Donna S. Gentry and Lisa M. Fultz

Integrating winter cover crops into an agricultural system is not new, but the potential for improved soil health and crop production have renewed producer interest over the past decade. According to a recent U.S. Department of Agriculture survey, cover crop acreage increased by 50% between 2012 and 2017 across the United States and includes expansion of acreage in the Mid-South. This has prompted university research to evaluate cover crops and soil conditions specific to that region. It has been well documented that winter covers can improve various soil properties. However, questions regarding their impact on crop yield still remain.

To address some of these questions, a two-year study was conducted at the Dean Lee Research and Extension Center in Alexandria, Louisiana, from 2016-2018 to evaluate the impact of winter cover crop seeding rates and soil type in a soybean production system. Three broadcast seeding rates of tillage radish, cereal rye and crimson clover were planted into a Moreland clay soil and Coushatta silt loam soil in addition to a standard, nonplanted fallow treatment. Cover crops were broadcast using recommended seeding rates (Table 1).

Cover crops were planted in October 2017 and 2018 and terminated in March of the following year. Maturity group 5 soybeans were planted at a seeding rate of 123,000 per acre in early May of 2017 and 2018 with a replant in June 2018 of the clay soil plots because of poor emergence. Pest management practices were followed according to LSU AgCenter recommendations, and soybeans were harvested on October 5, 2017, and October 3 and 24, 2018. Because dense plant populations and height variation can affect potential yield, data collected across the growing season included plant populations, plant height at harvest and soybean yield.

Results indicated that soybean plant population differed by soil type, with an interaction occurring between year and soil type. The plots in clay soil averaged 9,290 more plants per acre than those in the silt loam across two years. However, this did not correlate to higher yield. Though soybean seeding rates were equal for both soil types, Moreland clay plot plant populations increased from 2017 to 2018, where Coushatta silt loam plot plant populations decreased. Even though emergence was affected by soil type and year, cover crop seeding rate had no effect on soybean plant populations for either soil type for this study.

Plant heights also differed by year and soil type. Heights decreased from 40 inches in 2017 to 34.8 inches in 2018, a 13% reduction. Moreland clay plots had greater plant populations than Coushatta silt loam; however, plant heights were significantly greater for the silt loam compared to clay (38.2 inches and 36.6 inches, respectively). Similar to plant populations, cover crop seeding rate had no effect on plant heights, and no interactions occurred between other variables.

Soybean yield was different by soil type and year, with the silt loam plots yielding 41% higher than the clay plots across both years (52.1 and 30.9 bushels per acre, respectively). Although production year 2017 yielded 51.1 bushels per acre compared to 31.9 bushels per acre in 2018, the cover crop seeding rate had no significant impact on soybean yield in this study across soil types and years (Figure 1).

Although some research has reported significant increases in corn and soybean yields following radishes compared to fallow or other cover crops, seeding rates had no positive impact on soybean yield in this study but did not negatively impact yield either. Other studies have concluded cover crops like cereal rye did not significantly affect soybean yields after corn, which may indicate yield differences were due to environmental and other soil conditions. Rainfall fluctuation and cover crop biomass reduction (across all species) from 2017 to 2018 may have affected soil moisture levels and potential nutrient availability. Because soil type affected all variables, inherent differences in soil properties (structure, texture, water-holding capacity) seemed to have had more impact on soybean growth and yield than cover crop seeding rates. However, this was a short-term study, and there may not have been sufficient time to improve or change available soil nutrient levels and organic matter. Similar cover crop research trials are currently being conducted across the state on LSU AgCenter research stations to evaluate seeding rates and other variables and their impact on crop production. This additional data is needed to confirm seeding rate and specific soil type recommendations.

Donna S. Gentry is an extension agriculture and natural resources agent in the Southeast Region, and Lisa M. Fultz is an assistant professor in the School of Plant, Environmental and Soil Sciences.

(This article appears in the fall 2020 issue of Louisiana Agriculture.)

Donna S. Gentry is an extension agriculture and natural resources agent in the Southeast Region. Photo by Olivia McClure

These soybean plants are emerging in a high rate of cereal rye residue in a two-year study at the Dean Lee Research and Extension Center in Alexandria, Louisiana, from 2016-2018. Photo by Donna S. Gentry

PVC squares are used to measure cover crop biomass. Tillage radish was one of the cover crops planted in research at the Dean Lee Research and Extension Center. Photo by Donna S. Gentry

Soybeans are harvested in a field in which cover crops were used in research at the Dean Lee Research and Extension Center, Alexandria, Louisiana. Photo by Donna S. Gentry