Amanda J. Ashworth

Switchgrass yield and stand dynamics from legume intercropping based on seeding rate and harvest management

Intercropping legumes may reduce inputs and enhance sustainability of forage and feedstock production, especially on marginal soils. This approach is largely untested for switchgrass (Panicum virgatum L.) production, yet producer acceptance should be high given the traditional use of legumes in forage/agricultural systems. Our objectives were to evaluate three cool-season and two warm-season legumes and their required densities to influence yield and supply nitrogen (N) compared to three inorganic N levels (0, 33, and 66 kg N ha−1 [0, 30, and 60 lb N ac−1]) at three locations in Tennessee (Knoxville [Sequatchie Silt Loam], Crossville [Lilly Loam], and Milan [Loring B2 Series]). Fall of 2010 seeded, cool-season legumes (red clover [Trifolium pratense L.], hairy vetch [Vicia villosa L.], ladino clover [Trifolium repens L.]), arrowleaf clover (Trifolium vesiculosum L.), and a spring of 2011 seeded, warm-season legume (partridge pea [Chamaecrista fasciculate L.]) were interseeded into switchgrass at three (high, medium, and low) seeding rates each in two experiments. Harvest treatments were annual single, postdormancy biofuel (Experiment One) or integrated forage-biofuel (preanthesis and postdormancy; Experiment Two). Year one yield impacts were minimal. During the second harvest year, legumes increased yield versus Year 1; in general, yields for 33 and 67 kg N ha−1 did not differ from those for red clover, hairy vetch, ladino clover, or partridge pea (p < 0.05). Arrowleaf clover yields were not different from 0 kg N ha−1. Forage biomass yields were generally more responsive to legumes (p < 0.05) than the biomass regime. Legume persistence after three years was generally greatest for ladino clover and partridge pea. Forage quality (switchgrass only) in some cases was positively influenced by legume treatments, notably hairy vetch and partridge pea (p < 0.05). Intercropping selected legumes in switchgrass may enhance forage quality and yield while reducing nonrenewable inputs, fertilizer costs, and emissions/runoff to air and groundwater.

Biomass and integrated forage/biomass yields of switchgrass as affected by intercropped cool- and warm-season legumes

Switchgrass (Panicum virgatum L.) has potential as a biofuel feedstock for ethanol production on marginal soils not suitable for row crop production. Further, it is hypothesized that legumes may be interseeded into switchgrass to increase available soil nitrogen (N) and enhance switchgrass yields. Therefore the primary objective was to identify compatible legume species for intercropping with lowland switchgrass and determine if biomass yields and forage quality can be improved. Four cool- and two warm-season legume species were compared to application of 67 and 134 kg N ha−1 (59.8 and 119.6 lb N ac−1) during 2009 and 2010 over a range of soils at three research and education centers in Tennessee. Cool-season legumes were alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), crimson clover (Trifolium incarnatum L.), and hairy vetch (Vicia villosa L.), and warm-season legumes included Illinois bundle flower (Desmanthus illinoensis L.) and partridge pea (Chamaechrista fasciculata L.). Legumes were evaluated for establishment (plant densities) and their effects on switchgrass yield and forage quality under a one-cut biomass (single, postdormancy biofuel) and an integrated two-cut (biomass/forage [preanthesis]) system. In the one-cut system, switchgrass yields (16.6 Mg ha−1 [6.7 tn ac−1]) from the current recommended rate (67 kg N ha−1 [59.8 lb N ac−1]) exceeded (p < 0.05) legume treatment yields (average 13.5 Mg ha−1 [5.5 tn ac−1]). In the integrated harvest system, switchgrass yields from red (13.4 Mg ha−1 [5.4 tn ac−1]) and crimson clover (12.8 Mg ha−1 [5.2 tn ac−1]) intercrops were not different from 67 kg N ha−1 (14.5 Mg ha−1 [5.9 tn ac−1]). Crude protein levels were greater (p < 0.05) for 134 kg N ha−1 (119.6 lb N ac−1), compared to legume intercrops (except red clover). Partridge pea showed promise as a warm-season legume that can be grown compatibly with switchgrass for up to two years. Therefore, compatible legume-intercrop candidates, such as partridge pea and red clover, may enhance switchgrass yield and forage quality while displacing synthetic N in integrated biofuel/forage systems, but need to be further investigated in efforts to reduce nitrate (NO3) leaching and emissions from fertilizing.

Comparison of Near Infrared Reflectance Spectroscopy with Combustion and Chemical Methods for Soil Carbon Measurements in Agricultural Soils

ABSTRACT As interest in soil organic carbon (SOC) dynamics increases, so do needs for rapid, accurate, and inexpensive methods for quantifying SOC. Objectives were to i) evaluate near infrared reflectance (NIR) spectroscopy potential to determine SOC and soil organic matter (SOM) in soils from across Tennessee, USA; and ii) evaluate potential upper limits of SOC from forest, pasture, no-tillage, and conventional tilled sites. Samples were analyzed via dry-combustion (SOC), Walkley–Black chemical SOM, and NIR. In addition, the sample particle size was classified to give five surface roughness levels to determine effects of particle size on NIR. Partial least squares regression was used to develop a model for predicting SOC as measured by NIR by comparing against SOM and SOC. Both NIR and SOM correlated well (R2 > 0.9) with SOC (combustion). NIR is therefore considered a sufficiently accurate method for quantifying SOC in soils of Tennessee, with pasture and forested systems having the greatest accumulations.Abbreviations SOC, soil organic carbon; NIR, Near Infrared Reflectance Spectroscopy; MTREC, Middle Tennessee Research and Education Center; RECM, Research and Education Center at Milan; PREC, Plateau Research and Education Center; PLS, Partial least squares.

Yield and Stand Persistence of Switchgrass as Affected by Cutting Height and Variety

Switchgrass (Panicum virgatum L.) is a high-yielding forage species with low input demands on marginal soils. However, long-term response of prominent varieties to cutting heights is not well documented. To examine varietal type and cutting height impacts, three varieties of switchgrass (‘Cave-in-Rock’, ‘Kanlow’, and ‘Alamo’) that were established in 1992 for a previous study were evaluated in Knoxville, TN, from 2008 to 2011. Switchgrass plots were harvested at 10-, 20-, 30-, and 41-cm cutting heights in a two-cut system (June and September annually) to determine if lower harvest height affected forage yield and stand persistence. Plots were visually rated during the final year (2011) to assess stand vigor. Interactions were detected for total yield by cutting height and year (P < 0.05); shorter cutting heights appeared to negatively impact yields in subsequent years and the highest cutting height resulted in lower yields. However, there were no varietal yield responses to cutting heights. Shorter cutting height treatments (10 and 20 cm) had reduced Alamo and Kanlow presence (P < 0.05) by the end of the 4-yr study period. Consequently, lower switchgrass cutting heights (10 and 20 cm) may maximize short-term forage yield; however, stand persistence may be compromised long term, especially in

Grazing Management and Buffer Strip Impact on Nitrogen Runoff from Pastures Fertilized with Poultry Litter

Nitrogen runoff from pastures fertilized with animal manure, such as poultry litter, can result in accelerated eutrophication. The objective of this study was to evaluate the long-term effects of grazing management and buffer strips on N runoff from pastures fertilized with poultry litter. A 12-yr study was conducted on 15 small watersheds in Booneville, AR, using five management practices: continuous grazing, haying, rotational grazing, rotational grazing with an unfertilized buffer strip, and rotational grazing with a fenced unfertilized riparian buffer. Poultry litter was applied annually at a rate of 5.6 Mg ha. Concentrations and loads of total N, NO-N, NH-N, organic N, and total organic C in runoff varied intra- and interannually and coincided with precipitation trends. Overall, the greatest component of total N in runoff was organic N. Rotational grazing resulted in the highest concentrations and loads of all forms of N in runoff compared with other treatments, including the continuously grazed paddocks, which were grazed almost twice as much. Total organic C concentrations and loads in runoff were also higher from rotationally grazed watersheds than other treatments. Rotational grazing is considered a best management practice that typically reduces soil erosion; hence, the mechanism by which it caused higher N and C runoff is unclear. Nitrogen runoff losses from rotationally grazed pastures were reduced by 44% with unfertilized buffer strips, by 54% with fenced unfertilized riparian buffers, and by 52% by converting pastures to hayfields.

Crop Rotations and Poultry Litter Affect Dynamic Soil Chemical Properties and Soil Biota Long Term

Dynamic soil chemical interactions with conservation agricultural practices and soil biota are largely unknown. Therefore, this study aims to quantify long-term (12-yr) impacts of cover crops, poultry litter, crop rotations, no-tillage, and their interactions on dynamic soil properties and to determine their relationships with nutrient cycling, crop yield, and soil biodiversity (soil microbial and earthworm communities). Main effects were 13 different cropping sequences of soybean [ (L.) Merr.], corn ( L.), and cotton ( L.) at the Research and Education Center at Milan, TN, and eight sequences of corn and soybean at the Middle Tennessee Research and Education Center, Spring Hill, TN. Sequences were repeated in 4-yr phases from 2002 to 2014. Split-block cover crop treatments consisted of winter wheat ( L.), hairy vetch ( Roth), poultry litter, and a fallow control. Soil C and nutrient fluxes were calculated at surface (0-5 cm) and subsurface (5-15 cm) layers during Years 0, 2, 4, 8, and 12. After 12 yr, weighted means (0-15 cm) of soil pH, P, K, Ca, Mg, total N, and C were greater under poultry litter-amended soils compared with cover crops ( < 0.05). In addition, continuous corn sequences resulted in greater soil K, N, and C concentrations than soybean-soybean-corn-corn rotations ( < 0.05). Poultry litter treatments were positively correlated with greater soil fertility levels, as well as higher crop yield and soil biodiversity. These results underscore linkages between manure additions and cropping sequences, within the nutrient cycling, soil health, and crop production continuum.

A decision-support system for analyzing tractor guidance technology

Abstract A decision-support system was developed to assist small-scale producers, consultants, and extension agents with analyzing expected yield improvements, input cost savings, and equipment efficiency gains associated with global positioning system guidance on tractors using farm-specific details. Default parameters may be modified to perform partial budgeting and break-even analyses on a whole-farm basis. Findings suggest that this technology is profitable on as few as 49 ha, considered small-scale in the region, and under farm conditions assessed within. Further, tractor guidance is more feasible the more input-intensive the crops (e.g. cotton production vs. pasture maintenance) and the more expensive the equipment (e.g. using newer equipment). Changing input use affects greenhouse gas emissions that are reported as carbon equivalent footprint changes due to tractor guidance. For example, changing to tractors with lower horsepower to save on capital investment needs without changing the size of implement drawn, lowers fuel footprint as long as technically feasible and possible from a perspective of completing field work in a timely manner. Also, using manure instead of synthetic fertilizer, while economically advantageous, will increase the footprint of fertilizer applications given, among other factors, lesser nutrient density and thereby greater handling costs with manures. Quantifying these impacts across a whole farm is cumbersome since tractor guidance affects annual equipment use hours that are difficult to track and yet economically important. Hence, the decision support system was designed to capture farm-specific detail. The ability to perform whole-farm planning and sensitivity analyses in an automated, user-friendly, and flexible fashion is expected to increase technology adoption by small-scale producers.

Long-Term Soil Organic Carbon Changes as Affected by Crop Rotation and Bio-covers in No-Till Crop Systems

Soil organic carbon (SOC) sequestration is a potential negative-feedback for climate-warming gases in agriculture. The rate of no-tillage SOC storage is not well known due to large temporal and spatial biogeochemical and management variations. Therefore our objective was to compare long-term SOC fluxes at a no-till field site in Milan, Tennessee on Oxyaquic Fragiudalfs, in a split-block design with four replications. The whole-block was cropping sequences of corn, soybeans, and cotton with split-block bio-cover treatments of: winter wheat, hairy vetch, poultry litter, and a fallow control. Soil carbon flux was calculated at soil surfaces (0–5 cm) for years-0, 2, 4, and 8. During the first 2 years, small annual losses occurred in carbon over all treatments (1.40 Mg ha−1). During this time, cotton sequences lost significantly more surface SOC than other rotations. However, by year-4, SOC began to stabilize. By year-8, sequences with high frequencies of soybean and with greater temporal complexity generally gained greater SOC levels at 0–5 cm. Also, poultry litter bio-cover gained more surface SOC compared to wheat, vetch and fallow covers. Across all sequences and bio-covers, SOC had increased 1.47 Mg ha−1 after 8 years from pre-experimental levels of 9.20 Mg ha−1; suggesting long-term beneficial effects on C storage under no-till and diverse cropping sequences.