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Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department
2012
Optimizing Cover Crop Bene?ts with Diverse Mixtures and an Optimizing Cover Crop Bene?ts with Diverse Mixtures and an
Alternative Termination Method Alternative Termination Method
Samuel E. Wortman
University of Nebraska-Lincoln
, swortman@unl.edu
Charles Francis
University of Nebraska-Lincoln
, cfrancis2@unl.edu
Mark L. Bernards
University of Western Illinois
Rhae A. Drijber
University of Nebraska-Lincoln
, rdrijber1@unl.edu
John L. Lindquist
University of Nebraska-Lincoln
, jlindquist1@unl.edu
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Wortman, Samuel E.; Francis, Charles; Bernards, Mark L.; Drijber, Rhae A.; and Lindquist, John L.,
"Optimizing Cover Crop Bene?ts with Diverse Mixtures and an Alternative Termination Method" (2012).
Agronomy & Horticulture -- Faculty Publications
. 615.
https://digitalcommons.unl.edu/agronomyfacpub/615
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Agronomy Journal • Volume 104, Issue 5 • 2012 1425
Organic Agriculture & Agroecology
Optimizing Cover Crop Benefi ts with Diverse Mixtures
and an Alternative Termination Method
Sam E. Wortman,* Charles A. Francis, Mark L. Bernards,
Rhae A. Drijber, and John L. Lindquist
Published in Agron. J. 104:1425–1435 (2012)
Posted online 1 Aug. 2012
doi:10.2134/agronj2012.0185
Copyright © 2012 by the American Society of Agronomy, 5585 Guilford
Road, Madison, WI 53711. All rights reserved. No part of this periodical may
be reproduced or transmitted in any form or by any means, electronic or
mechanical, including photocopying, recording, or any information storage
and retrieval system, without permission in writing from the publisher.
C
been shown to provide many envi-
ronmental and agronomic services within agroecosystems.
ese include reduced soil erosion, increased biological diversity
(e.g., microbes, insects, and birds), increased nutrient cycling and
biological N
2
xation, increased soil organic matter, improved weed
control, and increased crop yield (Pimentel et al., 1992; Pimentel
et al., 1995; Sainju and Singh, 1997; Williams et al., 1998; Altieri,
1999; Reddy et al., 2003; Teasdale et al., 2007). While cover crops
have traditionally been used as a soil conservation tool (Pimentel et
al., 1995), there is increasing interest in using cover crops to enhance
agronomic crop performance. However, maximizing agronomic
bene ts associated with cover crops will depend on appropriate
species choice and residue management (Ashford and Reeves, 2003;
Wortman et al., 2012). Selecting a single species is o en popular
among farmers due to the ease of planting, uniform development,
and predictable termination e cacy of the cover crop (Creamer et
al., 1995; Mirsky et al., 2009). However, multi-species mixtures may
increase productivity, stability, resilience, and resource-use e ciency
of the cover crop community (Tilman, 1996; Tilman et al., 1997,
2001; Trenbath, 1999; Wortman et al., 2012).
Despite the demonstrated bene ts, on-farm adoption remains
limited due to farmer concerns about the potential cost and
management implications of cover crop use. One of the top
concerns among farmers is the amount of soil water used by cover
crops, potentially reducing available soil moisture for the cash crop.
During seasons with average and above-average rainfall conditions,
di erences in available soil moisture among cover crop species
and mixtures are o en undetectable. However, when cover crop
productivity is high and precipitation becomes limiting, species can
di er greatly in their e ects on soil moisture (Unger and Vigil, 1998;
Daniel et al., 1999). While transpiration demands will undoubtedly
vary among species, the method of cover crop termination and
residue management may have a greater impact on available soil
moisture during main crop growth. Daniel et al. (1999) found that
volumetric soil moisture (%) was increased by as much as 2.4% to a
depth of 61 cm when cover crops were terminated with herbicides in
a no-till system compared to conventional termination with a eld
disk. Soil water savings associated with no-till practices have been
well documented (Blevins et al., 1983; De Vita et al., 2007), but the
additional bene ts of cover crop residue in a conservation tillage
system are not as clear. Liebl et al. (1992) found that transpiration
reduced available soil moisture during dry periods, but following
no-till termination cover crop residue conserved soil moisture
relative to a no-till system without cover crops. Given that the driest
portion of the growing season in the western Corn Belt typically
occurs a er cover crop growth (i.e., June–August), potential
soil moisture savings o ered by the residue (post-termination)
throughout the growing season may negate moisture de cits
observed during cover crop growth.
ABSTRACT
Previous studies have demonstrated bene ts of individual cover crop species, but the value of diverse cover crop mixtures has
received less attention. e objectives of this research were to determine the e ects of spring-sown cover crop mixture diversity
and mechanical cover crop termination method on cover crop and/or cash crop productivity, soil moisture and N, and pro tability
in an organic cropping system. An experiment was conducted between 2009 and 2011 near Mead, NE, where mixtures of two
(2CC), four (4CC), six (6CC), and eight (8CC) cover crop species, or a summer annual weed mixture were included in a sun ower–
soybean–corn rotation. Cover crops were terminated in late May using a eld disk or sweep plow undercutter. Undercutting
cover crops increased soil NO
3
–N (0–20 cm) by 1.0 and 1.8 mg NO
3
–N kg
–1
relative to disk incorporation in 2010 and 2011,
respectively. Cover crop mixtures o en reduced soil moisture (0–8 cm) before main crop planting, though cover crop termination
with the undercutter increased soil moisture content by as much as 0.024 cm
3
cm
–3
compared to termination with the disk
during early main crop growth. Crop yields were not in uenced by cover crop mixture, but termination with the undercutter
increased corn and soybean yield by as much as 1.40 and 0.88 Mg ha
–1
, respectively. Despite di erences in productivity between
spring cover crop mixtures and weed communities, crop yield was not di erent among these treatments; thus, pro tability of the
weed mixture–undercutter treatment combination was greatest due to reduced input costs.
S.E. Wortman, C.A. Francis, R.A. Drijber, and J.L. Lindquist, Dep. of
Agronomy and Horticulture, Univ. of Nebraska- Lincoln, Lincoln, NE
68583; M.L. Bernards, School of Agriculture, Univ. of Western Illinois,
Macomb, IL 61455. Received 23 May 2012. *Corresponding author (sam.
wortman@huskers.unl.edu).
Abbreviations: CC, cover crop mixture; DAT, days a er termination; DOY, day
of year; NC, weed-free and cover crop-free control; WD, weedy mixture and cover
crop-free.
1426 Agronomy Journal • Volume 104, Issue 5 • 2012
Despite concerns about water use, many farmers are interested in
cover crops because of the potential for improved nutrient cycling
and biological N
2
xation. As a result, species in the Fabaceae
(legume) family are among the most popular and expensive cover
crops. Legumes (e.g., green manures) have been shown to reduce
synthetic N input demands by 50 to 100% depending on species, the
duration of cover crop growth, and subsequent crop N requirement
(Biederbeck et al., 1996; Burket et al., 1997). While legume species
have the potential to biologically x N, faster growing cover
crop species (e.g., grass and mustard spp.) may be more useful in
scavenging nitrates and nutrient cycling (Dabney et al., 2001).
A mixture of legume and nonlegume species may maximize the
bene ts of biological N
2
xation and nutrient cycling, as legumes
can increase N availability to other species in mixture leading to
increased productivity (Kuo and Sainju, 1998; Mulder et al., 2002).
In addition, termination method and residue management can
in uence N mineralization, soil availability, and crop uptake (Sainju
and Singh, 2001). Incorporation of cover crop residue via eld disk
or plow o en results in rapid N mineralization and plant availability,
but management of residue on the soil surface has been shown to
result in greater crop N uptake and yield (Sainju and Singh, 2001).
erefore, residue management on the soil surface with conservation
tillage methods may be e ective in syncing N mineralization and
availability with crop demand and uptake (Parr et al., 2011).
Overall, the agronomic objective for cover crop management is to
minimize soil water loss and increase the quantity and availability
of soil N to promote increases in crop yield. However, improper
management of cover crops can lead to substantial yield loss. e
timing and method of cover crop termination have both been
shown to a ect yield in uencing factors including: soil moisture
availability, weed communities, cover crop and soil N content,
and crop N uptake (Daniel et al., 1999; Mirsky et al., 2009; Parr
et al., 2011; Wortman, 2012). Yield loss associated with cover crop
use is typically attributed to incomplete cover crop termination,
soil moisture de cit, or nutrient immobilization and de ciency
(Wagger, 1989; Unger and Vigil, 1998; Mischler et al., 2010);
thus, management of cover crop residue should be focused toward
termination e cacy, moisture conservation, and optimum soil N
availability during peak crop growth. To this end, conservation
tillage implements like the sweep plow undercutter may have great
potential (Creamer et al., 1995). In contrast to conventional tillage
systems, the undercutter leaves intact residue on the soil surface,
minimizes soil inversion, and presumably reduces evaporative loss
from the soil. Moreover, the undercutter may be an improvement
on conservation implements like the roller-crimper, which is o en
inconsistent in termination e cacy (Mischler et al., 2010). Despite
these production challenges, many cover crop systems have been
shown to maintain or increase crop yield (e.g., Clark et al., 1994;
Davis, 2010; Mischler et al., 2010). Demonstrating predictable
yield and economic bene ts associated with cover crop use will be
necessary in increasing on-farm adoption.
e objectives of this research were to determine the e ects of
spring-sown cover crop mixture diversity and mechanical cover crop
termination method on cover crop and/or cash crop productivity, soil
moisture, soil N, and cropping system pro tability. We hypothesized
that increasing cover crop diversity will increase total cover crop
biomass, and subsequent grain yield, while soil moisture content will
not di er among mixtures despite di erences in productivity. With
regard to cover crop termination, we hypothesized that terminating
cover crops with the sweep plow undercutter will increase soil
moisture content, soil N availability, crop yield, and pro tability
compared to termination with a eld disk.
MATERIALS AND METHODS
Experimental Site and Design
A eld experiment was conducted in 2009, 2010, and 2011 at
the University of Nebraska-Lincoln Agricultural Research and
Development Center (ARDC) near Mead, NE. Dominant soil
type at the site is a Sharpsburg silty clay loam ( ne, smectitic,
mesic Typic Argiudoll) with 0 to 5% slopes. e experiment was
conducted in a 2.8-ha eld that is certi ed for organic production
(OCIA International, Lincoln, NE), and is managed without
irrigation. is eld was in organic alfalfa (Medicago sativa L.)
hay production between 2004 and 2008. In the fall of 2008 the
experimental area was amended with 50 Mg ha
–1
of liquid beef (Bos
taurus) feedlot manure (approximately 1.2% total N content) and
incorporated via eld disk. In the spring of 2009, the entire eld
(excluding a no cover control treatment) was seeded with 8.1 kg ha
–1
of velvetleaf (Abutilon theophrasti) seed, 2.6 kg ha
–1
of common
lambsquarters (Chenopodium album) seed, 1.2 kg ha
–1
of redroot
pigweed (Amaranthus retro exus) seed, and 3.7 kg ha
–1
of green
foxtail (Setaria viridis) seed to establish a common weed seedbank
throughout the eld for a concurrent weed management study.
e experiment was designed as a split-plot randomized complete
block design within a 3-yr crop rotation with four replications. e
rotation sequence consisted of confectionery sun ower (Helianthus
annuus L. ‘Seeds 2000 Jaguar’)–soybean (Glycine max L. Merr.
‘Blue River Hybrids 2A71’)– corn (Zea mays L. var. ‘Blue River
Hybrids 57H36’). Within each crop species, whole-plots (9.1 by
21.3 m; 12 crop rows spaced 0.76 m apart) were de ned by cover
Table 1. Cover crop species and seeding rates used in individual cover crop mixtures for 2009 and 2010–2011 (2CC = two species
mixture; 4CC = four species mixture; 6CC = six species mixture; 8CC = eight species mixture).
Common name
Scientifi c name
Cover crop seeding rate
2CC 4CC 6CC 8CC
________________________
kg ha
–1 ________________________
Hairy vetch Vicia villosa Roth 22.4 11.2 7.5 5.6
Buckwheat (2009) Fagopyrum sagittatum Moench 28.0 14.0 9.3 7.0
Idagold mustard (2010–2011) Sinapis alba L. 6.7 3.4 2.2 1.7
Field pea Pisum sativum L. 28.0 18.7 14.0
Pacifi c Gold mustard Brassica juncea (L.) Czern. 2.2 1.7 1.1
Oilseed radish Raphanus sativus L. 2.8 2.1
Crimson clover Trifolium incarnatum L. 4.7 3.5
Dwarf essex rape Brassica napus L. 1.7
Chickling vetch Lathyrus sativus L. 8.4
Agronomy Journal • Volume 104, Issue 5 • 2012 1427
crop mixture, while split-plots (4.6 × 21.3 m; 6 crop rows spaced
0.76 m apart) were de ned by cover crop termination method.
Each “crop × cover crop mixture × termination method” treatment
combination was replicated within each block so that each phase
of the 3-yr crop sequence was present each year within each block.
ere were six whole-plot cover crop treatments: (i) two-species cover
crop mixture (2CC), (ii) four-species cover crop mixture (4CC),
(iii) six-species cover crop mixture (6CC), (iv) eight-species cover
crop mixture (8CC), (v) weedy mixture and cover crop-free (before
main crop planting) (WD), and (vi) weed-free and cover crop-free
(before main crop planting) control (NC). e NC whole-plots
were eld disked and hand-hoed twice before main crop planting,
while the WD whole-plots were le unmanaged until cover crop
termination. e goal for the WD treatment was to manage existing
weed populations as a cover crop. Details on the individual species
and seeding rates included in each cover crop mixture whole-plot are
included in Table 1.
Split-plot cover crop termination methods included either disking
or undercutting. Termination method was randomized within the
rst replication (southernmost) and duplicated in the remaining
three replications (north of the rst replication) to facilitate adequate
speed for e ective tillage operations driving north–south through
the eld. Disking was conducted with a 4.6 m wide Sun ower 3300
(Sun ower Mfg., Beloit, KS) disk to an approximate depth of 15 cm.
Undercutting was conducted with either a Bu alo 6000 (Bu alo
Equipment, Columbus, NE) cultivator (modi ed for undercutting)
with seven overlapping 0.75 m wide sweep blades (2009) or a
Miller Flex-Blade sweep plow undercutter (2010 and 2011) with
three overlapping 1.5 m sweep blades. e undercutter sweeps are
designed to cut a level plane through the soil at an approximate
depth of 10 cm, severing plant roots and minimizing soil inversion,
resulting in a layer of intact surface residue. Details on the design of
the undercutter can be found in Creamer et al. (1995).
Cover crop mixtures were planted via hand-crank broadcast
seeding followed by light incorporation with a John Deere 950
cultipacker (Deere and Company, Moline, IL). Generally, cover
crops were planted in late March, terminated in late May, and the
main crop was planted within 1 wk of termination. Speci c dates for
eld operations across all years are detailed in Table 2. While fall-
sown cover crops (e.g., hairy vetch [Vicia villosa Roth] and winter rye
[Secale cereale L.]) are more commonly used in the U.S. Corn Belt,
there is increasing interest among farmers in spring-sown species.
Much of this interest has stemmed from integrated crop– livestock
farmers who o en struggle to establish fall-sown cover crops in
elds where crop residue is grazed in the winter months. Moreover,
many farmers cite di culties in establishing fall-sown cover crop
species (e.g., timing and winterkill) as major obstacles to cover crop
adoption. us, spring-sown species may increase the exibility
of cover crop use in cropping systems leading to greater on-farm
adoption.
Seeding rates for confectionery sun ower (Helianthus annuus L.),
soybean, and corn (Zea mays L.) were 62,000, 556,000, and
86,000 seeds ha
–1
, respectively. All crops were inter-row cultivated
approximately 1 mo a er planting the cash crop each season. In
2010 and 2011, all crops were cultivated a second time within 10 d of
the rst cultivation in an e ort to improve intra-row weed control.
Surface residues in the undercutter split-plot experimental units
were su ciently dried and decomposed (due to low C/N ratio of
the cover crop residues) by this point in the growing season and did
not interfere with the cultivation. Seeds of all legume cover crop
and crop species were inoculated with appropriate rhizobia bacterial
species before planting in 2009 and 2010.
Data Collection
Monthly precipitation (mm) and temperature (°C) for April to
September was determined for each growing season by summing
daily precipitation and temperature measurements from the High
Plains Regional Climate Center station located on the University
of Nebraska Turf Farm near Mead, NE (41°10'12" N, 96°28'12" W,
elevation = 366 m), located 1 km northwest of the experimental
site (Table 3). Climate data for the 30-yr mean was obtained from a
di erent climate center near Mead, NE (41°8'24" N, 96°28'48" W)
between 1971 and 2000 (long-term data from the University of
Nebraska Turf farm was unavailable).
ree (2009) or four (2010 and 2011) aboveground biomass
samples were taken from each whole-plot experimental unit before
cover crop termination to determine productivity of the cover
crop mixtures and weed communities. Samples were combined
within each experimental unit, dried at 60°C to constant mass
Table 2. Timing of fi eld operations and data collection for
each year of the study.
Operation
Year
2009 2010 2011
Cover crop planting 20 March 30 March 21 March
Cover crop sampling 19–21 May 24 May 1 June
Cover crop termination 22 May 28 May 3 June
Main crop planting 28 May 1–3 June 6 June
First interrow cultivation 1 July 28 June 30 June
Second interrow cultivation 1 July 8 July
First soil sampling 6–7 July 29–30 June 28 June
Second soil sampling 11–12 August 26–27 July 27–28 July
Table 3. Monthly precipitation (precip.) total (mm) and average air temperature (temp.) (°C) for April to September in 2009, 2010, and
2011, and the 30-yr mean from the University of Nebraska Turf Farm near Mead, NE (41°10'12" N, 96°28'12" W, elevation = 366 m).
Month
Year
2009 2010 2011 30-yr mean
Temp. Precip. Temp. Precip. Temp. Precip. Temp. Precip.
April 9.0 28 12.8 85 9.9 76 10.1 70
May 16.9 34 15.6 53 16.2 164 16.3 106
June 21.4 135 22.5 217 22.3 139 22.0 101
July 21.1 68 24.4 156 26.5 80 24.3 84
August 20.9 135 24.3 71 23.2 78 22.9 85
September 17.2 31 17.4 134 15.7 9 18.2 73
Total 17.8 432 19.5 717 19.0 547 19.0 519
1428 Agronomy Journal • Volume 104, Issue 5 • 2012
and weighed. e biomass harvest area included three 0.3 by 0.3 m
samples per experimental unit in 2009, and was increased to four 0.3
by 0.6 m samples per experimental unit in 2010 and 2011.
Surface soil moisture (0–8 cm) was measured weekly from
cover crop planting through the vegetative growth of the main
crop. Measurements were taken at three random points within
each whole-plot (before cover crop termination) or split-plot (a er
cover crop termination) experimental unit using a eta Probe soil
water sensor (SM 200 Soil Moisture Sensor, Delta-T Devices Ltd,
Cambridge, UK). Accuracy of the soil water sensor was veri ed
against 21 gravimetric soil samples in 2010 and the ratio between
method outputs was approximately 1:1. Linear regression analysis
indicated a positive relationship between outputs from the two
methods (p = 0.003, F = 11.68, df
n
= 1, df
d
= 19, R
2
= 0.38).
Soil samples were collected twice during each year to characterize
early (between 25 and 42 days a er termination, DAT) and late
(between 55 and 81 DAT) growing season NO
3
–N availability.
A composite soil sample of three (2009) or four (2010 and 2011)
soil cores (3.18 cm diam. by 20 cm) per split-plot experimental unit
were taken. Composite soil samples were then air-dried and sent to
Ward Laboratories (Ward Laboratories, Kearney, NE) for extraction
and analysis of soil NO
3
–N. Soil NO
3
–N was extracted with a Ca
solution and analyzed by the Cd reduction procedure (Ward, 2011).
Crop yield was determined for each main crop by harvesting
seed or grain from the middle three (corn) or four (soybean and
sun ower) rows of each split-plot experimental unit. Contents
were weighed using a combine scale (Model 400, Weigh-Tronix,
Fairmont, MN) and adjusted for moisture content in the lab. Corn
grain yields were adjusted to 0.155, soybean to 0.130, and sun ower
to 0.10 g kg
–1
moisture. A er crop yields were determined,
economic costs and returns were calculated for each crop– cover
crop mixture– termination method treatment combination for
each year. e di erence of returns less costs was considered pro t
and calculated on an average annual basis for each crop and the
entire rotation (Table 4). Cost estimates (e.g., seed, custom planting,
cultivation, and harvest, etc.) were obtained from a variety of sources.
Costs were considered xed across crops and years and only varied
due to the cost of each cover crop mixture and manure applied
before the 2009 cropping season (Tables 4 and 5). Similarly, one
price estimate was used for each crop return; thus, annual returns
only varied according to the yield of each crop among treatments
where signi cant yield di erences occurred.
Data Analysis
Values for cover crop biomass, soil moisture, soil NO
3
–N, and
crop yield were analyzed with a linear mixed model analysis of
variance using the GLIMMIX procedure in SAS 9.2 (SAS Institute,
Cary, NC). Fixed e ects in the model included main crop, cover
crop mixture, termination method, and all possible interactions
of these e ects. e random e ects were block and the interaction
of block × current crop × cover crop mixture. e model for data
taken before cover crop termination (i.e., cover crop biomass and
soil moisture) excluded xed e ects for main crop and termination
method. In addition, models for soil NO
3
–N and soil moisture
analysis included a xed e ect for and interactions including day
of year (DOY). E ects were o en tested within individual years
due to experimental changes in the cover crop mixture (buckwheat
[Fagopyrum sagittatum Moench] was replaced in all mixtures with
Idagold mustard [Sinapis alba L.] a er 2009 due to poor growth
of buckwheat) and interactions with year when initially included
as a xed e ect (data not shown). Least square means and standard
errors were calculated for all signi cant xed e ects at an α level of
Table 4. Economic costs, returns, and average annual profi t (U.S. dollars ($) ha
–1
) for the 11 different cover crop mixture by termi-
nation method treatment combinations in corn, soybean, and sunfl ower for the years 2009, 2010, 2011, and for the entire rotation.
NC = no cover control; WD = weedy mixture; 2-, 4-, 6-, and 8CC = two, four, six, and eight cover crop species mixtures, respec-
tively (Table 1); D = disk termination; U = undercutter termination.
Crop/year
Cover crop mixture and termination method
NC
WD 2CC 4CC 6CC 8CC
DU DU DU DU DU
________________________________________________
U.S. dollars ($) ha
–1 ________________________________________________
Costs
2009 1514 1472 1470 1667 1665 1656 1653 1690 1688 1690 1688
2010 771 731 729 900 897 912 909 954 951 961 959
2011 771 731 729 899 897 912 909 954 951 961 959
Returns
Corn
2009 3884 3404 4055 3404 4055 3404 4055 3404 4055 3404 4055
2010 2193 2674 3212 2674 3212 2674 3212 2674 3212 2674 3212
2011 4609 4211 4547 4211 4547 4211 4547 4211 4547 4211 4547
Soybean
2009 1933 1179 1836 1179 1836 1179 1836 1179 1836 1179 1836
2010 508 578 783 578 783 578 783 578 783 578 783
2011 1755 1475 2069 1475 2069 1475 2069 1475 2069 1475 2069
Sunfl ower
2009 1591 1394 1540 1394 1540 1394 1540 1394 1540 1394 1540
2010 401 540 540 540 540 540 540 540 540 540 540
2011 1065 1021 1109 1021 1109 1021 1109 1021 1109 1021 1109
Avg. annual profi t
Corn 2543 2451 2962 2274 2785 2270 2781 2230 2741 2225 2736
Soybean 380 99 587
−78
410
−82
406
−122
366
−127
361
Sunfl ower 1 7 87
−171 −90 −175 −94 −214 −134 −219 −139
3-crop rotation 975 853 1212 675 1035 671 1031 631 991 626 986
