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a Agronomy Dep., Purdue Univ., West Lafayette, IN 47907-1150 USA
b Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada N1G 2W1
c Land Resource Science Dep., Univ. of Guelph, Guelph, ON, Canada N1G 2W1
tvyn{at}purdue.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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The quantity of cover crop N available to a subsequent corn crop is species-dependent and usually associated with greater availability of N from legumes than from nonlegumes (Dekker et al., 1994; Torbert et al., 1996; Vyn et al., 1999). Considerable variation in N availability can occur, even among legume species (Hesterman et al., 1992; Wagger, 1989; Dekker et al., 1994) and environmental factors such as precipitation, temperature, length of growing season, and soil productivity can affect the amount of cover crop N accumulated and availability to succeeding corn crops (Hesterman et al., 1992; Dekker et al., 1994; Stute and Posner, 1995). In addition to environmental factors, management decisions such as tillage practices (Dou et al., 1994; Wilson and Hargrove, 1986; Sarrantonio and Scott, 1988) and timing of chemical kill (Wagger, 1989; King, 1994) can also affect cover crop N availability and the quantity of fertilizer N required for the maximum economic yield of the subsequent crop.
Cover crops can be readily incorporated into crop rotations that include cereals. Even in northern corn-producing regions, sufficient growing season remains after cereal harvest for cover crop species to produce a substantial quantity of biomass (Stute and Posner, 1993; Swanton et al., 1996; Vyn et al., 1999). Studies focusing on legume cover crop establishment following cereals have reported considerable N contribution to a subsequent corn crop (Bruulsema and Christie, 1987; Stute and Posner, 1995). However, grain corn yield responses to additional fertilizer N continue to occur in some environments and certain management systems (Dou et al., 1994; Hesterman et al., 1992).
In light of the species, environment, and management effects on the amount of cover crop N potentially available to a subsequent corn crop, simple N credits cannot reliably estimate the actual N contribution. Soil NO3–N tests may provide a relatively accurate estimate of the cover crop N contribution to a subsequent corn crop. Presidedress NO3–N tests indicated the potential for predicting when supplemental N was not required following forage alfalfa (Medicago sativa L.) (Bundy and Andraski, 1993) or legume cover crops (Meisinger et al., 1992). However, due to the lack of information on soil mineral N accumulation patterns following cover crops under the climatic and cultural practices typical for Ontario, soil NO3–N tests are currently not recommended. Therefore, a series of field studies were initiated to evaluate the effect of rye, oat, oilseed radish, and red clover cover crops established following winter wheat in autumn plow and no-till tillage systems on: (i) soil NO3 levels in autumn after wheat harvest; (ii) cover crop accumulation of biomass and aboveground N; (iii) spring soil NO3 accumulation patterns; and (iv) N availability to, and estimated supplemental fertilizer N requirements of, the subsequent corn based on yield response.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Wheat (cv. Harus) was grown using recommended production practices for Ontario (Ontario Ministry of Agriculture and Food, 1993). Wheat was seeded in early October following soybean [Glycine max (L.) Merr.] harvest using a no-till drill with a 18-cm row spacing. Fertilizer N was surface broadcast as urea on winter wheat during late March at a rate of 90 kg N ha-1. Wheat was harvested within the first week in August using commercial harvesting equipment. The straw was baled soon after harvest, leaving a standing stubble height of approximately 20 cm.
The experimental design was a randomized block split-plot with four replications where the subunit treatments were arranged in strips across each replication or block. Individual plot dimensions were 6.1 m (eight rows) wide by 12 m long at Ayr and 4.6 m (six rows) wide by 15 m long at Kirkton.
The main plots consisted of 12 cover crop–tillage combinations including four cover crops and a control, which were duplicated such that each of these cover crop treatments occurred in two tillage systems: Autumn plow and no-till. The cover crop species evaluated were red clover (cv. Walter), rye (cv. Danko), oilseed radish (common), and oat (cv. Ogle). The rye cultivar utilized in this study expresses a winter annual lifecycle and is capable of surviving winter conditions common for southern Ontario. No-till treatments involving red clover and rye were also duplicated to evaluate autumn and spring chemical kill dates. Fertilizer N rates (0 or 150 kg N ha-1) were initiated just prior to corn planting by surface broadcasting ammonium nitrate on the appropriate randomly assigned subunit treatment strip within each block.
Red clover was seeded by surface broadcasting at a rate of 15 kg ha-1 when the fertilizer N was applied to wheat in March. Cover crops were established in a disked seedbed after wheat harvest by drilling in 18-cm row widths at seeding rates of 115 kg ha-1 rye, 120 kg ha-1 oat, and 20 kg ha-1 oilseed radish.
Autumn-kill red clover and rye in the no-till system were killed with 2,4-D (2,4-dichlorophenoxyacetic acid) (1.0 kg a.i. ha-1) and glyphosate [isopropylamine salt of N-(phosphonomethyl)glycine] (1.1 kg a.i. ha-1). The oat and oilseed radish cover crops in the no-till system were not chemically killed but were allowed to grow until a killing frost, which usually occurred during the latter half of October. Plowing in the tilled treatments occurred during the first week of November using a moldboard plow (15 cm deep) at Kirkton and a chisel plow (12 cm deep) at Ayr.
Seven to 10 d before corn planting, spring-killed red clover and rye and all other no-till treatments were chemically killed with 2,4-D amine (1.0 kg a.i. ha-1) and glyphosate (1.1 kg a.i. ha-1). Secondary tillage on the plowed treatments consisted of two passes of a field cultivator plus a packer within 1 d of corn planting.
Corn (cv. Pioneer 3790) was planted in 76-cm wide rows at a seeding rate of 74000 seeds ha-1. Corn was planted at Ayr using a John Deere model 7000 conservation-till planter (Moline, IL) equipped with a single 2.5-cm wavy coulter per row positioned directly in front of the seed disc openers. At Kirkton, corn was planted using a Kinze model 2000 no-till row-crop planter (Williamsburg, IA) equipped with two 2.5-cm wavy coulters per row and unit-mounted spoked-wheel row cleaners. The planting dates were 8 May 1993 and 21 May 1994 at Ayr, and 11 May 1993 and 16 May 1995 at Kirkton. Granular starter fertilizer was applied at planting in a band 5 cm to the side and 5 cm below seeding depth at Ayr at a rate of 11 kg ha-1 of elemental N, 22 kg ha-1 of elemental P, and 0 K. Liquid starter was applied in the seed furrow at Kirkton at rates of 4 kg ha-1 of elemental N, 6 kg ha-1 of elemental P, and 3 kg ha-1 of elemental K.
Weed control consisted of a pre-emerge application of cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl)amino]-2-methylpropionitrile) (2.0 kg ai ha-1), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methyl-1-methylethyl) acetamide] (2.6 kg a.i. ha-1) as a tank mix. Whenever necessary, dicamba (3,6-dichloro-2-cethoxybenzoic acid) was applied postemergence at a rate of 0.3 kg a.i. ha-1 to control broadleaf weeds. At Ayr during 1994, pendimethalin [N-(1,ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] (at 1.0 kg a.i. ha-1) was also applied with dicamba.
End-of-season cover crop biomass was determined by collecting cover crop herbage present in two 0.5-m2 quadrats per plot after the autumn spray date. Similarly, spring regrowth biomass of red clover and rye were determined immediately before chemical kill. Dry biomass of cover crop herbage was determined after drying for at least 3 d in forced-air ovens at 80°C. Nitrogen content of cover crop herbage was determined using a modified Kjeldahl method (Thomas et al., 1967). The oven-dried cover crop herbage was ground to pass through a 1.0-mm sieve before being digested. Nitrogen content of the digested solution was determined using a Technicon Auto Analyzer (Technicon Corp., Tarrytown, NY). Cover crop biomass-N was calculated as the product of aboveground cover crop biomass and N concentration.
Corn grain yields were determined by hand harvesting two adjacent 5-m long rows (appropriately bordered) and are reported based on a moisture content of 155 g kg-1.
The concentration of soil NO3–N was determined at depths of 0 to 30 cm and 30 to 60 cm. During the cover crop establishment year, soil samples were taken in early September and early November. Spring soil samples were collected within 6 d after planting (at-plant sample) and 30 to 35 d after planting when corn was 15 to 30 cm high (presidedress sample). Soil samples were collected using a standard 25-mm diameter soil probe. A minimum of five cores were taken per plot.
Soil samples were frozen until NO3 extraction occurred. After thawing, a 5.0-g subsample of the mixed sample was added to 25 mL of 2 M KCl and shaken on a rotary shaker for 30 min. Concentration of NO3–N in the filtered extract was determined using a colorimetric technique outlined by Keeney and Nelson (1982) with a Braun and Lube TRAACS 800 autoanalyzer (AlfaLaval., AB., Stockholm, Sweden).
Parameters that were collected only on a main-plot basis (e.g., cover crop biomass and soil NO3–N) were analyzed using an analysis of variance appropriate for a randomized complete block design. Grain corn yields were analyzed using an analysis of variance appropriate for a randomized block split-plot design where the subunit treatments are arranged as strips across the blocks (replicates), perpendicular to the arrangement of the main treatments. For this particular variant of the split-plot design, the subtreatment and main x subtreatment interaction are tested with unique error terms, in contrast to a single error term that is commonly used to test these two effects when the split-plot treatments are randomly assigned to each main-plot (Cochran and Cox, 1957). Unless otherwise stated, differences among cover crop, tillage, or N rate effects were identified using a protected LSD test at the 0.05 level of probability. Each site originally consisted of four replicates; however, one replicate at Ayr during 1993–1994 was omitted from analyses due to poor drainage, which prevented timely planting.
Maximum economic nitrogen rate (MERN) was estimated from the grain yield responses to 150 kg N ha-1 based on equations developed by Kachanoski and von Bertoldi (1997). Kachanoski et al. (1996) observed that, in a 30-yr Ontario database of corn yield response equations to fertilizer N, there was a strong relationship between the linear and quadratic coefficients that served as the basis of the derivation of the following equation:
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is the slope of the relationship of the quadratic coefficients regressed onto the linear coefficients in a 30-yr database of Ontario yield response equations to fertilizer N (0.003125 ha kg N-1); and R is the fertilizer N/corn price ratio on an equal mass basis (5 kg corn kg N-1). The calculations were conducted on a plot basis with calculated values that were less than 0 being assigned a value of 0. The PSNT method was evaluated by regressing corn yields without fertilizer N, expressed as a percent of the yields obtained with 150 kg N ha-1, with soil NO3–N concentrations. The regression analyses were performed using treatment means within each location and year. |
Results and discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Cover Crop Growth, Nitrogen Content, and Nitrate Sequestration
Each of the cover crop species were successfully established at each site, resulting in relatively uniform stands. By the end of the establishment year, biomass production by red clover was greater than other cover crops, particularly at Ayr (Table 1) . Especially during the autumn of 1992 and 1993, oilseed radish and oat cover crops did not produce large quantities of biomass, which can be in part attributed to low soil NO3–N concentrations following wheat harvest. In early September, soil NO3–N concentrations to a depth of 30 cm where a cover crop was not established were 5.4 mg kg-1 at Ayr 1992, 4.4 mg kg-1 at Ayr 1993, 4.8 mg kg-1 at Kirkton 1992, and 8.3 mg kg-1 at Kirkton 1994. Low soil NO3 concentrations in 1992 and 1993 may have limited the growth potential of nonlegume cover crops.
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When actively growing, red clover was as effective as rye, oilseed radish, and oat in reducing residual soil NO3–N concentrations. By November of the establishment year, all cover crops reduced residual NO3–N concentrations in the surface 60 cm, relative to where a cover crop had not been established (Table 2) . Therefore red clover appeared to be effective at sequestering soil NO3 in autumn.
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On the spring-kill date, regrowth biomass yields in the spring-kill treatments were 1.5-fold to 2-fold greater for red clover compared with rye (Table 1). Red clover total N yields were 2.5-fold to 4-fold greater than rye due to a combination of higher biomass yields and N concentrations associated with red clover.
On the early spring sample date (i.e., at corn planting), soil NO3–N concentrations at Kirkton were similar for spring-killed red clover and rye, and 35 to 44% less than where a cover crop had not been established (Table 3) . The latter suggested that both red clover and rye are capable of sequestering soil NO3 in the spring as well.
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Spring Soil Nitrate-Nitrogen Concentrations
Rainfall occurred throughout the spring period (April–June) at each of the four sites (Fig. 1)
, indicating that soil moisture conditions were likely not limiting N mineralization potential. Except for a somewhat cooler-than-normal period from mid-May to early June during 1993 and 1994, spring temperature profiles (April–June) were similar to the long-term normals (Canada Atmos. Environ. Service, 1993).
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For Kirkton, at-plant NO3–N concentrations following red clover and oat in the moldboard plow system were 65% greater than in the no-till system. The PSNT concentrations following all four cover crops ranged from 29 to 41% higher after plowing than in no-till. Higher NO3–N concentrations in moldboard (conventional) tillage systems, relative to no-till, have been observed by other researchers as well (Dou et al., 1994; Wilson and Hargrove, 1986; Sarrantonio and Scott, 1988).
Highest spring soil NO3–N concentrations in the autumn plow tillage system occurred following red clover; these were 32% higher than no cover at Ayr and 52% higher than no cover at Kirkton (Table 3). By presidedress time, soil NO3–N concentrations following red clover were greater than the mean of all other treatments. At Kirkton, 20% greater soil NO3–N concentrations at the presidedress sample date were observed following oilseed radish compared with following rye, oat, and no cover in the plowed treatment. Rye, oat, and oilseed radish cover crops did not significantly affect soil NO3–N concentrations—relative to where a cover crop had not been established—at Ayr on either sampling date, nor in at-plant sampling date at Kirkton.
By the presidedress dates, soil NO3–N concentrations in the no-till system following autumn-killed red clover at Ayr and Kirkton were 36 to 58% greater than where a cover crop had not been established (Table 3). Soil NO3–N concentrations following oilseed radish and autumn-killed rye were similar to values observed where a cover crop was not established. Following oat at Kirkton, soil NO3–N concentrations were lower than where a cover crop had not been established in the no-till system but not after autumn plowing.
The lowest soil NO3–N concentrations for both sampling periods and locations occurred following spring-killed rye (Table 3). At plant and presidedress, soil NO3–N concentrations at Kirkton following spring-killed rye were lower than after red clover, autumn-killed rye, oilseed radish, and control treatments. This relationship was more pronounced at 35 DAP. At Kirkton, spring-killed red clover tended to have lower soil NO3–N concentrations compared with where it was autumn-killed and similar to where a cover crop had not been established. At Ayr, soil NO3–N concentrations following spring-killed red clover were similar to the autumn-killed treatment.
Corn Response
Generally, rainfall was evenly distributed and occurred in sufficient quantities throughout the whole growing season (May–September) at each site to ensure that moisture availability was not limiting corn yield potential (Fig. 1). Total rainfall (mm) from May through to the end of August at Ayr was 330 in 1993 and 275 in 1994 and at Kirkton was 275 in 1993 and 310 in 1995.
For the first 4 wk after corn planting (latter half of May and early June) during 1993 and 1994, air temperatures were generally below the long-term normals (Canada Atmos. Environ. Service, 1993) (Fig. 1). Throughout the rest of the 1993 and 1994 growing seasons (mid-June to end of September), air temperatures usually were near the long-term normals. The 1995 growing season at Kirkton often had air temperatures that were above long-term normals.
In spite of cool late spring weather during 1993 and 1994, sufficient thermal units were accumulated at each site to ensure that the corn hybrid utilized achieved physiological maturity. The actual thermal unit accumulation (Ontario Crop Heat Units), from planting until 30 September, at Ayr was 2830 in 1993 and 2910 in 1994, and at Kirkton was 2920 in 1993 and 3060 in 1995.
With the exception of a slow desiccation for spring-killed red clover at Kirkton during 1993, chemical control measures successfully controlled cover crops and weeds ensuring that observed corn responses to the various treatments were not affected by competition with actively growing cover crops or weeds.
Results of analyses across years indicated that interaction effects were significant for year x cover crop/tillage and year x N rate. Also, the cover crop/tillage x N interaction effect was significant, regardless of whether analysis was conducted combined over years or on a single-year basis. Therefore, grain corn yields presented in Tables 5 and 6 were not averaged over years or treatment levels.
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Cover crop and tillage effects on grain corn yield were more pronounced at the zero fertilizer N rate. At Kirkton, no-till yields following each cover crop were less than those obtained in the autumn plow system; the yield difference between the no-till and moldboard systems at the zero fertilizer N rate averaged about 1 Mg ha-1 (Table 5). At Ayr, tillage effects on corn yield were less consistent and often smaller (Table 6).
Highest corn yields without fertilizer N consistently occurred after red clover, regardless of tillage practice (Tables 5 and 6). Cereal cover crops (oat and rye) exhibited a trend of reduced yields relative to where a cover crop had not been established, and, in the no-till system at Kirkton, yield reductions after the cereal cover crops usually exceeded 1 Mg ha-1. Grain corn yields at zero N rate were lower following spring-killed rye compared with autumn-killed rye at Ayr 1994 (P = 0.10) and both years at Kirkton. Oilseed radish had minimal effects on corn yields on N fertilized plots.
The smallest grain corn yield increases to 150 kg ha-1 of fertilizer N typically occurred following red clover, and the largest increases were apparent after cereal cover crops (Tables 5 and 6). Larger yield responses following cereal cover crops relative to where a cover crop was not established were statistically significant (P = 0.05) only within the no-till system at Kirkton. Delaying desiccation of rye until the spring consistently resulted in a trend of greater yield responses to fertilizer N application at both sites. Within-cover-crop tillage effects on the yield response to fertilizer N were significant only at Kirkton (1993), where corn yield increases to N fertilizer addition following rye, oilseed radish, and oat cover crops were 1.1 to 1.5 Mg ha-1 higher with no-till production.
Lower yields following oat and rye cover crops compared to the control, and a greater yield response to added fertilizer N following these cover crops (Table 5), suggest that these cover crops reduce N availability to corn compared with when a cover crop was not established. Similar results have been reported when no-till corn had been planted into wheat (Dekker et al., 1994) and rye (Torbert et al., 1996) cover crops; both authors suggested that nonleguminous cover crop residues can immobilize N, resulting in reduced availability to corn. Similar effects were observed in spring plow systems by Vyn et al. (1999), who reported that annual ryegrass (Lolium multiflorum L.) cover crops were associated with lower spring soil NO3 –N concentrations and non-N fertilized corn yields when compared with controls. The results of the present study support observations of other researchers, which suggest that if the goal of including cover crops in the rotation is to enhance N availability to corn, then cereal or grass type cover crops are not appropriate for this purpose.
Estimation of Corn Fertilizer Nitrogen Requirements
Maximum economic nitrogen rate (MERN) following red clover in autumn plow systems, averaged over all 4 site-years, was estimated to be 54% (70 kg ha-1) of the estimated MERN requirement for the control (130 kg ha-1). In the no-till system where red clover was autumn-killed, MERN was estimated to be 77% (100 kg ha-1) of the control. The MERN estimate for the control was similar in both tillage systems. Kachanoski et al. (1996) suggested that MERN could be estimated based on the grain corn yield response to a known quantity of fertilizer N. These MERN estimates are based on the assumptions inherent in the prediction equations described by Kachanoski and von Bertoldi (1997) and are included in this paper to indicate the potential fertilizer N requirements following the various cover crops based on the size of the observed grain yield response to fertilizer N. The estimated MERN following the other cover crops did not differ by more than 10 kg N ha-1 from the MERN requirement for the control.
Delaying kill of red clover until spring did not enhance N availability to no-till corn. Only at Kirkton 1993 was the yield response to fertilizer N when red clover was spring killed less than when autumn killed (P = 0.10). Because zero N yields for the autumn- and spring-kill dates were similar, the amount of potentially mineralizable N was not the cause of this result. It is more likely that the slow desiccation of red clover when spring killed resulted in delayed early growth and low corn yields, even when N was not limiting. Tiffin and Hesterman (1998) reported that mid-May kill dates for red clover in Michigan did not result in greater N availability to succeeding no-till corn than when it was killed 2 wk earlier (early May). They concluded that delaying red clover kill in the spring will not enhance N availability to no-till corn in the northern Corn Belt. The present study suggests that N availability from red clover to no-till corn in the northern Corn Belt will not be reduced if chemically controlled in the autumn (late October) instead of 1 to 2 wk before corn seeding.
Evaluation of Soil Nitrate-Nitrogen Tests
Grain corn yields with zero N and the magnitude of yield response to 150 kg N ha-1 were both more strongly correlated with soil NO3–N concentrations at presidedress compared with at-plant (Table 7)
. The strength of the correlations was not substantially affected by sample depth, suggesting that estimates of potential N availability from nonfertilizer sources to corn based on soil NO3–N concentrations in the surface 30 cm were as reliable as estimates obtained using a sample depth of 60 cm.
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Relationships developed between soil NO3–N concentrations and relative yield (i.e., yields with zero N as a percentage of non-N limited yield) when combined over all site-years, suggest that the PSNT using a 30-cm sample most accurately reflected corn N requirements (Table 8) . Relative yields, rather than actual yields without fertilizer N, have been used by other researchers to evaluate the soil N test sensitivity to predicting corn N requirements when combined over site years (Blackmer et al., 1989; Fox et al., 1989; Meisinger et al., 1992). Critical soil NO3–N concentrations, above which yield responses are not expected, have been estimated using linear-plateau (Blackmer et al., 1989) or quadratic-plateau (Meisinger et al., 1992) relationships between relative yields and soil NO3–N concentrations. In our study, a plateau could not be established for any sample date and depth because there were few, if any, instances where sufficient N was available to ensure that N availability in the absence of additional N was not limiting yields. Neither was there evidence of a quadratic response between relative yields and soil NO3–N concentrations (data not shown). However, a statistically significant (P < 0.01) linear response could be identified for both sample times and depths, with more variability explained by a 30-cm, compared with a 60-cm, sample depth and by a presidedress, compared with at-plant, sample date (Table 8).
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The regression of relative yield on presidedress soil NO3–N concentrations (Fig. 2) excludes the spring-kill data for red clover and rye, and thus is somewhat less variable than when all cover crop treatments are included. However, the regression equation (RY = 28.5 + 3.1sN, R2 = 0.57, n = 40, P = 0.0001) and critical concentration at RY = 95% (22 mg NO3–N kg-1) was similar to the equation and critical value determined in Table 8 with all data included.
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Received for publication August 2, 1999.
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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