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Nitrogen Management

Effects of a Grass-Selective Herbicide in a Vetch–Rye Cover Crop System on Nitrogen Management

^a USDA Sustainable Agriculture Network, Beltsville, MD 20705-2351
^b USDA-ARS, Animal and Natural Resources Inst., Beltsville, MD 20705
^c Dep. of Natural Resource Sciences and Landscape Architecture, Univ. of Maryland, College Park, MD 20742
^d Lower Eastern Shore Research and Education Center, Salisbury, MD 21801

^* Corresponding author (aclark{at}sare.org)

Received for publication December 30, 2005.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Cover crop kill date affects N fixation by hairy vetch (Vicia villosa Roth), N uptake by cereal rye (Secale cereale L.), residue C/N ratio, and subsequent N availability. Data are needed on spring management of vetch–rye cover crop mixtures, compared to pure stands, to estimate fertilizer nitrogen (FN) equivalents. A 2-yr study evaluated spring management of hairy vetch (HV), pure rye, a vetch–rye mixture, and a no-cover check on N accumulation and subsequent no-till corn N uptake following corn FN rates of 0, 45, 90, 180, and 270 kg ha^–1. A grass-selective herbicide (GSH) was applied in late March to the pure rye and the vetch–rye mixture, leaving HV to accumulate N until early May. These treatments were compared to the same covers killed in early May. Cover crop N uptake was lowest for rye, intermediate for the mixtures, and highest for HV. The N content in the pure rye and vetch–rye mixture was significantly increased if the previous year's corn had received excess FN. The cover crop mixture produced greater rye growth if fall soil nitrate N was high, while low soil nitrate N resulted in greater yield of HV in the mixture. There was no difference in corn N uptake for the late- vs. early kill pure rye, or of the rye component in the vetch–rye mixture. A vetch–rye mixture functioned like a "dual purpose" cover by conserving fall residual N, producing a lower C/N ratio residue than pure rye, and supplying more N to the succeeding corn than pure rye, although the N supplied was still less than pure vetch.

Abbreviations: CKL, no-cover check • DM, dry matter • FN, fertilizer N • GSH, grass selective herbicide • HV, hairy vetch • LSD, least significant difference • MXL, late-killed vetch–rye mixed cover • MXP, early killed rye in vetch–rye mixed cover • RCB, randomized complete block • RYL, late-killed pure rye cover • RYP, early killed pure rye cover • VTL, late-killed vetch cover

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
LEGUME COVER CROPS function quite differently than grass cover crops. Hairy vetch has been shown to regularly supply large amounts of N for a subsequent corn crop (Ebelhar et al., 1984; Holderbaum et al., 1990; Hargrove, 1986; Clark et al., 1995, 1997a) but vetch conserves relatively little fall residual NO₃–N (Shipley et al., 1992). Cereal rye planted early in the fall (Brinsfield and Staver, 1989) has been shown to take up residual FN that can reduce winter NO₃–N leaching. Rye usually scavenges more N than other cover crops (Brinsfield and Staver, 1991; Meisinger et al., 1991; Shipley et al., 1992; Ranells and Wagger, 1997a). However, corn following small-grain cover crops often requires more FN than if no cover crop were grown (Mitchell and Teel, 1977; McCracken et al., 1989; Wagger, 1989b; Munawar et al., 1990; Eckert, 1988; Clark et al., 1997b). Maximum yield following small grain covers is often less than following legume covers (Decker et al., 1994). Vetch–rye cover crop mixtures have the potential to combine some of the advantages of both legume and grass cover crops, if properly managed. With no FN, corn yields following vetch–rye mixtures (Clark et al., 1994, 1997b) or legume–wheat (Triticum aestivum L.) mixtures (Holderbaum et al., 1990) were less than pure vetch, but greater than pure rye or no cover crop.

Optimizing the management of a vetch–rye cover crop mixture is complicated due to differences in spring growth of the two components. Hairy vetch can accumulate N at a rate of 2 kg N ha^–1 d^–1 from April to early May (Clark et al., 1994, 1997a). However, rye quickly matures during this same period resulting in a high C/N ratio that may reach 50:1 (Clark et al., 1997a) or 66:1 (Clark et al., 1994). An early kill of the mixture can lower the C/N ratio of the rye, but would limit vetch N accumulation. In contrast, a late kill of a vetch–rye mixture can result in greater vetch N accumulation. The downside of this practice is the resulting higher C/N ratio of the rye and the reduced N availability for the following crop.

Synchronization of cover crop N release with corn N uptake is also important to the success of a cover crop system. Wagger (1989a) and Vaughan and Evanylo (1998) reported that early killed rye resulted in a faster N release, while N release from vetch was similar when killed either early or late. Early kill of a vetch–rye mixture has also been reported to release N faster (Vaughan and Evanylo, 1998), but would likely sacrifice total residue production, potentially resulting in less moisture conservation (Clark et al., 1997b; Vaughan and Evanylo, 1998).

Limited data are available on N mineralization rates from cover crop mixtures. Davis et al. (1990) and Ranells and Wagger (1996) reported that the rate of N release from vetch–rye mixtures was intermediate to pure vetch and pure rye. Although more N was measured in the topgrowth of a vetch–rye mixture than in pure vetch or pure rye killed late, corn grain yield following the mixture was intermediate to pure vetch and pure rye (Clark et al., 1994, 1997a; Ranells and Wagger, 1997b). Holderbaum et al. (1990) and Ranells and Wagger (1997a) concluded that greater N immobilization was responsible for the intermediate corn yield response following grass–legume mixtures, compared to the pure legume.

Data are needed on spring management of grass–legume mixtures to optimize kill-date effects and to more accurately estimate corn FN needs. Our general objective was to evaluate cover crop management practices aimed at reducing excessive rye growth, yet allowing high vetch N production. The specific objectives were to evaluate management strategies of: an early (mid-March) application of a GSH to a vetch–rye mixture or a pure rye cover, and a late-kill (early May) of all vegetation in a vetch–rye mixture, a pure rye cover, a pure vetch cover, and a no-cover control. Effects of these treatments on crop N cycling were evaluated by documenting the cover crop yield, N content, C/N ratio, and the N uptake of the succeeding corn crop receiving four rates of FN. Corn grain yield, soil moisture, and economic interpretations are presented in a companion paper (Clark et al., 2007).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A 2-yr study (1989–1991) was conducted on a Mattapex silt loam soil (fine-silty, mixed, mesic, typic Hapludult) on the Atlantic Coastal Plain of Maryland at the Poplar Hill Research Farm, 38° N lat. Cover crop treatments of: no cover, hairy vetch, cereal rye, or a vetch–rye mixture occupied whole plots of a split-plot design with whole plots measuring 24.4 m by 3.1 m wide. Common hairy vetch was seeded at 28 kg ha^–1, ‘Abruzzi’ rye at 94 kg ha^–1, and the mixture seeded with 21 kg ha^–1 vetch and 47 kg ha^–1 rye as recommended by Clark et al. (1994). In the fall of 1989, covers were seeded into a plowed seedbed on 6 October following plow-tillage corn. Corn (Pioneer 3241) was seeded no-till into killed cover crop mulches on 12 May 1990 and 13 May 1991 in 76-cm rows running the entire length of the whole plots. Following 1990 corn harvest, the area was disked and the cover crops were re-seeded into the same whole plots on 6 October. The six whole-plot treatments were randomly assigned to their experimental units and consisted of cover crop and spring kill-date combinations of: no cover check (CKL), early killed pure rye (RYP), late-killed pure rye (RYL), early killed rye in vetch–rye mixture (MXP), late-killed vetch–rye mixture (MXL), and late-killed vetch (VTL). The two early kill treatments consisted of a single application of "Poast" {2-[1-(ethoxyimino)butyl]-5-[2(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} herbicide broadcast sprayed on 21 Mar. 1990 and 20 Mar. 1991 to kill the rye. All plots were also sprayed with a tank mixture of gramoxone (1,1' dimethyl-4. 4-bipyridinium ion), atrazine (2-chloro-4 ethylamino-6-isopropylamino-S-triazine), cyanazine [2-chloro-4-(1-cyano-1methylethylamino)-6-ethylamino-S triazine], and 2,4-dichloronhenoxy-acetic acid on 2 May 1990 and 1 May 1991 (late kill) at recommended application rates of: (kg a.i. ha^–1) 0.35, 1.5, 1.6, and 0.6, respectively. The May herbicide application killed all existing cover crop vegetation and provided residual weed control for the succeeding corn crop.

Corn FN rates made up the subplots of the experimental design and were 0, 45, 90, and 180 kg N ha^–l for vetch; and 0, 90, 180, and 270 kg N ha^–l for the no-cover, rye and the vetch–rye mixture treatments. Ammonium nitrate was broadcast by hand on 22 May l990 and 31 May 1991, when corn was at the two- to three-leaf stage. The complete experimental design was a split-plot, randomized complete block, with four replicates.

Hand samples of cover crop growth were taken on all whole plots just before herbicide applications by clipping two 0.6-m square areas per plot. Mixtures were separated into vetch and rye components. Samples were dried in a forced-air oven for 72 to 96 h at 60°C, weighed for dry matter (DM) yield determination, ground to pass a 1-mm screen, and analyzed for C and N content using a Leco CHN-600 Analyzer (LECO, St. Joseph, MI). Corn N uptake was measured 20 Sept. 1990 and 17 Sept. 1991 by hand-harvesting 3.7 m of one of the two center rows. Three representative whole plants from each plot were chopped in the field, and processed by drying and grinding as outlined above for the cover crop samples.

Soil samples were taken to a depth of 90 cm after corn harvest in the fall of 1990. Six 1.9-cm diam. cores were taken per plot in 20-cm increments through 60 cm plus a final 30-cm increment from 60 to 90 cm. Soils were rapidly air-dried at room temperature, ground to pass a 2-mm screen, and mixed. Ten-gram samples of soil were extracted with 50 mL of 2 M KCl and analyzed for NO₃–N and NH₄–N on a Technicon Autoanalizer (Technicon Instruments, Emeryville, CA) using the Cadmium reduction method for nitrate and the indo-phenol method for ammonium as described by Bundy and Meisinger (1994).

Data were analyzed using SAS (SAS Institute, 1988). In 1990, cover crop data were analyzed as a randomized complete block (RCB) because prior FN treatments had been uniform over the new study area. In 1991, these data were analyzed as a split-plot, RCB, to account for the variable corn FN rate in 1990. Corn data were analyzed as a split-plot RCB. For the combined analysis over years, year was considered a fixed effect (McIntosh, 1983). Soils data were analyzed for each depth, and totaled over the 90-cm depth, using a split-plot randomized complete block analysis. Quadratic response functions were fit to the corn N uptake data using the SAS general linear models procedure.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Cover Crop Yield, Nitrogen Uptake, and Soil Nitrate Content
First Year Data
Cover crop total DM yields were not significantly different for rye, vetch, or the vetch–rye mixture for the early harvest of 1990 (Table 1). However, total N content of the rye was significantly lower in mid-March. Total N content of the mixture (MXP) was not significantly lower than pure vetch, although only 70% of the N in the mixture was from the vetch.

View larger version (39K): [in this window] [in a new window]   Fig. 1. Twenty-year average monthly rainfall at Poplar Hill research station and monthly rainfall distributions during the study period of September 1989 to August 1991.

The 90-cm deep soil samples collected at the end of the 1990 corn season showed significant treatment effects on soil NO₃–N levels, but no significant effect on soil NH₄–N levels. Therefore, the low quantities of soil NH₄–N are not presented. Within cover crop, soil NO₃–N increased with each increment of FN applied to the preceding corn crop (Table 2). With the exception of RYL, residual soil NO₃–N was low (17–38 kg NO₃–N ha^–1) for nonfertilized treatments, moderate (38–67 kg NO₃–N ha^–1) for the first increment of FN, and greatest (85–141 kg NO₃–N ha^–1) for the highest FN rate. The low residual NO₃–N following RYL at all rates of corn FN probably reflects continued N immobilization caused by the high C/N rye residue, while the higher levels of residual N following vetch or mixtures reflect excess N from the combination of FN plus large amounts of N from the legume. High levels of fall residual N following surplus inputs of FN and drought have also been reported by Ranells and Wagger (1997c). The highest rate of FN for each cover crop treatment (180 kg ha^–1 for vetch, or 270 kg ha^–1 for the others) did not result in significantly greater grain yield in 1990 (Clark et al., 2007). Thus, the high residual N levels were due to surplus inputs from corn FN and cover crop N. These varying levels of fall residual NO₃–N set the stage for growth effects on the 1990 fall-seeded cover crops.

For both mixtures, as corn FN increased, rye DM yield increased and vetch decreased from 4950 to 3100 kg ha^–1 for MXP, and 4200 to 1900 kg ha^–1 for MXL. Using the GSH on the vetch–rye mixture (MXP) allowed more vetch DM to be produced than if the rye was allowed to grow until early May (MXL).

The N content of the covers (Table 3) was also influenced by previous cover crops and corn FN rates, although some noteworthy differences occurred. There were generally no significant differences in N content of the vetch in either mixture treatment, except for the high FN rate of MXL that contained only 73 kg ha^–1 compared to 124 kg ha^–1 for the high FN vetch in MXP. The cover crop vegetation in the high residual N mixture that was allowed to grow until the late-kill data (MXL) contained 47% of its total N in the vetch component, while the corresponding value for the early kill mixture was 76%. Controlling rye growth early with the GSH allowed the vetch to accumulate more N than when it was competing with the rye. This difference in composition of the mixture was also reflected in the corn N uptake (below) and in the corn grain yields the following year (Clark et al., 2007).

Corn Nitrogen Uptake
Rainfall Distribution
Corn N uptake, in this nonirrigated study, was influenced by summer rainfall distributions. The rainfall patterns differed somewhat between 1990 and 1991 (Fig. 1). In 1990, April and May were very wet but June and July were below normal, although the total rainfall from April through August was close to the 20-yr average. In 1991, April, May, and June rainfall was below normal followed by an above-normal July. The rainfall pattern in both years was quite ample for good corn growth due to the absence of prolonged dry periods.

Corn N uptake, calculated as N concentration of silage DM yield at grain harvest, is often a more sensitive indicator of N availability than corn grain yield. Average N uptake was 30 kg ha^–1 greater in 1991 than in 1990, attributed to better July rainfall. In 1990 (Fig. 2a ), corn N uptake for CKL without FN indicated soil N availability of about 71 kg ha^–1. The N uptake curve for both rye treatments with 90 kg FN ha^–1 is displaced to the right of the control curve by about 20 kg N ha^–1, indicating a lowered N availability of about 15 to 30 kg of the FN, but this lower availability was not significant above 180 kg FN ha^–1. In 1991, N uptake by CKL without FN indicated soil N availability of about 87 kg N ha^–1. The higher value in 1991 is probably attributable to better rainfall distributions that promoted higher soil organic N decomposition and/or lower N losses (Fig. 2b). Corn N uptake by the two rye treatments indicated about the same amount of N immobilization as in 1990 for the first 90 kg FN applied, i.e., about 15 to 30 kg N ha^–1. Early GSH treatment of the pure rye plots did not have a marked impact on corn N uptake.

View larger version (26K): [in this window] [in a new window]   Fig. 2. Corn N uptake (kg N ha–1) in 1990 (2a), 1991 (2b), and 1990–1991 average (2c) after cover crops of: rye killed early (RYP) or late (RYL), vetch killed late (VTL), vetch–rye mixture killed early (MXP) or late (MXL), or no cover (CKL).

Corn N uptake from the vetch cover crop was typical of results reported in earlier studies (Decker et al., 1994; Clark et al., 1994, 1997a). The vetch N response curve was offset to the left of the control plot N uptake curve by about 100 to 140 kg N ha^–1, which agrees with the average vetch FN equivalent of about 155 kg N ha^–1 reported by Seo et al. (2000). There were also indications of higher maximum N uptakes with vetch that could not be completely explained by FN additions. This has also been observed in earlier studies (Decker et al., 1994; Clark et al., 1994). The exact nature of these benefits has not been documented but can likely be attributed to surface "mulching effects" of water conservation and lower temperature, or reduced disease and insect pressure due to "rotational effects" resulting from the break in grass monoculture. The companion paper in this series (Clark et al., 2007) documents some of the water conservation benefits of these cover crops.

The corn N uptake response over the 2 yr of this study (Fig. 2c) clearly demonstrates that N availability to corn was greatest following hairy vetch, least following rye, and intermediate following the vetch–rye mixture. The FN equivalents of the cover crops in this study can be estimated from Fig. 2c at about 100 to 140 kg N ha^–1 for hairy vetch, 60 to 80 kg N ha^–1 for vetch–rye mixtures, and a FN debit of about 15 to 30 kg N ha^–1 for cereal rye. The early application of a GSH to the rye and vetch–rye mixtures did not have a marked impact on the corn N uptake curve.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The data of this study demonstrate that a vetch–rye cover crop mixture has the potential to function as a "dual purpose" cover crop. The rye component serves to conserve fall residual N, especially when residual N levels are high, and the vetch component serves to supply N to the subsequent corn crop. The botanical composition of the mixture is responsive to residual soil N levels and this simplifies cover crop selection for optimizing N management. The use of a GSH to control rye growth was most successful in vetch–rye mixtures and resulted in a mixture containing more desirable traits, such as a narrow C/N ratio and a greater proportion of vetch within the mixture. The N availability to corn (in FN equivalents) was greatest following hairy vetch (100–140 kg N ha^–1), least following rye (debit of 30 kg N ha^–1), and intermediate following the vetch–rye mixture (60–80 kg N ha^–1). The early treatment of the rye and vetch–rye mixtures with a GSH did not have a marked impact on corn N uptake. The GSH was most useful in changing the botanical composition of the vetch–rye mixture at high levels of residual N.

	ACKNOWLEDGMENTS

 
This research was funded in part by the USDA Sustainable Agriculture Research and Education (SARE) program.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Mention of trade names, brand names, or trademarks does not constitute endorsement of the product by the University of Maryland or the USDA and does not imply its approval to the exclusion of other products that may also be suitable.

	REFERENCES

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