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© 2005 Plant Management Network.
Accepted for publication 14 June 2005. Published 18 July 2005.

Grain Sorghum Response to Row Spacing, Plant Density, and Planter Skips

Shawn P. Conley, Assistant Professor, Department of Agronomy, Purdue University, Lafayette, IN 47907; W. Gene Stevens, Extension Associate Professor, Department of Agronomy, Delta Center Research Center, University of Missouri, Portageville 63873; and David D. Dunn, Soil Test Manager, Delta Center Research Center, University of Missouri, Portageville 63873

Corresponding author: Shawn P. Conley. conleysp@purdue.edu

Conley, S. P., Stevens, W. G., and Dunn, D. D. 2005. Grain sorghum response to row spacing, plant density, and planter skips. Online. Crop Management doi:10.1094/CM-2005-0718-01-RS.

Abstract

Adverse weather and soil conditions may prevent grain sorghum (Sorghum bicolor L. Moench) growers from achieving an evenly spaced, optimum plant density. In these situations, growers must decide whether to replant or manage a grain sorghum crop with uneven and/or lower stands. Research indicated that grain sorghum grain yield response to row spacing was variable and dependent upon environment. In uniform stands, grain sorghum was able to partially compensate for densities below 60,000 plants per acre by producing additional grain heads per plant. Results also indicated that in uniform grain sorghum stands, N at 100 to 150 lb/acre produced the highest grain yield under both high and low plant density conditions. In non-uniform stands with frequent 6- to 9-ft skips, sorghum had significantly reduced grain yield when compared to a uniform stand or 3-ft skips. These results do not support reduced N rates in either uniform or uneven grain sorghum stands with less than optimal plant densities.

Introduction

Stand establishment is often a concern with grain sorghum producers. Poor seedling vigor coupled with cool, wet soils may substantially decrease crop density. Though optimal plant densities for grain sorghum production differ among geographic regions, research indicates that grain yield generally increases as plant density increases (3,4,5,10,13). At sub-optimal plant densities grain sorghum head number per plant or seed number per head increased when compared to the recommended plant density (2,4,6,7,10,13). Larson and Vanderlip (8) suggested that grain sorghum’s ability to compensate for decreased plant density was related to plant space uniformity. Therefore, when assessing a grain sorghum stand, it is important to characterize both stand density and stand uniformity.

Crop row spacing may also impact crop yield potential (1,5,9,14). Staggenborg et al. (14) stated that crop row spacings < 30 inches will increase grain yield in high yield environments with little risk of reduced yield in low yield environments. Increased crop yield in narrow row spacings may be related to decreased soil water depletion or increased evapotranspiration efficiency (11,15,16). The objectives of these experiments were to quantify the affect of plant density, row spacing, planter skips, and N fertilizer rate on grain sorghum grain yield.

Field Procedures

Plant density experiments with grain sorghum were conducted in fields at Columbia (39°N, 92°W) and Portageville, Missouri (36°N, 90°W). Interactions between row spacing and plant density were studied at Columbia in 2002 and 2003. Interactions between nitrogen fertilization and plant density and in-row plant uniformity were evaluated at Portageville from 2001 to 2003. At all locations, plots were planted on the first week of June at high seeding rates (150,000 seeds per acre) and hand thinned when seedlings were 4 inches tall to achieve the desired plant density for treatments. Prior to planting, soils were tilled using a field cultivator. All experiments used a randomized complete block design with four replications. In each year, grain sorghum experiments followed soybean (Glycine max L. Merr.). Phosphorus and potassium were applied to the grain sorghum crop according to soil test recommendations provided by the University of Missouri Soil and Plant Testing Laboratory. Weed control was accomplished with a pre-emergence application of dimethenamid-P and atrazine.

Row spacing by plant density experiment. Field experiments were conducted at the University of Missouri Bradford Research and Extension Center located near Columbia, MO. The soil type was a Mexico silt loam (fine, smectic, mesic Aeric Vertic Epiaqualfs) with 2.5% organic matter and pH of 5.7. The experimental design was a randomized complete-block, 5-×-3 factorial with four replications. The main plot factors included plant density and crop row spacing. The plant densities in this experiment were 30,000, 60,000, 90,000, 120,000, and 150,000 plants per acre each in crop row spacings of 7.5, 15, and 30 inches. Sorghum Partners ‘Brand KS 735’ grain sorghum was planted in each year. Nitrogen (ammonium nitrate) was applied at 120 lb/acre to all plots. The experimental plot size was 10 × 30 ft.

Nitrogen fertilization and planter skips experiments. Two field experiments were conducted at the University of Missouri Lee Research Farm located near Portageville, Missouri to evaluate nitrogen response in uniform and uneven grain sorghum plant densities. Tests were conducted on a Tiptonville silt loam soil (fine-loamy, mixed, thermic Typic Argiudolls) with 1.7% organic matter and pH of 6.1 and a Sharkey clay soil (very fine, montmorillonitic, calcareous, thermic Vertic Haplaquepts) with 4.1% organic matter and pH of 5.7. Croplan Genetics 514 grain sorghum was planted.

In the uniform density test, plants were evenly thinned to low and high densities (35,000 and 105,000 plants per acre). This experiment was conducted on both the Tiptonville silt loam and Sharkey clay soils. Five early-season N treatments were applied to each 10 by 50 ft plot at N rates of 0, 50, 100, 150, and 200 lb/acre (ammonium nitrate) when grain sorghum was 4 inches tall.

The uneven plant density experiment was only conducted on the Tiptonville silt loam soil. Plots were first evenly thinned to 105,000 plants per acre. Plants were then removed by hand from each row to produce random 3-ft, 6-ft, or 9-ft long skips in intervals of 1, 2, and 3 times per 50 ft of row. Fertilizer was applied to each skip treatment at N rates of 45, 90, and 135 lb/acre (ammonium nitrate) following plant thinning. The experimental plot size was 10 × 50 ft.

Data collection and statistical analysis. Grain sorghum head number per plant, test weight, percent grain moisture, and grain yield were recorded at harvest at Columbia. Head number per plant was determined from 3 randomly selected plants per plot. Only grain yield was collected at the Portageville location. Grain weight was adjusted to 14.0% moisture. Precipitation and average monthly air temperatures in 2001, 2002, and 2003 are reported in Table 1.

Table 1. Precipitation and average monthly air temperatures in 2001, 2002, and 2003 at the University of Missouri Bradford Research and Extension Center located near Columbia, MO and the University of Missouri Lee Research Farm located near Portageville, MO.

 ^x Precipitation amounts were unavailable for the Portageville location in 2001, 2002, and 2003.

For each experiment, analysis of variance was performed using the PROC MIXED procedure of SAS (SAS Institute Inc., Cary, NC) (12). The Mixed Model procedure provides Type III F values but does not provide mean square values for each element within the analysis or the error terms. All effects except replication were considered fixed. Year was treated as a fixed effect to determine interactions involving year. Mean separation was evaluated through a series of pair-wise contrasts among all treatments. Main effects and all interactions were considered significant when P ≤ 0.05.

Row Spacing by Plant Density

Year by main effect and row spacing by plant density interactions were not significant for head number per plant or test weight (P ≥ 0.05); therefore data were pooled (Table 2). Head number per plant was greatest at 30,000 plants per acre (2.6 heads per plant) and nearly double the head number per plant of any other plant density. Averaged over years and row spacings crop test weight was greater at 90,000, 120,000, and 150,000 plants per acre when compared to 30,000 and 60,000 plants per acre.

Table 2. Grain sorghum percent grain moisture, head number per plant, and test weight response to plant density at the University of Missouri Bradford Research and Extension Center located near Columbia, MO. (2002-2003).

 ^x Means within a column and treatment group were considered significantly different at P ≤ 0.05.

 ^y Year by plant density interactions were significant (P ≤ 0.05); therefore data were reported separately for each year.

 ^z Year by main effect interactions were not significant (P ≥ 0.05); therefore data were pooled.

Year by plant density interactions were significant for percent grain moisture (P ≤ 0.01); therefore data were reported separately (Table 2). Percent grain moisture at harvest was higher at 30,000 plants per acre than at any other plant density in 2002. In 2003, percent moisture was greater at 30,000 plants per acre than at 90,000, 120,000, and 150,000 plants per acre. These results suggest that crop development and harvest may be delayed in decreased stand densities. Row spacing did not affect head number per plant or percent moisture.

Year by row spacing interactions were significant for grain yield (P ≤ 0.05); therefore data were reported separately by year (Table 3). Within year row spacing by plant density interactions were not significant (P ≥ 0.05). Grain sorghum grain yield was greater in the 7.5-inch row spacing than in the 15- and 30-inch row spacings in 2002. Grain yield was similar among row spacings in 2003. Grain yield was lowest at 30,000 plants per acre when compared to all other plant densities in 2002 and when compared to 60,000, 90,000, and 150,000 plants per acre in 2003. Yield was lower in 2003 than in 2002 due to lack of moisture in July and August (Table 1). Drought conditions may also have contributed to the lack of yield differences among row spacings in 2003. Staggenborg et al. (14) reported similar row spacing results in high versus low yield environments.

Table 3. Grain sorghum grain yield response to row spacing
and density at the University of Missouri Bradford Research
and Extension Center located near Columbia, MO. (2002-2003).

 ^x Year by row spacing interactions were significant (P ≤ 0.05);
therefore data were reported separately.

 ^y Means within a column and main effect were considered
significantly different at P ≤ 0.05.

Nitrogen Fertilization and Planter Skips

In the uniform plant density experiment there was a significant (P ≤ 0.05) soil by year by N interaction; therefore data were reported separately (Table 4). Averaged across plant density, grain sorghum response to N varied by soil and year, but maximum yields were usually achieved with N at 100 to 150 lb/acre (Table 5). The average grain yield based on plant density was 58.7 and 64.4 bu/acre at 35,000 and 105,000 plants per acre, respectively.

Table 4. Analysis of variance for grain sorghum response to

N fertilizer at plant densities of 35,000 and 105,000 plants per
acre at Portageville, Missouri on Tiptonville silt loam and
Sharkey clay soils in 2001 and 2002.

 ^x NS = not significant.

Table 5. Effect of N fertilizer rate on grain sorghum grain yield averaged across plant densities at Portageville, Missouri on Tiptonville silt loam and Sharkey clay soils in 2001 and 2002.

 ^x Means compared within a column, soil type, and year were considered significantly different at P ≤ 0.05.

In the uneven plant density experiment, no year by main effect interactions were detected (P ≥ 0.05); therefore data were pooled (Table 6). Averaged across skip number and skip length, grain yield was greater at 90 and 135 lb of N per acre than at 45 lb of N per acre (Table 7). Averaged across N fertilizer rate, grain yield was similar between the uniform stand and the 3-ft skip treatments (Table 8). Grain yield was decreased in the 6- and 9-ft skip treatments when crop stand loss resulted in ≤ 67,200 surviving plants per acre.

Table 6. Analysis of variance for grain sorghum response to
N fertilizer with different planter skip lengths and skip numbers
per row at Portageville, Missouri on Tiptonville silt loam in
2002 and 2003.

 ^x NS = not significant.

Table 7. Effect of three nitrogen fertilizer rates on grain
sorghum grain yield averaged across row skip numbers and
length on a Tiptonville silt loam soil at Portageville, Missouri
in 2002 and 2003.

 ^x Means within a column were considered significantly
different at P ≤ 0.05.

Table 8. Effect of skip number and length on grain sorghum grain yield averaged across N rates and years at Portageville, Missouri in 2002 and 2003.

 ^x Means within a column were considered significantly different at P ≤ 0.05.

Conclusions and Recommendations

Grain sorghum grain yield response to row spacing was variable and dependent upon environment. In a uniform stand, grain sorghum yield was not decreased until plant densities were reduced to < 60,000 plants per acre. Though grain yield was lower at 30,000 plants per acre the sorghum plant was able to partially compensate by developing > 1 additional head per plant. However, in most field settings grain sorghum plant densities are rarely uniform. In our uneven plant density experiment, frequent 6- to 9-ft skips decreased grain yield when compared to the 3-ft skips or uniform stand treatments. Plant densities of 67,200 plants per acre or lower decreased grain yield in the uneven stands. The N results indicated that 100 to 150 lb of N per acre was needed to maximize yields, regardless of plant population. Our results suggest that grain sorghum was somewhat flexible to stand density and stand uniformity and replanting may not be necessary in some situations.

Acknowledgments

This work was funded in part by the Missouri Fertilizer and Agricultural Lime Committee and the Missouri Agriculture Experiment Station.

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