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Agronomy Journal 95:253-259 (2003)
© 2003 American Society of Agronomy

PRODUCTION PAPER

Seeding and Nitrogen Rates Required to Optimize Winter Wheat Yields following Grain Sorghum and Soybean

S. A. Staggenborg*,a, D. A. Whitneyb, D. L. Fjellb and J. P. Shroyerb

a Northeast Area Ext., Kansas State Univ., 1007 Throckmorton Hall, Manhattan, KS 66506
b Dep. of Agron., Kansas State Univ., Manhattan, KS 66506

* Corresponding author (staggen{at}ksu.edu)

Received for publication January 28, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
No-till planting winter wheat (Triticum aestivum L.) following summer crops requires different crop management than continuous wheat. A 3-yr study was conducted to determine if increased seeding rates and N fertilizer rates were required to maximize wheat grain yields following grain sorghum [Sorghum bicolor (L.) Moench] and soybean [Glycine max (L.) Merr.]. Wheat seeding rates of 67, 101, 134, and 168 kg ha-1 and N treatments of 0, 45, 90, and 134 kg N ha-1 were applied to areas previously planted to grain sorghum and soybean. Grain yield increased as seeding rate increased in all 3 yr, with yield optimized at seeding rates of >=134 kg ha-1 regardless of the previous crop. Wheat response to N varied with previous crop, with wheat following grain sorghum requiring 21 kg ha-1 more N to maximize grain yields compared with wheat planted after soybean. These previous-crop effects were attributed to grain sorghum producing higher levels of residue and this residue immobilizing a greater amount of available N than soybean residue. Leaf N content decreased as seeding rates increased and increased as N rates increased. Leaf N content had a similar response to N rates and previous crops as grain yields. Grain N content increased as applied N increased. Results of this study indicate that different seeding and N rates are required to optimize wheat yields when no-till planted after grain sorghum and soybean.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
HARVESTED WINTER WHEAT in Kansas has declined 21% from 1990 to 2000 (NASS, 2001a). Despite this decline, winter wheat still remains an important crop in most cropping systems throughout the state. Including winter wheat in crop rotations with summer crops improves control of problem summer annual and perennial weeds, reduces the incidence of residue-borne fungal diseases, and is an excellent source of residue cover for reduced tillage systems.

Improved economics associated with intensifying crop rotations have been a motivating factor in the adoption of no-till systems in Kansas (Schlegel et al., 1999). Adoption of no-till planting of winter wheat immediately following summer crop harvest was one of the first changes made to intensify crop rotations. Planting winter wheat immediately after summer crop harvest eliminates an 11-mo fallow period, thus reducing the duration of the transitional period from summer crops to winter wheat. However, planting no-till winter wheat behind summer crops presented problems, such as later planting dates and managing heavy summer crop residue if the previous crop was corn (Zea mays L.) or grain sorghum. Using soybean as the previous crop addresses the residue issue, but with the dominance of corn and grain sorghum in Kansas, management strategies are needed that address issues impeding successful wheat production following these summer crops in no-till systems.

Proper management of late-planted wheat after a summer crop is complicated by factors that are influenced by the previous crop as well as the environment that the wheat crop is subjected to as a result of delayed planting. Dahlke et al. (1993) reported that increasing seeding rates compensates for reduced tiller growth that typically occurs under the cooler temperatures encountered at later planting dates. The recommended winter wheat planting window for Manhattan, KS, is from 25 September through 20 October (Shroyer et al., 1996). When following a summer crop, harvest often delays wheat planting through early November, suggesting that higher seeding rates are needed to maximize yields at these later planting dates.

Wheat yields may also be influenced by several factors such as soil water content (Norwood et al., 1990), allelopathy (Roth et al., 2000; Knowles et al., 1993), and N availability. Previous-crop influence on N availability to the following wheat crop in a no-till system complicates N management. Increased residue levels of grain sorghum compared with soybean have the potential to decrease N availability to the subsequent wheat crop through N immobilization. Hargrove et al. (1983) and Sanford and Hairston (1984) reported lower tissue N, lower yields, and a higher fertilizer N requirement for wheat planted after grain sorghum compared with soybean. Sanford and Hairston (1984) reported that N uptake by wheat following soybean was 1.3 to 1.9 times greater than wheat following grain sorghum. Both studies attributed these differences to the low residual N content (<10 g kg-1) of sorghum residue, which produced a sink for N immobilization and reduced the amount of N available for uptake by the wheat crop. Knowles et al. (1993) reported that wheat yields and N uptake were 39 and 36% lower, respectively, when wheat no-till planted after grain sorghum was compared with wheat yield and N uptake in a continuous wheat system. They also attributed the lower N use efficiency to N immobilization by the grain sorghum residue. Kissel et al. (1977) reported that grain sorghum residue can immobilize as much as 62 kg N ha-1. Smith and Sharpley (1993) reported that grain sorghum residue was a poor contributor of mineralized N to the system after a long incubation period, contributing approximately 18 kg N ha-1 after 168 d. They also found that leaving the residue on the surface reduced the amount of N mineralized by a factor of three after 14 d and by 60% after 168 d. Vigil et al. (1991) reported severe N deficiency in wheat crops following grain sorghum each year of a 3-yr study. They attributed these results to greater recovery of mineralized N by sorghum plants and release of this N by the sorghum residue later than the critical uptake periods by the subsequent wheat crop.

The relative N contribution by soybean to the subsequent wheat crop is less clear than the effects of available N by sorghum residue. Knapp and Harms (1988) reported that 0 to approximately 18 kg N ha-1 from soybean residue in a no-till system was available to the subsequent wheat crop, depending on the rate of soybean residue decomposition in the spring. Mays et al. (1998) reported that unfertilized wheat following soybean produced lower yields than unfertilized wheat following a fallow period. Varco et al. (1993) found that availability of N from legume residues was lower than that of commercial fertilizers as a result of legume mineralization being dependent on residue decomposition. Green and Blackmer (1995) reported that 95% of the N immobilized in soybean residue was released after approximately 18 d at 24°C. Under the cooler, dryer conditions experienced in Kansas, it is possible that N released by mineralization of soybean residue may occur after winter wheat has headed, late enough to have minimal impact on yields. In fact, Soon et al. (2001) found that tillage system had a greater influence on wheat N requirements than did previous crops, including wheat, field pea (Pisum sativum L.), or red clover (Trifolium pratense L.).

Little data exists to guide producers in managing double-crop winter wheat planted in a no-till system in Kansas. Therefore, the objective of this study was to determine if the optimal seeding rates and N rates for wheat are different when no-till planted immediately following grain sorghum compared with soybean.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
To initiate the previous-crop residue treatments, grain sorghum and soybean were planted in a randomized complete block design on the Agronomy Farm near Manhattan, KS, in the spring of 1997, 1998, and 1999. Soil type at this location is a Reading silt loam (fine, mixed, mesic, Typic Agriudoll). Recommended cultural practices for these crops were followed, but grain yields were not recorded. The winter wheat variety ‘2137’ (Sears et al., 1997) was no-till planted into the existing summer crop residue on 20 Oct. 1997, 30 Oct. 1998, and 25 Oct. 1999. Seeding rates of 67, 101, and 134 kg ha-1 were used in 1997 and an additional seeding rate of 168 kg ha-1 was added in 1998 and 1999. These seeding rates correspond to approximately 1.8, 2.7, 3.6, and 4.5 million seed ha-1 for the four seeding rates of 67, 101, 134, and 168 kg ha-1, respectively, based on an average seed weight of 37.3 g 1000-1 seed (Roozeboom, 1998, 1999, 2000). All plots were planted with a plot drill (Hege Model 10, Hege Manufacturing, Colwich, KS) in 0.25-m row widths. Nitrogen rates of 0, 45, 90, and 134 kg ha-1 were applied as ammonium nitrate after planting in the fall each year. A split-split plot arrangement of a randomized complete block design with four replications was used with previous crops as main plots, seeding rates as subplots, and N rates as sub-subplots. Sub-subplots were 6 m long and 1.5 m wide.

Soil test results indicated that levels for pH, P, and K were within the optimal ranges for winter wheat production. Therefore, no additional soil amendments were applied throughout the duration of the study. No herbicide applications were required for the first two growing seasons, but 26 g a.i. ha-1 flucarbazone-sodium {4-5-dihydro-3-methoxy-4-methyl-5-oxo-N-[[2-(trifluoromethoxy)phenyl]sulfonyl]-1H-1,2,4,triazole-1-carboxamide sodium salt} (Bayer, Kansas City, MO) was applied 6 Mar. 2000 for control of winter annual grasses.

To assess N uptake by the plant and to determine if final uptake levels were adequate, plant N content was determined from whole-plant samples taken at anthesis (Feekes 10.5.1) (Large, 1954), and grain N levels were determined from grain samples taken at harvest. Plant and grain samples were dried at 65°C and ground to pass a 0.10-cm screen. A 0.25-g subsample of ground material was digested with sulfuric acid and hydrogen peroxide (Isaac, 1977). Nitrogen content was determined on the digest with a Technicon Auto Analyzer II (Technicon Ind. Syst., Tarrytown, NY). Grain yields were determined at maturity by harvesting the center 0.5 m of each plot the entire plot length. Yields were adjusted to 13% moisture.

Because the number of seeding rate treatments differed in 1998 from 1999 and 2000, a single-year analysis of variance was conducted for data from 1998. An F test indicated homogenous variances among 1999 and 2000 data; therefore, a combined-year analysis of variance was conducted. Single degree-of-freedom contrasts were used to test N rates and seeding rate effects, as described by Swallow (1984).

Significant linear or quadratic responses were characterized using regression analysis. Nitrogen or seeding rates that produced maximum yield, leaf N content, or grain N content for all quadratic responses were determined by solving the first derivative for zero.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Growing conditions varied from year to year throughout this study (Table 1). Precipitation was below normal and temperatures near normal for the 1997–1998 growing season. Rainfall received in early June during grain fill improved yields, which averaged 3848 kg ha-1. During the fall of 1998, rainfall and temperatures were above normal. Above-average rainfall was received in the spring of 1999 with near-normal temperatures. These spring conditions resulted in leaf rust infestations that reduced yields, resulting in an average yield of 1895 kg ha-1. Above-average temperatures and extremely low rainfall amounts in the fall of 1999 resulted in poor fall growth. Above-average temperatures during grain fill in late May and early June of 2000 reduced grain yields, resulting in an average yield of 2678 kg ha-1.


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  		Table 1. Monthly temperature means and precipitation totals for Manhattan, KS, for three winter wheat growing seasons.

 
Grain Yields
Grain yields were influenced by seeding rate in 2 of 3 yr of the study as indicated by the significant seeding rate main effect in 1998 and significant seeding rate x year interaction in 1999 and 2000 (Table 2). Because no seeding rate x N rate or seeding rate x previous crop interactions were found, data are presented as main effects within each year. Grain yields responded in a linear manner to increasing seeding rates in 1998, in a nonlinear manner in 1999, and did not respond to seeding rates in 2000 (Table 3). In 1998, grain yield increased at a rate of 5.1 kg ha-1 per kilogram per hectare of seed (Table 3). The quadratic yield response to seeding rates in 1999 was the result of low yield at the 67 kg ha-1 seeding rate compared with the three higher rates. Grain yield increased at a rate of 23.1 kg ha-1 per kilogram per hectare as seeding rates increased from 67 to 101 kg ha-1 and 3.5 kg ha-1 per kilogram per hectare as seeding rates increased from 101 to 168 kg ha-1 (Table 3). The optimal seeding rate in 1999 was determined to be 150 kg ha-1 (first derivative set to zero).


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  		Table 2. Analysis-of-variance results for yield, leaf N, and grain N for wheat harvested in 1998, 1999, and 2000 in Manhattan, KS.

 

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  		Table 3. Effects of three seeding rates in 1998 and four seeding rates for 1999 and 2000 on wheat grain yield at Manhattan, KS, averaged across all N rates and previous-crop treatment levels.

 
Based on the 2 yr in which wheat yield responded to seeding rates, seeding rates of >=134 kg ha-1 were needed to reach maximum yields. This is approximately 35 kg ha-1 higher than the recommended seeding rate for continuous wheat in Kansas (Shroyer et al., 1996). Wheat yield response to increasing seeding rates was lower than expected, especially considering the late planting dates in this study. Dahlke et al. (1993) reported maximum wheat yields at seeding rates of approximately 90 kg ha-1 when planted in early September. However, when planting was delayed until late September, seeding rates from approximately 120 to 170 kg ha-1 were needed to maximize wheat yields. In 1998 and 1999, a seeding rate of >=101 kg ha-1 was needed for maximum yield with a late-October seeding date.

Others have reported inconsistent wheat yield responses to seeding rates as well (Ferguson et al., 1989; Frederick and Marshall, 1985; Koscelny et al., 1990, 1991). They reported that when early-season growing conditions were unfavorable, tiller production was limited and unable to compensate at the lower plant densities. As a result, yields increased as seeding rates increased as a result of higher spikes per square meter at the higher seeding rates.

Early-season growing conditions varied throughout this study and influenced yield responses to seeding rates. October and November temperatures were near average in 1997 and above average in 1998 and 1999 (Table 1). Adequate early-season growing conditions occurred in the fall of 1997 and 1998, the two years when yields responded to seeding rates. October and November precipitation was above average in 1998, and despite being below average in 1997, the precipitation was received over a period from 13 d before planting through 7 d after planting. Coupled with the near-normal temperatures in 1997, early-season growing conditions were adequate.

The 1999–2000 growing season began with a precipitation deficit in October (Table 1). November precipitation was near normal but was received in one large storm. This coupled with above-average temperatures reduced overall growth. Under these conditions, a seeding rate response would have been expected. However, above-average temperatures and high-velocity winds during late May in 2000 hastened maturity and reduced overall yield potential. This stress and reduction in yield potential may have masked any seeding rate differences.

Wheat yield response to N fertilizer was influenced by the previous crop in this study, as indicated by the significant N x previous crop interactions (Table 2). Both linear and quadratic responses were significant for each previous crop, except following soybean in 1999 when only the quadratic response was significant (Table 4). In 1998 and 1999, wheat planted after grain sorghum required higher N rates to maximize yields but produced lower maximum yields than wheat planted after soybean (Fig. 1) . In 1998, maximum wheat yield after grain sorghum of 3760 kg ha-1 occurred at 112 kg N ha-1, whereas maximum wheat yield after soybean was 4059 kg ha-1 and required 94 kg N ha-1. In 1999, the maximum wheat yield of 2043 kg ha-1 after grain sorghum required 94 kg N ha-1, and maximum wheat yield after soybean of 2333 kg ha-1 occurred at 70 kg N ha-1. Hargrove et al. (1983) reported a similar trend with wheat following soybean requiring approximately 30 kg ha-1 less N to maximize yields compared with wheat following grain sorghum.


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  		Table 4. Probability of exceeding F for contrasts regarding responses to N rates for wheat following two crops during three growing seasons at Manhattan, KS, averaged across all seeding rate treatments.

 


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  		Fig. 1. Wheat grain yield response to N rates following grain sorghum and soybean for 3 yr.

 
In 2000, wheat yield response to N fertilizer also varied by previous crop, with wheat yields after soybean exceeding those after grain sorghum by 675 kg ha-1. However, the optimal N rates for each previous crop were inverted compared with the previous 2 yr, with the yields maximized at 128 kg N ha-1 following soybean and 85 kg N ha-1 following grain sorghum. One possible explanation for these results may be the differences in soil available water at wheat planting as a result of the previous crop. The 1999–2000 growing season began with a precipitation deficit that continued throughout the growing season (Table 1). It is reasonable to assume that less soil water was available to the wheat crop following grain sorghum compared with soybean for several reasons. In Kansas, soybean matures (leaf drop) approximately 14 d earlier than grain sorghum (maturity) (NASS, 2001b). Also, grain sorghum's perennial growth habit results in it continuing to use water until the plant is terminated by subfreezing temperatures (Stone et al., 2002). The difference in cessation of grain growth between soybean and grain sorghum coupled with grain sorghum's perennial growth habit would reduce the amount of water available for the subsequent wheat crop following sorghum. During a dry year such as 1999–2000, these differences would likely result in lower yields and a potentially different response to N applications between the two crops. Under such conditions, the higher N rate treatments following grain sorghum may have developed a denser canopy during early spring, which resulted in greater stress in late May when above-average temperatures and high-velocity winds were experienced.

Less N was required to maximize wheat yields after soybean compared with grain sorghum in 1998 and 1999, which was expected. The expectations are that soybean contributes N to the system that is beneficial to the subsequent wheat crop and/or grain sorghum reduces N availability for the subsequent crop.

It is not likely that soybean contributes N to the subsequent wheat crop. Based on soil temperatures required to release 95% of the N immobilized in soybean residue reported by Green and Blackmer (1995), organic N release by soybean residue would occur most years in mid to late May in Kansas, which is late enough to have minimal impact on the subsequent wheat crop. In fact, current recommendations in Kansas for wheat following soybean do not consider N credits from the soybean on the subsequent wheat crop (Lamond et al., 1988).

The more plausible explanation for higher N requirements for wheat following grain sorghum compared with wheat after soybean would be associated with grain sorghum residue and N immobilization. Hargrove et al. (1983) and Sanford and Hairston (1984) reported lower tissue N, lower yields, and a higher fertilizer N requirement for wheat planted after grain sorghum compared with soybean. Both studies attributed these differences to the low residual N content (<10 g kg-1) of sorghum residue, which produced a sink for N immobilization and reduced the amount of N available for uptake by the wheat crop. Knowles et al. (1993) reported that wheat yields and N uptake were 39 and 36% lower, respectively, when wheat no-till planted after grain sorghum was compared with wheat yield and N uptake in a continuous wheat system. An additional 15 kg N ha-1 was required to maximize wheat yield following grain sorghum compared with wheat grown in the absence of grain sorghum residue. In 1998 and 1999, our differences between the two crops averaged 21 kg N ha-1. They also attributed the lower N use efficiency to N immobilization by the grain sorghum residue. Kissel et al. (1977) reported that grain sorghum residue could immobilize as much as 62 kg N ha-1.

Leaf and Grain Nitrogen Content
Wheat leaf N concentration was affected by seeding rate in 1999 and 2000 (Table 2). Leaf N concentration declined as seeding rates increased (Y = 2.12 - 0.00543x + 0.0000154x2, P < 0.05) (data not shown). Nitrogen application rates consistently increased N content in leaf (Fig. 2) and grain (Table 5). A quadratic response best described leaf N content at heading (Feekes 10.1) as a function of applied N in 1998 (P < 0.05) and 1999 (P < 0.05). In 1999, differences in leaf N content following soybean and grain sorghum increased as applied N rates increased, with leaf N content being greater following soybean. In 1998, the maximum leaf N rate of 17 g kg-1 occurred at 120 kg N ha-1. In 1999, maximum leaf N level following soybean was 22 g kg-1 and occurred at 134 kg N ha-1, and the optimal leaf N level following grain sorghum was 21 g kg-1 and also occurred at 134 kg N ha-1. The rate of leaf N increased more rapidly after soybean than after sorghum. Based on the equations derived, the calculated maximum leaf N contents for wheat occurred at 140 kg after soybean and 170 kg after grain sorghum. Although these values exceed the limits of the data collected, they do illustrate the relative differences in the amount of N fertilizer needed to achieve maximum leaf N values.



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  		Fig. 2. Wheat leaf N content response to N rates following grain sorghum and soybean for 3 yr.

 

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  		Table 5. Wheat grain N concentration for 1998, 1999, and 2000 at Manhattan, KS. All values are averaged across three seeding rates in 1998 and four seeding rates in 1999 and 2000 and two previous crops.

 
In 2000, differences in wheat leaf N content following soybean and grain sorghum also increased as applied N increased, with leaf N content after grain sorghum being greater. The leaf N content response to applied N was linear, rather than quadratic as in 1998 and 1999. Kelley (1995) and Hargrove et al. (1983) reported increased leaf N content with increasing applied N rates. Both reported higher leaf N content following soybean than grain sorghum, as did our result in 2 of the 3 yr of the study (1998 and 1999).

Grain N was only influenced by applied N (Table 2) and increased as applied N rates increased all 3 yr (Table 5). In 1998 and 2000, grain N increased in a quadratic manner as applied N increased, whereas in 1999, the response was linear. In all 3 yr, the applied N rate required to maximize grain N was greater than the N rate required to optimize grain yields. Several studies report increased grain N as applied N increased (Kelley 1995; Woodard and Bly, 1998; Hargrove et al., 1983; Howard and Lessman, 1991; Zebarth and Sheard, 1992; Knowles et al., 1993). Kelley (1995) and Woodard and Bly (1998) also reported higher N rates required to optimize grain N than N rates required to maximize grain yield.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
No-till planting winter wheat immediately after summer crops such as soybean and grain sorghum requires different management practices for each previous crop. Seeding rates of >=134 kg ha-1 were required to maximize grain yields, regardless of the previous crop. This is approximately 35 kg ha-1 higher than the recommended seeding rate for continuous wheat. Previous crop influenced N management, with wheat following grain sorghum requiring approximately 21 kg ha-1 more N fertilizer to maximize yields than wheat following soybean. The higher N requirement following grain sorghum was attributed to the higher residue levels produced by grain sorghum and greater N immobilization by the residue. However, allelopathy cannot be completely dismissed. Leaf and grain N levels were affected by applied N fertilizer rates throughout the study, with tissue N levels increasing with increasing N rates. Previous crop affected leaf N content in 2 of the 3 yr, but the results were inconsistent. These results suggest that when winter wheat is planted immediately after summer crop harvest, seeding rates should exceed 134 kg ha-1 and N rates should be increased an additional 24 kg ha-1 following grain sorghum compared with N rates used following soybean.


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




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