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© 2003 Plant Management Network.
Accepted for publication 21 October 2003. Published 26 November 2003.

Nitrogen Fertilizer Requirement for Inbred Corn Following Corn or Soybean

W. Bart Stevens, Department of Renewable Resources, Powell Research and Extension Center, University of Wyoming, 747 Road 9, Powell 82435; Robert G. Hoeft, Department of Crop Sciences, 1102 S. Goodwin Ave., University of Illinois, Urbana 61801; and W. Richard Peterson, Syngenta Seeds, Inc., 510 N 12th Avenue, Washington, IA 52353

Corresponding author: W. Bart Stevens. wstevens@uwyo.edu

Stevens, W. B, Hoeft, R. G., and Peterson, W. R. 2003. Nitrogen fertilizer requirement for inbred corn following corn or soybean. Online. Crop Management doi:10.1094/CM-2003-1126-01-RS.

Abstract

There is a need for information regarding the N benefit provided to inbred corn (Zea mays L.) grown in rotation with soybean (Glycine max L. Merr.). Our objective was to evaluate response of irrigated inbred corn to N fertilization in corn-following corn (C-C) and corn-following-soybean (C-S) cropping sequences. Fertilizer N was applied at rates from 0 to 144 lb/acre to three inbred corn lines grown following hybrid corn or soybean. Corn yields averaged 29% higher following soybean than following corn. In the C-C cropping sequence, seed yield increased with increasing N application rate in seven of nine cases, with the yield-based optimum N rate (N_Y) varying from 67 to 119 lb of N per acre. In the C-S cropping sequence, yield response to fertilizer N was observed in only four of nine cases, with one inbred line failing to respond in any of the three study years. Where responses occurred in the C-S sequence, N_Y ranged from 53 to 96 lb/acre. Growing seed corn in rotation with soybean decreased N_Y by an average of 42 lb/acre compared to the C-C sequence. Results provide no evidence that the soybean N credit used for inbred corn should be any different than that used for hybrid corn.

Introduction

Hybrid corn seed production provides farmers in the U.S. Corn Belt with a high-return alternative crop that requires little investment in specialized equipment. The value of seed corn has been reported to be 6 to 10 times higher than that of commercial hybrid corn (14), thus yield reductions caused by nutrient deficiencies reduce economic returns substantially. Contractual penalties for below-average yield combined with low-cost N fertilizers, have led to cases of excessive N application (15; Jim Morrison, Extension Educator, 1995, personal communication). Increasing fertilizer costs and growing awareness of environmental consequences resulting from excessive N use have caused growers, researchers, and industry personnel to call for more information about N requirements of inbred corn under various production conditions (8,9,14,15).

As with commercial hybrid corn production, yield response to fertilizer N by inbred corn lines varies considerably. In a study where inbred corn followed a non-leguminous crop, two of ten lines did not respond at all to N fertilizer and the optimum N application rate for the other eight varied from 54 to 108 lb/acre (1). Peterson and Corak (8) reported variable response to N fertilization in field-scale seed corn production, with yield improvement observed in only 21% of 71 site years. When responses were observed, seed yield was maximized with 100 lbs of N per acre or less, except for three site years where yield was improved by N applications up to 180 lbs/acre. Rasse et al. (9) observed that N applications of 90 and 180 lb/acre increased NO₃-N leaching by 73 and 300%, respectively compared to an unfertilized check. The authors also concluded that variability of N response among the inbred lines indicated a need to tailor N fertilizer recommendations for individual inbred lines.

Nitrogen contributed by the previous crop may also need to be considered when making N recommendations for inbred corn. When hybrid corn follows a legume, the N requirements are usually less than when following a non-legume. Nitrogen fertilizer recommendations for hybrid corn rotated with soybean are usually reduced by up to 45 lb/acre compared to continuous corn (5,12). However, inbred corn may respond differently than hybrid corn because inbred lines have less extensive root development, lower plant vigor, and are commonly grown on soils where the risk of N leaching is high.

Few studies have compared the yield response of inbred seed corn to N fertilization in a corn-following corn (C-C) sequence with a corn-following-soybean (C-S) sequence. In two such studies, Wilhelm et al. (13) suggested that N application rates should be reduced following soybean while Russell (10) concluded that the increased yield potential following soybean compensated for any increase in N availability; however, in neither case were the different cropping sequences evaluated in the same year, confounding the results. Peterson and Corak (8) compared the effects of C-C and C-S cropping sequences on response of inbred corn to N fertilization and concluded that optimum N application rate was not affected by cropping sequence. Optimum N application rate fell primarily between 0 and 100 lb/acre, but N fertilizer was applied in increments of 60 lb/acre, resulting in response curves defined by only two or three data points. Moreover, the research of Peterson and Corak was conducted on predominantly medium textured soils, but much of the corn seed production in the U.S. Midwest has been moved to irrigated fields, which usually have well-drained soils that are susceptible to NO₃-N leaching losses. Our objective was to evaluate response of irrigated inbred corn to N fertilization in C-C and C-S cropping sequences.

Field Experiments

An irrigated site in an area with extensive seed corn acreage was selected for the study. Soil type within the study area located near Prophetstown, IL is a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed, mesic Typic Hapludolls) with 1 to 2% slope. Soil chemical properties of the 6-inch tillage layer were: pH, 6.0; organic C, 1.8%; organic N, 0.17%; available P (Bray P₁), 73 ppm; and ammonium-acetate-extractable K, 280 ppm. Soil texture varied from silt loam at the surface to loam at a depth of 24 inches, and abruptly changed to coarse sand between the 24- and 36-inch depths.

The experiment consisted of two separate but adjacent areas representing C-C and C-S cropping sequences. Within each area, inbred lines and N treatments were randomly assigned to experimental units according to a split-plot arrangement of a randomized complete block design with inbred as the main plot and N rate as the subplot. Response of three inbred lines (HB4, AG6, and J90) to six N application rates (0, 36, 54, 72, 90, and 144 lb/acre) was evaluated within each cropping sequence and each treatment combination was replicated four times. Inbred lines were randomly selected from those widely used in the industry at the time the research was conducted. Corn-following-corn and C-S cropping areas were moved each year to new, randomly selected but adjacent locations within a 40-acre parcel of land. Data were analyzed using the GLM and NLIN procedures of the SAS statistical software (11) treating the two cropping sequence study areas as fixed "locations" based on the procedure of McIntosh (6) for analysis of combined experiments. Years were considered random. Response curves and optimum fertilizer N application rates were derived using linear, quadratic, quadratic-plus-plateau, or linear-plus-plateau models (2,3) with the best-fitting model being selected based on the coefficient of determination (R²).

Soybean and commercial hybrid corn were grown in 1993, 1994, and 1995 on the study areas selected for 1994, 1995, and 1996, respectively. Crop residues were incorporated with fall chisel plow and spring field cultivator operations. Pre-plant insecticide and herbicide applications were broadcast prior to spring tillage. Inbred lines were planted in early to mid-May using a four-row, tractor-drawn planter equipped with seed cone attachments. Prior to seedling emergence, N treatments were established by broadcasting fertilizer-grade NH₄NO₃. Experimental units measured 12.5 Ś 60 ft in size and contained five 30-inch rows. The same inbred was planted in all five rows, but one outside row was treated as a pollen source while the remaining four were detasseled in late July of 1994 and 1995. In 1996, plots contained only four rows, none of which was detasseled due to a trend in the seed corn industry toward using male sterile inbred lines.

Harvest areas were thinned to a uniform population of 25,000 plants per acre at about the V4 growth stage in 1994 and 1995. The thinned plant population was increased to 31,000 plants per acre in 1996 to simulate an industry trend towards higher populations. Post-emergence weed control was accomplished with herbicide applications and inter-row cultivation as needed. Supplemental irrigation was applied with a center-pivot sprinkler system according to the scheduling procedure of the cooperating farmer.

Seed was harvested in mid-October from 5.0-Ś-17.4-ft areas in the center of each plot using a mechanical plot harvester. Samples of the seed from each plot were collected so that seed physical characteristics could be determined. Kernel size was determined by shaking a 1-lb sample for approximately 2 minutes in a series of four sieves with openings of decreasing size (26/64, 22/64, 18.5/22, and 16/64 inch). Average seed mass was determined by weighing 100 randomly selected seeds from each plot.

Seed Yield Response To Nitrogen

Corn seed yield response to fertilizer N varied significantly by year, but was consistently greater when inbred lines were grown following soybean than when following corn. Rotating inbred corn with soybean resulted in an average increase of 20 bu/acre (29%) compared to the C-C sequence. Nitrogen fertilization led to higher seed yields in 7 of 9 cases (inbred-year combinations) in the C-C cropping sequence, but in only 3 of 9 cases in the C-S sequence (Table 1). The average yield-based optimum N application rate (N_Y) was 65 lb/acre for the C-C cropping sequence and 23 lb/acre for the C-S cropping sequence, resulting in an apparent 42-lb/acre N benefit from soybean residue. This is within the 40- to 45-lb/acre range of N credit from soybean typically reported for commercial hybrid corn. However, the soybean benefit was inconsistent among inbred lines. For example, N_Y for AG6 was greater for the C-S than for the C-C cropping sequence in two of three years (Table 1). Moreover, the interaction between cropping sequence and nitrogen rate (SŚN) was not significant (P < 0.05) for the three-year averages, suggesting that response to N was not affected by cropping sequence; however, when data were analyzed by year, the S Ś N interaction was significant for 1996. The lack of interaction within the pooled data may be due to atypical growing conditions during the 1994 and 1995 growing seasons.

Table 1. Yield-based optimum N application rate (N_Y), maximum yield (Y_MAX), and regression equations for yield response curves shown in Figure 1.

^a  C-C = corn-following-corn cropping sequence,
C-S = corn-following-soybean cropping sequence

* Significant at P = 0.05.

† Significant at P = 0.01.

‡ Significant at P = 0.001.

NS = not significant at P = 0.05.

Unusually favorable growing conditions during the 1994 season resulted in some of the highest yields of the three-year study, yet there was little yield response to fertilizer N regardless of cropping sequence. This lack of response was especially evident with the C-S sequence where neither HB4 nor J90 exhibited a yield response to applied N fertilizers, and AG6 showed only a small increase of 15 bu/acre between N application rates of 0 and 54 lb/acre (Fig. 1b). Surprisingly, neither HB4 nor AG6 respond to added N in the C-C sequence in 1994 and J90 responded to only the first 36-lb/acre increment of fertilizer N (Fig. 1a). The N_Y ranged from 0 to 59 lb/acre for the two cropping sequences and three inbred lines (Table 1). Except for HB4, the inbred lines produced higher yields in the C-S cropping sequence in 1994 than in the C-C cropping sequence. Inbred J90 produced an additional 30 bu/acre and AG6 an additional 16 bu/acre as a result of the rotation with soybeans.

Fig. 1. Seed yield of three corn inbred lines as influenced by N application rate and cropping sequence. Regression equations and their corresponding statistics are listed in Table 1.

During 1995, a wet spring and hot summer temperatures contributed to low seed yields for all three inbred lines in the C-C cropping sequence and for J90 in the C-S cropping sequence (Table 1). Yield response of HB4 and AG6 to fertilizer N was greater in 1995 than in 1994 (Fig. 1c and 1d), presumably because of lower yields and conditions less favorable for N mineralization. In the C-S cropping sequence, N_Y was 53 and 96 lb/acre for HB4 and AG6, respectively, and maximum yields (Y_MAX) were nearly equal to those produced during 1994, when N_Y was 0 and 59 respectively, for the same two hybrids (Table 1). As in 1994, no response to fertilizer N was observed when J90 was grown following soybean. All three inbred lines yielded more seed with the C-S cropping sequence than with the C-C cropping sequence in 1995; however, this rotation effect may have been somewhat confounded by differences in weed competition. Despite mechanical and chemical weed control measures, poor crop growth led to substantial weed competition in the C-C cropping sequence. Weed competition was negligible in the plots where soybean was the previous crop because of better weed control in the previous year and more vigorous crop growth in 1995.

In 1996, all three inbred lines responded significantly to N fertilizer application in the C-C cropping sequence (Fig. 1e). Maximum yields for AG6 and J90 were as high or higher in 1996 than in 1994, but while N_Y for the C-C sequence was 36 lb/acre or less in 1994, it ranged from 67 to 119 lb/acre in 1996 (Table 1). When soybean was the previous crop, no yield response was observed (Fig. 1f). As in the previous growing seasons, J90 did not respond to the application of fertilizer N when preceded by soybean, but still produced more than 100 bu of seed per acre. Seed yield of AG6 was 19 to 40 bu/acre higher in 1996 than in 1994 or 1995. This yield increase is likely a result of changes in plant population and/or tassel management, but it is impossible to separate the effects of these cultural practice changes from each other and from seasonal weather variations. Other researchers have reported that both population and tassel management may affect yield. Wilhelm et al. (14) found that for one inbred line, each leaf that was removed with the tassel decreased seed yield by 7 bu/acre, which is consistent with our results based on an estimated removal rate of three leaves per plant. Modarres et al. (7) observed that, in a short growing season environment, small-stature inbred lines yielded about 34% more at a population of 36,000 plants per acre than at 26,000 plants per acre, while yield of normal-stature inbred lines increased only about 8.5%. However, these results are not consistent with our observations because AG6 has a normal stature while J90, which was apparently less affected by tassel and population management, is small in stature.

Seed Characteristics

Seed size, kernel mass, and salable fraction were evaluated as indicators of seed quality and salability. The number of kernels produced per plant was estimated from seed mass, seed yield, and plant population data. Salable fraction is the proportion of the total seed harvested that is 16/64- to 26/64-inch in diameter. Seed corn is sold in units of 80,000 seeds rather than on a weight or volume basis. Consequently, from the perspective of commercial seed companies, smaller seed size leads to reduced packaging and shipping costs. Conversely, there is evidence that excessively small seed produces low-vigor seedlings (4).

Nitrogen fertilizer application rate did not affect salable fraction in the C-S rotation and there were no significant differences in kernel mass and salable fraction between the C-C and C-S cropping systems (data not shown). In the C-C sequence, J90 and HB4 produced 12.3% and 2.2% less salable seed, respectively, when no N was applied than when 36 lb of N per acre or more was applied (Fig. 2a). These differences in salable fraction may be explained by the effect of N application rate on kernel mass, which decreased with decreasing N rate for J90, but increased slightly for HB4 as N rate decreased to 0 lb/acre (Fig. 2b). Thus, the low salable fraction of seed produced by J90 at the 0 lb/acre N application rate in the C-C cropping sequence was likely the result of an increase in the proportion of small seed, while the reduction in salable fraction produced by HB4 under the same conditions resulted from a slight increase in the proportion of large seed as the N application rate was reduced to 0 lb/acre (Fig. 2b). HB4 characteristically has a large number of aborted embryos and consequently produces larger seed than do J90 and AG6. When HB4 is under N deficiency stress, the number of embryo abortions increases, which in turn increases the number of seeds larger than 26/64 inch in diameter.

The number of kernels per plant was significantly increased in the C-S compared with the C-C sequence (Fig. 3). The number of kernels produced per plant generally increased with increasing N application rate, with the exceptions of HB4 in the C-C cropping sequence and J90 in the C-S cropping sequence (Fig. 3). AG6 and J90 produced more kernels per plant when grown following soybean than when grown following corn. Consequently, differences in yield among N application rates and cropping sequences were generally the result of differences in the number of kernels produced per plant, while differences among inbred lines were a result of differences in both kernel mass and number of kernels per plant.

Fig. 3. Relationship between N application rate and number of kernels per plant for three corn inbred lines grown in two cropping sequences. Regression equations and statistics follow the corresponding inbred numbers in the legend of Table 1. Yield-based optimum N application rate (NY), maximum yield (YMAX), and regression equations for yield response curves shown in Fig. 1.

Summary and Conclusions

In this study, 120 lb of N per acre was adequate to maximize seed corn yield under the most N-responsive conditions, and 100 lb/acre was adequate in all other cases. Averaged over years and corn inbred lines, optimum N fertilizer application rates were 23 and 65 lb/acre for the C-S and C-C cropping sequences, respectively. This difference of 42 lb/acre is similar to the 40- to 45-lb/acre N credit typically used in making N fertilizer recommendations for hybrid corn. Though N fertilizer requirements were usually less for the C-S than C-C cropping sequence, there were two cases where N requirements were greater when soybean was the previous crop. Despite these inconsistencies, results from this study do not provide any evidence that the N credit following soybean should be different for inbred corn than for hybrid corn.

Increasing N application rate generally increased the number of kernels produced per plant, which in turn led to increases in amount of salable seed. Differences among inbred lines in the amount of salable seed produced were a result of differences in the size of kernels produced as well as the number of kernels per plant.

Acknowledgments

Funding for this research was provided by a grant from Pioneer Hi-Bred International, Inc. The authors thank Pioneer Hi-Bred and University of Illinois personnel for providing assistance with treatment implementation, plot maintenance, and sample collection and analysis.

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