance due to a diluted concentration of glyphosate in the plant. However, another factor is likely related to the mechanism of resistance within the R biotype that is contributing to a 3-fold difference in glyphosate sensitivity between the two growth types, such as alterations in EPSPS gene expression or changes in undescribed metabolism genes.