Published in Agron J 100:432-439 (2008)
DOI: 10.2134/agrojnl2007.0141
© 2008 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
COVER CROPS
Establishment and Growth of Self-Seeded Winter Cereal Cover Crops in a Soybean–Corn Rotation
Paul B. McDonalda,
Jeremy W. Singerb,* and
Mary H. Wiedenhoeftc
a NRCCS, Webster County Field Office, 1202 Banning Street, Marshfield, MO 65706
b USDA-ARS, National Soil Tilth Lab., 2110 University Blvd.., Ames, IA 50011
c Dep. of Agronomy, 1126DAgronomy Hall, Iowa State Univ., Ames, IA 50011
* Corresponding author (jeremy.singer{at}ars.usda.gov).
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ABSTRACT
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Perpetuating cereal cover crops through self-seeding may increase adoption by reducing risk and cost. Winter rye (Secale cereale L.), wheat (Triticum aestivum L.), and triticale (x Triticosecale Wittmack) were used to develop self-seeding cover crop systems in a soybean [Glycine max (L.) Merr.]–corn (Zea mays L.) rotation. Cereals were planted and managed chemically and mechanically in varying configurations. The objectives were to (i) quantify temporal establishment patterns after one cycle of self-seeding, (ii) quantify shoot biomass, N uptake, and seed production growing concurrently with corn, and (iii) quantify cover crop establishment after two cycles of self-seeding. Final plant densities for most species by treatment combinations were fully established within 1 wk after soybean (Cycle 1) harvest. Fall green ground cover after soybean was consistently higher with wheat and ranged from 16 to 61%. Straw biomass the following July ranged from 50.4 to 79.1 g m–2 in wheat, 20.1 to 39.3 g m–2 in triticale, and 0.0 to 52.7 g m–2 in rye. Combined spring and maturity maximum N uptake was 20.7, 21.2, and 35.0 kg ha–1 for triticale, wheat, and rye. Cycle two cover crop seed production was greatest in wheat and ranged from 559 to 1280 seeds m–2. Wheat also consistently had greater self-seeding plant establishment after two cycles than rye and triticale, which ranged from 5 to 21% of the original plant densities and 19 to 64% of the cycle one plant densities. Future research on self-seeding cereal cover crops should focus on efficient technologies for seed dispersal.
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NOTES
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All rights reserved. No part of this periodical m–ay 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.
1 Mention of trade names or commercial products is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. 
Received for publication April 17, 2007.
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INTRODUCTION
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COVER CROPS provide important environmental functions that include reducing soil erosion (Zhu et al., 1989; Kaspar et al., 2001) and nitrate leaching (Kladivko et al., 2004; Strock et al., 2004; Kaspar et al., 2007). Nevertheless, cover crop adoption in agronomic farming systems is low. Singer et al. (2007a) reported cover crop use among producers in the U.S. Corn Belt was 11% between 2001 and 2005. Reasons reported by producers in this region for not using cover crops included too much time involved (34.8%), too costly (27.4%), do not have a runoff problem (28.1%), already use no-tillage practices (38.6%), and do not know enough about them (39.5%). Innovative approaches that address producers' reasons for not using cover crops may increase their adoption in agronomic farming systems.
Interspecific competition in self-seeding cover crop systems occurs when growth of the cover crop overlaps with growth of the cash crop. Viable alternatives to the conventional approach of planting cover crops in the fall and terminating their growth in the spring in farming systems dominated by summer annual crops must minimize this competition. Westgate et al. (2005) reported that relying solely on mechanical control to suppress rye lowered soil water content and light interception by soybean compared with soybean growing without rye. Singer and Kohler (2005) in the same study reported 30 to 60% yield loss in soybean using mechanical control to suppress a rye cover crop at the second node, boot, or anthesis growth stages. In contrast, De Bruin et al. (2005) reported that rye and soybean growing concurrently had no effect on soil water content from the 0- to 45-cm soil depth after 6 June compared to a no rye check in 1 yr. However, significant differences in another year during mid-July from the 30- to 60-cm soil depth decreased soybean yield.
Singer et al. (2007b) compared competitiveness and self-seeding of winter cereal cover crop species using different planting configurations and management options while growing concurrently with soybean. They reported yield losses from 15 to 45% among management treatments, while no interaction was detected between the management treatments and the three winter cereal species.
Studies have demonstrated that legumes can be used in a self-seeding system (Kumwenda et al., 1993; Myers and Wagger, 1991; Ranells and Wagger, 1991), but few studies have used winter cereals as a self-seeding cover crop (Singer and Kohler, 2005; Singer et al., 2007b). Singer et al. (2007b) reported final self-seeding of winter wheat, triticale, and rye after one cycle of self-seeding but provided no information on the fate of the self-seeded winter cereals through the winter or the second cropping season when grown concurrently with corn. The current work reports on the continuation of the research first described by Singer et al. (2007b). The objectives were to quantify: (i) the establishment of the three winter cereal species through the fall and winter and following corn crop after the first cycle of self-seeding in soybean; (ii) quantify the shoot biomass, N uptake, seed production, and establishment of the self-seeded winter cereals when grown concurrently with corn using different cover crop management approaches.
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MATERIALS AND METHODS
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Field studies were conducted at the Agricultural Engineering Research Center near Ames, IA (42°01' N, 93°45' W; 341 m asl), from October 2003 through November 2006. The current study was a continuation of the original research that was published after the soybean phase was completed (Singer et al., 2007b). This research followed the original cover crop treatments into the fall after soybean harvest and through the subsequent corn production season. The soil for both field experiments was a Spillville loam (fine-loamy, mixed, superactive, mesic, Cumulic Hapludolls). The field site was managed in a soybean–corn rotation using no-tillage. Soil test levels in the surface 20-cm soil depth in 2004 were 17 mg kg–1 P, 80 mg kg–1 K, and a pH of 6.6 and in 2005 were 20 mg kg–1 P, 115 mg kg–1 K, and a pH of 6.5. Nitrogen, P, and K were surface applied on 2 Apr. 2005 and 17 Apr. 2006 at a rate of 35, 39, and 74 kg ha–1, respectively.
The experimental design was a randomized complete block with treatments arranged in a split-plot with four replicates. Species main plots were wheat (‘Karl 92’), rye (‘Rymin’), and triticale (‘Décor’ in 2004 and ‘Kitaro’ in 2005) that self-seeded through seed shatter after maturity and the physical disturbance caused by combining soybean on 29 Sept. 2004 and 3 Oct. 2005. Subplots were the residual of the previous cover crop management systems (Table 1
) and a no cover crop check. The original experiment was planted at 2,470,000 seeds ha–1 on 25 Sept. 2003 and 9 Oct. 2004 using a grain drill with 19-cm row widths. Cover crops were drilled with either two or four 19-cm rows between each 76-cm soybean row. In the four row treatments, cover crop management included chemically killing two 19-cm cover crop rows adjacent to the future soybean row early or late in the spring compared to no chemical control. In the two row treatments, the primary difference was the use of a rolling stalk chopper to suppress the two rows of cover crop drilled in the interrow. Cycle 1 self-seeding refers to the cover crops that self-seeded during the soybean production year after the original drilled cover crops reached maturity and commenced seed shatter to perpetuate the next cycle. Cycle 2 refers to the cover crops that established after the cycle 1 cover crops commenced seed shatter and established during the corn production year.
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Table 1. Cover crop treatment description and chemical and mechanical control dates during the soybean and corn production seasons. The initial cover crop treatments were seeded mechanically in the fall of 2003 and 2004. Cover crop treatments that self-seeded after soybean were followed in corn the following growing season to assess the residual effect of the initial treatments.
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Cover crop emergence was counted weekly from 6 Oct. through 11 November in 2004 and 5 October through 18 November in 2005 in a 2.3 m2 permanent quadrat. Quadrats were randomly placed between two former 76-cm nontrafficked soybean rows. Cover crop shoot biomass and tiller density were determined on 20 Nov. 2004 and 21 Nov. 2005 from two 0.25 m2 quadrats in each subplot. Shoot biomass was clipped at the soil surface and dried in a forced-air oven at 70°C until constant weight. Digital images were taken on 14 and 21 Nov. 2004 and 19 Nov. 2005 1 m above the soil surface and analyzed for green ground cover using a grid matrix with 100 sample points in each image.
Cover crop tiller density and shoot biomass were also measured on 13 to 14 Apr. 2005 and 17 Apr. 2006 in two 0.25 m2 quadrats in each subplot. Spring biomass sampling occurred at Feekes growth stage 3.0 for all species in 2005 and 4.0, 5.0, and 4.0 for wheat, rye, and triticale in 2006 (Zadoks et al., 1974). Shoot biomass was dried in a forced air oven at 70°C until constant weight and ground to pass through a 1-mm sieve. Total N concentration was determined using the Dumas combustion method (AOAC International, 2000).
Chemical control of cover crops was performed on 15 Apr. 2005 and 24 Apr. 2006 using glyphosate [N-(phosphonomethyl)glycine] in a 25-cm band over the corn row at an application rate of 1.1 kg a.i. ha–1. Dekalb brand ‘DKC 53–33’ corn was planted on 18 Apr. 2005 and 20 Apr. 2006 using a no-tillage planter equipped with row cleaners at a population of 86,487 seeds ha–1 using a 76-cm row spacing. Mechanical control of cover crops was achieved using a Buffalo rolling stalk chopper (Fleischer Manufacturing Inc., Columbus, NE)1 with one pass in the corn interrow on 23 May 2005 and 19 May 2006 at Feekes growth stages 10.0, 10.5, and 10.0 for wheat, rye, and triticale, respectively. The primary difference between the mechanical (before soybean) and self-seeding (before corn) cover crop systems is that the self-seeded cover crops were not in rows. The plant distribution was random. Broadleaf chemical control occurred in November 2004 with dicamba (3,6-dichloro-O-anisic acid) and 2,4-D amine (2,4-dichlorophenoxyacetic acid) at application rates of 0.28 kg a.i. ha–1 and 0.27 kg a.i. ha–1, respectively. Chemical control of broadleaf weeds also occurred on 31 May 2005 and 2006 using Buctril (3,5-dibromo-4-hydroxybenzonitrile) at an application rate of 0.42 kg a.i. ha–1. Nitrogen was injected as urea ammonium nitrate at 212 kg N ha–1 on 13 June 2005 at the V5 growth stage of corn (Ritchie et al., 1992) and 24 May 2006 at V2.
At Feekes growth stage 11.4 (Zadoks et al., 1974) shoot biomass and spike counts were obtained in a 0.76 m2 area in each subplot. Wheat, triticale, and rye were sampled at grain maturity on 10, 13, and 28 July in 2005, and 13, 31, and 31 July in 2006, respectively. Cover crop shoot biomass and grain were separated, dried in a forced-air oven at 70°C, ground to pass a 1-mm sieve and analyzed for total N concentration. Cover crop densities were recorded on 19 Nov. 2005 and 4 Nov. 2006 using a 2.3 m2 quadrat.
Daily rainfall and mean air temperature were recorded at a weather station about 2 km from the experimental site and presented by month for each growing season (Table 2
). Statistical analysis was conducted using PROC MIXED (SAS Institute, 2002) version 9.1 with block and block by species as random variables and cereal species, treatment, and years as fixed variables. A first order autoregressive model was used for the repeated measures fall cover crop establishment data. A Fisher's protected LSD (alpha = 0.05) was used to separate species, treatment, and interaction means.
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RESULTS AND DISCUSSION
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Self-Seeding—Cycle 1
Cover crop establishment data are presented by year because of a significant year effect (data not presented). Date, species, and treatment main effects were significant, but no species by treatment interactions were detected in 2004. A three-way interaction was detected in 2005. Both years, plant stand for most of the cover crop management systems was fully established at the first sampling point after soybean harvest, which occurred 1 wk after harvest in 2004 and 2 d after harvest in 2005. The consistent exception was the rye 2RBNC treatment, which increased until 5 wk after harvest in 2004 and just over 3 wk after harvest in 2005 (Fig. 1
).
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Fig. 1. Self-seeding cover crop establishment in the fall of 2004 and 2005. The LSD compares plant density within year, date, and species. Letters represent differences among dates, within year, species, and treatment. Only lines with letters were significantly different. Treatments in soybean in 2004 and 2005 were four rows with early (4REB) and late (4RLB) spring glyphosate to eliminate two 19-cm rows and mechanical control, four 19-cm rows with mechanical control (4RNB) only, two rows with mechanical control (2RB), and two rows with no mechanical control (2RBNC).
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Treatment 2RBNC had the highest self-seeding compared to most treatments because of two to eight times greater seed production than the next closest treatment (Singer et al., 2007b). The increase in plant density over time in this treatment probably resulted from improved seed-to-soil contact from settling or rainfall that resulted in seed germination. The 2RBNC treatment had greater self-seeding than all other treatments in rye (49.0 vs. 3.5 plants m–2) and triticale (58.0 vs. 12.5 plants m–2), while the 2RBNC, 4RLB, and 4RNB treatments in wheat were similar in 2005, except at the first sampling time. Breeding efforts for wheat may have increased reproductive partitioning that were not observed in the other species. Self-seeding systems in soybean using winter wheat and triticale that rely on natural seed shatter to establish the next cycle are likely to be fully established at soybean harvest regardless of cereal seed production.
Tiller Density
Fall 2004 and spring 2005 tiller densities had significant species and treatment effects but no species by treatment interaction (Table 3
). Averaged across treatment, wheat had the highest tiller density in the fall and spring at 67 and 339 tillers m–2, while rye and triticale tiller densities were similar in the fall and spring at 37 and 166 tillers m–2, respectively. In the fall, averaged across species, 4RLB (26 tillers m–2) was lower than 4REB (40 tillers m–2) and all treatments were lower than the 2RBNC (100 tillers m–2). In the spring, averaged across species, the same pattern was evident among treatments. Singer and Kohler (2005) reported rye self-seeded fall tiller density of 237 tillers m–2 in 1 yr, averaged across treatments, and 43, 86, and 118 tillers m–2 in another year after mechanical control at the second node, boot, and anthesis growth stages, respectively.
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Table 3. Cover crop species, treatment, and interaction means for late fall cover crop biomass (FCCB), spring cover crop biomass (SCCB) at corn planting, fall tiller density, spring tiller density, and late fall ground cover near Ames, IA, in the fall of 2004 and spring of 2005.
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In the fall of 2005 and spring of 2006, a species by treatment interaction occurred for tiller density (Table 4
). Wheat 4RLB, 4RNB, and 2RBNC were similar in the fall at 187 tillers m–2, while 4REB and 2RB were similar and lower at 105 tillers m–2. In the spring, wheat tiller response was generally similar among treatments, although the 4REB had similar tiller production as the 4RLB and 4RNB. Rye and triticale 2RBNC had the highest densities among treatments in the fall and spring, except for triticale 4RLB in the spring. Not surprisingly, the tiller density data closely followed plant density treatment response. Higher plant densities generally produced higher tiller densities. This is not consistent with Whaley et al. (2000), who reported decreasing tiller production as wheat plant density increased from 19 to 338 plants m–2. These self-seeding plant densities were below the critical threshold required to affect tiller prolificacy, which Whaley et al. (2000) reported occurred between plant densities of 30 and 59 plants m–2.
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Table 4. Cover crop species, treatment, and interaction means for late fall cover crop biomass (FCCB), spring cover crop biomass (SCCB) at corn planting, fall tiller density, spring tiller density, and late fall ground cover near Ames, IA, in the fall of 2005 and spring of 2006.
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Ground Cover
Ground cover was affected by species and treatment in 2004 (Table 3). Averaged across treatment, wheat had greater ground cover (24%) than rye and triticale, which had similar ground cover (15%). Averaged across species, 2RBNC had the greatest ground cover (29%), while 4REB, 4RLB, 4RNB, and 2RB were similar at 15% ground cover. A species by treatment interaction occurred for ground cover in 2005 (Table 4). Wheat 2RBNC and 4RLB had similar ground cover, while 2RBNC in rye and triticale had greater ground than the rest of the treatments. Using the revised universal soil loss equation (RUSLE) to calculate erosion rates, erosion can be reduced by 30% with as little as 10% residue cover and can be reduced by as much as 80% with 50% residue cover (Moldenhauer and Langdale, 1995).
Cover Crop Biomass
Cover crop shoot biomass had a species by treatment interaction in the fall of 2004 and spring of 2005 (Table 3). In the fall, wheat 2RBNC had the highest biomass at 27 g m–2, while other treatments ranged from 7 to 12 g m–2. Treatment 2RBNC (12 g m–2) was similar in rye and triticale, while 4REB, 4RLB, 4RNB, and 2RB were similar at 3 g m–2. Wheat spring biomass was highest in 2RBNC (23 g m–2), while 4REB, 4RNB, and 2RB were similar at 15 g m–2 and 4RLB (8 g m–2) was the lowest. Wheat was similar to rye and triticale in the 4REB, 4RLB, 4RNB and 2RB treatments at 12 g m–2. Rye 2RBNC had the highest biomass at 40 g m–2, while wheat and triticale were lower at 22 g m–2.
Fall 2005 shoot biomass only exhibited treatment effects (Table 4). The 2RBNC treatment had the highest biomass at 19 g m–2, while 4REB, 4RLB, 4RNB, and 2RB were similar at 9 g m–2. Spring 2006 shoot biomass had a species by treatment interaction (Table 4). Wheat biomass was similar among treatments (27 g m–2). Rye 2RBNC produced the greatest biomass (64 g m–2), while other treatments produced 6 g m–2 or less. In triticale, 2RBNC and 4RLB had similar biomass.
Singer and Kohler (2005) reported self-seeded rye biomass of 27.8 g m–2 in the spring, averaged over treatments, in 1 yr and 52.0 (second node), 27.5 (boot), and 17.9 (flowering) g m–2 in another year immediately before corn planting. The original mechanical control occurred about a year earlier at Feekes growth stages 7.0 (second node), 9.8 (boot), and 10.51 (flowering) with biomass production ranging from 134 to 604 g m–2 depending on treatment and year (Westgate et al., 2005). Tollenaar et al. (1993) reported a corn yield reduction of 2 to 16% with cover crops compared to the no cover crop check, but found no correlation between yield reduction and the quantity of spring cereal biomass before corn planting. Consequently, spring cover crop management should be balanced to encourage biomass production to increase nutrient capture and ground cover with timely planting of cash crops.
Spring Nitrogen Uptake
A species by treatment interaction occurred in 2005 for spring cover crop N uptake at corn planting (Table 5
). Wheat 2RBNC, 4RLB, 4RNB, and 2RB had similar N uptake (7.8 kg ha–1) and were higher than 4REB. Rye 2RBNC had the highest N uptake at 16.1 kg ha–1, while 4REB, 4RLB, 4RNB, and 2RB were similar (5.8 kg ha–1). Triticale 2RBNC, 4RLB, and 4RNB had similar N uptake (6.9 kg ha–1). A species by treatment interaction also occurred in 2006. All wheat treatments had similar uptake (12.5 kg N ha–1). Rye 2RBNC had the highest N uptake at 28.8 kg ha–1, while all other treatments were similar (1.3 kg ha–1). Triticale 2RBNC, 4REB, and 4RLB had similar N uptake (12.6 kg ha–1). De Bruin et al. (2005) in Minnesota reported N uptake ranging from 5.7 to 16.2 kg ha–1 for a rye cover crop controlled by mowing on 1 May across sites and years that was mechanically planted with higher stand densities and biomass than in this study.
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Table 5. Cover crop species, treatment, and interaction means for spring cover crop N uptake near Ames, IA, in 2005 and 2006.
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Maturity Biomass and Seed Density
Straw biomass had a species by treatment interaction in 2005 (Table 6
). In wheat and triticale, no differences were detected among treatments and averaged 59.1 and 26.9 g m–2. Rye straw biomass was similar among 2RBNC, 4RNB, and 4RLB treatments. Straw biomass had a species effect in 2006 (Table 7
). Wheat and triticale had similar biomass and rye and triticale were similar, but rye had lower biomass than wheat. No species (P value = 0.131), treatment (P value = 0.144), or interaction (P value = 0.119) differences occurred for spike density in 2005, which averaged 74 spikes m–2 (Table 6). Wheat had higher spike density in 2006 (107 spikes m–2) compared to rye and triticale (31 spikes m–2)(Table 7). Year was not significant for seed mass, however, a species by treatment interaction occurred (data not presented). Wheat 2RB had the highest seed mass (15.1 g m–2) compared to 4REB and 2RBNC (6.8 g m–2). Rye 2RBNC had the highest seed mass (8.4 g m–2) compared to all other treatments (0.9 g m–2), while all triticale treatments were similar (4.5 g m–2).
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Table 6. Cover crop species, treatment, and interaction means for straw biomass, seed mass, seed density (SD), and spike density (SPD) of cover crops growing concurrently with corn near Ames, IA, in 2005.
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Table 7. Cover crop species, treatment, and interaction means for straw biomass, seed mass, seed density (SD), and spike density (SPD) of cover crops growing concurrently with corn near Ames, IA, in 2006.
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Year was not significant for seed density, although a species by treatment interaction was detected (data not presented). Averaged across year, wheat 2RB had the highest seed density (1001 seeds m–2), 4RLB and 4RNB were intermediate (743 seeds m–2), and 4REB and 2RBNC were lower (539 seeds m–2). Rye 2RBNC had the highest seed density (628 seeds m–2) compared with all other treatments, which were similar (51 seeds m–2). All triticale treatments were similar (348 seeds m–2).
Nitrogen Uptake at Maturity
Year was not significant for straw N uptake, although a species by treatment interaction occurred (Table 8
). Averaged across year, wheat 4REB had lower N uptake (5.9 kg ha–1) than 2RB (9.8 kg ha–1). Rye 4REB, 4RLB, 4RNB, and 2RB had similar uptake (2.3 kg ha–1) compared to 2RBNC (9.7 kg ha–1). Triticale 2RB and 4RNB had less N uptake (2.5 kg ha–1) compared to 4RLB (7.6 kg ha–1). Grain N uptake also exhibited a species by treatment interaction but no year effect (Table 8). Averaged across year, wheat 2RB had greater N uptake (5.2 kg ha–1) than 2RBNC and 4REB (2.6 kg ha–1). Rye 4REB, 4RLB, 4RNB, and 2RB had similar N uptake (0.3 kg ha–1) and were all lower than 2RBNC (2.8 kg ha–1). Triticale treatments had similar grain N uptake (1.4 kg ha–1).
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Table 8. Cover crop species, treatment, and interaction means for grain and straw N uptake at maturity for cover crops growing concurrently with corn near Ames, IA, averaged across 2005 and 2006.
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Self-Seeding—Cycle 2
Year, species, and treatment affected cycle two self-seeding after corn but no interactions were detected. Wheat had greater self-seeding than rye both years and triticale in 2006 (Table 9
). Rye and triticale had similar plant densities both years. In 2005, 4REB, 4RLB, 4RNB, and 2RB were similar at 3.9 plants m–2, while 2RBNC was higher (7.1 plants m–2). In 2006, 2RBNC, 4RLB, and 2RB had similar plant density (15.2 plants m–2), while 2RBNC and 4RLB were higher than 4REB and 4RNB (9.8 plants m–2). Year did not effect cover crop seed production, nevertheless, year did influence cover crop plant establishment. The difference between the highest and lowest seed production in wheat was 462 seeds m–2, yet plant establishment did not respond to increasing seed production.
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Table 9. Cover crop species and treatment means for cycle 2 cover crop plant density in November following corn near Ames, IA in 2005 and 2006.
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These results indicate that seed production may not be limiting plant establishment. Plant establishment in the fall of 2005 was probably lower because only 9 mm of rainfall occurred in October, while 63 mm rainfall fell in October 2006. We did not measure the temporal establishment pattern of the cover crop after corn, but visual observations support this statement. The efficiency of self-seeded systems relying on natural seed shatter was <0.5% using these winter cereals after one cycle of self-seeding (Singer et al., 2007b). Singer et al. (2007b) reported 46 to 64% of drilled seed established plants at the beginning of this field experiment. After two cycles of self-seeding, wheat plant density was 20% of the average plant density established after drilling wheat to commence this study. Wheat self-seeding cover crop systems comparing these species and varieties exhibits the greatest promise to develop viable alternatives to the standard cover crop practice. Additional research should focus on minimizing interspecific competition between the cover crop and cash crop and improving cover crop seed distribution. Competition between the least competitive cover crop system and corn lowered corn grain yields about 9% and will be reported in a separate manuscript.
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CONCLUSIONS
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Winter cereals are capable of self-seeding in soybean and corn using natural seed shatter to establish the next plant cycle. Wheat exhibited the greatest promise among the cereals tested, although this may only reflect breeding improvements for reproductive partitioning. Nevertheless, most of the cover crop treatments had adequate seed production to establish the next plant cycle. Future research should focus on chemical and mechanical control to balance interspecific competition and seed production. Moreover, self-seeded plant establishment can likely be increased by developing technologies to improve seed dispersal during the soybean phase of the rotation. These improvements ultimately may lead to increased cover crop use because the risk and cost of establishing them will be lower and the environmental benefits may be enhanced.
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ACKNOWLEDGMENTS
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The authors thank Keith Kohler for managing the field study and providing input to make management decisions to achieve experimental objectives.
All rights reserved. No part of this periodical m–ay 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.
1 Mention of trade names or commercial products is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
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ABSTRACT
INTRODUCTION
Thirty-year mean is from 1977 to 2006.