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© 2006 Plant Management Network. Uniform Stand and Narrow Rows are Needed for Higher Double-crop Soybean Yield David L. Holshouser, Tidewater Agricultural Research & Extension Center, Virginia Polytechnic Institute and State University, Suffolk 23434; Robert D. Grisso, Jr., Biological Systems Engineering, Virginia Polytechnic Institute and State University, Blacksburg 24060; and Robert M. Pitman, Eastern Virginia Agricultural Research & Extension Center, Virginia Polytechnic Institute and State University, Warsaw 22572 Corresponding author: David L. Holshouser. dholshou@vt.edu Holshouser, D. L., Grisso, R. D., Jr., and Pitman, R. M. 2006. Uniform stand and narrow rows are needed for higher double-crop soybean yield. Online. Crop Management doi:10.1094/CM-2006-0417-01-RS. Abstract Double-crop soybean producers do not always realize a yield benefit when they convert from a 15-inch planter to a 7.5-inch grain drill. Poor seed singulation with drills was suspected to limit narrow-row yield response. Experiments were conducted to determine if a precision drill with 7.5-inch row spacing and accurate soybean seed singulation would improve stand uniformity and/or increase yield over a standard grain drill with poor seed singulation or a vacuum-meter planter with 15-inch row spacing and accurate soybean seed singulation in a double-cropped soybean system. The effects of planter speed on stand uniformity and yield were also investigated. Stand uniformity with the precision drill was equal to the vacuum-meter planter and better than the standard drill. Soybean yield with the standard drill was equal to the vacuum-meter planter in three of four years and less than the vacuum-meter planter in one year. Soybean yields were greater when planted with the precision drill than when planted with the vacuum-meter planter in 2 of 3 years, and averaged 10% over three years of study. Planting speeds of 5 or 7 mph did not affect stand uniformity or yield. The Implications of Stand Variability and Yield Narrowing row spacing from 30 or more inches to 20 or fewer inches consistently increases double-crop soybean yield (3,4,6,7). Yield benefits of narrowing row spacing to less than 15 to 20 inches are less clear. Comparing 10- versus 20-inch row spacing in Louisiana, Boquet et al. (5) measured a 11 and 5% increase in yield with the 10-inch rows when soybean were planted on June 15 and July 1, respectively. Averaged over 56 North Carolina locations, 10-inch row spacing yielded 5% greater than 20-inch row spacing (7). Soybean planted in 7.5-inch rows yielded greater than in 15-inch rows in three of five Maryland sites (13). In contrast, soybean grown in 7.5-inch rows did not yield greater than 19-inch rows when planted in June or July in an Arkansas study (2). Double-crop soybean growers in Virginia have commented that they receive little to no yield benefit from narrowing rows below that which can be obtained with a narrow-row (15 to 24 inches) planter (D. L. Holshouser, personal communication, 2001). Some of the discrepancy between the Arkansas data or Virginia grower experience and other research may be in the type of planter used. When yield increases occurred, either the same planter was used for both row spacings or plots were hand-planted to insure uniformity between plots. In the Arkansas study, soybean in 7.5-inch rows was planted with a grain drill and a rotary-plate planter was used for the 19-inch row spacing. Likewise, growers use grain drills when narrowing row spacing to 7.5 to 10 inches. Vacuum-meter, finger-pickup, and rotary-plate planters will meter, or singulate, seed well, while seed singulation with grain drills that use fluted wheels is usually not as good. Gaps created by poor seed singulation with drills may be responsible for the lack of yield response. This implies that uniform placement within a row is important in realizing higher soybean yield with 7.5- to 10-inch row spacing. Most research investigating stand uniformity has been conducted in corn. Crowded plants and gaps increased corn spacing variability, but gaps reduced yield most (12,14,16,17). Liu et al. (15) determined that planters with lower plant spacing variability and more uniform emergence achieved the highest yield. Those authors also found that planting speed affected plant spacing variation with the finger pickup and air seeder more than the vacuum meter planter. Nevertheless, with the exception of the air seeder under no-till conditions, planting speed had no effect on corn yield. With 22 on-farm trials, Nielsen (19) evaluated planter speeds of 4, 5, 6, and 7 mph on corn plant population and yield. Plant spacing variability increased with planting speed in nine of those trials and grain yield decreased in only five of those trials. At the yield responsive sites, yield decreased from 1.9 to 4.7 bu/acre per mph increase in planting speed above 4 mph. Few researchers have investigated within-row spacing of soybean or soybean plant population variation with grain drills. Naeve et al. (18) found no yield differences between uniform distribution, random distribution, 25% aggregation, and 50% aggregation of soybean plants. Soybean was planted in May using 30-inch row spacing; therefore that study may not relate to narrow-row double-crop systems. In another study, a grain drill equipped with belt metering system produced more consistent distribution of soybean seed and numerical reduction in plant stand variability compared to a conventional fluted wheel drill, but yields with the two metering systems did not differ (8). In 2000, Great Plains Manufacturing Inc. (Salina, KS) placed a metering wheel close to the disk opener on their grain drill. Until that year, most drills continued to use fluted wheels to meter seed. This new technology offered an opportunity to investigate soybean plant population variability and the consequent effect on yield. Our hypothesis was that, in a double cropping system, soybean grown in 7.5-inch row spacing would yield higher than soybean grown 15-inch row spacing if seed were distributed more uniformly within the row. In addition, we hypothesized that faster planting speed would decrease stand uniformity, resulting in lower soybean yields. Therefore our objectives were to: (i) determine if a grain drill (7.5-inch row spacing) with accurate soybean seed singulation would improve stand uniformity and/or increase yield over a grain drill with poor seed singulation or a planter (15-inch row spacing) with accurate soybean seed singulation in a double-cropped soybean system; and (ii) determine whether increased planter speed decreased stand uniformity or yield using the equipment described. Field Study Field experiments were conducted in 2001 through 2004 on soybean producer farms in Richmond County, Virginia. Soil type in 2001 through 2003 was a Kempsville loam (fine-loamy, siliceous, thermic Typic Hapludult). Soil type in 2004 was a Suffolk fine sandy loam (fine-loamy, siliceous, thermic Typic Hapludult). Experimental design was a randomized complete block in a strip plot arrangement with planter type being the vertical factor and planting speed as the horizontal factor. There were four replicates of each treatment. Three planters were chosen to represent the planter technologies available to the typical double-crop no-till soybean producer. For the purpose of description, the three planters are referred to as (i) vacuum-meter planter, (ii) standard drill, and (iii) precision drill (Table 1). The Precision Seeding System developed by Great Plains (Salina, KS) is new technology that is designed to provide accurate seed singulation and placement at 7.5- row spacing (Fig. 1). Table 1. Type, make and model, seed singulation method, and row spacing of seeding equipment used in this study.
* 1520P used in 2001 and 2004; 1510P used in 2002.
Plot width was 30 ft wide for the vacuum meter planter (one pass), 24 ft wide for the standard drill (two passes) and 30 ft wide for the precision drill (two passes). All plots were 450 ft long. Soybean was planted no-till into wheat residue on 3 July 2001, 1 July 2002, 1 July 2003, and 15 June 2004. Soybean was planted at speeds of 5 or 7 mph to represent low and high speeds used by farmers in Virginia. The soybean cultivar used in 2001 was UniSouth Genetics (Nashville, TN) brand USG 7528RR. Delta and Pine Land Company (Scott, MS) brand DP4690RR was used in 2002 through 2004. The vacuum meter planter and precision drill were calibrated to deliver 195 to 205 thousand seeds per acre. This seeding rate is recommended by Virginia Cooperative Extension and is a common seeding rate used for double-crop soybean planting. The standard drill was calibrated to deliver 220 to 240 thousand seeds per acre. We used the higher seeding rate because Virginia farmers commonly increase the seeding rate with standard drills by 10 to 15% in an attempt to compensate for gaps created by drills. We assumed that gaps would no longer be a problem with the precision drill; therefore seeding rate for that equipment was equivalent to the planter. Although different seeding rates combined with different planter systems confound the yield outcome, the seed rates chosen best reflect producer practices and the resulting outcomes directly address farmer practices. The precision drill was unavailable for our use in 2003; therefore only the vacuum meter planter and the standard drill were tested. At 2 to 3 weeks after emergence, soybean stands were randomly counted at 16 locations within each plot using 34-inch diameter hula hoops. For consistency, two 15-inch rows were carefully centered in the hoop. With the 7.5-inch rows, five rows were counted and the middle row was centered in the hoop. The number of plants contained within each hula hoop was converted to plants per acre. Plant population variability was determined by calculating the standard deviation (SD) within each plot. Glyphosate herbicide was applied two to three weeks after planting for weed control. Plots were harvested 31 October 2001, 2 December 2002, 3 November 2003, and 10 November 2004. Either three 15-inch rows or seven 7.5-inch rows running the entire length of the plot were harvested with a small plot combine equipped with a scale and moisture meter. Data were subjected to analysis of variance using the PROC MIXED procedure of the SAS software package, version 9.1 (SAS Institute Inc., Cary, NC). Planter and planting speed treatments were considered fixed effects, and the years and blocks were treated as random effects. Experimental treatments were considered significant if P ≤ 0.05. Treatments means were separated using Fisher’s protected LSD comparisons. Stand Uniformity and Yield Results varied between years, therefore data were not combined. Stand uniformity, as measured by SD of plant population, with the precision drill was better than the standard drill and equal to the vacuum planter (Table 2). The SD averaged 28, 49, and 30 thousand plants per acre for the vacuum meter planter, standard drill, and precision drill, respectively. In general, we observed few skips within a row with the vacuum meter planter or precision drill; however, skips within a row were very noticeable in the standard drill plots. Table 2. Planting equipment effects on seeding and plant population and seed yield, 2001-2004.
x Plant population, standard deviation, and seed yields within a year and in the same column followed by the same letter are not significantly different (P > 0.05). y SD = standard deviation. Soybean yields were similar when planted with the standard drill in 7.5-inch rows and vacuum meter planter in 15-inch rows in 3 of 4 years and less for the standard drill in 2004 (Table 2). The reason for the standard drill yielding less in 2004 may be due a mistake in seed calibration and very good emergence, which subsequently led to significantly higher populations in the standard drill plots. In addition, plant growth was very good in 2004 due to an earlier planting date and timely rainfall events, resulting in relatively tall plants. The high populations and good growth did increase lodging in some parts of the plot, and may have been the underlying cause of the lower yield. Regardless of year differences, these results are in contrast with past research showing yield advantages with 7- to 10-inch rows versus 15- to 24-inch rows (5,6,7,13). Those researchers used the same planter for all treatments or insured that the small plots contained no in-row skips. Our results are similar to findings by Beatty et al. (2) that showed no significant yield differences between 7.5-inch row width soybean planted with a standard grain drill and 19-inch row width soybean planted with a planter. Our data also verify observations from Virginia farmers that there is no yield advantage with narrow-row drills in double-crop soybean when less precise grain drills are used. On the other hand, when the precision drill was used to plant soybean, yields were greater than the vacuum meter planter in 2 of 3 years (Table 2). These data verify previous double-crop small-plot research showing yield advantages to reducing row spacing to that of the typical drill spacing (5,6,7,13). These data also support our hypothesis that 7.5-inch row spacing may yield higher than 15-inch row spacing if seed are distributed more uniformly within the row. Yield advantages with the precision drill were observed in 2001 and 2002, but not in 2004. Emergence of the seed in 2001, weather patterns in 2002 and planting date in 2004 all may have played a role in the effects of the precision drill. In 2001, the precision drill yielded 11% higher than the planter. Emergence averaged across all planters was only 59% due to dry weather after planting (Table 2); therefore, plant populations were lower than that recommended for double-crop planting. The relatively low plant populations achieved in this experiment might have allowed for greater response to row spacing. In row spacing studies that included plant population as a factor, plant population did not affect soybean yield response to row spacing (11,13). In our experiment, plant populations from precision drill and vacuum planter were not significantly different, therefore only row spacing influenced the findings. Under drought conditions prevalent in 2002, soybeans seeded with the precision drill yielded 25% higher than the planter. The greater difference in yield in a drought year is likely due to limited vegetative growth as a result of early season moisture stress. The soybean canopy did not close until the late pod to early seed fill development stages. In contrast, the soybean canopy closed in all treatments by full flower in the other three years. These data compare well to other research that suggest yield increases are greater under the low leaf-area conditions caused by early season or intermittent drought stress (1,9,11,13). The higher emergence rate (Table 2) for the precision drill resulted in a significantly higher plant population than the planter; therefore, higher plant population might have contributed to the yield differences. Some studies suggest that plant population responses are more common during periods of intermediate drought stress (1,11,13) while others indicate the opposite (10,20). Regardless, row spacing likely contributed more since the plant population difference was only 23,000 plants per acre. Past research indicates that this level of increase in plant population would not have solely accounted for a 25% increase in yield (1,11,13). In 2004, yields from the precision drill and vacuum meter were not significantly different. A distinction between this year and others was planting date. The planting date was June 15, which is a week to 10 days earlier than most double-crop planting in Virginia. Planting dates for other years were 3 July and 1 July in 2001 and 2002, respectively. Some research indicates yield benefits from narrowing row spacing are less with earlier planting dates (3,4), but other research does not (2,5). In addition to an earlier planting date, the crop experienced little stress through the growing season. Under less stressful conditions that allow high leaf area and light interception levels to develop early, there is less yield advantage for narrowing row spacing (9,11). In our experiment, the canopy closed in all plots before first flower, suggesting that minimal leaf area and light interception requirements were met early. Therefore, earlier planting date and less plant stress may have contributed to the lack of difference in yield between the precision drill and vacuum planter. Averaged over all three years, the precision drill resulted in a yield 10% higher than the vacuum planter. Although stands were more uniform, soybean planted with the precision drill did not yield higher than soybean planted with the standard drill (Table 2). This suggests that uniformly placing seed in 7.5-inch rows does not by itself result in significantly higher yields. Our data does suggest that drill-seeded soybean will yield higher than soybean seeded with planters only if seed are more uniformly distributed within the row and in-row gaps are minimized. Influence of Planting Speed We found no significant difference between the 5 mph and 7 mph planting speed in regard to variation in plant population or yield (data not shown), regardless of the seeding equipment used. These results differ somewhat from studies comparing corn planters, where yields were lower if planters were not capable of good seed singulation (16,19). In contrast to the corn studies, poor soybean seed singulation did not affect the response to planter speed in our experiments. Discussion and Conclusions This research supported the hypothesis that soybean planted in 7.5-inch rows would yield higher than soybean planted in 15-inch rows if seed are distributed more uniformly within the row. The precision drill yielded higher than the vacuum meter planter in 2 of 3 years and equal to the planter in the third year. Although the precision drill yielded numerically higher than the standard drill, these differences were not statistically significant. This indicates that a combination of uniform seed placement plus narrow rows is needed to optimize yield in double-crop soybean systems. Uniformity of plant spacing in the row is evidently the reason for contrast between row spacing research and farmer experience. This research verified farmers’ experience that planting soybean in 7.5 inch rows with a standard drill does not yield better than with a 15-inch vacuum-meter planter. The hypothesis that faster planting speed would decrease stand uniformity, and therefore result in lower soybean yields, was not supported. Evidently, newer equipment that is set properly can cut residue, place the seed at the proper depth, and obtain good soil-to-seed contact at the speed tested in these experiments. Negative effects of planting speed can occur; therefore Nielsen’s (19) words are echoed: "Because the potential for yield loss exists, growers concerned about the effects of excessive planting speeds should determine their own planter’s response to planting speeds with on-farm trials." This research was conducted to verify farmers’ experience and to test new equipment under actual farm conditions. The information gained is very practical and can be used immediately with confidence by the double-crop farmer. More research is needed to evaluate the variation in actual distance between plants and the influence of plant-to-plant spacing on soybean yield. Small-plot research with more detailed data collection on stand uniformity would be valuable. For instance, variation in the actual distance between plants could be determined and related to yield. The influence of soil type on the response of soybean to uniform stands and row spacing needs investigating. More productive soils may respond differently than less productive soils. Understanding environmental influences, especially soil moisture, would enhance the knowledge and allow educators to make more site-specific recommendations. Finally, plant population × row spacing × planting date research is needed to better understand how plant population and planting date interact with row spacing. Acknowledgments The authors appreciate the assistance from Lloyd Mundie, Bobbie Yeatman, Mike Yeatman, Rob Hall, and John Fleet for the use of land, equipment, and/or labor, and for supplemental funding provided by the Virginia Soybean Board. Literature Cited 1. Ball, R. A., Purcell, L. C., and Vories, E. D. 2000. Optimizing soybean plant population for a short-season production system in the southern USA. Crop Sci. 40:757-764. 2. Beatty, K. D., Eldridge, I. L., and Simpson, A. M., Jr. 1982. Soybean response to different planting patterns and dates. Agron. J. 74:859-862. 3. Boerman, H. R., and Ashley, D. A. 1982. Irrigation, row spacing, and genotype effects on late and ultra-late planted soybean. Agron. J. 74:995-999. 4. Boquet, D. J. 1990. Plant population density and row spacing effects on soybean at post-optimal planting dates. Agron. J. 82:59-64. 5. Boquet, D. J., Koonce, K. L., and Walker, D. M. 1982. Selected determinate soybean cultivar yield responses to row spacings and planting dates. Agron. J. 74:136-138. 6. Coale, F. J., and Grove, J. H. 1990. Root distribution and shoot development in no-till full-season and double-crop soybean. Agron. J. 82:606-612. 7. Dunphy, E. J. 1988. 1987 soybean on-farm test report. Ext. Circ. AG-53. North Carolina Agric. Ext. Service, Raleigh. 8. Ess, D. R., Hawkins, S. E., Young, J. C., and Christmas, E. P. 2005. Evaluation of the performance of a belt metering system for soybeans planted with a grain drill. Appl. Engin. Agric. 21:965-969. 9. Heatherly, L. G., Elmore, C. D., and Spurlock, S. R. 2001. Row width and weed management systems for conventional soybean plantings in the midsouthern USA. Agron. J. 93:1210-1220. 10. Heitholt, J. J., Farr, J. B., and Eason, R. 2005. Planting configuration × cultivar effects on soybean production in low-yield environments. Crop Sci. 45:1800-1808. 11. Holshouser, D. L., and Whittaker, J. P. 2002. Plant population and row-spacing effects on early soybean production systems in the Mid-Atlantic USA. Agron. J. 94:603-611. 12. Johnson, R. R., and Mulvaney, D. L. 1980. Development of a model for use in maize replant decisions. Agron. J. 72:459-464. 13. Kratochvil, R. J., Pearce, J. T., and Harrison, M. R., Jr. 2004. Row-spacing and seeding rate effects on glyphosate-resistant soybean for Mid-Atlantic production systems. Agron. J. 96:1029-1038. 14. Lauer, J. G., and Rankin, M. 2004. Corn response to within row plant spacing variation. Agron. J. 96:1464-1468. 15. Liu, W., Tollenaar, M., Stewart, G., and Deen, W. 2004. Within-row plant spacing variability does not affect corn yield. Agron. J. 96:275-280. 16. Liu, W., Tollenaar, M., Stewart, G., and Deen, W. 2004. Impact of planter type, planting speed, and tillage on stand uniformity and yield of corn. Agron. J. 96:1668-1672. 17. Nafziger, E. D. 1996. Effects of missing and two-plant hills on corn grain yield. J. Prod. Agric. 9:238-240. 18. Naeve, S. L., Kurle, J. E., and Potter, B. D. 2001. Soybean planting patterns: Effects on microclimate, sclerotinia stem rot incidence and yield. 2001 Annual Meeting abstracts. ASA-CSA-SSSA, Madison, WI. 19. Nielsen, R. L. 1995. Planting speed effects on stand establishment and grain yield of corn. J. Prod. Agric. 8:391-393. 10. Taylor, H. M. 1980. Soybean growth and yield as affected by row spacing and by seasonal water supply. Agron. J. 72:543-547. |
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