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Published in Agron. J. 96:1021-1028 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

PRODUCTION PAPER

Living Mulches of Legumes in Imidazolinone-Resistant Corn

Sjoerd W. Duiker* and Nathan L. Hartwig

Dep. of Crop and Soil Sci., The Pennsylvania State Univ., 116 ASI Bldg., University Park, PA 16802-3504

* Corresponding author (swd10{at}psu.edu).

Received for publication October 10, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Living mulches of legumes are permanent cover crops that can fix atmospheric N and improve soil quality. The objectives of this study were to determine effects of living mulches on corn (Zea mays L.) yield and N fertilizer response and on soil quality. In this split-plot design, living mulches were main plots and N rates (0–225 kg ha–1 N) subplots. Living mulches were crownvetch (Coronilla varia L.) cultivars Penngift and Pennmulch, flatpea (Lathyrus sylvestris L.) cultivar Lathco, birdsfoot trefoil (Lotus corniculatus L.) cultivars Empire and Steadfast, hairy vetch (Vicia villosa Roth), and galega (Galega officinalis L.). The crop was rainfed imidazolinone (IMI)-resistant corn on a Murrill silt loam (Typic Hapludult) in central Pennsylvania. Only crownvetch, flatpea, and birdsfoot trefoil (BFT) cultivars survived the herbicide program. Living mulches did not reduce corn yields, except in the dry year of 1995. The average N fertilizer equivalency of Penngift, Pennmulch, Empire, Steadfast, and Lathco was 71, 45, 44, 13, and 50 kg ha–1 yr–1, respectively, at 0 N rates. Their N fertilizer equivalency decreased to zero, however, with increasing N fertilizer rates. Bulk density, soil organic C content, and infiltration rate were not significantly improved after 10 yr of Penngift living mulch. When suppressed severely, crownvetch, BFT, and flatpea can be managed without yield reduction to IMI-resistant corn, but then N contribution to corn and soil quality benefits will be limited. For maximum corn yield, full N rates are required with these living mulches.

Abbreviations: BFT, birdsfoot trefoil • IMI, imidazolinone • UAN, urea ammonium nitrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
LEGUMINOUS COVER CROPS have the potential to reduce dependence on fossil fuel and reduce negative environmental effects of corn production. Some functions these cover crops can perform are (i) fixing atmospheric N that is made available to corn, (ii) protecting nitrate from leaching in the fall/winter, (iii) protecting soil from erosion during the winter and during the corn growing season, (iv) improving soil quality, (v) suppressing weeds, and (vi) reducing evaporation and increasing infiltration during the corn growing season (SAN, 1998). Unfortunately, leguminous cover crops cannot be established after corn grain harvest in Pennsylvania (Curran et al., 1996; SAN, 1998). Establishment of relay-planted cover crops before corn harvest is often difficult because of equipment limitations, limited access to fields, and dry soil conditions during late summer. To avoid cover crop establishment problems in summer, farmers and researchers have experimented with a variety of living mulches that occupy the land permanently (Hartwig and Hoffman, 1975; Hartwig, 1987).

Intercropping research has shown that most legumes do not compete strongly with cereals for light, N, P, and K, whereas they compete equally for water (Ofori and Stern, 1987; Vandermeer, 1990). The low stature of most legumes and their horizontally positioned leaves reduce competition for light with tall, erect cereals. Since many legumes are C3 crops with low light saturation points and low temperature optima, one might expect these legumes to complement a C4 crop such as corn that has a high light saturation point and high temperature optimum (Ofori and Stern, 1987). Instead of competing for N, legumes may instead contribute N to the main crop (Fujita et al., 1992). Because of their different root systems (less fibrous and often having a taproot), competition for the immobile nutrients P and K can be expected to be limited (Ofori and Stern, 1987; Vandermeer, 1990). Legumes are therefore promising candidates for living mulches in corn.

The symbiotic relationship of legumes with rhizobia bacteria allows them to use N from the atmosphere. Of major interest is whether some of the fixed N will be available to a cereal grown simultaneously with the legume. If this is the case, living mulches of legumes could reduce the need for fertilizer N. The primary mechanism of N transfer from a legume to a nonlegume is decomposition of leaves, roots, and stems of the legume (Fujita et al., 1992). If this were the only mechanism, some parts of a living mulch of legumes would have to die before N would be transferred to the cereal. Observations in cereal–legume intercrops have confirmed, however, that N is also excreted from legume roots and leached from leaves, thus becoming available to the cereal immediately (Fujita et al., 1992).

Although legumes compete weakly with cereals for light, N, P, and K, they can compete strongly for water. If water stress is eliminated by irrigation, living mulches of legumes rarely reduce and sometimes increase main-crop yields (Grubinger and Minotti, 1990; Fischer and Burrill, 1993; Costello, 1994). Major benefits of the living mulches in these studies included reduced aphid infestation, increased leaf N content, and reduced soil erosion. Without irrigation, it becomes more challenging to implement a living mulch system. There are successful examples of annual or biennial cover crops established after emergence of the main crop, which gives the main crop a competitive advantage (Scott et al., 1987; Wall et al., 1991). If cover crops are established before or after the main crop is planted, competition of the living mulch for water may reduce crop yields (Echtenkamp and Moomaw, 1989; Eberlein et al., 1992; Masiunas et al., 1997; Teasdale et al., 2000). We found only one example where a living mulch system without irrigation never reduced main-crop yields (Ilnicki and Enache, 1992). In this study, subterranean clover (Trifolium subterraneum L.) in rainfed corn and other crops provided excellent weed control without the use of tillage or herbicides. We experimented with subterranean clover as a living mulch, but it did not survive Pennsylvania winters (results not reported here).

Management of living mulch becomes critical to reduce competition with the main crop for water while allowing the mulch to grow sufficiently to reap potential benefits. Different methods have been employed to suppress the living mulch, such as tillage, mowing, and herbicides (Grubinger and Minotti, 1990; Fischer and Burrill, 1993; Costello, 1994; Martin et al., 1999; Zemenchik et al., 2000). In this paper, we report results of an evaluation of several living mulches in IMI-resistant corn in Pennsylvania. The use of IMI-resistant corn opened new management options to adequately control the living mulch to reduce competition for water. Hypotheses were that the leguminous living mulches (i) could be suppressed in IMI herbicide programs without eradicating them, (ii) would not reduce corn yields, (iii) would provide N to the corn, and (iv) would improve soil quality.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field
The living mulches for this experiment were established in 1990 on the Russell E. Larson Agricultural Research Center in Rock Springs, PA. Nitrogen treatments were started in 1995. The soil was a Murrill silt loam (Typic Hapludult) containing 160 g kg–1 sand (2–0.05 mm), 670 g kg–1 silt (0.05–0.002 mm), and 140 g kg–1 clay (<0.002 mm) in the surface 15 cm and an organic matter content of 25 g kg–1 at the beginning of the experiment. Corn was grown from 1990–2000 with the exception of 1993 and 1997 when oat (Avena sativa L.) and soybean [Glycine max (L.) Merr.] were planted, respectively. Oat in 1993 allowed for a straightforward weed control program when the main investigator was on sabbatical. Soybean in 1997 was planted to attempt Steadfast BFT establishment in Treatment 5. Another objective for use of soybean was to reduce carryover effects of excess N fertilizer that might have been possible on plots with high N rates applied. Corn height and yield and the biomass of living mulches were collected in 1994, 1995, 1996, 1998, 1999, and 2000, and soil quality parameters were measured in 2001 and 2002.

The treatments consisted of a factorial combination of six living mulches and six N fertilizer rates in a split-plot design with five replications. Plots were 4.5 m wide and 7.5 m long. All living mulches were established without tillage. The living mulch treatments were (Table 1)

  1. Control without living mulch.
  2. Penngift crownvetch. Crownvetch was seeded at a rate of 6 kg ha–1 in the spring of 1990 and was overseeded in 1993.
  3. Lathco flatpea. Flatpea was seeded at 22 kg ha–1 in the spring of 1990 and overseeded in 1993, 1995, and 1998 at the same rate.
  4. Empire BFT. Birdsfoot trefoil was seeded at 6 kg ha–1 in the spring of 1990 and overseeded in 1993, 1994, 1995, and 1997 (seeding rate was 11 kg ha–1 since 1993).
  5. In 1990, 1991, and 1993, this living mulch treatment was hairy vetch seeded in August. No living mulch was present in 1992 and 1995, and galega was seeded at 2 kg ha–1 in May of 1996. This treatment was seeded to Steadfast BFT (11 kg ha–1) in 1997.
  6. Pennmulch crownvetch (a prostrate selection of crownvetch) was established in spring of 1993 (seeding rate 6 kg ha–1) and overseeded at the same seeding rate in 1995. Before 1993, this treatment was subterranean clover that failed as a living mulch.


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  		Table 1. Living mulch treatments 1994-2000.

 
Living mulches were overseeded when their stands became thin. Thin stands were usually the result of addressing a weed problem with a herbicide that proved to eliminate too much of the already established living mulch.

The whole experiment received a blanket application of fertilizer based on Penn State recommendations until 1995. In 1995, 1996, 1998, 1999, and 2000, the N rates were 0, 45, 90, 135, 180, 225 kg ha–1 N. Urea ammonium nitrate fertilizer (UAN, 30% N) was applied by hand. One-half of the total N was applied mid-May and the other half mid-June. Sidedressed N was banded on the surface, which minimizes volatilization losses of urea (Fox and Bandel, 1986). In addition to the sidedressed N fertilizer, all treatments received a nominal amount of 112 kg ha–1 10–30–10 starter fertilizer. The starter fertilizer was applied 5 cm besides and 5 cm below the seed through fertilizer openers on the planter. In 1997, when soybean was planted, no fertilizer was applied. In 2001, a blanket application of urea (143 kg ha–1 N) was applied to all plots in April. In 2002, all plots received equal amounts of ammonium nitrate (54 kg ha–1 N applied in May) and UAN (73 kg ha–1 N applied in June).

Imidazolinone-resistant corn hybrids were Pioneer ‘3343IR’ (planted 28 Apr. 1994), Pioneer ‘3751IR’ (planted 9 May 1995), Funks ‘4393IMR’ (planted 15 May 1996), Pioneer ‘35A19IR’ (planted 20 May 1998 and 4 May 1999), Pioneer ‘36D14IT’ (planted 12 May 2000), DeKalb ‘471IMI’ (planted 14 May 2001), and DeKalb ‘595IMI’ (planted 24 May 2002). All hybrids were planted at a target density of 70000 seeds per hectare. Trefluthrin R soil insecticide was applied with the corn seed at planting time at a rate of 0.10 to 0.15 kg ha–1 every year. On 23 May 1997, soybean cultivar Asgrow STS A2704 was planted at a target density of 430 000 seeds per hectare.

Herbicide programs changed over the years of the experiment for two reasons: (i) The weed spectrum changed over the years, dictating the need to adjust the weed control program, and (ii) whenever improved herbicide programs were discovered for the management of living mulches, they were introduced in the experiment. All herbicides used are labeled for corn. A blanket application of pre-emergence herbicide was used in all years, but postemergence herbicide applications varied per treatment in some years. The aim of pre-emergence herbicide applications was to maintain 5% aboveground living mulch cover before corn canopy closure. This would forestall undue competition of the living mulch with corn but allow recovery of the living mulches in July. Pre-emergence herbicide applications in 1994 were a mixture of imazethapyr {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid} (0.07 kg ha–1 a.i.), pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] (1.12 kg ha–1 a.i.), and 2,4-D LVE (0.11 kg ha–1 a.i.) applied on 5 May. In 1995, glyphosate [N-(phosphonomethyl)glycine] (0.84 kg ha–1 a.i.) mixed with a nonionic surfactant (NIS) (0.25% v/v) was sprayed on 3 May. In 1996, glyphosate (1.12 kg ha–1 a.i.) mixed with NIS (0.25% v/v) was applied on 13 May. In 1998, imazethapyr (0.035 kg ha–1 a.i.) and rim-sulfuron {N-[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]-3-(ethylsulfonyl)-2-pyridinesulfonamide} + thifensulfuron {3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid} (2:1) (0.014 kg ha–1 a.i.) mixed with Chaser (1.5% v/v) were applied on 15 May. In 1999, glyphosate (0.56 kg ha–1 a.i.) was applied in a 1% v/v UAN solution. In 2000, glyphosate (0.56 kg ha–1 a.i.) was applied to all treatments except to the flatpea treatment (the flatpea treatment was chisel-plowed in 2000). Postplant herbicide applications are summarized in Table 2.


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  		Table 2. Postplant herbicide programs for cover crop treatments.{dagger}

 
Plant and Soil Analyses
Living mulch weight (reported on an oven-dry basis) was determined by harvesting aboveground biomass in a 0.5-m2 area in each plot (on 2 Sept. 1994, 2 Sept. 1995, 6 Aug. 1996, 26 July 1999, and 26 July 2000). Moisture content was determined by drying a sample for 24 h at 70°C in a convective oven. Corn grain yields are reported as containing 155 g kg–1 moisture.

Soil bulk density and infiltration rate were measured in 2001–2002 in the Penngift crownvetch and the control treatments that had received 135 kg ha–1 N. We focused on the effect of Penngift crownvetch because this was the most vigorous living mulch and also because the other living mulches were mostly present at very low densities in 2001. The 135 kg ha–1 N was chosen because it represents a typical fertilization rate a farmer would use. Soil bulk density was determined in three replicates by the core method, using rings with a height and diameter of 7.62 cm (Blake and Hartge, 1986). Two cores were taken per plot from the 0- to 12-, 12- to 24-, and 24- to 36-cm depths. Infiltration rate was measured in August of 2002 in all five replicates with a single, 36-cm-diam. infiltration ring in which a constant head of 5 cm was maintained (Bouwer, 1986). The pH, soil organic C content, and exchangeable P, K, Ca, and Mg contents were determined of the Penngift and control treatments at all six N rates (0–225 kg ha–1 N). Samples for these analyses were collected from the 0- to 15-cm depth in all five replicates. Soil organic C content was determined with a Carlo-Erba NA 1500 analyzer (Nelson and Sommers, 1996). The soil pH was measured in a 1:1 soil/water mixture (Eckert and Sims, 1995). Available P, K, Ca, and Mg were determined by the Mehlich 3 method (Wolf and Beegle, 1995).

Statistical Analyses
Analysis of variance, separation of means (LSD), and regression analyses were determined with PROC GLM and PROC STEPWISE in SAS (SAS Inst., 2001). Nitrogen response curves of corn height and yield were fitted to a quadratic function (Cooke, 1982). Data were analyzed within years only because of different establishment dates of the living mulches.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Climatic Conditions
The average precipitation received from May–October is 570 mm at the research location. Total precipitation received during the months May–October was 655 mm in 1994, 569 mm in 1995, 778 mm in 1996, 524 mm in 1997, 464 mm in 1998, 435 mm in 1999, and 549 in 2000. Low precipitation in July–August of 1995 resulted in greatest yield reductions due to drought stress in that year.

Living Mulch Performance
Analysis of variance of biomass produced by the living mulches in the fall showed significant differences between living mulches in every year that measurements were taken. Additionally, there was an N effect in 1996, 1998, 1999, and 2000 and a living mulch x N rate interaction in 1998. Penngift crownvetch always produced the greatest amount of fall biomass (Table 3). Lathco flatpea produced much biomass at the beginning of the experiment but largely disappeared toward the end of the experiment. The use of glyphosate since 1995 probably explains the decline of flatpea that proved to be easily eradicated by this herbicide. Empire BFT produced more biomass than flatpea except in 1994 and 1996. Greater biomass production of Empire BFT compared with flatpea was especially evident in 1999 and 2000, indicating it survives the herbicide suppression (especially glyphosate) better than flatpea. These results suggest that crownvetch and BFT can also be successfully managed as living mulches in a glyphosate-based herbicide program, potentially broadening the applicability of the results to glyphosate-resistant crops.


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  		Table 3. Living mulch dry matter yield in early fall.{dagger}

 
In Living Mulch Treatment 5, hairy vetch was successfully established and survived the winter (see footnote in Table 3), but it did not qualify as a living mulch because it was killed before it could set seed to re-establish itself. Galega, seeded in 1996 in Treatment 5, did not germinate. Treatment 5 was then seeded to Steadfast BFT in 1997. It produced less biomass than Empire BFT in the first year after establishment but was at an equal par in 2000. If we disregard differences between cultivars, this suggests it takes approximately 2 to 3 yr before BFT is firmly established under this management regime. Pennmulch crownvetch never produced as much biomass as Penngift crownvetch. This may be an indication that Pennmulch is suppressed more by the herbicides than Penngift, possibly due to its more prostrate nature, which causes more herbicide to be absorbed by the leaves. Other factors that probably influenced Pennmulch performance include the high dose of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] applied in 1994 (by mistake) and an extra postplant herbicide application in 1996 to control heavy weed pressure in this treatment (Table 2).

Table 4 presents the p values of the linear and quadratic orthogonal comparisons of N rate on biomass production of the living mulches from 1998–2000, years when N rate had a significant effect on living mulch biomass production. On average over these 3 yr, biomass production of Penngift crownvetch and Lathco flatpea decreased linearly with increasing N rate, that of Empire and Steadfast BFT was unaffected by N rate, and that of Pennmulch crownvetch followed a quadratic curve. The latter may be considered an anomaly, with biomass first decreasing with increasing N rate and then increasing at the highest N rate. The bottom line is that when N is supplied to the system, crownvetch and flatpea biomass production decrease due to increased competition of corn for limited resources. Most likely, reduced light infiltration through the canopy of a more vigorous corn crop produced this decrease although competition for water, P, and K may also have played a role. Birdsfoot trefoil seems to be affected less by this competition, suggesting BFT may be more tolerant of shading than crownvetch and flatpea.


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  		Table 4. Nitrogen rate effects on living mulch biomass.

 
Because N rate was not varied in 1994, it is logical that living mulch biomass did not vary with N rate in 1994. The plausible explanation for an absence of N rate effect in 1995 is that water, not N, was the limiting factor for corn growth. The corn was therefore less competitive with the living mulches at all N rates in 1995. In 1996, total living mulch biomass was small for all living mulches (Table 3) because of excessive suppression by herbicides, causing limited living mulch growth at all N rates. The rate of imazethapyr was therefore more than halved after 1996, allowing more living mulch growth.

Effects of Living Mulches on Corn
Analysis of variance (not shown) indicated that corn height at tasseling was significantly affected by living mulch treatments from 1996–2000 and by N rates whenever they varied (1995–2000). Living mulch x N rate interactions occurred from 1996–2000. The average N response curve of corn height for the period 1996–2000 is presented for Penngift and Pennmulch crownvetch, Lathco flatpea, Empire BFT, and control treatments in Fig. 1 and Table 5. The effects of Steadfast BFT on corn height are not shown in Fig. 1 because Steadfast BFT was established in 1997 (regression equation is given in Table 5). Lathco flatpea had a positive effect on corn height at low N fertilization rates and a neutral to positive effect at high N fertilization rates compared with the control. Both crownvetch cultivars and Empire BFT had a positive effect on corn height at zero N fertilization but a neutral or negative effect when more than 50 kg ha–1 N was applied.



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  		Fig. 1. Living mulch and N rate effects on corn height at tasseling (1996–2000). BFT, birdsfoot trefoil.

 

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  		Table 5. Coefficients of N-response curves for corn heights at tasseling (1996–2000).{dagger}

 
Analysis of variance (not shown) of corn yields showed significant living mulch effects in all years except 1994 and N rate effects in all years with variable N fertilization rates (1995–2000). Significant interactions between living mulch and N rate occurred in 1996, 1998, 1999, and 2000. Of most interest are the N response curves of living mulch treatments with the control in the years when N rates varied (1995–2000). We will present here average response curves for the period 1996–2000 and separate response curves for the year 1995. The reason for this way of presentation is that 1995 was a year when the crop experienced significant water stress. The average yield of the experiment was only 3296 kg ha–1 in 1995 due to the drought compared with 5459, 5390, 4678, and 6602 kg ha–1 in the years 1996, 1998, 1999, and 2000, respectively. Separate analysis of 1995 yield data provides us with the opportunity to investigate the effect of the living mulch on crop yield in a year of extreme water stress. Corn yield analysis of Steadfast BFT could only be done for the period 1998–2000 since this cover was established in 1997.

In years with adequate rainfall, Penngift and Pennmulch crownvetch, Lathco flatpea, and Empire BFT had a positive effect on corn yields that decreased with increasing N rates (Fig. 2 and Table 6). The N response curve of corn was not influenced by the late established Steadfast BFT (Fig. 3 and Table 6). The fertilizer N equivalent of the living mulches at 0 kg ha–1 fertilizer N rate was calculated by equating the N response curve of the control with the y intercept of the response curve of the respective living mulch. The fertilizer N equivalent of the living mulches was Penngift crownvetch, 71 kg ha–1 N; Pennmulch crownvetch, 45 kg ha–1 N; Empire BFT, 44 kg ha–1 N; and Lathco flatpea, 50 kg ha–1 N. Calculated for the period 1998–2000, the N fertilizer equivalent was 13 kg ha–1 N for Steadfast. In the dry year of 1995, Penngift crownvetch and Empire BFT reduced corn yields at most N rates (Fig. 4) . Pennmulch crownvetch and Lathco flatpea had a slight positive or neutral effect on 1995 corn yield at low N rates.



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  		Fig. 2. Living mulch and N rate effects on corn yields (1996–2000).

 

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  		Table 6. Coefficients of N-response curves for corn yields (1996–2000).{dagger}

 


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  		Fig. 3. Steadfast birdsfoot trefoil (BFT) and N rate effects on corn yields (1998–2000).

 


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  		Fig. 4. Living mulch and N rate effects on corn yields in a dry year (1995).

 
These results suggest that:
  1. At low N rates, the living mulches provide N to corn. The most vigorously growing living mulches (i.e., Penngift) provide the highest amount of N to the main crop. The N supplied by the living mulches is less than what would be needed to reach an optimum yield goal for this soil. The typical capability of a Murrill silt loam is 9400 kg ha–1, or 150 bu ac–1 (Duiker, 2002), which would require an N rate of 185 kg ha–1 N at Penn State University recommendations (Beegle, 2002). The maximum N supplied by the living mulches in this experiment was 71 kg ha–1.
  2. For maximum corn yield, fertilizer N rates cannot be reduced due to the presence of a living mulch of legumes. As N rates increase, N contribution of the legume to corn gradually decreases to zero. So instead of adding to symbiotically fixed N, fertilizer N starts to replace it. One effect of fertilizer N application was increased competition of corn with the living mulch for light and other resources, reducing photosynthesis of the legume and therefore biomass production (Table 4). Because N fixation requires photosynthate, less N fixation by rhizobium bacteria is the likely result (Fujita et al., 1992). It has also been observed that fertilizer N application to a legume–cereal mix decreases nodulation (Patra and Poi, 1998). In years with adequate precipitation, corn height was somewhat negatively affected in the presence of Penngift, Pennmulch, and Empire BFT, but this did not result in a yield decrease in those years. Lathco flatpea had a positive or neutral effect on corn height in these years.
  3. Competition of a vigorous living mulch for water is likely to reduce corn yield in a drought year. Competition of Penngift crownvetch and Empire BFT for water reduced corn yields in the drought year of 1995. Pennmulch crownvetch and Lathco flatpea did not produce a yield dip in 1995. The reason for the absence of yield reduction with flatpea and Pennmulch was probably their low biomass production in 1995 (Table 3).

Soil Improvement
Penngift crownvetch or N fertilizer rate did not have a significant effect on soil organic C content (average 17.0 g kg–1), pH (average 6.58), or exchangeable P (average 57 mg kg–1), Mg (average 221 mg kg–1), and Ca (average 1436 mg kg–1) concentration of the topsoil (Table 7). The statistical analysis indicated lower K concentration in the control than in the Penngift crownvetch treatment. The average K concentration was 149 mg kg–1 in the crownvetch treatment and 129 mg kg–1 in the no-cover treatment. A legume such as crownvetch with different root architecture than corn might absorb relatively immobile K from deep zones in the soil that would not be accessed by corn (Vandermeer, 1990). Upon degradation of leaves, stems, and roots of crownvetch, some of this K might become available to corn. Absence of such an effect with P can be explained by the fact that it is less mobile than K, and little leaching to deeper layers can be expected. Since no detailed analyses were performed, this conclusion needs reconfirmation in additional research.


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  		Table 7. F values of soil analyses of Penngift crown vetch and control plots.

 
N rate had a significant effect on pH (not shown) due to the acidifying effect of the N fertilizer (the pH of the 225 kg ha–1 N rate had a pH 0.6 units lower than the 0 kg ha–1 N rate). Bulk density in the surface 36 cm of the soil was not significantly affected by Penngift crownvetch. Average bulk density was 1.36, 1.46, and 1.50 Mg m–3 in the 0- to 12-, 12- to 24-, and 24- to 36-cm depths, respectively. The values are in the ideal range for a silt loam surface horizon in both treatments (USDA, 1999). Cumulative infiltration in the Penngift treatment was 164 mm h–1, whereas the cumulative infiltration in the control treatment was 110 mm h–1. The final infiltration rate was 2.0 mm min–1 in the Penngift and 1.35 mm min–1 in the control, but these differences were not statistically significant. There is therefore some tentative indication of improved infiltration with crownvetch. Both infiltration rates fall in the moderately rapid rate (USDA, 1999).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This research has shown that it is possible to adequately suppress crownvetch, flatpea, and BFT living mulches with an IMI-based herbicide program in rainfed field corn. Crownvetch was most successfully managed. Flatpea and BFT had to be reseeded a number of times because the herbicide program was sometimes too severe for these living mulches. Flatpea seemed to be very sensitive to glyphosate that was used from 1995 onwards, leading to its demise. The results suggest, however, that crownvetch and BFT can be successfully managed in a glyphosate-based herbicide program as well as in an IMI herbicide program.

The N contribution of living mulches to corn was present only at low N fertilizer rates and disappeared at high N rates. Farmers can therefore not decrease N fertilizer rates with living mulches of legumes if they strive for maximum corn yields. At high N rates, competition for light and water reduced biomass production of the living mulches and probably also reduced N fixation by rhizobium (Fujita et al., 1992). Application of N fertilizer has been found to reduce nodulation and nodule growth (Singh and Nair, 1995; Patra and Poi, 1998).

Corn yields were not negatively affected by competition from the crownvetch, BFT, and flatpea living mulches in years with adequate precipitation. In 1995, a year with very low rainfall in July and August, Penngift crownvetch and Empire BFT reduced corn yields. Pennmulch crownvetch and Lathco flatpea did not produce much biomass in this year and did not have a yield-reducing effect. The more vigorous a living mulch, therefore, the greater the risk that yields will be reduced in a drought year.

We did not observe a significant improvement of soil organic C content or bulk density with Penngift crownvetch, the most vigorous living mulch in this experiment. Although infiltration rate was improved somewhat, the increase was not significant. It is evident that in no-till corn, where all crop residue is left in the field, the benefits of leguminous living mulches for soil quality improvement will be limited. In a corn silage system, where all corn residue is harvested, there might be a greater potential for soil quality benefits of a living mulch of legumes.

A dilemma is faced in management of crownvetch, BFT, and flatpea. On the one hand, maximum biomass production of the living mulches is needed to obtain potential benefits such as N fixation and soil quality benefits. On the other hand, the living mulches need to be severely suppressed to reduce risk of corn yield loss. Greater potential benefits might be expected from living mulches with a very different active growth period than corn (for example, early spring or late fall). Future research should try to identify such living mulches.


   ACKNOWLEDGMENTS
 
The authors thank Gerald Rogers for doing most of the field work and Mr. Carl Reidler, a conservation-minded businessman/farmer, for financial support for this research. Many thanks also to Michael Poteet for taking the bulk density and infiltration measurements. Finally, we wish to acknowledge USDA funding through the Hatch Act of 1887, without which this type of long-term research would be impossible.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
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