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a Dep. of Soil & Crop Sci., C4 Plant Sciences Building, Colo. State Univ., Ft. Collins, CO 80523 USA
b Dep. of Crop & Soil Environ. Sci., Smyth Hall, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061-0404 USA
rkhosla{at}lamar.colostate.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsNOTESResults and discussionREFERENCES |
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Abbreviations: ANOVA, Analysis of variance • DAP, days after planting • UAN, urea ammonium nitrate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResults and discussionREFERENCES |
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One factor that continues to be a problem in high-residue (no-till) farming systems is N fertilizer management (Lamond et al., 1991). No-till systems often exhibit suppressed yields because of lesser N availability (Rao and Dao, 1996). This occurs because of slower N mineralization (Phillips et al. 1980), greater N immobilization (Rice and Smith, 1984), denitrification (Rice and Smith, 1982), and NH3 volatilization (Terman, 1979). Also, below-optimum soil temperatures in no-till environments cause lower nutrient availability in the early part of the growing season (Gordon and Whitney, 1995). All these complexities with N fertilizer management in no-tillage systems indicate the need for more research for improved and efficient utilization of fertilizer N.
Opportunities to incorporate N fertilizer below the residue layers in reduced-tillage systems are limited (Mengel et al., 1982). Consequently, the most common application method used in no-tillage systems is broadcasting either solid ammonium nitrate or urea, or spraying urea ammonium nitrate (UAN) solutions on the soil surface immediately before or after planting (Mengel et al., 1982). However, surface application of N fertilizer can result in significant N losses through ammonia volatilization.
Several studies (Eckert, 1987; Fox and Piekielek, 1987; Fox et al., 1986; Maddux et al., 1984; Bandel et al., 1980 and 1984; Mengel et al., 1982) have examined placement methods for no-tillage corn production in the mid-Atlantic region, the Corn Belt and the Great Plains. They reported that similar N application rates of broadcast UAN produced lower yields than either injected or surface-banded UAN. Possible N loss mechanisms noted with broadcast UAN include volatilization and immobilization (Lamond et al., 1991). Much less work has been done on N fertilizer management for grain sorghum in no-tillage systems.
Studies with grain sorghum (Lamond, 1987; Sweeney, 1989) have shown that knifed N–P–K applications at planting increased grain sorghum yields relative to broadcast applications in high-residue systems. These results for grain sorghum, coupled with those reported for other crops [i.e., corn, barley (Hordeum vulgare L. subsp. vulgare) (Tomar and Soper, 1981; Malhi and Nyborg, 1990), and wheat (Diebert et al., 1985; Rao and Dao, 1992 and 1996)] have consistently shown that surface-band-applied N fertilizer is more efficient than surface broadcast-applied N. However, in the mid-Atlantic region, the majority of crops are grown on sandy coastal plain soils with low organic matter content (generally <20 g kg-1) and sandy to sandy loam surface (Gilliam and Boswell, 1984). Therefore, it is not advisable to band-apply the total amount of fertilizer N needed by the crop at the time of planting, because of the potential for high N leaching and denitrification losses.
A possible means to increase the fertilizer N efficiency for humid regions is to split-apply the fertilizer N. The sidedress application, N fertilization several weeks after corn emergence, has maximized the efficiency of fertilizer N in most situations (Piekielek and Fox, 1992; Fox et al., 1986; Aldrich, 1984; Olson and Kurtz, 1982). Also, the presence of plants at the time of side-dressing application reduces NH03 volatilization loss by shading and absorption of some of the evolved NH03 (Harper et al., 1983).
The period of rapid growth and nutrient uptake by grain sorghum plants occurs about 35 d after emergence (Vanderlip, 1993) at the eight-leaf growth stage. Sidedress application at this stage is feasible and would be beneficial for the crop. However, the need for N fertilizer application at the time of planting grain sorghum under a no-tillage system should not be ignored. The layer of crop residue on the soil reduces soil temperature (Unger, 1978; Thomas et al., 1973) and may sometimes lower the nutrient availability in the early part of the growing season (Gordon and Whitney, 1995). Application of starter-band fertilizer N within the rooting zone of the young seedlings has been shown to be efficient and beneficial to the crop (Lamond and Whitney, 1991). In a more recent study, Gordon and Whitney (1995) reported an 18% grain yield increase by application of fertilizer N in a starter band. Similar research is lacking in the mid-Atlantic Coastal Plain region.
Our overall objective was to evaluate the influence of multirate N fertilization on dryland grain sorghum production, and to determine the optimum N fertilization rates for maximum economic grain yields. Specific objectives were to: (i) determine the optimum rate of band-placed starter N fertilizer needed in combination with sidedress N applications to achieve maximum economic grain yields; (ii) evaluate whether preplant broadcast N applications are as efficient as band-placed and sidedress N applications used together; and (iii) estimate profit associated with N fertilizer use as a function of starter-band and sidedress N rates.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResults and discussionREFERENCES |
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Nitrogen treatments consisted of factorial combinations of four starter-band N rates at planting and four sidedress N rates at the eight-leaf growth stage to supply a total of 10 different N fertilization rates (11, 34, 56, 78, 101, 123, 146, 168, 190, and 213 kg N ha-1) and a broadcast rate of 67 kg N ha-1 at the time of planting. Urea ammonium nitrate solution was used as the N fertilizer source. Starter-band treatments were placed 5 cm below and to the side of the seed with a standard single disk opener using a carbon dioxide-pressurized system mounted on a John Deere Max Emerge 2 Conservation tillage planter (Deere and Co., Moline, IL). Phosphorus fertilizer solution (in the form of 10-34-0 ammonium polyphosphate) was also placed with the starter-band N. Potassium chloride was broadcast at each site at 67.2 kg K2O ha-1 rate to ensure adequate K availability.
Sidedress N was applied approximately 35 days after planting (DAP) at the eight-leaf growth stage of grain sorghum plants. The UAN solution was applied with a carbon dioxide-pressurized backpack sprayer whose spray nozzles were fitted with Teejet raindrop spray tips (Spraying Systems, Wheaton, IL). Flow rates for each tip size were measured at each experimental location prior to N application. Proper walking speed to obtain the desired application rate was calculated, and a stopwatch and metronome were used to calibrate and maintain proper walking speed during N application. Broadcast treatment application of 67 kg N ha-1 was done in the same manner at the time of planting.
Harvesting was done between the third week of September and the first week of October every year. Grain sorghum yield was determined by harvesting the four middle rows of each plot with a plot combine. Grain moisture contents were measured on all samples with a GAC II grain moisture meter (Dicky John, Auburn, IL). Grain yields are reported at 140 g kg-1 moisture content. On-site weather data including daily rainfall and daily maximum and minimum temperatures were recorded with Weather Monitor II automated weather systems (Spectrum Technologies, Plainfield, IL).
Analysis of variance (ANOVA) on grain yield was done using the SAS software package (SAS Inst., 1993) to test for significant treatment effects. When ANOVA results indicated significant effects at the 0.05 probability level, mean separation of grain yield was performed by Duncan's multiple range procedure (SAS Inst., 1993). Data were further analyzed via quadratic regression procedures with SAS (SAS Inst., 1993) and Sigma-Stat (Jandel Scientific, San Rafael, CA) to determine the optimum rate of starter-band N fertilizer in combination with sidedress N applications, and to compare starter-band with broadcast N application at planting. To determine the estimated profit associated with N fertilizer use as a function of starter-band N rate and sidedress N rate, the least square quadratic response surface regression (RSREG procedure) was calculated for each location using SAS (SAS Inst., 1993). The rate corresponding to the highest point on this response surface was the economic optimum (Scharf and Alley, 1993). The economic optimum N rate at planting (starter-band N) in combination with the sidedress N rate was calculated from this regression equation. Profit was estimated as grain sorghum value (yield x grain price) - N fertilizer cost (total N rate x N price) - other production costs (estimated as $345 ha-1 for all experiments). Current price was taken as $9.56 Mg-1 of grain sorghum and as $0.55 kg-1 of fertilizer N.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResults and discussionREFERENCES |
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125 mm water m-1 soil) to very low (Bojac sandy loam 50 mm water m-1 soil) (Khosla and Persaud, unpublished data, 1998). These soils are deep and well drained and are susceptible to leaching of nitrates.
Climatic conditions varied during the three years (1995, 1996, and 1997) of this project (Fig. 1)
. Rainfall during two (1995 and 1997) out of three years was much below average (average rainfall ranges between 100 and 150 mm per month based on 30 yr of rainfall data, Norris, 1985). The early part of the growing season (May to June) in 1995 was conducive to germination, emergence, and vegetative growth (i.e., temperatures were above the critical temperature of 
10°C for grain sorghum growth) (Anda and Pinter, 1994). Grain sorghum stands on both soils, the Pamunkey sandy loam and the Conetoe loamy sand, were excellent. However, the months of July and August were extremely hot and dry (32–38°C daytime temperatures) and plants were severely water stressed. In 1997, conditions were dry throughout the growing season with very little rainfall [203 mm and 260 mm at two sites, Atlee and Kempsville, respectively (Fig. 1)]. Temperatures were below normal during the month of May and planting was therefore delayed by about 2 wk. However, on Bojac soil, temperatures stayed below or around the 10°C critical level through the week after the emergence of grain sorghum. This low temperature induced tiller growth in grain sorghum later in the season, consequently increasing the water requirement of the crop. Water stress symptoms in plants were more prominent in 1995 and 1997 and were particularly evident in the plants growing on the Conetoe loamy sand, Bojac sandy loam, and Atlee very fine sandy loam. In 1996, conditions were good overall, with normal to above-average rainfall (550 mm) during the growing season at all locations (Fig. 1).
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70 kg N ha-1 of mineral N in the surface 0.3 m, out of a total >105 kg N ha-1 of profile mineral N (Table 3)
. This level of residual mineral N evidently provided sufficient N to support the early-season growth of the grain sorghum plants prior to side-dressing.
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130 kg N ha-1 sidedress N on other sandy textured soils. Also, the total profile mineral N in the Appling-Cecil complex was 131 kg N ha-1, which is 48 kg N ha-1 higher than the level in the Suffolk soil (Table 3).
Residual mineral N present in the top 0.3 m of soil at planting is crucial in deciding whether or not starter-band N should be applied. Residual mineral N levels of >45 kg N ha-1 in the surface 0.3m of soil in this study were sufficient to support the crop growth until side-dressing. The optimum rate of starter-band N fertilizer needed in combination with sidedress N applications to optimize grain yields therefore depends on the amount of soil mineral N at planting. Based on the data from these experiments, for soils testing high in mineral N (50 kg N ha-1 in the surface 0.3 m of soil) at planting, zero starter-band N should be applied in conjunction with sidedress N applications of 130 kg N ha-1. For soils testing low (<50 kg N ha-1 in the surface 0.3 m of soil) in mineral N at planting, starter-band N application of 40 kg N ha-1 in conjunction with 130 kg N ha-1 sidedress should optimize the grain sorghum yields under conditions similar to those in our study.
The efficiency of preplant broadcast N application as compared with starter-band N plus sidedress N applications could not be evaluated extensively here. Lack of yield response to N fertilization on four sites in 1995 and 1997, and to starter-band N on three out of four sites in 1996, prevents any assessment of the efficiency of the fertilizer placement methods. However, the Suffolk fine sandy loam experiment conducted in 1996 showed that the grain yield response to broadcast N applications and starter-band applications were not significantly different (Table 2). The average grain yields on the Suffolk soil from starter-band N and broadcast N at planting were 8.05 and 7.95 Mg ha-1, respectively, suggesting that broadcast N applications were as efficient as band-placed and sidedress N applications. This finding contradicts those reported by other researchers (Lamond, 1987; Sweeney, 1989; Malhi and Nyborg, 1990; Lamond and Whitney 1991; Lamond et al., 1991; Rao and Dao, 1996). However, the results on the Suffolk soil can probably be attributed to significant rainfall that occurred soon after planting and continued for the next several days. Consequently, little or no NH3 volatilization losses or fixation in the surface mulch would be expected. The success of broadcast N applications probably depends on rainfall shortly after N applications. Chichester and Morrison (1995) reported similar results from their 2-yr study in Temple, TX.
Response surfaces describing the estimated profit due to N application as a function of N fertilizer rates at planting (starter-band) and side-dressing are shown for four experimental locations (Fig. 3). The highest point on the response surface for Suffolk soil (Fig. 3a) corresponds to a maximum estimated profit of $341 ha-1, obtained by application of 39 kg ha-1 of starter-band N and 135 kg ha-1 of sidedress N fertilizer. There is an increasing trend in the yield response to applied fertilizer N for both starter-band and sidedress applications on Suffolk soil (Fig. 3a). Conversely, on the Bojac soil there is a decreasing trend in the estimated profit with the increase in the application rates of starter-band fertilizer (Fig. 3b). Starter-band N did not increase the estimated profit because the grain yield increases due to starter band were not great enough to cover the cost of the added fertilizer N. However, each additional sidedress N application significantly increased the estimated profit. The highest point on the response surface corresponds to the maximum estimated profit of $402 ha-1 obtained in this experiment by application of 0 kg ha-1 of starter-band N and 135 kg ha-1 of sidedress N fertilizer (Fig. 3b). Trends in estimated profit similar to those reported for the Suffolk and Bojac soils were found for the Appling-Cecil complex and Wheeling soils (Fig. 3c and 3d).
Nonresponsiveness or negative-responsiveness of grain sorghum yields in four out of eight site-years of this study can be attributed to high levels of residual mineral N present in the soil profile at planting. Also, erratic rainfall patterns that promoted early season luxuriant crop growth on low plant-available water holding capacity soils caused severe water stress conditions later in the season. These conditions translated growth into early leaf senescence and poor head development and grain filling. Consequently, the grain yields were lower and nonresponsive to applied N fertilizer.
Residual mineral N is usually neglected in the humid mid-Atlantic region in making fertilizer N recommendations due to the widely accepted perception that over-winter soil mineral N losses are high (Bundy and Malone, 1988; Gilliam and Boswell, 1984). However, residual soil mineral N was found in significant amounts in the top 0 to 0.9 m of the soil profile at planting for four nonresponsive sites. In light of the above-mentioned results, it would be unwise to ignore soil residual mineral N at planting. There were 147 and 112 kg of mineral N ha-1 in the Pamunkey and Conetoe soils, respectively, in 1995. Similarly, there were 129 and 87 kg of mineral N ha-1 in the Atlee and Kempsville soils, respectively, in 1997. These high residual N levels probably reflect N that mineralized from the previous soybean crop and native organic matter. Mineral N in the deeper (0.6–0.9 m) horizon could mostly have come from previous N fertilizer applications. Lack of yield response to applied N on four sites is therefore reasonable because of below-average rainfall conditions during the growing season and relatively high residual mineral N levels that were present at these four sites at the time of planting. Even with normal rainfall conditions on these four sites, it is highly unlikely that grain sorghum will respond to starter-band N application when soil mineral N levels exceed 50 kg N ha-1 in the surface 0.3 m at planting as observed at other sites in this study. These agronomically significant levels of soil profile mineral N warrant consideration in developing improved and more efficient N fertilizer recommendations for no-till grain sorghum production on sandy soils in the mid-Atlantic region. A system for incorporating soil mineral N as an integral part of the N recommendation system for no-till grain sorghum is presented in an associated paper (Khosla and Alley, 2000).
The challenge of dryland grain sorghum production in the mid-Atlantic region is to synchronize the plant N need with plant-available water. This can perhaps be achieved by partitioning the N application of the crop into several doses. Our findings indicate that little (
30 kg N ha-1) or no starter-band N should be applied to soils testing high (50 kg N ha-1) in residual mineral N at the time of planting, while the remaining N fertilizer should be applied in smaller amounts as sidedress N applications at various growth stages of the crop. This will restrict plants from accumulating excessive biomass early in the season and may reduce the risk of severe water stress later in the season. Sidedress N in this experiment was applied at the eight-leaf growth stage that occurs about 35 DAP. Another period of rapid growth and nutrient uptake in grain sorghum occurs at the midbloom growth stage, around 60 d after emergence (Vanderlip, 1993). Should rainfall occur during this period, sidedress N application to the crop at midbloom stage can be applied with a high-clearance applicator. Although the window of opportunity that exists at this stage is relatively short, it could promote proper head development and enhance grain yield and N use efficiency of the crop.SAS Institute 1993
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResults and discussionREFERENCES |
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Received for publication January 25, 1999.
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TOPABSTRACTINTRODUCTIONMaterials and methodsNOTESResults and discussionREFERENCES |
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