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

View the project final report

Commodities

• Agronomic: triticale

Practices

• Crop Production: cover crops, no-till
• Pest Management: cultural control, integrated pest management, weed ecology

The Central Great Plains (CGP) is characterized by a semi-arid climate with relatively low annual precipitation (~300 to ~1200 mm) (Lenssen et al. 2007; NOAA 2024). To conserve soil moisture and to prevent soil erosion by wind, no-tillage (NT-) and fallow-based cropping systems are widely adopted in the region. Successful adoption of these soil conservation practices was achieved utilizing chemical-based weed control (Hansen et al. 2012; Kumar et al. 2020). However, the adoption of NT-based production systems has resulted in weed species representing smaller-seeded weeds like kochia [Bassia scoparia (L.) A.J. Scott], Palmer amaranth (Amaranthus palmeri S. Watson), horseweed [Erigeron canadensis (L.) Cronquist], common lambsquarters (Chenopodium album L.), Russian thistle (Salsola tragus L.), downy brome (Bromus tectorum L.), wild oat (Avena fatua L.), foxtail species (Setaria spp.), and tumble windmill grass (Chloris verticillata Nutt.) (Jha et al. 2016; Nichols et al. 2015).

Winter wheat (Triticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]-fallow (W-S-F) is a dominant crop rotation in the CGP region (Holman et al. 2022). This 3-year crop rotation includes a fallow period of approximately 10 months between winter wheat harvest and sorghum planting as well as 10 months of fallow period between sorghum harvest and the next winter wheat planting (Kumar et al. 2020). Continuous reliance on herbicides with the same site(s) of action for weed control has resulted in the evolution of herbicide-resistant weeds, including B. scoparia, A. palmeri, and E. canadensis (Heap 2024). For instance, glyphosate resistance is widespread among B. scoparia and A. palmeri populations in Kansas and other neighboring states in the CGP region (Heap 2024; Kumar et al. 2019a, 2019b, 2020; Westra et al. 2019). Evolution of glyphosate-resistant (GR) weed populations and limited availability of alternative effective herbicide options pose a serious production challenge for grain sorghum producers in the region. Previous researchers have reported that season-long weed interference can result in an average grain yield loss of 47% in sorghum, which is an estimated loss of around US $953 million annually (Dille et al. 2020). Therefore, alternative integrated weed management strategies are needed to achieve effective control of herbicide-resistant weed populations in grain sorghum.

Integration of cover crop (CC) in crop rotations has been proven as one of the effective tools to suppress herbicide-resistant weeds in the CGP region (Kumar et al. 2020; Mesbah et al. 2019; Obour et al. 2022a; Petrosino et al. 2015). Growing CC in the semi-arid CGP also provides several other benefits, including reduced soil erosion, enhanced nutrient cycling, increased microbial activity, improved soil health, and increased plant diversity and pollinator resources (Blanco-Canqui et al. 2011, 2013; Simon et al. 2022). Additionally, CC residue left on the soil surface after termination reduces soil temperature and soil moisture evaporation, thereby contributing to increased soil water storage (Holman et al. 2020, 2021). However, replacing the fallow period with CC in the semi-arid cropping systems sometimes reduces the yield of successive crops because of the reduced plant available water (Holman et al. 2018; Nielsen et al. 2016). However, the United States Department of Agriculture Natural Resources Conservation Service (USDA NRCS) provides some financial support to growers under the Environmental Quality Incentives Program (EQIP) to pay some of the cost of growing CC and improve net returns (Anonymous 2024). Previous studies have evaluated the effect of spring-planted CC on weed suppression and winter wheat yields when CCs replaced the fallow phase of W-S-F rotation in this region (Holman et al. 2022; Mesbah et al. 2019; Obour et al. 2022a). For instance, Obour et al. (2022a) reported that spring-planted CC (oats/triticale/spring peas) in W-S-F rotation can reduce weed biomass by 86 to 99% compared to weedy fallow. Holman et al. (2022) reported that spring-planted CC had no significant effect on wheat and grain sorghum yields when conditions were either extremely dry with poor yields or very wet with above-average yields, however, replacing fallow with CC increased the cost of production by 16 to 97% compared to fallow.

Farmers are currently relying on residual herbicides to manage GR weeds in the dryland W-S-F rotation (Kumar et al. 2020). Several researchers have previously documented the importance of residual herbicides in combination with CC to achieve season-long weed control (Perkins et al. 2021; Whalen et al. 2020). For instance, Whalen et al. (2020) reported that CC terminated with glyphosate plus 2,4-D in combination with residual herbicides (sulfentrazone plus chlorimuron) resulted in greater waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer.] control (73 to 84%) compared to no residual herbicide (44 to 65%). Most CC weed suppression research studies were conducted in cotton (Gossypium hirsutum L.), corn (Zea mays L.), or soybean [Glycine max (L.) Merr.] in greater precipitation environments (Weisberger et al. 2023; Whalen et al. 2020). However, limited information exists regarding the integration of fall-planted CC in combination with soil residual herbicides at the termination of CC on weed suppression in subsequent grain sorghum in the semi-arid CGP region.

For fall-planted CC, a mixture (triticale/winter peas/radish/rapeseed) was planted after wheat harvest and terminated at triticale heading stage before sorghum planting. Treatments included nontreated control, chemical fallow, CC terminated with glyphosate (GLY), and CC terminated with GLY+ acetochlor/atrazine (ACR/ATZ). Across three years, CC terminated with GLY+ACR/ATZ reduced total weed density by 34-81% and total weed biomass by 45-73% compared to chemical fallow during the sorghum growing season. Average grain sorghum yield was 786 to 1432 kg ha^-1 and did not differ between chemical fallow and CC terminated with GLY+ACR/ATZ. However, net returns were lower with both CC treatments (USD -$275 to $66) in all three years compared to chemical fallow (USD -$111 to $120). These results suggest that fallow replacement with fall-planted CC in the W-S-F rotation can help suppress GR B. scoparia and A. palmeri in the subsequent grain sorghum. However, the cost of integrating CC exceeded the benefits of improved weed control and lower net returns were recorded in all three years compared to chemical fallow.

For spring planted CC, a mixture of oats (Avena sativa L.)–barley (Hordeum vulgare L.)–spring peas (Pisum sativum L.) was spring-planted in no-till sorghum stubbles and terminated at oats heading stage. Four treatments, including (1) weedy fallow (no CC and no herbicide), (2) chemical fallow (no CC but glyphosate + flumioxazin/pyroxasulfone + dicamba), (3) CC terminated with glyphosate, and (4) CC terminated with glyphosate + flumioxazin/pyroxasulfone were tested. Across 3 yrs, CC at termination reduced total weed density by 78 to 99% and total weed biomass by 93 to 99% as compared to weedy fallow. Weed suppression by the CC terminated with glyphosate plus flumioxazin/pyroxasulfone continued for at least 90 days with reduced total weed density of 52 to 80% and total weed biomass reduction by 70% compared to weedy fallow across 3 yrs. No differences in subsequent 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 glyphosate + flumioxazin/pyroxasulfone had greater wheat yield than CC terminated with glyphosate only. These results suggest that integration of spring-planted CC with residual herbicide may help suppress GR B. scoparia and A. palmeri in the CGP.

The main objectives of this study were (1) to determine the effect of fall-planted cover crop (CC) in combination with soil residual herbicides on weed suppression (density and biomass) in subsequent grain sorghum, grain yield, and net returns, (2) to determine the combined effects of spring-planted CC and soil residual herbicide on weed suppression during fallow period, and its impact on subsequent winter wheat yield.