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ฉ 2007 Plant Management Network.
Accepted for publication 9 November 2006. Published 23 March 2007.

Nitrogen Management Affects Sorghum Grown for Grain and Forage

Daniel W. Sweeney and Joseph L. Moyer, P.O. Box 316, Southeast Agricultural Research Center, Kansas State University, Parsons 67357

Corresponding author: Dan Sweeney. dsweeney@oznet.ksu.edu

Sweeney, D. W., and Moyer, J. L. 2007. Nitrogen management affects sorghum grown for grain and forage. Online. Crop Management doi:10.1094/CM-2007-0323-01-RS.

Abstract

Producers have to maximize crop production to stay economically viable. After grain harvest, sorghum [Sorghum bicolor (L.) Moench] stalks may be utilized for animal feed. The objective of this study was to determine the effect of fertilizer N rate and timing on yield, N concentration, and N removal in grain and the remaining stover of a stay-green sorghum hybrid harvested immediately after grain or left until a killing frost. Sorghum grain yield responded to N rates up to at least 120 lb/acre, but was unaffected by fertilizer N timing. Nitrogen removed in the grain increased linearly with N rate and was about 25% of the applied fertilizer N. Stover yield and N removal was reduced by applying two-thirds of the fertilizer N as a sidedress at the eight-leaf stage compared with applying two-thirds or more N preplant. Stover yield was greater when cut immediately after grain harvest than after a killing frost partly because of poor post-harvest growth. Small amounts of post-harvest plant growth resulted in greater stover N concentration, especially at higher N rates. Total N removal by stover and grain responded linearly to increasing N rates and was greater if stover was harvested after grain harvest and if post-harvest growth did not occur. Nitrogen management is important for sorghum grain, as well as for stover used for feed.

Introduction

With increased economic constraints, producers need to find ways to increase revenue by improving production efficiency with minimal additional inputs. Sorghum is a major crop in the eastern Great Plains partially because it can tolerate the extreme climatic conditions often encountered without total crop failure. Moderate yields and low prices, however, may result in revenue from grain to be $100/acre, or less. Because sorghum has perennial characteristics, it is often ratooned in the southern United States to give two grain crops (13,17). Although two grain crops are impractical at the latitudes of the eastern Great Plains, the remaining sorghum stover after grain harvest often remains green and may regrow if frost is delayed. This stover left after sorghum grain harvest has the potential to be used as livestock feed and, thus, increase the value of the crop. Cow-calf operations are common in much of the eastern Great Plains and provide a potential feed outlet for sorghum stover after grain harvest. Crop residues can be utilized by grazing cattle or by harvesting stalks for drylot feed (25), but quality (9), especially crude protein (26), may be low. Sorghum stover could be harvested immediately after grain harvest or left to continue to acquire photosynthate until a killing frost and then cut for hay, chopped for silage, or grazed.

Nitrogen is generally the most limiting nutrient in sorghum production. Increasing N fertilizer rates generally results in improved grain yield (5,11,20). However, effective N management may be equally important if the remaining sorghum stover is to be used for livestock feed. Forage sorghum, harvested as whole plants, responds to increasing N rates, but the yield response diminishes at higher rates (6,24). Timing of N fertilizer applications may affect yields and N uptake during the season. Lafitte and Loomis (10) found that the timing of a second N application may improve grain yield, especially when N is limiting. Nitrogen application split between planting and the 8-leaf growth stage can improve grain yield (16), but also increases N uptake in stover (15). Data are lacking, however, on optimum N rates and timing for sorghum grown not only for grain but also to utilize the remaining stover as quality forage. Thus, the objective of this research was to determine the effect of fertilizer N rate and timing on yield, N concentration, and N removal in sorghum grain and the remaining stover harvested immediately after grain or left until a killing frost.

Field Experiment Description

The experiment was conducted during 2003 and 2004 near Parsons, KS, at the Southeast Agricultural Research Center of Kansas State University. The topsoil was a Parsons silt loam (fine, mixed, thermic, Mollic Albaqualf) of approximately 12 inches, overlying a claypan B horizon. The topsoil has an available water-holding capacity of approximately 2 inches, and the subsoil has a low percolation rate of < 0.06 inch/h (19). Selected background soil chemical analyses in the 0- to 6-inch depth were 5.6 pH (1:1 soil:water), P (Bray-1) at 42 ppm, and K (1 M NH₄C₂H₃O₂ extract) at 120 ppm analyzed by the procedures recommended by North Central Region Agricultural Experiment Stations (4). Monthly rainfall and average daily maximum air temperature during the growing season for 2003, 2004, and the 30-year average are shown in Table 1.

Table 1. Precipitation and Average Maximum Daily Temperature for April through November for 2003, 2004, and the 30-year average at the Southeast Agricultural Research Center, Parsons, KS.

The experimental design for the grain harvest was a randomized complete block with a 4 ื 3 factorial arrangement of treatments in four replications. Fertilizer N rates were 40, 80, 120, and 160 lb/acre. Fertilizer application timings were (i) 100% preplant, (ii) two-thirds of the amount applied preplant and one-third applied as a sidedress at the eight-leaf growth stage, and (iii) one-third of the amount applied preplant and two-thirds applied sidedress. A no-N control treatment was included in each replication. The sorghum variety was Pioneer 84Y00, chosen because of yield potential and stay-green characteristics. Sorghum was planted in 30-inch rows at approximately 62,000 seeds/acre on 28 April 2003 and on 7 May 2004. In 2004, initial stand was poor and sorghum was replanted on 24 May. In this region, planting date effects on sorghum yield are often small and inconsistent (7,18). Individual plot size was 10 ื 40 ft. The center two rows of each plot were harvested for grain yield with a small-plot combine on 27 August 2003 and 30 September 2004. Grain was weighed, moisture was determined, and yield was adjusted to 12.5% moisture. After grain harvest, soil was sampled from the surface six inches of each plot and analyzed for NH₄-N and NO₃-N in a 1:10 soil to 1 M KCl extraction followed by use of a rapid flow analyzer (1,2).

After grain harvest, the plots were split by cutting and removing a 3-ft-wide section across the middle of the 40-ft plots perpendicular to the planting direction. The stover from 18.5 ft of the two grain harvest rows was cut with a flail-type forage harvester on 28 August 2003 and 4 October 2004 after grain harvest. The remaining stover was left until killing frosts on 7 November 2003 and 25 November 2004 and then the stalks in the remaining 18.5 ft from the two harvest rows were harvested with a flail harvester on 13 November 2003 and delayed by wet conditions until 13 December 2004. The areas for stover harvest were randomly assigned, thus, the experimental design for the stover harvest was a split-plot arrangement of a randomized complete block. The N rate and timing factorial treatments described above were whole plots and forage harvest timings (immediately after grain harvest or after a killing frost) were subplots. Stover harvests were fresh weighed and a subsample of the fodder was dried, weighed, and yield was adjusted to 12% moisture. Sorghum grain and stover subsamples were dried at 140ฐF, ground, and analyzed for N (8) in a H₂SO₄-H₂O₂ digest (12). Nitrogen removal was calculated as the product of dry matter production and N concentration.

Data were analyzed using the Proc Mixed procedure of the Statistical Analysis System (SAS Institute Inc., Cary, NC). All factors except replicate were considered fixed. Year was treated as a strip-plot fixed effect, so that across years the data were analyzed as a strip-split plot. Regression of dependent variables as a function of N rate was done using Proc Reg of SAS (SAS Institute Inc.).

Sorghum Grain Yield and N Removal

Sorghum grain yield, grain N concentration, and N removal in grain were primarily affected by fertilizer N rate, but not by fertilizer N timing (Table 2). Grain yield and grain N concentration were also affected by year ื N rate interactions. In 2003, grain yield was increased 22 bu/acre by applying N at 120 lb/acre compared with the control (Fig. 1). In 2004, grain response to applied N was greater than in 2003 resulting in more than 50 bu/acre greater yield with N at 160 lb/acre than with the control. Weather conditions were hotter and drier in 2003 than in 2004 during July and August, except for the last three days of August 2003 when nearly 90% of the month’s rainfall occurred (Table 1). Buah et al. (5) found that N fertilizer increased grain yield and N concentration, but reduced nitrogen use efficiency for dry matter and grain production. In our study in 2003, when yield response to N rate was less, grain N concentration increased linearly with increasing N rate (Fig. 1). In contrast, in 2004 when the yield response to N was greater, N concentration in the grain was affected by N rate only as the rate approached 160 lb/acre. As a result, the N removed in the grain was only affected by N rate and not by the interaction with year (Table 2). The amount of N removed in the grain increased linearly with increasing N rate (Fig. 1). The slope of 0.243 suggests that about 25% of applied fertilizer N is being translocated to the grain.

Fig. 1. Effect of fertilizer N rate on grain yield, grain N concentration, and N removed in sorghum grain. For regression equations: ** denotes significance at the 0.01 level of probability.

Table 2. Analysis-of-variance significance levels for the effect of N rate and N application timing on grain yield, grain N concentration, and N removed in grain of sorghum grown during 2003 and 2004.

 * = Significant at the 0.05 level of probability.

** = Significant at the 0.01 level of probability.

NS = nonsignificant.

Stover Yield and N Removal

Stover yield, N concentration, and N removal were primarily affected by N rate and harvest timing and interactions involving year, whereas interactions among treatments were not prevalent (Table 3). Split-N applications have been found to increase N uptake in sorghum in early growth stages (15) and at anthesis and physiological maturity (16). In our study, the effect of N fertilizer timing on stover yield and N removal was marginal and interacted with the time of the forage harvest but not with year (Fig. 2). When forage was harvested immediately after grain harvest, two-year average forage yield was less when two-thirds of the fertilizer N was applied as a side-dress compared with treatments where more N was applied preplant. In contrast, when stover was left until a killing frost, not only was yield lower than just after grain harvest, there were no differences in yield because of N timing. Nitrogen removed in the stover was greater following grain harvest than when left until after a killing frost. Immediately after grain harvest, stover N was greater when two-thirds of the fertilizer N was applied before planting and one-third as a side-dress at the 8-leaf stage than when one-third was applied preplant and two-thirds followed as a side-dress. Applying all the fertilizer N preplant resulted in an intermediate value for N removal in the stover and was not significantly different than either of the split application treatments. After a killing frost, stover N removal appeared to be increased as more of the fertilizer N was applied as a side dress, but, similar to yield, the differences were not significant. At grain harvest, there were no significant differences in total soil inorganic N (sum of NH₄-N and NO₃-N) in the 0- to 6-inch surface layer (Fig. 3). It was expected that, when two-thirds of the fertilizer N was applied as a side-dress at the 8-leaf stage, soil N levels may have been greater after grain harvest than when more of the N was applied preplant, but this was not the case and may partially explain the post grain-harvest stover yields and N removal.

Fig. 2. Effect of fertilizer N timing and stover harvest time on yield and N removed in sorghum stover. Timing of N applications: Preplant – 100% applied preplant; Split-A – two-thirds applied preplant, one-third applied as sidedress at the 8-leaf stage; Split-B – one-third applied preplant, two-thirds applied as sidedress at the 8-leaf stage. Sorghum stover harvest times: Grain – stover harvested immediately after grain harvest; Frost – stover harvested after die-down following a killing frost. Bars with the same letter are not statistically different at P < 0.10 according to the LSD test.

Fig. 3. Effect of fertilizer N timing and rate on total soil inorganic N (sum of NH4-N and NO3-N) after grain harvest. Timing of N applications: Preplant – 100% applied preplant; Split-A – two-thirds applied preplant, one-third applied as sidedress at the 8-leaf stage; Split-B – one-third applied preplant, two-thirds applied as sidedress at the 8-leaf stage. For regression equations: ** denotes significance at the 0.01 level of probability.

Table 3. Analysis-of-variance significance levels for the effect of N rate, N application timing, and stover forage harvest timing on stover yield, stover N concentration, N removed in stover, and total N removed by sorghum in grain and stover during 2003 and 2004.

 † = Significant at the 0.10 level of probability.

 * = Significant at the 0.05 level of probability.

** = Significant at the 0.01 level of probability.

NS = nonsignificant.

Stover yield, N concentration, and N removed in stover were affected also by year ื N rate ื harvest timing interactions (Table 3). In 2003, the amount of stover after grain removal was little affected by N rate and the response to fertilizer N was marginal for stover left until a killing frost (Fig. 4). In 2004, the yield of stover remaining after grain harvest was increased by more than 50% when fertilizer N rate was increased to 80 lb/acre or more compared with the no-N treatment. However, the stover yield response to N after a killing frost was similar to that in 2003. In both years, increasing fertilizer N rates did not prevent lower stover yield, likely from leaf loss, measured after a killing frost compared with yield obtained after grain harvest. The value of baled sorghum stover will vary with year, but with yield levels after killing frosts similar to those in this study, it is unlikely to prove economical. Even though stover yield was lower, N concentration in the stover was greater after a killing frost than immediately after grain harvest in 2003 (Fig. 5) and was visually evident by green tissue. After grain harvest, stover N concentration increased linearly with increasing N rate, but the N concentration in stover that was allowed to continue to grow until a killing frost increased rapidly at greater N rates. This may be partially explained by a similar response of increased inorganic N in the soil at greater N rates in 2003 (Fig. 3). Although stover nitrate concentrations were not measured in this study, high soil inorganic N and soil moisture deficits can result in nitrate concentrations in sorghum plants that are potentially toxic to animals (14). After grain harvest in 2004, soil N level responses to increasing N rates were small (Fig. 3) and this may have accounted for marginal increases in N concentration in stover with increasing N rate (Fig. 5). In 2003, N removal in stover generally ranged from 10 to 25 lb/acre (Fig. 6). The increased N concentration in stover grown until after a killing frost at the highest N rate resulted in a removal of greater than 25 lb/acre. In contrast, N removal in stover immediately after grain harvest was much greater in 2004 and increased more rapidly at lower N rates than at rates approaching 160 lb/acre. Sotomayor-Rios et al. (24) also found that crude protein yield increase was greatest for the first fertilizer N increment. But in our study, sorghum plants did not remain green in 2004 when left until a killing frost and this resulted in much lower N removal and little response to fertilizer N rates (Fig. 6). Additionally, leaf loss exacerbates low N removal in the stover because N concentrations are greater in the leaf blades than the stalks (23).

Fig. 4. Effect of fertilizer N rate and stover harvest time on sorghum stover yield in 2003 and 2004. Sorghum stover harvest times: Grain – stover harvested immediately after grain harvest; Frost – stover harvested after die-down following a killing frost. For regression equations: †, **, and NS denote significance at the 0.10 and 0.01 levels of probability or non-significant.

Fig. 5. Effect of fertilizer N rate and stover harvest time on sorghum stover N concentration in 2003 and 2004. Sorghum stover harvest times: Grain – stover harvested immediately after grain harvest; Frost – stover harvested after die-down following a killing frost. For regression equations: † and ** denote significance at the 0.10 and 0.01 levels of probability.

Fig. 6. Effect of fertilizer N rate and stover harvest time on N removed in sorghum stover in 2003 and 2004. Sorghum stover harvest times: Grain – stover harvested immediately after grain harvest; Frost – stover harvested after die-down following a killing frost. For regression equations: † and ** denote significance at the 0.10 and 0.01 levels of probability.

Total N Removal and Apparent Recovery

In 2003, total N removal was about 50 lb/acre with no fertilizer N applied, but in 2004 total N removal was less than 50 lb/acre without N fertilizer (Fig. 7). This difference is likely because of N contribution from the 2002 soybean crop, but in 2004 it was the second year of sorghum. The response of total N removal by sorghum to increasing N rates was linear in both years, regardless of stover harvest timing. The maximum N rate in our study was 160 lb/acre, but, as shown by Powell and Hons (22), greater N rates may result in quadratic relationships between fertilizer N rate and N uptake that suggest that maximum N uptake occurs below 200 lb of N per acre. In our study, total N removal in 2003 was similar whether the stover was removed immediately after grain harvest or left until a killing frost (Fig. 7) because of some green tissue until frost. In contrast, in 2004 total N removal was greater when stover was removed after grain harvest because of little, if any, viable plant growth and N uptake until frost. The slopes of the lines in 2004 suggest that the apparent fertilizer N recovery was greater than 30% when stover was cut after grain harvest compared with less than 30% apparent N recovery when stover was left until a killing frost.

Fig. 7. Effect of fertilizer N rate and stover harvest time on total N removed in grain and stover in 2003 and 2004. Sorghum stover harvest times: Grain – stover harvested immediately after grain harvest; Frost – stover harvested after die-down following a killing frost. For regression equations: ** denotes significance at the 0.01 level of probability.

Conclusion

In the eastern Great Plains, sorghum grain yield will vary with year and, thus, response to fertilizer N rates. Yield responses were found up to 120 lb of N per acre, but splitting N applications did not improve yield. Nitrogen removed in grain responded linearly to increasing fertilizer N rates since N concentration appeared to dilute when yield increased under more favorable growing conditions. If sorghum stover is to be utilized as animal feed, it appears that harvesting the stalks immediately after grain harvest may be preferred. When sorghum did not stay green or regrow, dry matter losses were large if the stover was left until a killing frost, even with increased N fertilizer. Even small amounts of post-harvest growth until frost reduced dry matter losses. Increasing N rates increased N concentration, and thus crude protein level in the feed. Total apparent N recovery levels by grain and stover were constant, ranging from 25 to 35% of that applied. These observations, however, are based on one hybrid. Even though stay-green hybrids retain more photosynthetically active leaves under drought and take up more N (3), yield and N uptake response by sorghum to fertilizer N rates can vary (21). Future research should explore additional stay-green hybrids to determine whether greater growth and N removal can be obtained between grain harvest and a killing frost than obtained with the hybrid selected for this study.

Acknowledgement and Disclaimer

Contribution no. 07-62-J, Kansas Agricultural Experiment Station. Product names are included for the benefit of the reader and do not imply any endorsement or preferential treatment by Kansas State University.

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