2017 FINAL STUDY REPORT Bridger Plant Materials Center, Bridger, MT

Evaluation of Two Cowpea Varieties in a Cover Crop Mix Mark Henning, Area Agronomist, Miles City, MT

Robert Kilian, (Retired) Rangeland Management Specialist, Bridger PMC, MT

ABSTRACT

Cowpea (Vigna unguiculata L.) is used in cover crop mixes in Montana to add plant diversity and provide ecosystem services such as nitrogen fixation. However, on-farm observations indicate that their performance, in terms of number of plants established and biomass produced in diverse, warm season mixes, has been weak. A 2016 study at the Bridger Plant Materials Center (BPMC) in Bridger, MT comparing cowpea and mung bean varieties as monocultures indicated that the cowpea variety ‘Red Ripper,’ due to its’ prostrate plant habit, may perform better in mixes than the upright ‘Iron & Clay’ variety, which is commonly used in cover crop mixes. Therefore, in 2017 a study was conducted at the BPMC to determine if there were any performance differences in these two cowpea varieties planted in a warm season cover crop mix. ‘Red Ripper’ and ‘Iron & Clay’ were added separately to a six-way warm season cover crop mix at 20 lb/acre and planted in a randomized complete block design under dryland conditions. There were no significant differences in stand count or biomass between ‘Red Ripper’ and “Iron & Clay.’ The six-way warm season cover crop mix in the ‘Red Ripper’ treatment had more biomass (7399.3 lb/acre) than the mix in the ‘Iron & Clay’ treatment (4904.5 lb/acre). This was likely due to variation within the experimental area. When cowpea biomass was added to the cover crop mix biomass, there were significant differences in total biomass between treatments, with the ‘Red Ripper’ treatment yielding higher (7514.4 lb/acre) than the ‘Iron & Clay’ treatment (5033.2 lb/acre). Available forage (dry matter basis) at 50% utilization was 2400 to 3800 lb/acre, a decent amount considering the degraded soil in a dryland system. The cowpea biomass, regardless of variety, was a very small percentage of the total cover crop mix biomass (2 to 3%). Poor performance of cowpeas in this experiment was likely due to competition from warm season grasses in the mix, including sorghum-sudangrass, millet, and corn. The soil in the plots contained high residual nitrate-N, which grasses can preferentially scavenge, thus providing them a distinct advantage over the cowpeas. Future research should focus on integrating cowpea into broadleaf-only cover crop mixes, which would allow for improved weed control options, enhanced pollinator habitat, and possibly enhanced ecosystem services such as nitrogen fixation.

INTRODUCTION Cover crop mixes in Montana are increasingly utilized by producers as a tool to improve soil health. Producers plant cover crop mixes for a variety of reasons, such as supplemental grazing, increasing nitrogen (N) for crop production, and reducing soil compaction. Mixes typically contain five or more species, and are often dominated by warm season species, as warm season crop types are often missing in crop rotations. While there are excellent choices for warm season grasses (millet, sorghum, corn, sudangrass, etc.), warm season legume choices are limited. Soybeans, cowpeas, and mung beans are all options and show promise in Montana. However,

Mark Henning, Area Agronomist, 3120 Valley Drive East, Miles City, MT, mark.henning@mt.usda.gov Robert Kilian, berto.rk84@gmail.com

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soybeans are not very drought tolerant, limiting their potential in dryland systems. Cowpeas are drought tolerant, and prefer higher temperatures (Davis et al., 2017).

Cowpeas have been tried in mixes with variable success, possibly due to use of poorly adapted varieties. Warm season mixes may also simply outcompete cowpea, specifically warm season grasses such as millet and sorghum-sudangrass. Using an adapted cowpea variety that is more compatible with warm season grasses may help to overcome this issue. In 2016 at the BPMC, a variety study was conducted testing two mung bean varieties and four cowpea varieties. All varieties, despite harsh soil and weather conditions, performed well (Figure 1). The cowpea variety ‘Red Ripper’ had a prostrate plant habit (Figure 2), in contrast to the upright habit of ‘Iron & Clay,’(Figure 3) possibly allowing it to be more compatible in a warm season cover crop mix.

Figure 1. Mung bean and cowpea variety trial plot, August 5, 2016, BPMC, Bridger, MT.

Figure 2. ‘Red Ripper’ cowpea variety, BPMC, Bridger, MT, 2016.

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Figure 3. ‘Iron & Clay’ cowpea variety, BPMC, Bridger, MT, 2016.

This study’s primary objective was to compare “Red Ripper’ and ‘Iron & Clay’ cowpea varieties in a six-way cover crop mix of BMR (brown midrib) sorghum-sudangrass, German millet, grazing corn, forage collards, turnips, and sunflower. Warm season legumes can add needed crop type diversity (warm season broadleaf component), but can substantially increase the cost of cover crop mixes, so finding an effective variety is important for economic reasons. Cost per acre can vary from $13.50 (no cowpea) to $30.50, depending on the cowpea rate (Table 1).

Table 1. Seeding rates and cost per acre of various cover crop mixes, 2017.1 Cover Crop Mix Seeding Rate (lb.) Seeds/Acre # Seeds/ft2 Cost/Acre cover crop mix only 16 823,500 19 $13.50 5 lb. cowpea + mix 21 844,000 19 $17.75 10 lb. cowpea + mix 26 864,500 20 $22.00 20 lb. cowpea + mix 36 905,500 21 $30.50

1 Based on a 100-acre seed order, Green Cover Seed, Bladen Nebraska.

MATERIALS AND METHODS Twenty pounds of ‘Iron & Clay’ and ‘Red Ripper’ cowpea varieties were added separately to a six-way cover crop mix (BMR sorghum-sudangrass, German millet, grazing corn, forage collards, turnips, sunflower; see Table 2 for rates). Cowpea varieties and the mix were procured from Green Cover Seed (Bladen, NE), and are representative of species planted as a full-season mix for grazing in eastern Montana.

Mixes were inoculated with the appropriate rhizobia strain just prior to seeding and no fertilizer was added to the plots. There were four replications in a randomized complete block design. Plots were seeded on June 1 at 14-inch row spacing using a four-row cone planter (Kincaid Equipment Manufacturing®, Haven, KS) equipped with double disk furrow openers, depth

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bands, and double packer wheels. Each plot measured 9.4 feet wide by 50 feet long and consisted of eight rows. The soil, a Heldt silty clay loam (Soil Survey of Carbon County Area, Montana), was moist at the planting depth of approximately 2 inches. There was no residue cover, and the site has a long history of conventional tillage due to seed production (Figure 4). The Bridger, Montana area is in a 10 to 11-inch annual precipitation zone with frost free dates of May 19 through September 20. No irrigation was used. Total precipitation for January through September was 9.39 inches (Table 3). Soil samples (0 to 6, 6 to 24 inches) were taken on June 14 and analyzed by AgVise Laboratories (Northwood, ND) for organic matter %, N, P, K, S, soluble salts, and pH. Table 2. Background cover crop mix seeding rate, legume seeding rate study, BPMC, Bridger, MT, 2017. Species Variety Seeding Rate

lb/acre BMR sorghum-sudangrass ‘Sweet Forever’ 5 German millet ‘Golden 88’ 2 Grazing corn ‘BMR84’ 5 Collards ‘Impact Forage’ 1 Purple top turnip VNS 1 Sunflower Black Oilseed 2

Total: 16

Figure 4. Plot seeding, cowpea variety study, June 1, 2017, BPMC, Bridger, MT.

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Table 3. Monthly precipitation, January through September, BPMC, Bridger, MT, 2017. Month Precipitation --inches-- January 0.32 February 0.72 March 0.67 April 2.47 May 0.89 June 0.46 July 0.31 August 0.28 September 3.27

Total: 9.39 Cowpea plant counts for 120 feet of row per plot (inner four rows x 30 feet long) were taken on August 17, 2017. Plant biomass was harvested to ground level September 18 and 19 in a 14.1 square foot area (middle four rows x 3 feet long), dried for 48 hours at 50°C, and weighed. Cowpeas were harvested and dried separately from the cover crop mix. Data was analyzed using the analysis of variance procedure in Statistix 10 (Analytical Software, Tallahassee, FL). Significant means were separated with Tukey’s HSD all-pairwise comparisons tests at P<0.05.

RESULTS AND DISCUSSION Considering the poor soil conditions at the test site, including compaction, low organic matter (2.5%), and no residue cover, the plots established well, growing vigorously and showing a healthy, green color. Plants emerged within one week of seeding. The cover crop mix was about 32 inches tall by August 9 (Figure 5), and over three feet tall by September 18 (Figure 6). Soil nitrate-N measured 139 lb/acre in the top two feet of soil. Some of this nitrate-N likely came from an alfalfa hay crop that was in the field a few years ago. Sorghum-sudangrass was prevalent in all plots, as were sunflowers. The cowpeas were less evident in August and September, and were not nearly as tall or vigorous as the monocultures in the 2016 BPMC experiment. There were no significant differences between ‘Iron & Clay’ and ‘Red Ripper’ in terms of cowpea stand count (Table 4) or cowpea biomass (Table 5). There were significant differences in cover crop mix biomass. The six-way cover crop mix biomass in the ‘Red Ripper’ treatment was 7399.3 lb/acre while the cover crop mix biomass in the ‘Iron & Clay’ treatment was 4904.5 lb/acre (Table 6). This indicates variation between the plots, possibly due to soil fertility. When cowpea biomass was added to the cover crop mix biomass the ‘Red Ripper’ treatment had significantly more biomass than the ‘Iron & Clay’ treatment, reflecting the higher biomass of the cover crop mix in the ‘Red Ripper’ treatment. (Table 6).

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Figure 5. Cowpea variety study, August 9, 2017, BPMC, Bridger, MT.

Figure 6. Cowpea variety study, September 18, 2017, BPMC, Bridger, MT.

~3 ft.

~3 ft.

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Table 4. Mean cowpea stand counts, BPMC, Bridger, MT. Treatment Stand Count1 ‘Iron & Clay’ + cover crop mix 164 a

‘Red Ripper’ + cover crop mix 123 a 1 Stand counts done on August 18, 2017. 2 Means in columns followed by different letters are significantly different according to Tukey’s HSD at P<0.05.

Table 5. Mean biomass of cowpea varieties, BPMC, Bridger, MT.

1 Harvested on September 18-19, 2017. 2 Means in columns followed by different letters are significantly different according to Tukey’s HSD at P<0.05.

Table 6. Mean cover crop biomass with and without the contribution of cowpea biomass, BPMC, Bridger, MT. Treatment Cover crop mix

(no cowpea) Cover crop mix (with cowpea)

-------------- lb/acre -------------- ‘Red Ripper’ + cover crop mix 7399 a1 7514 a

‘Iron & Clay’ + cover crop mix 4905 b 5033 b 1 Means in columns followed by different letters are significantly different according to Tukey’s HSD at P<0.05.

CONCLUSIONS There were no differences in ‘Red Ripper’ and ‘Iron & Clay’ for biomass. The prostrate plant habit of ‘Red Ripper’ did not result in an advantage in terms of number of cowpea plants or biomass. There was a statistical difference in the background cover crop mix biomass, with the ‘Red Ripper’ treatment higher than the ‘Iron & Clay’ treatment. This was likely due to variation within the experimental area, possibly in soil fertility. In terms of grazing value both treatments showed that this warm season mix, which favors warm season grasses, produces a decent amount of forage in a dryland system, even under degraded soil conditions. Assuming a 50% utilization rate, available forage would be 2400 to 3800 lb/acre. Cowpeas, which do well as a monoculture, did not perform well in terms of numbers or biomass in the cover crop mix. In terms of biomass, they contributed a minimal percentage of the total biomass (2-3%). This mix had warm season grasses including sorghum-sudangrass, millet, and corn, all of which grow extremely well and fast in warm conditions. They are also good nitrogen scavengers, and likely benefited from the high nitrate level in the soil from the previous alfalfa hay crop, which in turn provided sufficient N for growth that helped them outcompete the cowpeas. If a producer desires legumes in a cover crop mix to meet a specific objective, e.g. nitrogen fixation to boost fertility for the next crop, then there are alternatives:

Cowpea Biomass (lb/acre)1

‘Iron & Clay’ 129 a

‘Red Ripper’ 115 a

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1) Decrease the warm season grass seeding rate to decrease competition with legumes.

However, if the mix is to be grazed this is not advisable since grasses provide excellent grazing value. In addition, the C:N ration of the mix would be lowered, which may not always be desirable, especially if more soil cover is an objective.

2) Incorporate legumes as cash crops into the cropping system as a way to add nitrogen to the cropping system.

3) Choose legumes that will grow well with warm season grasses. Depending on producer objectives and their cropping system, peas or hairy vetch may be options. Both may be more collaborative species, being able to vine up plants such as warm season grasses. However, these are both cool season species, so when they are planted is important. A late May/early June planting time is not ideal. Either early in the season (which precludes a warm season mix), or late to mid-August, would work for peas and hairy vetch.

4) Grow a broadleaf only cover crop mix, which may allow the warm season legumes to perform better.

For a variety of reasons, the last alternative is worth exploring at the BPMC in 2018. An exclusively broadleaf cover crop mix allows for weed control options, promotes pollinator habitat (if appropriate species are selected), and may allow the legumes to grow better.

LITERATURE CITED

Davis, D.W., E.A. Oelke, E.S. Oplinger, J.D. Doll, C.V. Hanson, and D.H Putnam. 2017. Alternative Field Crops Manual- Cowpea. Available at: https://hort.purdue.edu/newcrop/afcm/cowpea.html (accessed December 20, 2017).

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