Journal of the NACAA
ISSN 2158-9429
Volume 13, Issue 1 - June, 2020


Evaluation of Cover Crop Seeding Rate and Soil Type on Soybean Production with Potential Economic Impacts

Gentry, D.S., Conservation Agronomy Agent, LSU AgCenter
Fultz, L., Assistant Professor, LSU AgCenter
Adusumilli, N., Assistant Professor, LSU AgCenter


The integration of winter annual cover crops into a cropping system can potentially improve soil health and crop production, however, information regarding seeding rates and specific soil types is limited. A two-year study was conducted at the Dean Lee Research Station and Extension Center in Alexandria, Louisiana to evaluate the effects of cover crop seeding rate and soil type on soybean (Glycine max L) growth and yield and their potential economic impacts. Three broadcast seeding rates of tillage radish (Raphanus sativus var. L), cereal rye (Secale cereale), and crimson clover (Trifolium incarnatum) were planted into a Moreland clay and Coushatta silt loam soil. Soybean yield was different by soil type and year, with Coushatta silt loam plots yielding 41% higher than Moreland clay (3,504 and 2,079 kg ha-1, respectively). Although production year 2017 (3,434 kg ha-1) yielded 39% greater than 2018 (2,147 kg ha-1), cover crop seeding rate had no impact on soybean yield in this study. Economic estimations were calculated based on cover crop inputs and soybean grain yield with high rates of tillage radish and cereal rye being less profitable compared with a fallow treatment (all other species and seeding rates were equal to fallow) for Coushatta silt loam soil. In contrast, all rates and species were equal to fallow in Moreland clay except for low rate of cereal rye. Under specific conditions and soil type, low and medium cover crop seeding rates may provide equal or greater monetary returns compared to fallow treatments, while helping to improve the soil.




Integrating winter cover crops into agricultural production systems is not a new practice. The use of alfalfa (Medicago sativa), clovers (Trifolium), and lupine (Lupinus) increased wheat (Triticum aestivum) yields date as far back as 29 B.C.E. (Fulk, 2014). The species of cover crop selected depends on the objectives of the grower, with each having its own characteristics that make them advantageous. Heavy nitrogen (N)-dependent crops like field corn (Zea mays) and cotton (Gossypium hirsutum) may benefit from a legume that can fix atmospheric-N or a grass species that can scavenge excess soil N, while reducing nitrate leaching. Brassicas are also scavengers and are known to help alleviate compacted soils, especially in drought-prone regions (Williams and Weil, 2004). Cash crops like soybeans (Glycine max L.) are a summer legume that are able to fix atmospheric-N, so winter covers are not typically planted in a soybean production system for the purpose of N fixation. Soybean rotation systems, however, may benefit from increased organic matter, improved water aggregate stability, and soil penetration resistance (Villamil et al., 2006).

Although it has been well documented that winter cover crops provide vegetative cover in erosion-prone areas in the winter (Frankenberger and Abdelmagid, 1985; Smith et al., 1987) and improve physical, chemical, and biological soil properties (Hoyt and Hargrove, 1986; Power and Doran, 1988; Vigil and Kisse, 1991), questions regarding impacts on crop yield still remain. Williams and Weil (2004) found soybean yields were significantly greater following a forage radish (Raphanus sativus var. oleiformis) and cereal rye (Secale cereale) mixture, mainly due to the conservation of water early in the season and reduction of soil resistance with root channels from the radishes. A study conducted across five locations in Iowa over four years indicated soybean grain yield was not affected positively or negatively by the cereal rye cover crop planted after corn, but corn yield was 5% lower compared to no rye cover crop (Sawyer et al., 2017). Delaney et al. (2016) reported greater soybean plant population counts in no-cover crop treatments compared with cover crop treatments in a fine-sandy loam soil but no differences in a Compass loamy sand. Results from that study concluded all cover crop treatments increased soybean yield by 188.3-627.7 kg ha-1 in a fine, sandy loam soil. On-farm comparisons in Maryland, Ohio, Pennsylvania, and Illinois reported significant increases in corn and soybean yields following tillage radishes compared to fallow or other cover crop species (Gruver et al., 2012), primarily due to early-season available soil-N. Because research shows inconsistent crop growth and yield responses to cover crops across soil types, species, seeding rates, and cropping systems, more data needs to be collected to address the potential challenges of these variables. 

Even with potential yield response challenges, producers across the country increased cover crop acreage nearly 60% from 2014 to 2016 (SARE, 2017), with acreage expected to continue to increase.  A wide-spread implementation may still be hindered due to the risk and inconsistent return on investments. Evaluating cover crop cost/benefit ratios are difficult, mainly due to upfront costs with management changes and hard-to-quantify benefits such as soil health improvements. Additional inputs that may prevent implementation include additional equipment, seed cost and planting, and pesticide applications, which can add a significant cost to production.   

Some benefits may be realized in a short period of time including reduced erosion and weed pressure, however increased soil organic matter and field productivity (increased cash crop yield) may take several years. Myers et al. (2018) reports that it can take three or more years for cover crops to pay off if no incentive payments are obtained and no special circumstances exist. Some studies show that cover crops become more profitable as the price of N increases (Clark, 2007), mainly due to reduced fertilizer applications with cover crop implementation. One objective of this study was to evaluate costs associated with various species and seeding rates, in addition to potential financial net returns. Because N is not typically applied to soybeans, cost-savings for reduced fertilizer applications was not included in the analysis. Additionally, an objective of this study was to determine the impact of cover crop seeding rates and soil type on soybean growth and grain yield. It is hypothesized that high seeding rates of all species planted will increase soybean growth, but may not have a significant yield impact.



Materials and Methods

Site Description

A two-year non-irrigated experiment was conducted at the LSU AgCenter Dean Lee Research and Extension Center, located 9.7 km south of Alexandria, Louisiana from 2016 to 2018. Three species of cover crops, Elbon rye (Secale cereale), Daikon radish (Raphanus sativus L.), and AU Sunrise crimson clover (Trifolium incarnatum) were evaluated at low, medium, and high seeding rates, in addition to a control plot with no cover crops planted. Cover crops were broadcast using seeding rates obtained from USDA-NRCS Cool Season Cover Crop Species & Planting Dates TX-PM-15-03 (USDA, 2015) and Alabama Extension ANR-2139 (Delaney et al., 2014) publications (Table 1).



Table 1. Seeding rate treatments for cover crop species in kg ha-1 based on recommended broadcast rates




Field locations included two areas with classified soil textures of clay and silt loam, specifically Moreland clay (MCl) and Coushatta silt loam (CSL) soils. These were located approximately 0.8 km apart on 0-1% slope. The Moreland clay soil was classified as a very-fine, semiotic, thermic Oxyaquic Hapluderts and considered a very deep, somewhat poorly drained, permeable soil (USDA, 2010). In contrast, the Coushatta silt loam classified as a fine-silty, mixed, superactive, thermic Fluventic Eutrudept soil that is very deep and well-drained. According to United States Department of Agriculture (USDA, 1997) both soil types are considered Prime Farmland and suitable for growing crops. Previous crop rotations for each soil type included soybean and cotton (Gossypium hirsutum).


Experimental Design and Field Management

The experimental design was a randomized complete block of ten treatments with three replications, for a total of 30 plots per soil type. Plots consisted of four, 96.5 cm rows x 12.2 m in length (~ 49 m2) with the two middle rows used for data collection. Nine seeding rate treatments were randomly assigned within each soil type, in addition to an untreated control plot (Figure 1). Initial field preparation included mechanically incorporating soybean residue from the previous crop as rows were conventionally prepared for cover crop planting using a Case International 235 Magnum tractor and cultivator equipment. According to initial soil test recommendations, P2O5 was broadcast at a rate of 35.1 kg ha-1 + K2O at 70.6 kg ha-1 to each plot on October 1. Year two of the study followed the same protocol with an increase in fertilizer application rates of 45.4 kg ha-1 P2O5 + 90.8 kg ha-1 K2O.


Figure 1. Field experimental design with cover crop species (crimson clover [CC]; cereal rye [RYE]; tillage radish [RAD]; fallow [FALL]) and randomized seeding rates (low [L]; medium [M]; high [H]).


Seeds were broadcast onto prepared beds with an Earthway 3400 Ergonomic Hand-Held Broadcast spreader on October 17, 2016 and November 6, 2017, respectively. Planting dates were one-week post and prior to soil sample collections (for year one and two, respectively) due to fall field preparation timing. Beds were immediately rolled with a culti-packer to ensure optimum seed-to-soil contact. Plots were not irrigated, and cover crops were planted into dry soil conditions after soybean harvest both years, which delayed emergence until approximately mid-November.


Year One: 2017

Approximately 155 days after planting, cover crops were chemically terminated on March 21, 2017 with a herbicide mixture of glyphosate at 0.95 L ha-1 + 2, 4-D at 0.95 L ha-1 using a Case International 235 Magnum tractor and 8-row broadcast sprayer. According to Copes et al. (2018), a spring burn down herbicide should be applied 4-6 weeks prior to planting to allow adequate plant decomposition and reduce winter pest carryover potential. Subsequently, a 4.9 maturity Liberty Link® soybean (Hornbeck HBK 4953 LL) was planted on May 10, 2017 in both MCl and CSL soils at a seeding rate of 325,040 seed ha-1. Field operations throughout the season included five herbicide applications, one fungicide, and one insecticide application. Agronomic data collected and analyzed included plant population, plant height, and grain yield. Five plant population counts were collected per plot at the V6 growth stage using a one-meter stick and plants were counted per meter of row and recorded. Plant heights were also taken at the R8 growth stage, immediately prior to harvest, using a meter stick. Soybeans from the middle two rows were harvested on October 5, 2017 with Massey-Ferguson 8XP plot combine and dry weight and moisture were recorded. The yield was calculated (kg ha-1) and adjusted to 13% moisture.


Year Two: 2018

The following year, to accommodate an optimum soybean planting date, cover crops were chemically terminated after only 141 days on March 18, 2018 with a combination of glyphosate at 0.95 L ha-1 + 2, 4-D at 0.95 L ha-1. A 5.1 maturity Roundup-Ready® soybean (Asgrow AG51X8 RR) was planted on May 4th at a seeding rate of 325,040 seed ha-1. Droughty conditions reduced soybean emergence in the MCl soil to approximately 10-15% of acceptable plant population (Spivey et al., 2018) and was chemically terminated on June 7. The plot was replanted the same day at a seeding rate of 325,040 seed ha-1. Six herbicide applications were made (including a termination application before re-plant) and three insecticide applications for 2018. Soybean plant populations and heights were again recorded at the V6 and R8 growth stages. The CSL plots were harvested on October 3rd with a Massey-Ferguson 8XP plot combine and dry weight and moisture were recorded. Due to re-planting, soybeans in the MCl plot were harvested approximately three weeks later, on October 24th. Grain weight and moisture were recorded, and yield was adjusted to 13% moisture. 


Economic Evaluation

Evaluation of the financial impact of planting cover crops into a soybean production system was completed using a Cover Crops Decision Making Tool that used a cover crop production cost estimator developed by the LSU AgCenter (Adusumilli et al., 2018). This Microsoft EXCEL program utilizes specific species, seeding rates, planting methods, fertilization, chemical applications, and labor costs in the calculation (Table 2). This information was used in estimating potential financial net returns based on soybean yields for each plot, with average direct and indirect costs of production and a projected market price of $351.27 per metric ton of soybeans (USDA, 2018).



Table 2. Cover crop production costs by species and seeding rate

†L= low seeding rate; M= medium seeding rate; H = high seeding rate



Statistical Analysis

Data were analyzed using the Mixed Model Analysis of Variance (%MMOV). Dependent variables included cover soybean plant heights, soybean plant population, and soybean grain yield, while independent variables were sampling date, cover crop seeding rate, and soil type. Replication was considered a random effect.  Soybean data were analyzed using Glimmix Procedure of SAS release 9.4, (SAS Institute Inc. 2013. SAS/STAT® 13.1 User’s Guide. Cary, NC: SAS Institute Inc.) and means were separated using the Fisher’s Least Significant Difference with the LSD option of the MEANS statement. An α ≤ 0.05 was considered significantly different for all procedures. Simulated financial net return data were analyzed using StataCorp. 2011. Stata Statistical Software: Release 12. College Station, TX: StataCorp LP.



Results and Discussion

Soybean plant population differed by soil type (P < 0.0001), with an interaction occurring between sample date and soil type (P = 0.0016). Moreland clay averaged 22,947 more plants ha-1 than CSL across two years, however, this did not correlate to higher grain yield. Interestingly, the MCl soil consistently had higher plant populations than CSL, and actually increased from 2017 to 2018, where CSL’s plant population decreased. LSU AgCenter (2018) recommendations for optimum soybean plant populations are 192,660-256,880 plants ha-1, which shows that the CSL had less than the recommended population in 2018 at 182,042 plants ha-1. Cover crop seeding rate had no effect on soybean plant populations for either soil type for this study (P=0.7397).

 Soybean plant heights differed for sample date (P<0.0001) and soil type (P=0.0185). Heights decreased from 101.6 cm in the fall of 2017 to 88.4 cm in 2018, a 13% reduction. Even though MCl had greater plant populations than CSL, plant heights were significantly greater for CSL than MCl (97 cm and 93 cm, respectively). Cover crop seeding rate had no effect on plant heights (P=0.4321) and no interactions occurred between other variables.

Integrating cover crops into a production system may positively impact soil health, but may not consistently increase crop growth and yield. Results indicated that there were differences in grain yield for sample date (P <0.0001) and soil type (P <0.0001). Soybean yield decreased by 39% across all soil types in 2018 (Figure 2). Coushatta silt loam yield averaged 1,418 kg ha-1 compared to 844 kg ha-1 for MCl soil across two years. Soybean yield was also impacted by an interaction between sample date and soil type for soybean yield, with FALL2017 yielding 3,793 kg ha-1 and FALL2018 yielding 3,215 kg ha-1 for CSL soils. Moreland clay soybean yields were 65% higher in FALL2017 versus FALL2018. This could be partially attributed to high rainfall accumulation during the last 60 days prior to harvest in 2018 versus 2017 (totals of 18.3 cm and 5.5 cm, respectively) and noted late-season disease pressure in MCl plots (Figure 3).



Figure 2. Soybean grain yield across sample dates. Bars with different superscripts are significantly different (α=0.05).




Figure 3. Precipitation for the last 60 days prior to soybean harvest in 2017 and 2018



Although other research has reported significant increases in corn and soybean yields following radishes compared to fallow or other cover crops (Gruver et al., 2016), cover crop species and seeding rate had no impact on soybean yield in this study across both soil types (P=0.739). Other studies have concluded cover crops like cereal rye did not significantly positively or negatively impact soybean yields after corn (Sawyer et al., 2017), which may indicate yield differences were due to environmental and other conditions. Even though some research has shown up to 11.6% yield increase for soybeans following cover crops (Myers et al., 2019), the data did not provide consistent results of increased yields.


Potential Economic Impacts and Estimated Net Returns

Net returns on investments are a major factor in cover crop implementation. All cover crop species used in this study were evaluated based on costs of implementation (Adumusilli et al., 2018), along with soybean grain yield for each soil type, to determine maximum potential economic profitability. The potential net return on investments for the CSL and MCl soil types ranged from $204-$383 ha-1 and $163-$273 ha-1, respectively, with all seeding rates compared to a standard winter-fallow treatment (only burndown herbicide costs incurred). Results indicated that in CSL soils, all seeding rates of RYE, CC, and RAD were equally profitable compared to the FALL treatment, with the exception of RADH and RYEH (Figure 4). Unlike the other two species, all seeding rates of CC were equally profitable (ranging from $290 - $304 ha-1). Interestingly, the low seeding rate for RYE was the least profitable in the MCl soil, with all other species and seeding rates being equally profitable to FALL (Figure 5).

Reddy (2001) reported that when evaluating winter cover crops in no-till (NT) and convention tillage (CT) systems for soybeans, it was determined that net return on investment was highest in both fallow treatments, NT at $105 ha-1, followed by CT at $76 ha-1, with negative net returns for all cover crop species. Results from this study estimated financial net return for FALL treatment in MCl soil was $231.81 ha-1, which was 35% lower than CSL soil at $358.55 ha-1, but still higher than some reported research results. When cover crop treatments were equally profitable to FALL, this would indicate the cover crop “paid for itself”, while likely providing intangible benefits described earlier. Because CSL soybean yields were significantly higher than MCl yields for this study both years, the majority of higher net returns were correlated to the CSL soil type as well.

Jiang and Thelen (2004) found that when comparing yield-limiting soil properties in corn and soybean cropping systems, soil variables such as base saturation, pH, clay content, and elevation were helpful in explaining yield variability, which may explain yield differences, and ultimately net returns, in this study. Other research has indicated that performance of production systems in terms of crop yields and net returns is influenced by location and production year, with conventional and fallow systems having higher net returns than no-till systems (Popp et al., 2002). Fallow treatments in this study did provide substantial financial net returns for both soil types, however, may not account for any potential soil health improvements or other benefits from cover crop implementation long-term.


  Figure 4. Simulated net returns for cover crop seeding rates (CC: crimson clover, RAD: tillage radish, RYE: cereal rye; L: low rate, M: medium rate, H: high rate) in Coushatta silt loam soil compared to fallow treatment. Bars with asterisks are significantly different (α=0.05) from the standard fallow treatment (S).



Figure 5. Simulated net returns for cover crop seeding rates (CC: crimson clover, RAD: tillage radish, RYE: cereal rye; L: low rate, M: medium rate, H: high rate) in Moreland clay soil compared to fallow treatment. Bars with asterisk are significantly different (α=0.05) than the standard fallow treatment (S).



Soil type affected soybean plant population, soybean plant heights, and soybean grain yield in this study, however, cover crop seeding rate did not. Differences in plant population indicated MCl soil consistently had higher populations than CSL for both growing seasons, but that did not correlate to higher grain yields. Wet field conditions at planting may have impacted emergence in 2017, with drought conditions in 2018 also affecting emergence and ultimately plant population in CSL. Plant heights were greater in CSL plots than MCl, but there were no differences for cover crop seeding rate, which suggests that cover crop residue may have provided early season N to the soybeans for additional plant growth. Yearly variations in environmental conditions also impacted yield for sample dates across years, with a significant reduction in yield from 2017 to 2018 (3434 and 2147 kg ha-1, respectively). While seeding rate had no effect on soybean yield, low and medium rates were equally profitable compared to FALL (with the exception of RYEL in MCl), suggesting that lower recommended seeding rates could reduce input costs while having minimal negative effect on production. Though not all potential benefits were evaluated in this study of cover crops, reduced erosion, N fixation and scavenging, and improved soil health at lower seeding rates may help producers manage their input costs and provide incentives while still maintaining cash crop yields and positive net returns.


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