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Published online 13 May 2005
Published in Agron J 97:854-863 (2005)
DOI: 10.2134/agronj2004.0263
© 2005 American Society of Agronomy
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Production Papers

Corn Yield, Nitrogen Use, and Corn Rootworm Infestation of Rotations in the Northern Corn Belt

Joseph L. Pikul, Jr.*, Leslie Hammack and Walter E. Riedell

USDA-ARS, Northern Grain Insects Res. Lab., 2923 Medary Ave., Brookings, SD 57006

* Corresponding author (jpikul{at}ngirl.ars.usda.gov)

Received for publication October 15, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Crop rotation may improve production efficiency and reduce fertilizer N requirements for corn (Zea mays L.). Objectives were to determine effect of rotation and N on corn yield, efficiency of water use (WUE) and N use (NUE), and corn rootworm (Coleoptera: Chrysomelidae) beetle populations (CR). Rotations (started in 1990) were continuous corn (CC), corn–soybean [Glycine max (L.) Merr.] (CS), and a 4-yr rotation of corn–soybean–spring wheat (Triticum aestivum L.) companion-seeded with alfalfa (Medicago sativa L.)–alfalfa hay (CSWA). Nitrogen treatments for corn were corn fertilized for a grain yield of 8.5 Mg ha–1 (highN), 5.3 Mg ha–1 (midN), and no N fertilizer (noN). Average yield (1992–2003) was greatest (p = 0.003) under CS and highN (7.0 Mg ha–1). Yield differences (p = 0.001) among rotations increased with decreased fertilizer N. Average (1992–2003) yield with noN fertilizer was 5.8 Mg ha–1 under CSWA, 4.5 Mg ha–1 under CS, and 2.8 Mg ha–1 under CC. Nitrogen use efficiency differed (p = 0.096) only under midN with CSWA = CS > CC. Soil water (upper 1.8 m) for corn measured on 1 June (average of N treatments) was 55, 54, and 45 cm for CC, CS, and CSWA, respectively. For CSWA under highN, available water limited yield in 3 of 6 yr. At highN, CR adult populations were greater under CS compared with CC and greater at higher N fertilizer levels within CC. Rotations have potential to improve production efficiency; however, there is potential for reduced corn yield after alfalfa due to less available soil water.

Abbreviations: CC, continuous corn • CR, corn rootworm • CS, corn–soybean • CSWA, corn–soybean–wheat/alfalfa–alfalfa • highN, high-nitrogen treatment • midN, mid-nitrogen treatment • NCR, northern corn rootworm • noN, zero-nitrogen treatment • NP, nitrogen prescription • NUE, nitrogen use efficiency • SD, standard deviation • TSN, total soil nitrate nitrogen • WCR, western corn rootworm • WU, water use • WUE, water use efficiency • YG, yield goal


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SUSTAINABLE AGRICULTURE has been defined as practice that over the long term: enhances environmental quality and the resource base on which agriculture depends, provides for basic human food and fiber needs, is economically viable, and improves the quality of life for farmers and society as a whole (White et al., 1994). Profit margins for production of most crops are very narrow, and producers seek sustainable cropping systems that provide consistent return on investment (Clegg and Francis, 1994). Generally, increased diversity of crops grown in rotation enhances sustainability of agriculture systems because crops grown in rotation, with similar off-farm inputs, have greater yield than those grown in monoculture (Mannering and Griffith, 1981; Dick et al., 1986; Higgs et al., 1990). Liebig and Varvel (2003) used a multiattribute ranking procedure to quantify agroecosystem performance in the western Corn Belt. They found that CC under 180 kg N ha–1 attained a high score solely through it's capacity to be highly productive but attained low scores for environment quality-related functions. Crop rotations that include legumes also increase soil N levels (Peterson and Varvel, 1989; Raimbault and Vyn, 1991). Nitrogen has been considered as one of the best crop-input investments that a farmer can make in terms of return on dollars spent. Bundy et al. (1999) estimated that in the 12 states of the North Central United States, at least 3.6 million tonnes of N fertilizer are applied annually to corn at a cost of about $800 million. Thus, there is substantial justification to improve N management in the North Central Region of the USA.

The literature is rich with reports that show varying degrees of yield benefit for rotated corn over CC but contradictory in respect to the benefits of tillage. For example, in Elora, Ontario, Raimbault and Vyn (1991) reported that first-year corn grown in rotation yielded 3.9% more than CC under fall moldboard plow and 7.9% more than CC under fall chisel (minimum tillage). These results show a yield benefit associated with reduced tillage intensity. At Mead, NE, under conventional tillage, Peterson and Varvel (1989) found that corn grown in a 4-yr rotation and fertilized with 180 kg N ha–1 yielded 22% more than CC fertilized at the same rate. At Aurora, NY, Katsvairo and Cox (2000a)( 2000b) showed that under high chemical inputs and chisel plow, CC yielded 16% less than a 3-yr rotation (that included a legume). Under moldboard plow, CC yielded 22% less than the 3-yr rotation. In the rotations at Aurora, NY, corn yields were greater under moldboard plow compared with chisel plow, and this is in contrast to the findings of Raimbault and Vyn (1991). On a 20-yr experiment with CC in Belleville, IL, Kapusta et al. (1996) found that corn yield was equal under moldboard plow, chisel plow, and no-tillage.

Crop rotation has been a good defense against CR damage because eggs laid in corn will typically hatch the next spring as larvae into a crop other than corn. Rotation is not always practiced, however, and some northern corn rootworm (NCR) and western corn rootworm (WCR) populations have adapted to survive 2-yr rotations by, respectively, extending the egg stage for a second winter (Krysan, 1986) and by laying eggs in crops grown in rotation with corn (Levine et al., 2002). These adaptations may further increase pesticide applications for CR, which already account for nearly 20% of the insecticide applied to U.S. field crops (Delvo, 1993). The widespread use of soil insecticides in a preventative application has undoubtedly developed from difficulties associated with monitoring CR populations, especially the larval populations, which are primarily responsible for crop damage (Bergman et al., 1981; Weiss and Mayo, 1983).

Regional long-term crop rotation experiments provide a way to identify performance of crop sequences within a unique soil and climate framework. Objectives of our research were to determine effect of rotation and fertilizer N on: (i) corn yield, (ii) WUE, (iii) NUE, and (iv) adult CR populations for northern Corn Belt conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
Our study was located on the Eastern South Dakota Soil and Water Research Farm near Brookings, SD (44°19' N, 96°46' W; 500 m elevation) on a Barnes sandy clay loam (275 g/kg clay, 466 g/kg sand, and 18.9 g/kg total C in the plow layer) soil (fine-loamy, mixed, superactive, frigid Calcic Hapludoll) with nearly level topography. Brookings is located in a transition zone between cool (frigid temperature regime) and warm (mesic temperature regime) prairies. Annual precipitation is 580 mm. Soils of this area are Udic Borolls to the north, Udic Ustolls to the south, and Typic Ustolls to the west. Before the start of the study (1990), the land was cropped to oat (Avena sativa L.) and barley (Hordeum vulgare L.) (1972–1978), corn rotated with soybean (1979–1987), soybean (1988), and spring wheat (1989).

Experimental Design and Management
Field plot design was a randomized complete block with three replications (blocks) where main plot was rotation treatment and split plot was N treatment. All crop phases of each rotation were present every year. Crop rotations were CC, a 2-yr rotation of CS, and a 4-yr rotation of CSWA. In the 4-yr rotation (CSWA), spring wheat was used as a grain crop and as a companion crop to establish alfalfa in Year 3, and alfalfa was cut for hay in Year 4. Main plots were 90 m long by 30 m wide, and subplots were 30 m long by 30 m wide.

Nitrogen treatments were corn fertilized for a yield goal (YG) of 8.5 Mg grain ha–1 (highN), corn fertilized for a YG of 5.3 Mg grain ha–1 (midN), and corn not fertilized (noN). Total soil nitrate (TSN) test was used to estimate fertilizer N prescription (NP) for corn (Gerwing and Gelderman, 1996).

At the start of the experiment, N fertilizer splits were called input levels. Inputs included fertilizer, tillage, herbicide, and insecticide. Management was changed after 1995 to reduce the number of experimental variables to just crop rotation and fertilizer N. Table 1 provides a brief comparison of the differences in management before and after 1996 using 1994 and 1999 as "typical" of the period 1992–2003. Briefly, input level treatments for the corn phase of the rotations included high input (where fertilizer N was applied for 8. 2 Mg ha–1 YG, preventative herbicide and insecticide applications were made, and fall moldboard plow/spring disk and cultivation operations were used), intermediate input (where fertilizer N was applied for 5.3 Mg ha–1 YG, pesticide applications based on pest survey and IPM principles were used, and fall chisel plow/spring disk and cultivation operations were used), and low input (where no fertilizer, herbicide, or insecticide applications were applied and tillage was fall chisel plow/spring disk and cultivation operations). Riedell et al. (1998) provide a detailed discussion of management before 1995.


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  		Table 1. Management for the corn year before and after 1996, using 1994 and 1999 as "typical years" for the period 1992 to 2003. Rotations were continuous corn (CC), corn–soybean (CS), and corn–soybean–wheat/alfalfa–alfalfa (CSWA).

 
Before 1996, primary tillage was with a moldboard or chisel plow in the fall of the year (conventional tillage). Primary tillage since 1996 was with a chisel plow. Tillage has been about 20 cm deep. In 1995 and 1996, wet weather conditions precluded fall tillage. Seedbeds for corn were prepared each spring using a tandem disk and field cultivator. In a given year, all soil preparation, fertilizer and pesticide applications, seeding, cultivation, and harvesting operations were conducted on about the same date (Tables 1 and 2). Corn was planted using an eight-row planter with 76-cm row spacing. Seeding rate before 1996 was at 65000 seeds ha–1. Average seeding rate (1996 to 2003) was at 87000 seeds ha–1. Stand counts (1996–2003) on each plot were made at about the V3 (Ritchie et al., 1993) stage of development (before the first cultivation) by counting the number of plants in 1 m of row in three different passes of the planter. Plots were cultivated twice during the early growing season for weed control (date of second cultivation shown in Table 2).


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  		Table 2. Corn planting date, variety (1992–2001, Pioneer; 2002–2003, Golden Harvest), date of second cultivation, harvest date, date of soil sample for nitrate N, growing season precipitation, and growing degree days (base 10°C) for 1992 through 2003. Weather data courtesy of Alan Bender, South Dakota State Climatologist, Brookings, SD.

 
Soil Measurements
Samples for soil nitrate N were collected in the fall or spring, depending on weather conditions (Table 2) for establishing yearly N fertilizer prescription for the corn crop. Samples for 1992–1996 crops were taken from 0- to 15- and 15- to 60-cm depths. After 1996, three 120-cm-deep soil samples were taken randomly on each subplot and divided into increments of 0 to 15, 15 to 30, 30 to 60, 60 to 90, and 90 to 120 cm. Core diameter was 3.2 cm. Samples were dried at 40°C immediately after sampling, ground through a 2-mm sieve, and subsampled. Measurements of nitrate N in samples collected in 1991–1995 were made using a nitrate electrode procedure (Gelderman et al., 1995). After 1995, nitrate N was measured using 2 M KCl extraction and copperized Cd reduction column procedure (Zellweger Analytics, 1992). Soil samples for TSN were not taken for the 2001–2003 crop years, and for these years, average TSN (1992–2000) for each rotation and N level was used to estimate NP.

A special soil sampling to measure soil NO3–N to a depth of 3 m was conducted in 1998. Three 3-m-deep soil samples were taken randomly on each subplot and divided into 30-cm increments.

Fertilizer Management
On each subplot under highN and midN, NP was calculated as NP = 0.022YG – TSN. Adjustment to NP for previous crop or sampling date was not made. Nitrogen prescription for each N treatment, expressed as an average of three replications, was met by applying starter fertilizer with the seed and sidedressing with appropriate amounts of urea as 46–0–0 (elemental N–P–K). Urea N was sidedressed using a Barber spreader (Barber Eng., Spokane, WA) with sidedressing attachment. Application of urea N was directly before the second corn cultivation (Table 2, date of second cultivation). Pikul et al. (2001) provide additional information of the experimental design, and Table 1 provides detail on management history. Starter fertilizer for corn was applied at seeding and placed 5 cm to the side and 5 cm deeper than seed. Starting with the 1996 crop year, 112 kg ha–1 of starter fertilizer as 14–16–11, 7–16–11, and 0–16–11 (elemental N–P–K) was applied on highN, midN, and noN subplots, respectively.

Soil P levels were elevated on all subplots before spring field work in 1996 with broadcast application of triple superphosphate as 0–20–0 (elemental N–P–K) equivalent to 89 kg ha–1 of elemental P. Soil samples were taken from the top 15 cm of each subplot in the experiment. Samples were oven-dried, ground to pass a 2-mm sieve, and the Olsen procedure (Gerwing and Gelderman, 1996) was used to measure extractable P. Application rate to all subplots was the same and was based on the amount of P required to bring subplots with the lowest soil test P to at least a high to very high soil test (Gerwing and Gelderman, 1996). Tillage for seedbed preparation was conducted after P application.

Available N for the corn crop was defined as mineral sources of N available through soil nitrate N and additions by N fertilization. Available N does not include N that may be potentially released through mineralization of organic N during the growing season. Nitrogen use efficiency was calculated as the ratio of corn grain yield to available N.

Crop Measurements
Grain yields were measured with a Massey Ferguson MF 8-XP research plot combine (Kincaid Equipment Manufacturing,1 Haven, KS) equipped with an electronic weigh bucket. On each subplot, eight rows, 30 m long (one-fifth of the subplot area), were harvested for grain yield. Before 1996, four rows, 30 m long, were harvested, and a weigh wagon was used to measure the quantity of grain. Subsamples of combine-harvested grain were retained for grain moisture and test weight. Grain moisture and test weight were measured with a Dickey-John GAC 2000 Grain Analysis Computer (Johnston, IA). Corn grain yields were adjusted to 155 g kg–1 moisture.

Yield data cover a period of time starting in 1992 although the first year when all crops were grown was in 1990. We operationally selected 1992 as the beginning year to report corn yield because it was in this year (on all rotations) that corn followed at least one full year of some previous crop. The CSWA is a 4-yr rotation that includes alfalfa. In our rotation sequence, corn follows a complete alfalfa cycle in 1992 on only one set of plots, and those plots were seeded to wheat–alfalfa in 1990 and cut for alfalfa hay in 1991. This set of plots (CSWA, corn in 1992) and associated corn yield for 1992, 1996, 2000, and 2004 provide a "measure" of a true rotation effect on one piece of ground.

Yield advantage (YA) of corn grown in rotation under CS or CSWA was calculated as a percentage of corn yield grown under CC. Yield advantage was determined as: YA = [(yield under rotation – yield under CC)/yield under CC] x 100.

Soil Water Content
Soil water content was measured from 1996–2001 using neutron attenuation equipment to determine water storage and use. Neutron equipment was calibrated in a manner described by Pikul and Aase (1998). On each subplot, a permanent access tube was installed, enabling volumetric soil water measurements to a depth of 1.8 m at 0.3-m increments. Soil water content was expressed as an average of three replications for each rotation and N management treatment. Measurements were made at seeding and at crop maturity. Water use (WU) was defined as beginning soil water content minus ending soil water content plus precipitation during the growing season. Operationally, this period was defined as 1 June through 30 September. For water balance calculations, runoff was assumed to be negligible because the experiment is located on nearly level topography. However, at least once per year, we might expect runoff from a high intensity summer storm. Water drainage beyond 1.8-m depth was assumed to be negligible among treatments during the growing season. Other researchers have made similar assumptions when estimating soil water depletion by corn and soybean in the northern Corn Belt (Copeland et al., 1993). Water use efficiency was calculated as the ratio of corn grain yield to WU.

Adult Corn Rootworm
Adult beetles were trapped as they emerged from the soil in four cages per subplot. Each cage, 0.76 m long by 0.6 m wide, covered the roots of three corn plants and extended 38 cm to each side of a corn row. Beetles were removed from cages and counted by species (NCR or WCR) every 3 to 4 d through peak emergence and every 6 to 8 d later in the season. Typically, sampling began before emergence started in early July and continued into late September. All fertilizer treatments of the CC rotation were sampled during 1998–2001. Additionally, CC and CS under highN were sampled during 1999–2003.

Data Analysis
Statistical comparisons of all measurements were made using one-way and two-way analysis of variance for each year (MINITAB, Release 12, State College, PA). All treatment factors (N and rotation) in the experiment were considered fixed effects. Years and blocks were treated as random effects in the combined analysis across year. Treatment means (one-way ANOVA) were separated using Fisher's LSD for all pair wise differences between level means. Effects were considered significant for p ≤ 0.10. Effect of rotation and N (two-way ANOVA) and interaction of block x rotation and rotation x fertilizer were evaluated using a general linear model.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Crop Yield, Nitrogen Use, and Water Use
These field trials cover one of the wettest and coolest periods in South Dakota history. Precipitation totals from 1991–1995 were the greatest in more than 100 yr of South Dakota climate records, and the 1992 and 1993 summers were the coolest (those years showing the lowest cumulative growing degree days in Table 2) consecutive summer seasons in the South Dakota climate record beginning in 1890 (Alan Bender, South Dakota State Climatologist, Brookings, SD). Corn yields were generally least in 1992 and 1993 compared with other years in the study (Table 3). Thus, the first years of the experiment (1990–1995) presented difficult conditions to establish reliable measures of N use and WU. Further, we found it impossible to control weeds on the "low input" plots (noN in Table 1) using only row cultivation. Corn yield under noN (low input) through 1994 reflects on our difficulties producing a crop with low input. Since 1996, weather has moderated allowing a careful examination of N, water, and insect pests within the corn phase of these rotations.


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  		Table 3. Corn yield (155 g kg–1 grain moisture) for continuous corn (CC), corn–soybean (CS), and corn–soybean–wheat/alfalfa–alfalfa (CSWA) rotations (R). Corn N fertilizer treatments were corn fertilized for a yield of 8.5 Mg ha–1 (highH), corn fertilized for a yield of 5.3 Mg ha–1 (midN), and corn not fertilized (noN).

 
In the first year of the experiment (1990), there was no significant difference in corn yield among rotations, but there was a significant response to N fertilizer. The highN treatment resulted in 2.2 Mg ha–1 more grain than did the noN treatment. Results were important because they showed that our test site was responsive to N fertilizer (data not shown).

For the 1992–2003 time interval, there was a significant corn yield response (in most years) to rotation within each N fertilizer treatment (Table 3, averages, one-way ANOVA within fertilizer treatment). Under highN, average (1992–2003) corn yield under CS was 16% greater than yield under CC and 8% greater than yield under CSWA (p = 0.003). By comparison, for a long-term crop rotation experiment at Lamberton, MN (located about 120 km east of Brookings, SD), under a fertility and tillage program (Crookston et al., 1991) similar to our study, Porter et al. (1997) reported average corn yield (1985–1995) under CS to be 13% greater than corn yield under CC. Under midN, there was no difference in average corn yield between CS and CSWA. Respective yields for CS and CSWA rotations under midN were 6.4 and 6.1 Mg ha–1, and these yields were 36% greater (p = 0.001) than CC (Table 3). Under noN, average corn yield was 5.8 Mg ha–1 under CSWA, and this yield was 28 and 107% greater (p = 0.001) than yield under CS and CC rotations, respectively. Average (all years and N levels) corn yield was significantly (p = 0.002, Table 3, means not shown) less under CC compared with CS and CSWA; respective yields for CC, CS, and CSWA were 4.5, 6.0, and 6.1 Mg ha–1.

It is difficult to determine when (or if) soil on which long-term cropping experiments are conducted reaches an "equilibrium" condition. That equilibrium should reflect previous crop sequence and tillage. Corn yield for 1992, 1996, 2000, and 2004 (all yield data for 2004 not shown) provides a "measure" of a true rotation effect of CSWA on one piece of ground. Yields under CSWA for midN, expressed as a percentage of corn yield under CC for midN, were 59, 35, 48, and 9% greater, respectively, for 1992, 1996, 2000, and 2004. Corn yields under CS midN, expressed as percentage of corn yield under CC for midN, were 35, 16, 74, and 45% greater, respectively, for 1992, 1996, 2000, and 2004. Average yields for both CSWA and CS do not suggest a trend in yield (upward or downward) as a function of time under rotation.

In most years, there was a positive yield advantage (calculated values not shown) for corn grown in rotation. Further, YA increased as fertilizer N decreased. Under highN, the average (all years) YA of CSWA over CC was 8.1% [standard deviation (SD) = 20.4%] and 16.8% (SD = 11.6%) for CS over CC. Under noN, the average (all years) YA of CSWA over CC was 136.5% (SD = 41.8%) and 53.9% (SD = 22.2) for CS over CC.

Yield advantage of rotation under midN for 12 yr is shown in Fig. 1 . There was a linear negative relationship between YA and yield of corn under CC. As the yield of corn increased under CC (presumably a consequence of favorable growing conditions), the yield advantage of both CSWA and CS decreased. The decrease in YA as a function of yield under CC was different for CSWA (slope = –24.5, Fig. 1) and CS (slope = –12.8, Fig. 1). Average (all years) YA under midN of CSWA was 43.3% (SD = 49.9%) and 44.2% (SD = 23.6%) under CS. Under highN, there was not a significant relation between YA and yield under CC for CSWA or CS. However, under noN, there was a significant relation (p = 0.005) between YA for CSWA and yield under CC. We do not attribute yield differences among rotation or N treatment to plant population. For the years 1996–2003, there were no significant differences in plant populations for either rotation or fertilizer treatment. Average plant population was 75900 plants ha–1. Further, the authors have not observed serious lodging of corn (with the exception of CC in 2000 and CS in 2001) related to CR damage.



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  		Fig. 1. Relationship between the yield advantage (YA) of corn in a corn–soybean (CS) rotation and corn in a rotation of corn–soybean–wheat/alfalfa–alfalfa (CSWA) to continuous corn (CC). Comparison is for corn fertilized for a yield of 5.3 Mg ha–1 (midN).

 
For the 1996–2001 time interval, yields varied significantly with rotation under midN and noN (Table 4). The nearly identical yields on the CSWA rotation of 7.1, 7.3, and 7.3 Mg ha–1 for highN, midN, and noN, respectively, likely caused much of the significant rotation x N interaction (Table 4). Under the CS rotation, average corn yield for highN and midN (7.6 Mg ha–1) was significantly (p = 0.001, statistical data not shown) greater than that for noN (6.1 Mg ha–1). Corn yield response to N fertilizer was significantly (p = 0.001, statistical data not shown) different for each N level under CC with yields of 6.5, 5.4, and 3.8 Mg ha–1 for respective fertilizer treatments of highN, midN, and noN. There were no significant differences in average (years) corn grain moisture at harvest for rotations under highN and midN (Table 5). However, under noN, CSWA had the lowest grain moisture (with the exception of 2002), and CC had the highest grain moisture.


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  		Table 4. Average (1996–2001) corn grain yield, applied N, autumn soil nitrate N, N use efficiency (NUE), water use efficiency (WUE), and total water used for June through September (precipitation and soil water from the top 1.8 m). Rotations were continuous corn (CC), corn–soybean (CS), and corn–soybean–wheat/alfalfa–alfalfa (CSWA). Corn was fertilized for a grain yield of 8.5 Mg ha–1 (highN), 5.3 Mg ha–1 (midN), and no N fertilizer (noN).

 

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  		Table 5. Corn grain moisture at harvest for continuous corn (CC), corn–soybean (CS), and corn–soybean–wheat/alfalfa–alfalfa (CSWA) rotations (R). Corn N fertilizer treatments were corn fertilized for a yield of 8.5 Mg ha–1 (highH), corn fertilized for a yield of 5.3 Mg ha–1 (midN), and corn not fertilized (noN).

 
There were no differences in N fertilizer applied to rotations within fertilizer treatments for the 1996–2001 time interval (Table 4). In addition, there were no significant differences in autumn soil nitrate N (Table 4). However, the greatest difference in autumn soil nitrate N among rotations was at the noN fertilizer level (Table 4) where soil nitrate N under CSWA was 47 and 26 kg ha–1 under CC.

There were significant differences in NUE among rotations only within the midN treatment (Table 4). As applied N decreased, NUE increased. A large NUE value suggests that significant amounts of N were transformed (mineralized) from organic to inorganic forms during the growing season. Within each N treatment and rotation, there was considerable variability in NUE from year to year. For example, under CSWA rotation with noN, greatest NUE was about 450 kg corn ha–1 per kg N ha–1 in 1997 and least was 92 kg corn ha–1 per kg N ha–1 in 1999. The lowest NUE was under CC regardless of N level. On average (years), corn yield under CSWA with highN increased only 11% over CSWA with noN. Corn yield under CS at highN increased 55% over CS with noN, and corn yield under CC with highN increased 116% over CC with noN.

Corn yield per unit of available N came close to that expected for South Dakota. Fertilizer N recommendation in South Dakota (Gerwing and Gelderman, 1996) is 0.021 kg N kg–1 corn (1.2 lbs N bushel–1 corn). Within the highN treatments, there were no differences in the amount of N fertilizer supplied in a given year among rotations (Table 4). Average (all rotations) available N (fertilizer N and soil nitrate) for the highN treatment was 183 kg N ha–1, and average corn yield was 7103 kg ha–1. Our ratio of available N to corn yield was 0.026 kg N kg–1 corn for 1996– 2001.

Soil N mineralization during the corn growing season could account for, or contribute to, the yield advantage of rotated corn over CC. In soil incubation studies designed to mimic field soil temperature under a growing corn canopy, we found that soil from CSWA had potential to mineralize about 56 kg ha–1 more N than did soil under CC (Carpenter-Boggs et al., 2000). Similarly, soil from CS had potential to mineralize 47 kg ha–1 more N than soil under CC. By comparison, Varvel and Wilhelm (2003) estimated that corn obtained approximately 65 kg N ha–1 from soybean in a 2-yr rotation with soybean in central and eastern Nebraska. Our field measurements of NUE suggest a ratio of 0.026 kg N kg–1 corn. Therefore, an additional 52 kg N ha–1 (average of CS and CSWA) has potential to produce about 2000 kg corn ha–1. Under midN (Table 3), the average (years) difference between rotated corn (CS and CSWA) and CC was 1600 kg corn ha–1. Under noN, the difference between rotated corn (CS and CSWA) and CC was 2400 kg corn ha–1. The difference in yield between rotated corn and CC for both midN and noN treatments is remarkably close to the estimated yield increase from potential mineralizable N.

Efficient use of N can minimize potential for groundwater contamination by leached nitrate. We found that increased N fertilization significantly increased NO3–N to a depth of 3 m following 8 yr of rotations and fertilizer treatments. (Samples were collected in 1998 and complete data set is not shown.) Under highN treatment, total NO3–N in the top 3 m was 200 kg ha–1 under CC, 143 kg ha–1 under CS, and 134 kg ha–1 under CSWA. Under noN treatment, total NO3–N in the top 3 m was 73 kg ha–1 under CC, 102 kg ha–1 under CS, and 130 kg ha–1 under CSWA. Measurement of soil NO3–N in the top 3 m of soil provided a point-in-time evaluation of the quantity of NO3–N remaining in the soil profile. We are uncertain if N has leached past the 3-m depth.

Water and available N are the most important factors that govern yield, and one or the other can limit growth. It is commonly known, especially in irrigated agriculture, that seasonal evapotranspiration of alfalfa hay may be at least twice that of corn. Corn yield under the CSWA rotation fertilized at highN was significantly reduced in 1998 compared with CC and CS (Table 3). Alfalfa grown in 1997 extracted more soil water than did corn on CC or soybean on CS, and the consequence of this can be seen in Table 6 for soil water at the start of the corn year. The CSWA rotation used only 11 cm of soil water in 1998 (data not shown) because there was less soil water in the profile on 1 June 1998 (Table 6). Average soil water (Table 6, all N treatments) on subplots following alfalfa (CSWA) was 37.6 cm (volumetric content of 21%) on 1 June. In contrast, average soil water for CC and CS rotations (average of all N treatments) was 52.4 cm (volumetric content of 29%). Soil WU, for the 1998 crop year, on CC was 14.5 cm, and WU on CS was 15.7 cm (data not shown). There were no significant differences among rotations in average (1996–2001) WUE under highN (Table 4). However, WUE of CSWA was greater than WUE under CC for midN and noN. On average (all N treatments), WUE of rotations (CS and CSWA) was 33% greater than CC. By comparison, Varvel (1994) reported precipitation use efficiency of corn (based on annual precipitation) in rotations as 22% greater than that of CC.


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  		Table 6. Soil water in the upper 1.8 m of soil at the start of the corn growing season for continuous corn (CC), corn–soybean (CS), and corn–soybean–wheat/alfalfa–alfalfa (CSWA) rotations (R). Corn N fertilizer treatments were corn fertilized for a yield of 8.5 Mg ha–1 (highH), corn fertilized for a yield of 5.3 Mg ha–1 (midN), and corn not fertilized (noN).

 
Corn Rootworm
There were no upward or downward trends of corn yield with time (1992–2003, data not shown) on the CC or CS rotations relative to the CSWA rotation. Average (all years) corn yield for CC (expressed as a percentage of corn yield from CSWA) was –4, –22, and –54% of yield from the CSWA rotation for highN, midN, and noN, respectively. Average corn yield for CS was 12, 7, and –24% of the yield from CSWA for highN, midN, and noN, respectively. The CSWA rotation is least conducive for CR survival, and consequently, this rotation serves as a base to evaluate trends in corn yield where yield loss associated with CR is suspected.

Continuous corn under noN yielded fewer adult beetles (Table 7) and usually fewer larvae (data not shown) than did midN or highN treatments. This finding was not unexpected because corn receiving N may show larger root systems and greater capacity for root regrowth (Spike and Tollefson, 1991a, 1991b; Riedell et al., 1996). There were no clear correlations between insect numbers and crop yields.


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  		Table 7. Cumulative number of adult corn rootworms per plant emerging from soil during the corn growing season for continuous corn (CC) and corn–soybean (CS) rotations under highN (fertilized for a yield of 8.5 Mg ha–1). Cumulative number of adult corn rootworms emerging from soil during the corn growing season for continuous corn (CC) rotation under highN (fertilized for a yield of 8.5 Mg ha–1), midN (corn fertilized for a yield of 5.3 Mg ha–1), and noN (corn not fertilized).

 
Intuitively, a large number of emerging adults should be related to a large number of larvae and, consequently, greater root damage and reduction in corn yield. However, corn tolerance to larval feeding damage is related to the size of the root system at the time of feeding (Riedell et al., 1996) and the plants ability to grow new roots after feeding damage (Riedell, 1994). A favorable growing season (timely precipitation and adequate heat units) can mask the effect of root damage caused by larval feeding. For example, in 1998, we found relatively large numbers of emerging adult beetles in CC under all N levels (Table 7). But, corn yield on CC rotations in 1998 under all N treatments exceeded the average yield during 1992–2003 (Table 3). Continuous corn rotation under highN was severely lodged in 2000 but not in 1999, 2001, or 2002. In 2000 and 2002, we found that the ratio of WCR/NCR was about 2:1 (species data not shown). However, this ratio was not consistent among years, and in 1998, 1999, 2001, and 2003, the numbers of NCR were greater than those of WCR (data not shown).

Lodging occurred in the CS rotation only in 2001 when severe lodging accompanied a NCR infestation of 35 emerging adults per corn plant. In that year, the number of adults (NCR and WCR) emerging under CS was 35 (Table 7) and significantly more (p = 0.019) than the 16 adults per plant from CC. Because numbers of NCR emerging from CC under highN remained fairly constant among years (not shown), the large jump in NCR emergence in CS 2001 could have been due to increased incidence of the extended diapause trait rather than to especially favorable weather or soil conditions. A second spike in adult emergence numbers under CS rotation appeared in 2003 (Table 7).

Emergence of CR adults from CC and CS under highN was nearly equal in 1999 and 2002 but differed by about twofold in 2000, 2001, and 2003 (Table 7). There were not consistent differences in the numbers of emerging adults between CC and CS. For example, CC produced more adults than CS in 2000 (21 compared with 11), whereas CS yielded more than CC in 2001 (35 compared with 16 per plant). Northern corn rootworm accounted for nearly all (98%) adult emergence from the CS rotation in 1999, 2001, 2002, and 2003 and 87% in 2000 (separation by species data not shown).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
On average (1992–2003), corn yield under the CS rotation exceeded both CC and CSWA rotations when corn was fertilized for a grain yield of 8.5 Mg ha–1. Under noN, corn yield on CSWA exceeded both the CC and CS rotations. At midN, there were no differences in yield between CS and CSWA, and both rotations held about a 40% corn yield advantage over CC. Greater NUE on CSWA and CS rotations compared with CC suggests that the timing and availability of N, presumably from microbial decomposition of organics during the growing season, on CSWA and CS rotations were superior to those of annually grown corn. Measurements of soil N mineralization (independent of the current work) found that the soil under CS and CSWA had potential to mineralize about 52 kg ha–1 more N than soil under CC, and this N could account for, or contribute to, the yield advantage of rotated corn over CC under midN and noN.

Soil WUE increased with increased rotation length in years with ample growing season precipitation. On average (6 yr), WUE of rotated corn (CS and CSWA) was 33% greater than CC. However, in dry years, excessive soil WU by alfalfa (before the corn year) in the CSWA rotation may be a liability to subsequent corn yield. Available water rather than available N seems to be the factor limiting corn yield under the CSWA rotation in some years.

We found no statistical difference in NUE among rotations fertilized at highN, but both CS and CSWA had greater NUEs than did CC. We speculate that the greater concentration of NO3–N in soil (depth of 3 m) under CC after 8 yr (1990–1998) is a consequence of inefficient use of applied fertilizer, and a simplistic conclusion might be that the overall risk associated with inefficient N use might be minimized by using longer rotations (that include legumes) and reduced fertilizer N.

Although CR populations fluctuated from year to year for reasons that we do not understand, results of our measurements to evaluate the influence of crop management practices on CR numbers suggested effects of both crop rotation and N fertilization. Under highN in both 2001 and 2003, emergence of CR adults from CS was nearly twofold greater than that from CC. Northern CR accounted for nearly all adults emerging from the CS rotation. Additionally, CR populations tended to be greater at higher N fertilization levels within the CC plots. Thus, management practices fostering higher yields also favored higher CR populations at least in those rotations conducive to CR survival (CC and CS).


   ACKNOWLEDGMENTS
 
The authors thank Dr. Michael Ellsbury, USDA-ARS, who supervised the maintenance of the long-term crop rotation study during 1990–1995. We thank Janet Fergen, Biological Science Technician, for her work in sample collection and laboratory work associated with corn rootworm. David Harris, Agricultural Science Research Technician Soils, and Max Pravecek, Biological Science Technician, are recognized for their careful maintenance of the experimental plots. David Harris is also recognized for work in sample collection and technical laboratory analysis.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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 USDA over other products not mentioned. Back


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




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