Integrating Fall- and Spring-Planted Cover Crops for Weed Suppression in semiarid Central Great Plains

Objective 1:

Variable precipitation amount and frequency were observed at KSU-ARCH during the experimental periods 2020-2021, 2021-2022 and 2022-2023 (Figure 1). The total amount of precipitation received during the CC growing season (September to May) in 2020-2021, 2021-2022 and 2022-2023 was 217, 99, and 130 mm, respectively (Figure 1). The 30-year average precipitation from September to May in the region is 347 mm (Figure 1). No difference was recorded in aboveground CC dry biomass at the time of termination across the years and was 1520, 1130, and 1470 kg ha^-1 in 2021, 2022, and 2023, respectively, with an average of 1370 ± 123 kg ha^-1. During the sorghum growing season (June through October), the total precipitation amount was 256, 171, and 237 mm in 2021, 2022, and 2023, respectively (Figure 1). The 30-year average precipitation from June to October in the region is 341 mm (Figure 1).

Total weed density and weed dry biomass

Across three years, four summer annual broadleaf weed species were observed at the study site, including B. scoparia, A. palmeri, Venice mallow (Hibiscus trionum L.), and puncturevine (Tribulus terrestris L.). Based on the relative abundance, B. scoparia and A. palmeri were the dominant weed species across three years (Tables 1, 3, and 4).

2021 growing season. Amaranthus palmeri was the most dominant weed species with a mean relative abundance of >40% across treatments at all monthly evaluation timings (Table 1). Prior to termination [0 days after termination (DAT)], CC reduced the total weed density by 86 to 95% compared to chemical fallow (no herbicide was applied in chemical fallow at this time) (Table 1). However, an application of GLY plus ACR/ATZ plus dicamba in chemical fallow at the time of CC termination reduced weed density at later evaluation timings. Total weed density at 60 DAT was dominated by A. palmeri (relative abundance = 90%) and was significantly greater (approximately 4 times) following the CC terminated with GLY only compared to the CC terminated with GLY plus ACR/ATZ (Table 1). This would be due to a lack of residual herbicide and CC residue was not enough to suppress the emerging A. palmeri seedlings. At the time of grain sorghum harvest (120 DAT), CC terminated with GLY plus ACR/ATZ reduced the total weed density by 50% compared to chemical fallow (Table 1). Consistent with total weed density, CC at termination reduced the total weed dry biomass by 93% compared to chemical fallow (Table 2). The CC terminated with GLY only and with GLY plus ACR/ATZ reduced the total weed dry biomass by 50 to 65% and 42 to 68% at 60 to 120 DAT, respectively compared to chemical fallow. It is interesting to note that no differences were observed in total weed density between chemical fallow and CC terminated with GLY only at 120 DAT; however, the same CC treatment significantly reduced the total weed dry biomass by 42% compared to chemical fallow, indicating the suppressive effect of CC on weed growth that would ultimately reduce the weed seed production (Baraibar et al. 2018).

2022 growing season. Mean relative abundance was 46 to 56% for B. scoparia, 0 to 21% for A. palmeri, and 29 to 49% for H. trionum before CC termination (Table 3). Similar to 2021, the CC at termination reduced the total weed density by 90 to 93% compared to weedy fallow. At 30 DAT, the CC terminated with GLY plus ACR/ATZ reduced the total weed density by 92% compared to chemical fallow and 96% compared to weedy fallow. At 60 DAT, the mean relative abundance was 41 to 75% for B. scoparia, 11 to 53% for A. palmeri, and 12 to 31% for H. trionum among all treatments. The CC termination with GLY only treatment did not produce enough CC biomass to suppress emerging weed seedlings and resulted in 21 more weed seedlings m^-2 than the CC terminated with GLY plus ACR/ATZ at 60 DAT (Table 3). These results indicate the importance of residual herbicide with CC to achieve effective weed suppression. Our findings are consistent with Wiggins et al. (2016), who also concluded that CC alone was not enough for season-long control of GR A. palmeri and suggested integrating residual herbicides to complement the suppressive effect of CC. In the current study, total weed density did not differ between chemical fallow and CC terminated with GLY plus ACR/ATZ treatments at 120 DAT; however, weed density was nearly 95% lower in CC terminated with GLY plus ACR/ATZ compared to weedy fallow and CC terminated with GLY only (Table 3). These results are consistent with Obour et al. (2022a), who previously reported 82% reduction in total weed density with spring-planted CC mixture (oat/triticale/pea) compared to weedy fallow. Petrosino et al. (2015) also reported a 78 to 94% reduction in B. scoparia density with fall-planted CC (triticale/triticale-hairy vetch mixture) compared to chemical fallow in winter wheat-fallow rotation. Similar to weed density reduction, CC at the time of termination provided >95% total weed dry biomass reduction compared to weedy fallow (Table 2). The presence of CC reduces the sunlight penetration to the ground for weed seed germination and also reduces their competitive ability for other resources, thereby resulting in lower weed biomass (Silva and Bagavathiannan 2023; Webster et al. 2016). The CC terminated with GLY only reduced total weed dry biomass by 94% at 30 DAT and 63% at 120 DAT compared to weedy fallow. In contrast, the CC terminated with GLY plus ACR/ATZ reduced total weed dry biomass by >95% as compared to weedy fallow throughout the sorghum growing season (Table 2). These results indicate the need of residual herbicide in combination with CC for a season-long weed control in grain sorghum. These results are consistent with Whalen et al. (2020), who previously reported that fall-planted CC, including Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot], oat (Avena sativa L.), and winter wheat provided 38 to 48% weed control without a residual herbicide; however, the control ranged from 72 to 85% under CC with a residual herbicide (sulfentrazone plus chlorimuron) application.

2023 growing season. No weed emergence was observed under both CC treatments at the time of termination (Table 4). In addition, no weed emergence was observed in the CC terminated with GLY plus ACR/ATZ treatment at 30 DAT. In contrast, a greater weed density of 91 plants m^-2 with a relative abundance of 70% for A. palmeri and 30% for B. scoparia was recorded in the CC terminated with GLY only treatment at 30 DAT (Table 4). This increase in A. palmeri and B. scoparia densities under CC terminated with GLY only was probably due to more precipitation within 30 DAT (78 mm, 33% of total precipitation received during the entire sorghum growing season), and lack of any residual herbicide applied at CC termination. Treatments, including the CC terminated with GLY plus ACR/ATZ and chemical fallow reduced the total weed density by 70% compared to weedy fallow at 90 DAT. Chemical fallow and CC terminated with GLY plus ACR/ATZ had much fewer weeds (7 to 8 plants m^-2) compared to weedy fallow (54 plants m^-2 with 73% relative abundance of A. palmeri) at 120 DAT (Table 4). Similar to weed density reduction, CC terminated with GLY plus ACR/ATZ resulted in 98%, 46%, and 57% total weed dry biomass reduction compared to weedy fallow, chemical fallow, and CC terminated with GLY only, respectively, at 60 DAT (Table 2). At 120 DAT, the CC terminated with GLY plus ACR/ATZ and chemical fallow reduced the total weed dry biomass by 83% and 65% compared to weedy fallow. Our results are consistent with Petrosino et al. (2015), who also reported that fall-planted triticale and a triticale-hairy vetch mixture reduced B. scoparia density by 78% and 94%, respectively, and biomass up to 98% compared to chemical fallow. Compared to chemical fallow, the CC terminated with GLY plus ACR/ATZ provided 5 to 18% more reductions in total weed dry biomass during the entire grain sorghum growing season (Table 2). Wiggins et al. (2016) reported that CC including cereal rye (Secale cereale L.), winter vetch, crimson clover (Trifolium incarnatum L.), or winter wheat had less than 65% control of A. palmeri, whereas the same CC in combination with PRE-applied acetochlor or fluometuron resulted in >87% control of A. palmeri.

Volumetric water content and grain sorghum yield

The soil water content at sorghum planting is directly related to grain yields in the dryland region (Holman et al. 2023; Obour et al. 2022b). In 2021, fall-planted CC did not affect volumetric water content at grain sorghum planting compared to chemical fallow across all soil depths (Figure 2). This is likely due to greater precipitation at pre- and post-CC termination time (Figure 1). The precipitation received closer to termination or post-termination of CC likely recharged the soil profile thereby diminishing the effects of the growing CC on water availability. Furthermore, the CC residue likely decreased soil water evaporation, increasing moisture storage (Holman et al. 2020, 2021). In contrast, fall-planted CC did reduce the volumetric water content at grain sorghum planting compared to chemical fallow at 0-30 cm and 30-60 cm depths in the 2023 growing season (Figure 2). No differences in the water content were observed between CC and chemical fallow treatments at deeper soil depths. Similar to CC, the weedy fallow treatment also resulted in relatively low volumetric water content (0.16 to 0.20 cm³ cm^-3) up to 60 cm soil depth compared to chemical fallow (0.20 to 0.22 cm³ cm^-3) indicating soil water depletion by weeds. Holman et al. (2021) reported no difference in the available soil moisture at time of wheat planting between fallow and spring-planted CC left standing during the fallow phase of W-S-F rotation.

No differences in grain sorghum yields were observed between chemical fallow (1876 kg ha^-1) and CC terminated with GLY plus ACR/ATZ (2072 kg ha^-1) in 2021 and the least grain yield (1456 kg ha^-1) was recorded under CC terminated with GLY only (Figure 3). During the 2022 growing season, the overall grain sorghum yield was low due to lower season precipitation (171 mm) compared to 2021 (256 mm). Effective weed suppression (both density and total weed dry biomass) achieved with the CC terminated with GLY plus ACR/ATZ resulting in a greater grain sorghum yield (1319 kg ha^-1) compared to chemical fallow (912 kg ha^-1). The CC termination with GLY only suppressed weeds up to 30 DAT (Table 3) and A. palmeri emerging after 30 DAT reduced the grain sorghum yield (472 kg ha^-1) (Figure 3). The total precipitation during the 2023 sorghum growing season was 237 mm, but the majority of this precipitation occurred in May and June. There was moisture stress at the boot stage of grain sorghum in September (only 17 mm of rainfall) that resulted in reduced grain yield (432 to 1323 kg ha^-1) (Figures 1 and 3). No difference in grain sorghum yield was observed between chemical fallow and CC terminated with GLY plus ACR/ATZ. The precipitation events in May and June resulted in the emergence of several cohorts of A. palmeri following the CC terminated with GLY only that resulted in competition with grain sorghum and reduced yield (432 kg ha^-1) compared to chemical fallow and CC terminated with GLY plus ACR/ATZ (Figure 3). Based on the precipitation amount and frequency, fall-planted CC had a variable impact on grain sorghum yield. Nielsen et al. (2016) reported a 10% reduction in winter wheat yield following a CC compared to fallow in the W-S-F rotation under the semi-arid CGP. Holman et al. (2022) reported that spring-planted CC (oat grain) after sorghum harvesting in the fallow phase of W-S-F rotation resulted in 29% less available soil moisture at sorghum planting and did not affect the wheat and sorghum yield compared to fallow. In south-central Kansas, Janke et al. (2002) reported no differences in grain sorghum yield following CC (hairy vetch/winter pea) in winter wheat-grain sorghum rotation compared to no CC in years with good rainfall; however, during years with low rainfall, CC establishment was poor, and grain sorghum yield was reduced because of water use by the CC compared to no CC treatment. Eash et al. (2021) also reported that CC had no impact on subsequent crop yields in a very low-yielding environment in Colorado, mainly due to low CC biomass. In this study, the chemical fallow treatment provided more available soil water in one of two years compared to CC treatments prior to sorghum planting. However, this did not translate into a higher grain yield as long as the residual herbicide ACR/ATZ was applied with GLY at CC termination.

Economics analyses

During 2021, the CC terminated with GLY plus ACR/ATZ resulted in greater gross returns of $497 ha^-1 compared to $450 ha^-1 following chemical fallow (Table 5). This was because of better weed control and greater sorghum yield under CC terminated with GLY plus ACR/ATZ compared to chemical fallow. However, CC seed and planting costs decreased net returns. Chemical fallow had the highest net returns by $55 ha^-1 compared to CC terminated with GLY plus ACR/ATZ. Net returns were negative under CC terminated with GLY only because of lower grain sorghum yield than chemical fallow and CC terminated with GLY plus ACR/ATZ.

Similar to 2021, CC terminated with GLY plus ACR/ATZ had greater gross returns ($317 ha^-1) followed by chemical fallow ($219 ha^-1) and CC terminated with GLY only ($113 ha^-1) in 2022 (Table 6). Weedy fallow had the lowest gross returns of $24 ha^-1. The net returns for 2022 were negative for all treatments because of low grain sorghum yield which suggests that gross return from grain sorghum was not enough to cover the variable input costs. However, net returns for CC terminated with GLY plus ACR/ATZ and chemical fallow were not significantly different (Table 6).

In 2023, the lower grain sorghum yield in CC terminated with GLY plus ACR/ATZ resulted in a lower gross return of $181 ha^-1 compared to $265 ha^-1 in chemical fallow (Table 7). Similar to 2022, the net returns in 2023 were also negative for all treatments. Chemical fallow had less negative net returns (-$65 ha^-1) compared to other treatments (-$138 to -$275 ha^-1). The cost of CC seed and planting increased the variable cost for both CC treatments and thus resulted in greater negative net returns. Janke et al. (2002) also reported lower net returns with fall-planted CC (hairy vetch/winter pea) before grain sorghum because of lower grain sorghum yield in the years with low rainfall. Obour et al. (2022a) also reported that integrating CC in the dryland cropping system resulted in negative net returns compared to fallow. These results indicate that growing CC only for weed suppression was not profitable compared to chemical fallow.

Based on the sensitivity analysis, a minimum grain sorghum yield of 2000 kg ha^-1 was needed to obtain a positive net return ($8 ha^-1) following CC terminated with GLY plus ACR/ATZ at average grain sorghum price ($0.22 kg^-1) during the 3-year study period (Table 8). In the present study, the average grain sorghum yield was low (786 to 1432 kg ha^-1). The estimated average grain sorghum yield in western Kansas ranged from 3800 to 5000 kg ha^-1 (Ciampitti and Carcedo 2022) therefore, based on this yield scenario, the expected net returns would be $404 to $668 ha^-1 following CC terminated with GLY plus ACR/ATZ and $506 to $770 ha^-1 following chemical fallow at $0.22 kg^-1 sorghum price (Table 8). At the lowest grain sorghum price ($0.09 ha^-1), a grain yield of 5000 kg ha^-1 was expected to cover the cost of CC seed and planting and obtain positive net returns ($18 ha^-1) under CC terminated with GLY plus ACR/ATZ, however, following chemical fallow, a lower yield of 3800 kg ha^-1 was sufficient to obtain positive net returns ($12 ha^-1) (Table 8). It is important to note that in the present study, greater weed control was observed following CC terminated with GLY plus ACR/ATZ over the grain sorghum growing season compared to chemical fallow. The POST herbicides were not applied in chemical fallow treatment but farmers generally apply POST herbicides to control later emerged weeds and this application would increase the cost of production and decrease the net returns following chemical fallow as compared to CC treatment. At the highest sorghum price ($0.24 ha^-1), CC terminated with GLY plus ACR/ATZ would result in $48 ha^-1 net returns at only 2000 kg ha^-1 sorghum yield (Table 8). These results indicate the cost of integrating fall-planted CC in the W-S-F exceeded the benefits of improved weed control.

Results from this 3-year study indicate that integrating a fall-planted CC mixture after winter wheat harvest and terminating it with GLY in combination with residual herbicide before grain sorghum planting in the W-S-F rotation can provide an effective weed suppression in grain sorghum. Results showed that fall-planted CC terminated with GLY plus ACR/ATZ reduced total weed density by 34 to 81% and total weed biomass by 45 to 73% compared to chemical fallow over the grain sorghum growing season. No sorghum yield penalty was observed after CC terminated with GLY plus ACR/ATZ. However, due to the CC seed and planting costs, lower net returns were recorded in all three years compared to chemical fallow. These results suggest that growing CC for only weed suppression in the semi-arid CGP would not be profitable at current commodity prices. If the CC were used for hay or forage then net returns would be increased due to alternative use income (Holman et al. 2018, 2021, 2022, 2023; Obour et al. 2022a). However, weed control during the sorghum growing season would likely be affected after the CC residue removal from the field. Therefore, future studies should focus on understanding the timing for the removal of CC residue from the field and its interaction with weed control during the grain sorghum growing period.

Table 1. Total weed density and mean relative abundance of weed species observed in the cover crop (CC) treatments at 0 to 120 days after CC termination (DAT) in 2021^{a, b}

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05

^bThe cover crop was terminated on May 13, 2021

Table 2. Total weed dry biomass in the cover crop (CC) treatments at 0, 30, 60, 90, and 120 days after CC termination during 2021 to 2023 growing seasons^a

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter within a column are not different according to Fisher’s protected LSD at P < 0.05.

Table 3. Total weed density and mean relative abundance of weed species observed in the cover crop (CC) treatments at 0 to 120 days after CC termination (DAT) in 2022^{a, b}

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05.

^bThe cover crop was terminated on May 11, 2022

Table 4. Total weed density and mean relative abundance of weed species observed in the cover crop (CC) treatments at 0 to 120 days after CC termination (DAT) in 2023^{a, b}

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05.

^bThe cover crop was terminated on May 22, 2023

Table 5. Economic analyses of grain sorghum after fall-planted cover crop in 2021 growing season^a

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter among treatments are not different according to Fisher’s protected LSD at P < 0.05

^bGrain sorghum yield is in kg ha^-1

^cGrain sorghum price is in $ kg^-1

Table 6. Economic analyses of grain sorghum after fall-planted cover crop in 2022 growing season^a

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter among treatments are not different according to Fisher’s protected LSD at P < 0.05.

^bGrain sorghum yield is in kg ha^-1

^cGrain sorghum price is in $ kg^-1

Table 7. Economic analyses of grain sorghum after fall-planted cover crop in 2023 growing season^a

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine. Means followed by the same letter among treatments are not different according to Fisher’s protected LSD at P < 0.05

^bGrain sorghum yield is in kg ha^-1

^cGrain sorghum price is in $ kg^-1

Table 8. Net returns to possible grain sorghum yield (700 to 7400 kg ha^-1) and prices ($0.09 to $0.24 kg^-1) in the region^a

^aCC + GLY + ACR/ATZ indicates cover crop terminated with glyphosate plus a premix of acetochlor/atrazine

Objective 2:

The total amount of precipitation received during the spring-planted CC growing season (March to June) was 332, 164, and 184 mm in 2021, 2022, and 2023, respectively (Figure 1). No difference was recorded in aboveground CC dry biomass at the time of termination in 2021 and 2022, however, significantly higher CC biomass was recorded in 2023 and was 1240, 1290, and 4060 kg ha^-1 in 2021, 2022, and 2023, respectively. Although cumulative precipitation in 2021 was high, it was not uniformly distributed. A few major events led to waterlogging and poor CC growth, resulting in lower CC biomass that year.

Total weed density and weed biomass

Bassia scoparia, A. palmeri, E. canadensis, Venice mallow (Hibiscus trionum L.), carpetweed (Mollugo verticillata L.), and grasses mainly tumble windmillgrass (Chloris verticillata Nutt.) were the major weeds at the study site across three yrs. Among all, B. scoparia and A. palmeri were the dominant weed species based on their relative abundance, across three yrs (Tables 2-4).

2021 growing season. Bassia scoparia and A. palmeri were two dominant weed species in 2021 across all treatments and evaluation timings with a relative abundance of 14 to 100% and 23 to 70%, respectively (Table 2). Before termination or 0 DATe, CC reduced total weed density by 97 to 99% compared to chemical fallow. At this evaluation, no herbicide was applied in chemical fallow plots. CC terminated with GLY and GLY plus FLU/PYR reduced total weed density by 74 to 77% and 33% compared to chemical fallow at 30 and 90 DATe, respectively. Consistent with total weed density, CC at 0 DATe reduced the total weed dry biomass by 97% compared to chemical fallow (Table 5). Similarly, at 30 DATe, CC terminated with GLY and GLY plus FLU/PYR reduced total weed dry biomass by 40 to 44% compared to chemical fallow. However, there was no difference in total weed dry biomass between CC treatments and chemical fallow at 90 DATe. These results indicated that over time, CC residue degraded and was no longer able to suppress weeds. Osipitan et al. (2018) conducted a meta-analysis and reported that CC significantly reduced total weed density and biomass at termination and provided early-season weed suppression.

2022 growing season. Bassia scoparia was the dominant weed in 2022 with a relative abundance of 54 to 100% across all treatments and evaluation timings (Table 3). This dominance could be attributed to low precipitation during the growing season and early emergence ability of B. scoparia than A. palmeri. Only 135 mm was received from CC termination to 90 DATe in 2022 compared to 225 mm in 2021 (Figure 1). As B. scoparia is drought-tolerant, it thrived under these conditions; however, lower precipitation probably limited the germination of A. palmeri. Before termination, CC reduced total weed density by 94% compared to weedy fallow (Table 3). At 30 DATe, CC terminated with GLY and GLY plus FLU/PYR, and chemical fallow resulted in a 65 to 95% reduction in total weed density compared to weedy fallow. At 60 DATe, CC terminated with GLY plus FLU/PYR provided 88 to 89% reduction in total weed density compared to weedy fallow or chemical fallow. Similarly, at 90 DATe, CC terminated with GLY plus FLU/PYR resulted in 80%, 72%, and 60% reduction in total weed density compared to weedy fallow, chemical fallow, and CC terminated with GLY only, respectively (Table 3). These results indicate the importance of CC in combination with residual herbicide for reducing the total weed density as compared to chemical fallow. Obour et al. (2022) also reported 82% reduction in total weed density with spring-planted CC mixture (oat/triticale/peas) compared to weedy fallow. Consistent with total weed density, CC reduced total weed dry biomass by 93 to 95% before termination compared to weedy fallow (Table 5). The CC reduced sunlight penetration to the soil, suppressing weed seed germination and their seedlings emergence, competitiveness for resources, ultimately lowering weed biomass (Silva and Bagavathiannan 2023; Webster et al. 2016). At 60 DATe, CC terminated with GLY plus FLU/PYR reduced total weed biomass by 98, 92, and 82% compared to weedy fallow, chemical fallow, and CC terminated with GLY only, respectively. Similarly, at 90 DATe, CC terminated with GLY plus FLU/PYR reduced total weed dry biomass by 70, 36, and 44% compared to weedy fallow, chemical fallow, and CC terminated with GLY only, respectively. A low reduction in total weed dry biomass under CC terminated with GLY only over time indicated that only CC was insufficient to suppress weeds and highlighted a need for residual herbicide with CC. Wiggins et al. (2016) also reported that CC alone was not enough for season-long control of GR A. palmeri and suggested integrating residual herbicides to complement the suppressive effect of CC.

2023 growing season. Bassia scoparia, A. palmeri, and grass weeds (mainly C. verticillata) were dominant in 2023 across all treatments and evaluation timings with a relative abundance of 41 to 100%, 0 to 59%, and 0 to 27%, respectively (Table 4). Prior to termination, CC reduced total weed density by 78 to 80% compared to weedy fallow (Table 4). At 30 DATe, CC terminated with only GLY reduced total weed density by 42% compared to weedy fallow, however, CC terminated with GLY plus FLU/PYR and chemical fallow resulted in 82 to 97% reduction in total weed density compared to weedy fallow. At 90 DATe, both weedy fallow and CC terminated with GLY only had statistically similar total weed density and CC terminated with GLY plus FLU/PYR and chemical fallow had 52 to 71% lower total weed density than weedy fallow. Consistent with total weed density, CC before termination resulted in 97 to 99% total weed dry biomass reduction compared to weedy fallow (Table 5). At 30 DATe, CC terminated with GLY only resulted in 65% reduction in total weed dry biomass compared to weedy fallow, however, CC terminated with GLY plus FLU/PYR increased the total biomass reduction to 95% which was similar to chemical fallow. Similar trend was observed at 60 and 90 DATe. It is important to note that at 90 DATe, there was no difference in total weed density between weedy fallow and CC terminated with GLY only, however, the same CC treatment reduced total weed biomass by 52% compared to weedy fallow. These results indicate the suppressive action of CC on weed growth that would ultimately reduce the weed seed production (Baraibar et al. 2018).

 Wheat yield

During the 2022-23 wheat growing season, particularly in October and November 2022, only 10 mm of precipitation was received (Figure 1), which led to poor germination and eventual failure of winter wheat, therefore, spring wheat was planted in the spring of 2023. Wheat grain yield was greater in 2021-22 and ranged from 2789 to 3073 kg ha^-1 compared to 2022-23 and 2023-24, where wheat grain yield ranged from 570 to 952 kg ha^-1 (Table 6). The greater winter wheat grain yield in 2021-22 compared to 2023-24 might be due to higher precipitation in 2021-22 at critical wheat stages, including the jointing stage (Mar - Apr) and booting to heading stages (April - May) (Figure 1). There were no significant differences in wheat yield between CC treatments and chemical fallow in 2021-22 and 2022-23, however, in 2023-24, chemical fallow and CC terminated with GLY plus FLU/PYR resulted in greater yield followed by CC terminated with GLY only and least yield was recorded in weedy fallow. Holman et al. (2022) reported that fallow replacement in W-S-F rotation with spring-planted CC had no significant effect on wheat yield when conditions were either extremely dry with poor yields or very wet with above-average yields.

Results from this study indicate that integrating spring-planted CC mixture after grain sorghum in the W-S-F rotation and terminating it with GLY plus FLU/PYR herbicide can provide weed suppression up to 90 DATe. Before termination (mid to late June), spring-planted CC reduced total weed density by 78 to 99% and total weed biomass by 93 to 99% compared to weedy fallow. It is critical for early-emerging weeds like B. scoparia and A. palmeri which can start emerging in March-May (Dhanda et al. 2024; Dille et al. 2017; Kumar et al. 2018). With time, CC residue started to degrade and at 90 DATe, CC terminated with GLY plus FLU/PYR reduced total weed density by 52 to 80% and total weed dry biomass by 70% compared to weedy fallow across yrs. No differences in wheat grain yield between CC treatments and chemical fallow were recorded in 2021-22 and 2022-23, however, in 2023-24, chemical fallow and CC terminated with GLY plus FLU/PYR had greater yield than CC terminated with GLY only. It is important to note that under the low wheat yield scenario observed in the present study, net returns could become negative. This is because the reduced returns from wheat yield may not be sufficient to offset the costs of CC seed and planting, as highlighted in our previous study (Dhanda et al. 2025b). Future studies should focus on integrating other weed control methods along with CC to control late-emerging weeds.

Table 1. Planting and termination dates for cover crop and planting and harvesting dates for grain sorghum and wheat during 2020-22, 2021-23, and 2022-24 seasons at Kansas State University Agricultural Research Center near Hays, KS.

*Winter wheat planted on September 30, 2022, did not germinate, therefore, spring wheat was planted

Table 2. Total weed density and mean relative abundance of weed species observed in the cover crop (CC) treatments at 0, 30, and 90 days after CC termination (DATe) in 2021-22 at Kansas State University Agricultural Research Center near Hays, KS^a.

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + FLU/PYR indicates cover crop terminated with glyphosate plus a premix of flumioxazin/pyroxasulfone. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05

Table 3. Total weed density and mean relative abundance of weed species observed in the cover crop (CC) treatments at 0, 30, 60, and 90 days after CC termination (DATe) in 2022-23 at Kansas State University Agricultural Research Center near Hays, KS^a.

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + FLU/PYR indicates cover crop terminated with glyphosate plus a premix of flumioxazin/pyroxasulfone. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05

Table 4. Total weed density and mean relative abundance of weed species observed in the cover crop (CC) treatments at 0, 30, 60, and 90 days after CC termination (DATe) in 2023-24 at Kansas State University Agricultural Research Center near Hays, KS^a.

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + FLU/PYR indicates cover crop terminated with glyphosate plus a premix of flumioxazin/pyroxasulfone. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05

Table 5. Total weed dry biomass in the cover crop (CC) treatments at 0, 30, 60, and 90 days after CC termination during 2021 to 2024 growing seasons at Kansas State University Agricultural Research Center near Hays, KS^a.

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + FLU/PYR indicates cover crop terminated with glyphosate plus a premix of flumioxazin/pyroxasulfone. Means followed by the same letter within a column at each timing are not different according to Fisher’s protected LSD at P < 0.05

Table 6. Spring-planted cover crop (CC) effect on winter wheat yield over three growing seasons at Kansas State University Agricultural Research Center near Hays, KS^a.

^aCC + GLY indicates cover crop terminated with glyphosate only and CC + GLY + FLU/PYR indicates cover crop terminated with glyphosate plus a premix of flumioxazin/pyroxasulfone. Means followed by the same letter within a column are not different according to Fisher’s protected LSD at P < 0.05

^bIn 2022-23, because of low rainfall, winter wheat did not germinate, therefore, spring wheat was planted

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