Cereal Rye Control and Biomass Reduction

The termination time by termination herbicide interaction was significant (P < 0.001) for cereal rye control at 28 DAT (Table 2). Therefore, data were pooled over the years. For cereal rye biomass reduction, due to the year-by-termination time by termination herbicide interaction being significant (P < 0.001), data were analyzed separately for the 2019 and 2020 seasons. Different weather conditions, when 2020 was dryer with warmer early season temperatures, may have affected the different levels of cereal rye biomass production (Figure 1).

Table 2. ANOVA results (significance of F-values) for the fixed effects of year, termination time, and termination herbicide on response variables.^a,b

^a Abbreviations: DAT, days after treatment; H, herbicide NS, nonsignificant, P-value > 0.05; T, termination time; Y, year.

^b This experiment was conducted as a split-plot design, with cereal rye termination time as the whole plot and cereal rye termination herbicide as the subplot, during the 2019 and 2020 growing seasons at the South Central Agricultural Laboratory, near Clay Center, NE.

Figure 1. Average daily air temperature and total precipitation during the 2019 and 2020 growing seasons compared with the 30-yr average at the South Central Agricultural Laboratory near Clay Center, NE.

The average of cereal rye biomass after each termination timing (15 DBSP, SPD, and 15 DASP) was, respectively, 394, 1,697, and 3,700 kg ha⁻¹ in 2019; and 330, 1,304, and 4,550 kg ha⁻¹ in 2020 (Figure 2). Cereal rye was controlled by ≥98% after glyphosate was used regardless of the termination timing (Table 3). Glyphosate efficacy on cereal rye has been extensively documented in the literature. For example, Palhano et al (Reference Palhano, Norsworthy and Barber2018b) reported that cereal rye that was 134 to 154 cm tall was controlled by 100% when glyphosate was used, while Noorenberghe et al. (Reference Noorenberghe, Sikkema, Cowbrough, Hooker, Soltani and Tardif2023), in a multilocation study, reported ≥99% cereal rye control at the Zadoks 31 stage (10–43 cm tall). Similarly, Cornelius and Bradley (Reference Cornelius and Bradley2017) reported 98% and 87% cereal rye control in early April and early May, respectively, when the cover crop was terminated with glyphosate. At 15 DBSP, 77% control of cereal rye was recorded after clethodim was used compared with ≥94% control when fluazifop-P, fluazifop-P plus fenoxaprop, quizalofop, and glyphosate were applied (Figure 3). Young et al. (Reference Young, Whaley, Lawrence and Burke2016) reported 60% control of feral cereal rye (Secale cereale L.) with clethodim at 105 g ai ha⁻¹ 42 d after spring application compared with 93% control when glyphosate at 866 g ae ha⁻¹ was applied, and 99% control when quizalofop was applied at 62 g ai ha⁻¹. Similar trends were observed when cereal rye was terminated at SPD and 15 DASP, when 66% and 31% control, respectively, was recorded after clethodim was used, whereas >92% control was recorded when other ACCase inhibitors were used (Figure 4). Similarly, Bushong et al. (Reference Bushong, Peeper, Boyles and Stone2011) in a multilocation study investigated control options for feral cereal rye in winter canola and reported that winter application of clethodim (88 g ai ha⁻¹) resulted in 15.3% feral cereal rye seed content of harvested canola compared with 0.1% and 0.2% content after applications of glyphosate (770 g ae ha⁻¹) and quizalofop (62 g ai ha⁻¹), respectively.

Figure 2. Cereal rye biomass harvested 15 d after each termination timing (15 d before soybean planting [DBSP], soybean planting day [SPD], and 15 d after soybean planting [DASP]) averaged over termination herbicides during the 2019 and 2020 growing seasons in field experiments conducted at the South Central Agricultural Laboratory near Clay Center, NE.

Table 3. Cereal rye control and biomass reduction at 28 d after termination.^a–e

^a Field experiments were conducted at the South Central Agricultural Laboratory, near Clay Center, NE, in 2019 and 2020.

^b Year by treatment interaction was significant for biomass reduction; therefore, data were analyzed separately. Herbicides were applied at various timings mentioned in column 1 and data were collected 28 d after treatment; therefore, comparisons were made separately for treatments applied and evaluated at the same time.

^c Means presented within the same column and with no common letter(s) are significantly different according to Fisher’s Protected Least Significant Difference test.

^d Significance levels: NS, nonsignificant at α = 0.05; *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

^e Different letters (A, a, a) indicate a separated analysis because herbicides were applied at different timings and data were collected 28 d after treatment.

^f Herbicides were applied in the following amounts: clethodim, 136 g ai ha^–1; fluazifop-P-butyl, 420 g ai ha^–1; fluazifop-P-butyl + fenoxaprop-P-ethyl, 314 g ai ha^–1; glyphosate, 1,260 g ae ha^–1; and quizalofop-P-ethyl, 92 g ai ha^–1.

Figure 3. Cereal rye terminated 15 d before soybean planting using herbicides (A) glyphosate at 1,260 g ae ha⁻¹, (B) fluazifop-P-butyl at 420 g ai ha⁻¹, (C) quizalofop at 92 g ai ha⁻¹, (D) fluazifop-P-butyl/fenoxaprop-P-ethyl at 314 g ai ha⁻¹, and (E) clethodim at 136 g ai ha⁻¹ in field experiments conducted at the South Central Agricultural Laboratory near Clay Center, NE. Photographs were taken at the soybean planting day, 15 d after herbicides were applied.

Figure 4. Cereal rye terminated 15 d after soybean planting with (A) glyphosate at 1,260 g ae ha⁻¹, (B) fluazifop-P-butyl at 420 g ai ha⁻¹, (C) quizalofop at 92 g ai ha⁻¹, (D) fluazifop-P-butyl/fenoxaprop-P-ethyl at 314 g ai ha⁻¹, and (E) clethodim at 136 g ai ha⁻¹ in field experiments conducted at the South Central Agricultural Laboratory near Clay Center, NE. Photographs were taken 28 d after herbicides were applied.

Cereal rye biomass reduction occurring from applications of fluazifop-P, fluazifop-P plus fenoxaprop, and quizalofop was mostly comparable to the reduction that occurred after glyphosate was used, and clethodim was the least effective herbicide at all termination timings (Table 3). For instance, at 15 DBSP, cereal rye biomass was reduced by 67% and 75% in 2019 and 2020, respectively, when glyphosate was applied. These percentages are comparable to those of other ACCase-inhibiting herbicides, except clethodim, for which reductions of 53% and 66% occurred in 2019 and 2020, respectively (Table 3). Furthermore, at 15 DASP, differences in biomass reduction between treatments were more subtle, which can be attributed to higher cereal rye biomass at later growth stages, and plants were no longer developing vegetatively, but were developing reproductive structures instead. Clethodim was the least effective herbicide, when its application resulted in a 27% and 15% biomass reduction in 2019 and 2020, respectively; however, the reduction was comparable to that when fluazifop (36%) and fluazifop plus fenoxaprop (38%) were applied in 2019, and to that of fluazifop (23%) in 2020 (Table 3). Few studies have investigated using ACCase-inhibiting herbicides for cereal rye termination as an alternative to glyphosate. Kumar et al. (Reference Kumar, Singh, Flessner, Haymaker, Reiter and Mirsky2023), while investigating chemical and mechanical cereal rye termination methods, reported that an application of clethodim resulted in 7% biomass reduction, which is comparable to that of quizalofop at 13%. In contrast, Felsman et al. (Reference Felsman, Arneson, DeWerff and Werle2023) reported that when terminating cereal rye on the day when soybean is planted, glyphosate (≥98%) was more effective than quizalofop and clethodim (<57%). However, those authors reported that cereal rye yielded 9,019 and 15,819 kg ha⁻¹ of biomass in 2020 and 2021, respectively. Therefore, more research is needed to investigate whether reduced efficacy of ACCase inhibitors against cereal rye is affected by cover crop biomass.

Palmer amaranth and Setaria species Control, Biomass, and Density

The main effect of termination time was significant for Palmer amaranth control, biomass, and density, and Setaria species control and density; therefore, means were pooled over termination time (Table 2). No differences were observed among the cover crop termination herbicides in their ability to control late-season Palmer amaranth and Setaria species, biomass, or density, which is expected because the entire study was treated with the same preemergence/postemergence herbicide program and none of the cereal rye termination herbicides have any soil residual activity. For instance, Palmer amaranth and Setaria species control ranged from 82% to 92% at the R5 soybean growth stage regardless of the herbicide that was used for cereal rye termination (Table 4).

Table 4. Effect of cereal rye termination timing and termination herbicides along with preemergence and postemergence herbicides on weed control and weed biomass at the R5 soybean growth stage.^a,b

^a Field experiments were conducted at the South Central Agricultural Laboratory, near Clay Center, NE, in 2019 and 2020.

^b The main effect of termination time was significant for Palmer amaranth control, biomass, and density, and Setaria species control and density; therefore, treatment means were averaged over within each main effect.

^c All the treatments received a premix of chlorimuron/flumioxazin/pyroxasulfone applied preemergence (Fierce® XLT; Valent USA LLC, Walnut Creek, CA) at 230 g ai ha⁻¹ and dicamba (XtendiMax®; Bayer Crop Science, St. Louis, MO) applied postemergence at 560 g ae ha⁻¹.

^d Means presented within the same column and with no common letters are significantly different according to Fisher’s protected least significant difference test.

^e Significance levels: NS, nonsignificant at α = 0.05; *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

Late-season Palmer amaranth control increased with delayed cereal rye termination timings, with 70%, 88%, and 96% control at 15 DBSP, at SPD, and 15 DASP, respectively. Additionally, Palmer amaranth control without cereal rye cover crop was 65%, which is comparable to 70% control observed at 15 DBSP (Table 4). This might be partially due to a premix of chlorimuron/flumioxazin/pyroxasulfone (Fierce XLT) applied preemergence that might have contributed to the residual control of Palmer amaranth. Webster et al. (Reference Webster, Simmons, Culpepper, Grey, Bridges and Scully2016) reported that Palmer amaranth biomass and density were inversely correlated to winter rye cover crop biomass, for which 5,200 kg ha⁻¹ of winter rye biomass resulted in a 50% reduction in Palmer amaranth emergence. This further highlights the importance of integrating cereal rye cover crop with herbicides for management of troublesome weeds.

In accordance with Palmer amaranth control ratings, reduced weed biomass and density were observed at later cereal rye termination timings; in addition, at 15 DBSP, Palmer amaranth biomass and density were similar to the conventional treatment without cover crops (Table 4). For example, Palmer amaranth density within cereal rye plantings was 1 and 2 plants m⁻² at 15 DASP and SPD, respectively, compared with 8 and 9 plants m⁻² when cereal rye was terminated at 15 DBSP and with conventional treatment, respectively. Vollmer et al. (Reference Vollmer, Besançon, Carr, VanGessel and Scott2020a) observed a similar result, in which the presence of cereal rye cover crop resulted in a reduced density of Amaranthus species. More specifically, those authors reported that terminating cereal rye at 3 and 5 wk after watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is transplanted resulted in a reduced density of Amaranthus species by 45 and 8 plants m⁻², respectively, whereas 193 plants m⁻² was observed when cereal rye was not planted as a cover crop.

The response of Setaria species to cereal rye termination timings was similar to that of Palmer amaranth. For instance, greater control of Setaria species in the late season was observed at SPD and 15 DASP termination timings, with 92% and 96% control, respectively, and control shifted to 82% when cereal rye was terminated 15 DBSP and to 71% when a conventional treatment without cereal rye was implemented. Vollmer et al. (Reference Vollmer, Besançon, Carr, VanGessel and Scott2020a) reported similar results when cereal rye was terminated 10 d before watermelon planting resulted in 81% large crabgrass [Digitaria sanguinalis L. (Scop.)] control compared with 72% control when cereal rye was terminated 20 d before watermelon was planted.

Although no differences in Setaria species biomass were observed (less than 2 g m⁻² for all treatments), density was reduced at SDP and 15 DASP (2 and 1 plants m⁻², respectively) compared with density at 15 DBSP (5 plants m⁻²). Mirsky et al. (Reference Mirsky, Curran, Mortenseny, Ryany and Shumway2011) reported that densities of multiple weed species were reduced when cover crop termination was delayed; for instance, Setaria species density when cover crops were terminated early (May 1) was 9 plants m⁻² compared with 2 plants m⁻² when termination occurred late (May 30). Essman et al. (Reference Essman, Loux, Lindsey and Dobbels2023) observed a lower density of late-season giant foxtail when cereal rye termination was delayed, and when cereal rye was terminated 7 DBSP, 7 DASP, and 21 DASP it resulted in 63, 46, and 9 plants m⁻², respectively.

Results from this study highlight the importance of an integrated weed management approach in which the greatest level of weed control was achieved when cereal rye termination was delayed to SPD or 15 DASP. Previous research has shown that the presence of a cereal rye cover crop typically reduces Palmer amaranth density; for instance, Palhano et al. (Reference Palhano, Norsworthy and Barber2018a) reported that when cereal rye was terminated 3 wk before cotton planting, Palmer amaranth density 8 wk after cotton planting was 3.8 plants m⁻² compared with 22.4 plants m⁻² when cereal rye was not used as a cover crop. Furthermore, in accordance with what we observed in this study, Hodgskiss et al. (Reference Hodgskiss, Young, Armstrong and Johnson2021) concluded that because of greater cereal rye biomass production from delayed termination timings, waterhemp density in plots where cereal rye was terminated on the day when soybean was planted (352 plants m⁻²) and after soybean planting (287 plants m⁻²) were reduced compared with 915 plants m⁻² before the soybean planting termination timing. In addition, the negative impact of cereal rye cover crop on weed density has been reported for other broadleaf weeds. Vollmer et al. (Reference Vollmer, Besançon, Carr, VanGessel and Scott2020a) reported that common lambsquarters (Chenopodium album L.) density was 18 plants m⁻² in a treatment without cereal rye and it shifted to 6 and 0 plants m⁻² at 3 and 5 wk, respectively, after watermelon transplant termination timings. Furthermore, Schramski et al. (Reference Schramski, Sprague and Renner2020), in a multilocation study, reported that horseweed density was minimized in plots where cereal rye and winter wheat were terminated 7 DASP (198 plants m⁻²) compared to 7 DBSP (582 plants m⁻²). Although weed suppression from cereal rye cover crop has been largely documented, it is not a stand-alone weed control tactic, and its ability to suppress weeds might vary by season and environment. For instance, Bish et al. (Reference Bish, Dintelmann, Oseland, Vaughn and Bradley2021), while investigating the effects of cereal rye seeding rates on waterhemp emergence, reported that in 2020 waterhemp cumulative emergence at 28 DASP ranged from 49 to 63 plants m⁻² regardless of the cereal rye seeding rate compared to 385 plants m⁻² in a treatment without cereal rye; however, in the 2019 season, all treatments had comparable waterhemp emergence, ranging from 927 to 1,412 plants m⁻².

Soybean Stand and Grain Yield

Soybean stand count was reduced to 17 plants m⁻¹ row when cereal rye was terminated 15 DASP compared with the 19 plants m⁻¹ row observed in other termination timings and the conventional treatment (Table 5). There were no differences in soybean stand observed among termination herbicides, ranging from 18 to 19 plants m⁻¹ row. Furthermore, soybean grain yields were greater when cereal rye was terminated at 15 DBSP, SDP, and conventional treatment with 4,460, 4,566, and 3,977 kg ha⁻¹, respectively, compared to 2,184 kg ha⁻¹ at 15 DASP. In addition, no differences in soybean yield were observed among the termination herbicides tested, suggesting that the reduced efficacy of clethodim in cereal rye did not impact cash crop performance.

Table 5. Effect of cereal rye termination timing and termination herbicides on soybean stand and yield.^a,b

^a Field experiments were conducted at the South Central Agriculture Laboratory near Clay Center, NE. Soybean stand was obtained from the 2019 and 2020 seasons; however, due to a hailstorm in 2019, only 2020 soybean yield data are reported.

^b All the treatments received a premix of chlorimuron/flumioxazin/pyroxasulfone applied preemergence (Fierce® XLT; Valent USA LLC, Walnut Creek, CA) at 230 g ai ha⁻¹ and dicamba (XtendiMax®; Bayer Crop Science, St. Louis, MO) applied postemergence at 560 g ae ha⁻¹ + labeled adjuvants.

^c The main effect of termination timing was significant for soybean stand and yield; therefore, treatment means were averaged over within each main effect.

^d Means presented within the same column and with no common letters are significantly different according to Fisher’s protected least significant difference test.

^e Significance levels: NS, nonsignificant at α = 0.05; *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

Results from this study suggest that, although delaying cover crop termination resulted in greater weed suppression, it may also negatively impact soybean stand and yield. Similarly, Hodgskiss et al. (Reference Hodgskiss, Young, Armstrong and Johnson2022) reported up to 28% soybean yield loss when cereal rye was terminated 9 to 14 DASP compared to termination before soybean planting. Nunes et al. (Reference Nunes, Arneson, DeWerff, Ruark, Conley, Smith and Werle2023) reported that planting soybean into actively growing green cereal rye cover crop alters preemergence herbicide dynamics and can reduce yield. However, other studies reported no soybean yield penalties in delayed cereal rye termination timings (Grint et al. Reference Grint, Arneson, Arriaga, DeWerff, Oliveira, Smith, Stoltenberg and Werle2022; Mischler et al. Reference Mischler, Curran, Duiker and Hyde2010; Reed and Karsten Reference Reed and Karsten2022; Vollmer et al. Reference Vollmer, VanGessel, Johnson and Scott2020b). Silva et al. (Reference Silva, Mourtzinis, McMechan, Carmona, Potter, Tilmon, Hesler, Seiter, Wright, Osborne, Hunt and Conley2024) in a comprehensive study investigating the impacts of delayed cereal rye termination on soybean yield across multiple locations in the Midwest, reported that delayed termination did not reduce soybean yield in 25 of 28 location-years. The outcome of integrating cover crops into a cropping system depends on multiple variables, such as rainfall, irrigation, temperature, planting conditions, soil nitrogen, etc. (Garba et al. Reference Garba, Bell and Williams2022). Consequently, more research is needed to make accurate predictions of the potential impact of cover crops on cash crop yields and potential long-term benefits within the various agronomic systems.