Root Biomass of Fall-planted Cover Crops in Northeast Nebraska

By Vesh R. Thapa and Bridget McKinley et.al

Background

Cover crops provide several benefits, including soil erosion control, nitrogen cycling, organic matter build up, weed suppression, and livestock forage. However, the extent of these benefits largely depends on biomass production — both above and below ground. While aboveground biomass has been extensively studied, root biomass often receives less attention due to field-variability and the labor-intensive nature of sampling and processing. Yet, the contributions of roots to soil health are just as significant as those of aboveground biomass.

For instance, it is estimated that grass cover crops transfer 33% or more of their fixed carbon into the soil through their dense root systems (Austin et al., 2017; Engedal et al., 2023). Root-derived carbon also contributes to soil organic matter formation at higher rates than aboveground biomass (Amsili and Kaye, 2021). In addition to carbon accumulation, the fibrous roots of grass cover crops improve soil structure by binding soil particles into stable aggregates. Depending on the species, other benefits may include the deep taproots of brassica cover crops, which can penetrate compacted soil layers and promote water infiltration, or nitrogen-fixing nodules formed by legume cover crops, which improve soil fertility.

In the northeastern Nebraska Loess Hills, which are characterized by a semi-humid climate and harsh winters, limited research has focused on root biomass production, particularly for inter-seeded (broadcast) cover crops. To address this knowledge gap, we evaluated the root productivity of six commercially available cover crop species, varieties and mixtures planted in fall under the region’s unique climatic conditions.

Study Site, Experimental Design and Treatments

This study was conducted during the 2022-2023 and 2023-2024 growing seasons at the UNL Haskell Ag Lab in Concord, Nebraska. The site experiences a semi-humid climate, with a 30-year mean precipitation of 6.7 inches from September to November (initial growth period), and 17.7 inches from September to May (entire cover crop growing season). The soil is classified as Moody silt clay loam with a pH of 7.4. 

The experiment followed a randomized complete block design (RCBD) with four replications. Each plot measured 5 feet × 30 feet. Cover crop treatments included two rye varieties (Elbon and Yankee), triticale (Montech), wheat (Gore), hairy vetch (AU-Merit), and a mixture of Elbon rye + AU-Merit hairy vetch. These species and varieties were selected for their adaptability to local conditions and their superior performance by the end of fall 2022. 

Cover crops were hand-broadcast into irrigated cornat the R5-R6 growth stage. Seeding rates were 150 lbs./acre for sole rye, triticale and wheat, 45 lbs./acre for hairy vetch, and 50% of the sole seeding rates for mixture (75 lbs./acre for rye and 22.5 lbs./acre for hairy vetch). These higher-than-standard rates ensured proper establishment to ensure meaningful comparisons among species. 

Planting dates were Sept. 9, 2022, and Sept. 1, 2023, with terminations in mid-May (May 13, 2023, for the 2022–2023 season and May 20, 2024, for the 2023–2024 season). The trials were conducted under no-tillage management. 

During the cover crop period, Concord experienced average high temperatures of 62°F in 2022–2023 and 66°F in 2023–2024, with average low temperatures of 15.5°F and 22.6°F, respectively. Fall precipitation (during initial growth period) totaled 1.8 inches in 2022 and 7.8 inches in 2023, while total growing season (planting to termination) precipitation was 10.2 inches in 2022-2023 and 23.8 inches in 2023-2024.

Root Sampling

At cover crop termination each spring, root samples were collected to a depth of 12 inches using a tractor-mounted Giddings hydraulic probe. Soil cores were extracted from two locations within each plot: an in-row sample (directly beneath the plant) and a between-row sample (4 inches away from the plant) (Figure 1). To obtain a representative estimate of root biomass, the in-row and between-row samples were combined into a composite sample. In the laboratory, roots were washed, air-dried, and then oven-dried at 140 ⁰F for approximately even days until a constant weight was achieved.

Results

At termination in the spring, root biomass for all cover crop species was higher in 2023 than in 2024, averaging 222% greater in 2023 (Figure 2). The drier 2022–2023 growing season, compared to the wetter 2023–2024 season (precipitation during initial growth period: 1.8 inches vs. 7.8 inches; entire season: 10.2 inches vs. 23.8 inches), likely led cover crops to allocate more resources to root growth for enhanced water uptake and survival. This was supported by the lower average shoot biomass observed in 2023 (726 lbs./ac) compared to 2024 (1648 lbs./ac). 

Additionally, reduced nutrient mobility in drier conditions may have prompted cover crops to develop a more extensive root system, leading to greater root biomass accumulation by spring 2023. 

In contrast, the higher rainfall in 2023–2024 may have caused periodic waterlogging or reduced soil aeration, potentially suppressing root growth. Moreover, greater nutrient availability in wetter conditions likely reduced the need for extensive root development.

Root biomass for grass species ranged from 410 to 1,039 lbs./ac in 2023 (average: 688 lbs./ac), compared to 203 to 245 lbs./ac in 2024 (average: 222 lbs./ac). In 2023, the top root biomass producers were Elbon rye (1039 lbs./ac) and Yankee rye (892 lbs./ac), followed by a mixture of Elbon rye and AU-Merit hairy vetch (697 lbs./ac). 

In contrast, root biomass in 2024 was lower across all species, with Elbon rye having the highest biomass (245 lbs./ac). 

Among all species, AU-Merit hairy vetch had the lowest root biomass in both years, with 363 lbs./ac in 2023 and 80 lbs./ac in 2024.

Key Takeaways

• Elbon rye and Yankee rye consistently produced the highest root biomass, demonstrating their adaptability to northeastern Nebraska’s growing conditions.
• AU-Merit hairy vetch produced the lowest root biomass, indicating lower belowground productivity potential compared to grass species.
• Soil moisture availability played a crucial role in root biomass production.
• Drier conditions promoted greater root biomass accumulation, as cover crops allocated more resources to root growth to enhance water uptake and nutrients acquisition for survival.
• In the drier year (2022-2023), for every pound of shoot biomass produced, the root biomass produced was:
• 0.5 lbs. for Elbon rye and Montech triticale
• 1.3 lbs. for Yankee rye
• 1.1 lbs. for Gore wheat
• 3.3 lbs. for AU-Merit hairy vetch
• 1.4 lbs. for Elbon rye + AU-Merit hairy vetch mixture
• In the wetter year (2023-2024), for every pound of shoot biomass produced, the root biomass produced was:
• 0.1 lbs. for Elbon rye, Yankee rye, Montech triticale, and Gore wheat
• 0.2 lbs. for AU-Merit hairy vetch and Elbon rye + AU-Merit hairy vetch mixture

Implications for Producers

Root biomass is a vital but often overlooked component of cover crops, contributing to soil organic matter, microbial activity and overall soil health. These findings highlight the importance of selecting species with greater root biomass, such as Elbon rye and Yankee rye, to enhance soil stability and long-term soil health in northeast Nebraska. While AU-Merit hairy vetch has nitrogen fixation potential, it produced low root biomass in this study, suggesting limited benefits for nitrogen inputs and soil structure improvement compared to grass species.

Soil moisture availability significantly influences root development. In drier conditions, cover crops allocate more resources to root growth, enhancing water uptake and nutrient acquisition. Deeper-rooting species can be particularly beneficial in dry years, while in wetter years, balancing root and shoot growth is necessary for optimal biomass distribution. To maximize soil health benefits, producers should consider species selection in relation to seasonal moisture conditions and long-term management goals.

This Cover Crop Initiative Project is a collaborative effort between the UNL and the Nebraska Natural Resources Conservation Service (NRCS) to promote the use of spring, summer and fall-planted cover crops across Nebraska. For more project outputs, visit the CropWatch Variety Testing page.