Cover Crop Management Options to Improve Weed Control, Crop Yield and Soil Health

On-farm field study results

Cover crop production and grazing days

In general, the grass species in the mixture dominated CC biomass at each location. Total CC biomass after grazing at Alexander, KS, (spring planted) and Marquette, KS (fall planted) were not different from biomass measured before grazing (Table 1). Suggesting significant regrowth from the cool season CCs.  However, the post-grazed biomass was less than that of non-grazed CC in most cases. In 2019 at Hays, KS, forage mass at the time of grazing the summer CC was similar to non-grazed CC treatment because there was no regrowth between grazing and the first killing frost in October that terminated CC growth. The total forage mass of the summer CC (2019 in Hays and 2020 at Alexander) were two to five-fold   greater than the spring or fall planted CCs. For example, spring CC biomass in 2021 at Hays averaged 1400 lb/a compared to average biomass of 6900 lb for summer CCs planted in 2019 in the same field.

Table 1. Cover crop biomass (grazed and non-grazed) at Alexander, Hays and Marquette, KS from 2019 to 2021.

^†Means in a row followed by different letters indicate significant differences among cover crop management at α < 0.05.

Spring CCs in Alexander, KS were grazed from May 14, 2019 to mid-June (6/14/2019) for 31 grazing days. Stocking rate was 354 lb/ac with an average daily gain of 3.1 lb/day. Similarly, summer CCs were grazed from August 7 to September 18, 2020 (41 grazing days). Stocking rate was 576 lb/ac with an average daily gain of 1.6 lb/day. Similarly, in Marquette, stocking rate averaged 550 lb/a and average daily gain was 1.2 lb/day for > 40 grazing days from 2019 through 2021 (Table 2). Grazing days and stocking rate varied at Hays because of the difference is calving time and when were turn. Notwithstanding, the lactating cows maintained their body weight by grazing on the summer CC in 2019 (Table 2).  Because of dry conditions and little forage production, cattle grazed for only 25 grazing days in 2021 at Hays.

Table 2. Grazing days and animal performance of cattle grazing cover crops in western Kansas.

†Fall calving pairs were turned out in groups as they calved.

Grazing effects on soil properties

At Hays and Alexander, soil bulk density, SOC, nutrient concentrations and mean weight diameter (MWD) of water stable aggregates were not different between grazed and non-graze CCs (Table 3 & 4). However, compared to the initial measurements, grazed and non-grazed CCs had significantly greater SOC and NO₃-N concentration particularly at Hays (Table 4).  The SOC measured in the spring 2019 at surface 0 to 2-inch depth was 1.4%, less than 2.1% SOC measured in 2021 following two cycles of grazing CCs at this location (Table 4). The MWD of water stable aggregates at Hays in 2021, averaged 1.45 mm with grazed and 1.96 mm for non-grazed CCs. Similarly, at Alexander, MWD measured in 2021 was unaffected by grazing CCs (Table 3). Wind-erodible fraction was not different between CC treatments. Penetration resistance at the 0- to 6-inch soil depth with grazed CCs was not different compared to non-grazed CCs and averaged 0.52 and 0.52 MPa with grazed and non-grazed CCs, respectively.

At Marquette, measured soil properties were not different between grazed and non-grazed CCs.  Soil bulk density measured within the top 0 to 2-inch in grazed plots averaged 1.23 g/cm³ compared to 1.43 g/cm³ for the non-grazed treatment at the Marquette in 2019 (Table 3).  The bulk density measured on this same field in 2021 averaged 1.38 g/cm³ for grazed and 1.36 g/cm³ for the non-grazed treatment in the 0–2-inch depth.  The bulk density measured at 2–6-inch depth was 1.53 g/cm³ for grazed and 1.49 g/cm³ for non-grazed treatment.  The penetration resistance measurements taking following grazing in 2021 averaged 0.36 and 0.34 MPa with grazed and non-grazed CCs, respectively, at Marquette. The measured penetration resistance across locations and CC management strategies was below the threshold of 2 MPa that will limit root growth. The SOC concentration after two cycles of CC measured in the top 0–2-inch depth in 2021 averaged 1.74% with grazed CCs and 1.62% for non-grazed.  The SOC at 2-6 depth was unaffected by CC management and averaged 1.22 % and 0.97% for grazed and non-grazed CCs, respectively (Table 4).

Table 3. Cover crop grazing effects on soil bulk density (BD), mean weight diameter (MWD) of aggregates, wind erodible fraction (WEF), and penetration resistance (PR) at Alexander, Hays, and Marquette, KS.

^†Means in a row followed by different letters indicate significant differences among cover crop management at α < 0.05.

Table 4. Cover crop grazing effects on soil organic carbon (SOC), nitrate-N, and phosphorus concentrations at Alexander, Hays, and Marquette, KS.

^†Means in a row followed by different letters indicate significant differences among cover crop management at α < 0.05.

Results at Kansas State University HB Ranch near Brownell, KS

Forage production and nutritive value

Forage mass produced varied over the study period because of variations in soil water availability and air temperature in the spring.  Average forage mass was greatest in 2018 and least in 2020 (Table 5). The lower CC forage mass production in 2020 was due to drier than normal spring conditions that delayed cover crop growth. Similarly, wet soils in March 2021 delayed planting which decreased forage production. The hay treatment was harvested at a greater height (6 inches) and therefore had relatively lower yields than cover treatments (clipped at 2 inches). In 2019 and 2021 when there was time for regrowth before CC termination, biomass left after grazing was similar to that measured pre-grazing. Residue left post-grazing ranged from 69 to 84% of that biomass of the non-grazed CC treatment across years. Therefore, careful grazing of CCs can leave adequate amount of residue to protect the soil to achieve soil health goals while providing a forage resource for livestock.

In general, grazed CC treatments had more CP, and IVDMD concentrations than CC hayed treatments. Similarly, the hayed treatment had significantly greater ADF and NDF concentrations compared to the grazed treatments (Table 5). This was expected because grazed treatments were usually sampled 7 to 10 days earlier than the hayed treatments. Delaying harvest resulted in more mature plants reducing forage digestibility and nutritive value. Nonetheless, in a production setting, grazing of forage would likely begin at a more immature stage of forage growth and the quality would match the needs of stocker cattle.

Table 5. Spring planted cover crop forage mass and nutritive content at heading, before grain fill over four years at the Kansas State University experiment fields at HB Ranch near Brownell, KS from 2018 to 2022.

¹CP = crude protein. ADF = acid detergent fiber (higher values reflect lower digestibility). NDF = neutral detergent fiber (higher values reflect lower animal intake). IVDMD = in vitro dry matter digestibility (reflects relative energy differences).^†Means in a row followed by different letters indicate significant differences among cover crop management at α < 0.05.

Wheat yields

Spring CCs reduced winter wheat yields between 25 and 31% compared to fallow in two out of five years with no yield differences between CC management strategies. However, when average across the 5-years (2018 to 2022), wheat yield were not different among treatments.  Yields averaged 47 bu/for fallow compared with 44 bu/a for standing or grazed CCs, 47 bu/ac when hayed, and 48 bu/a for flex-hayed CCs (Fig.2).  Yields of grain sorghum grown one year following CCs in the crop rotation were unaffected by CC treatments.  Grain sorghum yields averaged 75 bu/for fallow, 77 bu/a for grazed CCs, 71 bu/ac when hayed, and 74 bu/a for standing CCs. These resul ts showed CCs can be grazed or hayed in NT dryland cropping systems with no negative effects on wheat and grain sorghum yields compared to standing CCs.  Grazing or haying of CCs enrolled in NRCS cost-share programs can be effective strategies to offset losses when grain yields are reduced and could potentially increase CC adoption and overall dryland cropping system profitability. 

Weed suppression

Alexander in 2019

In 2019, weed density was compared in chemical fallow plots with grazed and non-grazed CCs after grazing in Alexander. Data on total weed density and weed biomass (primarily kochia and field bindweed species) were collected from each plot using eight 1-m² quadrats. Results indicated no differences in total weed biomass (40 to 48 g m^-2) in grazed vs. non-grazed plots (Fig. 3). The use of herbicide in fallow plots resulted in significant reduction of total weed biomass (11 g m^-2).

HB Ranch Study (2020 through 2022 at HB Ranch)

The on-going spring-planted CC grazing study at Kansas State University HB Ranch was monitored for weed suppression in 2020, 2021 and 2022.  The treatments were oat/triticale CC which were either hayed, grazed by yearling heifers, or left standing compared to chemical fallow. Approximately four herbicide applications were made at the recommended use rate to control weeds in the chemical fallow plots. Application of dicamba with glyphosate in March, and paraquat in May. Two more sequential burndown applications of glyphosate were made in July and September to control weeds prior to winter wheat planting in October.  Weed density measurements were done in mid-June at termination of CCs. In 2020, total weed density and weed biomass (primarily glyphosate-resistant kochia) were collected from each plot using six 1-m² quadrats before CC termination.  Results showed no differences in total weed biomass accumulation (0.8 to 1.7 g m^-2) in grazed and non-grazed CC plots (Fig. 4). The fallow plots had significant higher total weed biomass accumulation (140 g m^-2). Dry conditions prevented weed regrowth in fallow plots after herbicide application in May 2021. Similarly, significant weed suppression from CC resulted in no measurable weed biomass in CC grazed and non-grazed at termination. In 2022, growing a CC either grazed, hayed or non-grazed significantly reduce weed biomass compared to chem-fallow (Fig. 4).

Hays, KS 2021 and 2022

Another spring-planted CC (spring oats/barley/spring peas mixture) study in sorghum stubble was conducted at Kansas State University Agricultural Research Center in Hays to evaluate CC herbicide termination strategies on weed suppression. Cover crop was terminated with two different herbicide programs: (1) glyphosate only or (2) glyphosate plus soil residual herbicide (flumioxazine + pyroxasulfone) on June 26, 2021. Chemical fallow plots (where herbicides were applied to keep them weed free) were also maintained for treatment comparison. Data on total weed density and weed biomass (primarily glyphosate and dicamba-resistant kochia and Palmer amaranth) were collected from each plot using two 1-m² quadrats at CC termination and 40 days after termination. Results showed CC treatments reduced weed biomass more than 98% compared to chem-fallow at the time of cover crop termination (Fig. 5). Furthermore, CC plots terminated with glyphosate or glyphosate plus soil residual herbicide resulted in significant weed biomass reduction at 40 days after termination in comparison to chem-fallow plots (62 g m^-2) (Fig. 5). Similarly in 2022, CCs terminated with glyphosate or glyphosate plus soil residual herbicide provided more than 95% suppression at 60-day after termination compared to non-treated fallow (Fig.5).

Soil bulk density, organic matter and aggregate stability

Grazing or haying CCs had no significant effect on bulk density from 2018 through 2021.  Averaged across the four years, fallow, non-grazed CCs, and grazed CCs had soil bulk densities of 1.11, 1.15, and 1.15 g/cm³ at the 0- to 2-inch soil depth and 1.39, 1.40, and 1.37 g/cm³ at the 2- to 6-inch soil depth, respectively. However, a significant precipitation event ( > 76 mm) that occurred during grazing in earlier years of the study in 2015 (before the current study), resulted in a significant increase in soil bulk density at 0 to 2 inch  depth . No difference in bulk density was observed beyond the top 2 inches over the study period. The SOC stocks measured in 2019 with standing and hayed CCs were greater than fallow (24.79 Mg ha^-1) which was similar to grazed CCs. However, in 2020, stocks were less with hayed CCs (21.80 Mg ha^-1) compared to grazed or standing CCs (24.27 Mg ha^-1) and all were similar to fallow.

Mean weight diameter of water stable aggregates in the 0 to 5 cm soil depth  in 2019 and 2020 was greater with all CCs (standing, grazing, or hayed) compared to fallow (Fig. 6). Mean weight diameter was 2.79-mm for the standing CCs, 2.54-mm for the hayed CCs, 3.05-mm for the grazed CCs, and 1.78-mm for fallow. This indicates that CCs have the ability to increase soil aggregation similarly when standing, hayed, or grazed. Additionally, all CCs were found to increase the proportion of large macroaggregates (> 2-mm) compared to fallow (Fig. 7).

Garden City, KS: 2019 to 2021 growing season.

Crop data: In Garden City, forage productivity varied over the study period. Forage mass of the cocktail treatment was 1700 lb/a, 620 lb/ac and 908 lb/a in 2019, 2020 and 2021, respectively. Similarly, oat/triticale CC biomass were 2500, 617, and 1395 lb/ac in 2019, 2020, and 2021, respectively. The productivity of the flex-cover crop was like the oat/triticale treatment when it was planted in 2019 and 2020. 

Wheat yields following CCs were not different from fallow in 2019. Yields averaged 90 bu/ac for fallow compared to 89 bu/ac for oat/triticale hayed or 86 bu/ac for oat/triticale standing and 91 bu/for the cocktail. Dry conditions in 2020 significantly reduced wheat yields, which averaged 23 bu/a with fallow and 19 bu/a with oat/triticale hayed or standing.

Soil samples were also collected from the Garden City study in fall 2018 and summer 2019 before modifying the experiment to examined long-term CC management effects on soil properties in a no-till (NT) winter wheat- grain sorghum-fallow cropping system in southwest Kansas. Treatments were all spring-planted and included one-, three-, and six-species CC mixtures compared with chemically controlled NT fallow. Half of each CC treatment was harvested for forage. Soil organic carbon (SOC) stocks within the 0- to 15-cm soil depth were greater with CCs compared to fallow in 2012 after three cycles of CCs in the initial wheat-fallow rotation. In 2018 and 2019 after two cycles of the WSF rotation, SOC stocks were similar across all treatments including fallow likely, because CC residue inputs declined due to a succession of drought years. The significant residue contribution from grain sorghum in the WSF rotation increased SOC in 2018 and 2019 in all treatments compared to 2012. Soil aggregation was greater with CCs compared to the fallow treatment and was unaffected by CC diversity. Mean weight diameter (MWD) of water stable aggregates

 was 1.11 mm with standing CCs compared with 0.77 mm for pea.  The MWD of dry aggregates with standing (3.55 mm) and hayed CCs (3.62 mm) were greater compared to fallow (2.75 mm). Water infiltration rates and saturated infiltrability measured in summer 2019 were greater with CCs compared. These findings suggest simple CC mixtures and CCs managed for annual forage provide similar soil benefits as diverse CCs mixtures and CCs left standing.

Results of Flex-Cover Crop Study at Brownell and Garden City, KS

Crop residue cover at winter wheat planting in the fall of 2021 was unaffected by CC management practices compared to chemical fallow at Brownell, KS (Fig. 8). Crop residue cover measured on the fallow plots exceeded 80% at winter wheat planting. However, in the 2022 season, standing CCs increased crop residue cover 29% and 24% compared to fallow at Brownell (Fig 8.) and Garden City, KS (Fig. 9), respectively. Across locations, hayed and flex-hayed CCs had similar levels of crop residue cover compared to fallow. Although CC biomass was removed as hay, the 15 cm cutting height of the forage harvester maintained similar or somewhat greater residue cover compared to fallow.

Across locations in 2021, soil bulk density, SOC stocks, and wind-erodible fraction were unaffected by CC management practices compared to chemical fallow regardless of soil depth (0-5 and 5-15 cm) (Table 6). This indicates that haying or grazing CCs did not cause any soil compaction

 compared to standing CCs or fallow. Soil organic carbon is a soil chemical property that is slow to respond to changes in management, especially in semi-arid dryland cropping systems where crop residue inputs are relatively low. Results indicate that hayed and flex-hayed CCs did not affect SOC stocks compared to standing CCs, which suggests that CC biomass removal as hay did not limit organic carbon inputs from crop residue compared to fallow. The flex-hayed CCs were not different compared to the fixed rotations with hayed CCs or fallow. The wind-erodible fraction, which is the fraction of soil <0.84-mm diameter, was unaffected by CC management compared to fallow. This property indicates the susceptibility of the soil to wind erosion though actual soil loss will also be dependent on crop residue cover. In the 2021 growing season at Brownell, crop residue cover was unaffected by CC management compared to fallow (Fig. 8). However, in the 2022 growing season, standing CCs increased crop residue cover compared to fallow though hayed CCs and flex-hayed CCs were not different from fallow.

Table 6. Cover crop management effects on soil physical and chemical properties in the 0-5 and 5-15 cm soil depths at winter wheat planting in 2021 at Brownell and Garden City, KS.

^†Means are not significantly different (α = 0.05).

In the 2021 growing season at Brownell, all CC management practices (standing, hayed, and flex-hayed) increased the mean weight diameter of water stable aggregates in the 0-2-inch cm soil depth by 39 to 70% compared to chemical fallow (Fig 10). This indicates that CCs could increase mean weight diameter even when CC biomass is harvested for hay or when CCs are used in flexible cropping systems. At Brownell, flex-hayed CC plots were planted to CCs in 2021 but had not been planted in 2018, three years prior in the winter wheat-grain sorghum-fallow cropping system. This indicates that MWD was not limited by a legacy of fallow in this flexible cropping system.

In the 2-6 inch soil depth, mean weight diameter was 78 and 53% greater with standing and flex-hayed CCs compared to fal

low, respectively (Fig. 10). However, hayed CCs were not significantly different from fallow. This was surprising as past investigations of CC management effects on soil properties have shown treatment differences only in the surface 0-2-inch soil depth. These results suggest that standing CCs can increase mean weight diameter compared to fallow. However, hayed CCs may not always increase mean weight diameter compared to fallow.

In the 2021 growing season at Garden City, standing and hayed CCs increased the mean weight diameter of water stable aggregates in the 0-2-inch soil depth 98% compared to the traditional fallow (Fig. 11). However, flex-hayed CCs were unaffected compared to fallow. These results were similar to those observed at Brownell (Fig. 10) and indicate that CCs could increase mean weight diameter even when CC biomass is harvested for hay. At Garden City, flex-hayed CCs were not planted in 2021 nor in 2018 three years prior in the winter wheat-grain sorghum-fallow cropping system. In both growing seasons, criteria for CC planting were not met so the plots were management the same as the fixed fallow cropping system.

In the 2-6-inch soil depth, MWD was 23% lower under the flex-hayed CCs treatment compared to standing and hayed CCs though all CC management treatments were similar to fallow (Fig. 11). As previously mentioned, flex-hayed CCs had not been planted at Garden City in the 2018 growing season so that this treatment was managed the same as fallow. These results indicate that CCs, whether standing or hayed, may increase mean weight diameter in the 2 to 6-inch soil depth compared to fallow.

In conclusion, standing CCs may increase crop residue cover compared to fallow and hayed CCs may maintain crop residue

cover when 6-inch stubble remains after harvest. Bulk density, SOC, and wind-erodible fraction were unaffected by CCs compared to fallow. Hayed and flex-hayed CCs were not significantly different from standing CCs. Cover crop management practices may increase mean weight diameter of water stable aggregates compared to fallow in both the 0-5 and 5-15 cm soil depths. Harvesting CCs for hay may limit the effect compared to standing CCs. However, flex-CCs may increase mean weight diameter compared to fallow despite a legacy of fallow in past growing seasons when growing conditions were not favorable for planting a CC.