Agronomy Journal 94:1282-1288 (2002)
© 2002 American Society of Agronomy
PRODUCTION PAPER
Reduced Seeding Rate for Glyphosate-Resistant, Drilled Soybean on the Southeastern Coastal Plain
Jason K. Norsworthy*,a and
James R. Frederickb
a Dep. of Crop and Soil Environ. Sci., Clemson Univ., Edisto Res. and Educ. Cent., 64 Research Rd., Blackville, SC 29817
b Dep. of Crop and Soil Environ. Sci., Clemson Univ., Pee Dee Res. and Educ. Cent., Florence, SC 29506
* Corresponding author (jnorswo{at}clemson.edu)
Received for publication March 27, 2002.
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ABSTRACT
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Seeding-rate recommendations for narrow-row (<76 cm) soybean [Glycine max (L.) Merr.] on the southeastern Coastal Plain are almost double those of more traditional, wider row widths. A 2-yr field study was conducted on a Dothan loamy sand soil to assess how main-stem and branch yield fractions of four glyphosate-resistant cultivars, ranging from maturity group (MG) V through mid-MG VII, would respond to a lower-than-recommended seeding rate for narrow-row culture. The cultivars Pioneer 95B32 (early MG V), Hartz 6255 (early MG VI), Delta and Pine Land 6880 (late MG VI), and Hartz 7550 (mid-MG VII) were seeded at 370 000 seeds ha-1 and at the recommended rate of 620 000 seeds ha-1. Total seed yield of all cultivars was usually similar for the two seeding rates. Less seed yield from the main-stem fraction with the lower seeding rate was usually compensated for by a higher seed yield from the branch fraction, except when insufficient rainfall occurred during critical periods of the growing season. Branch seed yield was more closely correlated with total seed yield at the low seeding rate (r = 0.675) than at the recommended seeding rate (r = 0.453) while the correlation between total seed yield and seed yield from the main-stem fraction was similar for the two seeding rates (r = 0.579–0.613). In the absence of prolonged periods of drought stress, seeding rates for glyphosate-resistant soybean grown in full-season, narrow-row systems can be reduced below current recommendations, thereby lowering seeding costs without decreasing seed yields.
Abbreviations: MG, maturity group
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INTRODUCTION
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IN RECENT YEARS, growers in the Southeast have become interested in planting soybean using narrow row widths because of the potentially higher yields and better weed control that can be achieved (Boerma and Ashley, 1982; Frederick et al., 1998; Norsworthy, 2002). Additional advantages of narrow row widths are higher soybean pod placement, less moisture loss via soil evaporation, and less soil erosion (Palmer and Privette, 1992). Other production practices, such as planting herbicide-resistant cultivars, can compliment the weed-control benefits of narrow row widths.
Glyphosate [N-(phosphonomethyl)-glycine]-resistant technology has been widely accepted by soybean producers since its release in 1996, with the technology utilized on 68% of the U.S. soybean hectarage in 2001 (USDA-NASS, 2001). Adoption of this technology has occurred even faster in the southeastern USA, with 68% of South Carolina soybean producers employing the technology in 1999 (Norsworthy, 2002). Glyphosate, the broad-spectrum herbicide labeled for application over the top of glyphosate-resistant soybean, lacks residual weed control. Reduced weed interference (Stoller et al., 1987), a shortened weed-free requirement (Murdock et al., 1986), rapid canopy closure (Yelverton and Coble, 1991), greater herbicide effectiveness (Mickelson and Renner, 1997), and suppression of late-emerging weeds (Mickelson and Renner, 1997; Yelverton and Coble, 1991) are ideal characteristics of a narrow-row system that centers on a nonresidual herbicide, such as glyphosate. Although effective in terms of weed control, the relatively high costs associated with planting glyphosate-resistant soybean cultivars at currently recommended seeding rates may be a deterrent to their use in narrow-row systems (Norsworthy, 2002).
Since a technology fee is assessed on the purchase of glyphosate-resistant soybean, gross profit margins for drill-seeded soybean in the Midsouth are optimized at seeding rates lower than those recommended for conventional soybean (Norsworthy and Oliver, 2001). Recommended plant populations in South Carolina, when using row widths of 19 cm, are twice those of soybean seeded in row widths greater than 75 cm (Palmer, 1999). This seeding rate for narrow-row systems is recommended based on unpublished field trials conducted throughout the South (J. Palmer, personal communication, 2000). When planting with narrow row widths, producers who choose to drill-seed glyphosate-resistant soybean pay on average $22.24 ha-1 in technology fees alone (Norsworthy, 2002). The higher seed costs associated with producing transgenic cultivars under narrow-row culture makes seeding-rate recommendations an important determinant of whether farmers will adopt this practice. Therefore, more information is needed as to the accuracy of the seeding rates recommended for narrow-row soybean production on the sandy Coastal Plain.
Soil water availability throughout the soybean growing season directly influences the response of seed yields to seeding rates (Devlin et al., 1995). Therefore, recommended seeding rates may vary among regions of the country because of inherent differences in soil type. Soybean grown on the southeastern Coastal Plain routinely experience yield-reducing drought because of the coarse texture of the Ap soil horizons (Frederick et al., 1998). In this region, soil water depletion occurs more rapidly in narrow than in wide row widths, likely because of earlier canopy closer and greater water utilization by the crop with narrow row widths (Frederick et al., 1998). In drought-stressed environments, seed yields at low seeding rates are generally higher or equivalent to those at dense populations (Alessi and Power, 1982; Devlin et al., 1995; Elmore, 1998; Taylor, 1980). Thus, seeding rates need to be based on anticipated soil water and seed yield goals (Devlin et al., 1995).
Maturity groups V and VI soybean are commonly planted in South Carolina beginning in May while later-maturing cultivars are seeded usually in June after small-grain harvest (Palmer, 1999). When seeding early maturing cultivars at later planting dates, the length of vegetative development is shortened, limiting seed yields (Egli and Bruening, 2000). Except for early planting dates, planting an early-season soybean at lower-than-recommended seeding rates would likely restrict the amount of vegetative growth and, therefore, light interception before seed fill. Because of the limited branching ability of early maturing cultivars, it may not be feasible to reduce seeding rates without sacrificing seed yields at most planting dates (Akhter and Sneller, 1996). Conversely, because of the extended vegetative period of later-maturing cultivars, it may be possible to reduce seeding rates without limiting seed yields. Because seed yields are closely tied to seed number per square meter (Ball et al., 2000; Frederick et al., 1998, 2001), increased seed numbers on main stems and/or branches are essential for high yield at low plant densities. While main-stem seed yields of drill-seeded soybean are thought to be stable over environments, branch yields are highly contingent on the severity of drought stress that occurs during the growing season (Frederick et al., 2001). To maintain high seed yields at low populations, the plant must augment main-stem seed yields and/or branch seed yields to offset reductions in plant number.
Objectives of this research were to evaluate (i) the seed yield potential of four commonly grown glyphosate-resistant MG V through VII cultivars seeded at the currently recommended rate and a lower-than-recommended seeding rate for drilled soybean and (ii) the distribution of seed yield and yield components between main stems and branches to determine how soybean may compensate for differences in plant populations.
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MATERIALS AND METHODS
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Site Description and Cultural Practices
The determinate glyphosate-resistant cultivars Pioneer 95B32 (early MG V), Hartz 6255 (early MG VI), Delta and Pine Land 6880 (late MG VI), and Hartz 7550 (mid-MG VII) were seeded in 19-cm-wide rows at 370 000 and 620 000 seeds ha-1 using a small-plot cone drill (Almaco, Nevada, IA). The higher seeding rate is recommended to South Carolina soybean farmers for narrow row-width production (Palmer, 1999). The cultivars used in this study were selected based on their high seed yields in Clemson University's variety tests (http://cropweb.clemson.edu/Soybean/bean.htm; verified 1 Aug. 2002). The experiments were seeded on 27 May 2000 and 21 May 2001 at the Edisto Research and Education Center near Blackville, SC. Plant populations within 3 m of row were determined at three random locations within each plot at 2 wk after emergence. Individual plots were 1.8 m wide and 30 m long in 2000 and 1.8 m wide and 11 m long in 2001. Daily rainfall was recorded using an automated weather station located near the test site (Fig. 1)
. The test was conducted on the same experimental site each year under nonirrigated conditions on a Dothan loamy sand soil (fine-loamy, siliceous, thermic Plinthic Paleudult).
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Fig. 1. Rainfall pattern for the 2000 and 2001 growing seasons. Soybean was seeded on 27 May 2000 and 21 May 2001. Refer to Table 2 for specific reproductive dates of each of the four evaluated cultivars.
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Before crop establishment, the study site was disked twice and broadcast deep-tilled to a 40-cm depth using a 10-shank Terra Max II (Worksaver, Litchfield, IL). Fertilizer and lime were annually broadcast-applied based on Clemson University soil test recommendations. All plots were sprayed for weed control with 0.84 kg a.e. ha-1 glyphosate plus 0.0008 kg a.i. ha-1 chloransulam-methyl {3-chloro-2-[[[5-ethoxy-7-fluoro(1,2,4)triazolo(1,5-c)pyrimidin-2yl]sulfonyl]amino]benzoic acid} 3 wk after crop emergence. Uncontrolled and later-emerging weeds were hand-removed during the growing season.
Aboveground biomass was harvested 2 cm above the soil surface from 3 m of row either weekly or biweekly beginning 1 wk after emergence and continuing through 81 and 66 d after emergence in 2000 and 2001, respectively. All harvested sample material was immediately oven-dried at 55°C for a minimum of 2 wk and weighed.
Soybean reproductive growth stages (Fehr and Caviness, 1977) were recorded through R5 on five randomly selected plants per plot (Table 1). At R8 (full maturity), five randomly selected plants per plot were harvested and oven-dried, after which total shoot weight and seed yield from the main-stem and branch fractions determined. One hundred random seeds were counted, dried, and weighed for each main-stem and branch seed sample. Plots were trimmed to 8.5 m2 and harvested with a small-plot combine, and seed yields were expressed as 130 g kg-1 seed moisture. Seed yield data from the main-stem and branch fractions were summed to give total seed yield. Main-stem yields per plant were divided by total seed yield per plant and multiplied by whole-plot yield (g m-2), giving a main-stem yield per unit area. A similar method was used to calculate branch yields per unit area. Total, main-stem, and branch seed numbers per unit area were estimated by dividing the corresponding seed yields (g m-2) by individual seed weights (g seed-1).
Treatments consisted of a factorial combination of four cultivars and two seeding rates in a randomized block design with four replications. Because of significant differences in rainfall distribution between years, data were not pooled over years. An analysis of variance on the main effects and interaction of the independent variables cultivar and seeding rate was performed using a general linear model (SAS Inst., 1989). Means were compared using Fisher's Protected LSD test at P 
0.05 and correlation between yield and yield component variables assessed using the Pearson correlation statistic from PROC CORR in SAS.
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RESULTS AND DISCUSSION
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Crop Development and Rainfall Conditions
Rainfall distribution between and within years can explain many of the seed yield differences found among cultivars. From May through June, 14 cm of rainfall occurred in 2000 while twice that amount fell over the same period in 2001 (Fig. 1). Soil moisture was adequate at planting, resulting in 50% emergence of all cultivars by 6 d after planting in both years. However, it was another 23 d until the next rainfall >1 cm occurred (Fig. 1); hence, early-season soybean vegetative growth was less in 2000 than in 2001 (Fig. 2)
. Subsequent rainfall before growth stage R1 (initial flowering) of all cultivars caused a rapid increase in aboveground biomass in 2000. In 2001, sufficient rainfall occurred during June and July (Fig. 1), months in which all cultivars exhibited linear vegetative growth (Fig. 2). Rainfall that occurred in early September of 2001 coincided with seed fill of the early MG V cultivar, whereas dry conditions that occurred from mid-September throughout October coincided with seed fill of the later-maturing cultivars (Table 2).
Seeding-Rate Effects
Soybean densities were similar in each year for each seeding rate and ranged from 72 to 85% of the seeding rate (Table 1). Total seed yields per unit area were statistically similar for the two seeding rates although average seed yields were 8% less at the lower seeding rate over both years (Table 3). Seed yields from the main-stem fraction were greatest at the recommended seeding rate in both years because of the greater density of main stems at that seeding rate. Decreasing the seeding rate reduced seed yields from the main-stem fraction an average of only 23% over years, whereas plant density was 40% less at the lower seeding rate, indicating that the main stems compensated for the lower-than-recommended seeding rates by producing more seed yield. In 2000, a 42% increase in seed yield from the branch fraction at the low seeding rate offset the reduction in seed yield from the main-stem fraction, resulting in similar total seed yields for the two seeding rates (Table 3). This increase in seed yield from the branch fraction was a result of a 47% increase in branch-fraction seed number (Table 3). Seed yield from the branch fraction was similar for the two seeding rates in 2001, probably due to the favorable rainfall amounts that occurred from June through mid-August, the period when most soybean branch growth occurs in this region (Frederick et al., 2001). Over both years, seed yield of the branch fraction was more closely correlated with total seed yield at the low (r = 0.675; P < 0.01) than at the recommended (r = 0.453; P < 0.01) seeding rate, whereas the correlation between seed yield from the main-stem fraction and total seed yield was similar for the two seeding rates (r = 0.579–0.613; P < 0.01).
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Table 3. Effect of seeding rates on soybean total, main-stem, and branch yield and yield components averaged over four cultivars.
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Total (whole plant) seed number per square meter was highly correlated with seed yield per unit area at the low and high seeding rates (r = 0.956 and 0.829, respectively; P < 0.01) (Table 4). Average seed number from the main-stem fraction at the low seeding rate was 21% less than at the recommended seeding rate. However, seed number from the branches at the low seeding rate compensated for the reduced main-stem seed number, resulting in similar total seed number per square meter for the two seeding rates. Total seed yield per unit area at the low seeding rate was correlated with seed yield of the main stem (r = 0.613; P < 0.01) and branches (r = 0.675; P < 0.01). Conversely, at the recommended seeding rate, main-stem seed yields were more correlated with total seed yields (r = 0.579; P < 0.01) than were branch seed yields (r = 0.453; P < 0.01) (Table 4). Furthermore, individual seed weight for seeds from the main stem, branches, or whole plant did not differ between seeding rates in either year (Table 3) nor was it closely associated with seed yield per unit area (Table 4). Therefore, total, stem, and branch seed yield differences between seeding rates were primarily due to differences in seed number per unit area.
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Table 4. Pearson correlation coefficients (r) for total, stem, and branch yield and yield components for 370 000 and 620 000 seeds ha-1, over cultivars and years. Observed significance level is in parentheses, and n = 31 for both seeding rates.
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Cultivar Effects
Differences in total seed yield per unit area among cultivars closely followed differences in seed yield from the branch fraction in both years (Table 5). In the drier year of 2000, the contribution of branch seed yield to total seed yield per unit area was 31 to 35% for the two lowest-yielding cultivars (early MG V and mid-MG VII), whereas 40 to 45% of total seed yield was produced on the branches for early MG VI and late MG VI, the two highest-yielding cultivars (Table 5). Similar contributions of the branch-fraction seed yield to total seed yield have previously been reported for determinate soybean produced in a dryland environment (Frederick et al., 2001). Although rainfall amounts throughout vegetative and reproductive development in 2000 differed among cultivars (due to differences in stages of development), seed yields from the main-stem fraction were less variable than those from the branch fractions, and they provided the greatest contribution to total yields. Seed yields of the main stems accounted for 56 to 69% of the total seed yield for the cultivars studied. In other research conducted on the southeastern Coastal Plains, main-stem seed yields of narrow-row, determinate soybean were stable over irrigated and nonirrigated environments, with main-stem seed yields contributing more to total yield in moisture-limited environments and branch seed yields contributing more with irrigation (Frederick et al., 2001).
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Table 5. Effect of cultivar on soybean total, main-stem, and branch seed yield and yield components averaged over seeding rates.
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The early MG V cultivar had the greatest numerical yield of 292 g m-2 in 2001 while the mid-MG VII cultivar produced the lowest seed yield, presumably because of the lack of late-season rainfall from mid-September throughout October in that year. The numerical yield ranking of cultivars was consistent with their relative maturity, which is likely a result of favorable early-season rainfall and the occurrence of late-season moisture stress. The contribution of main-stem seed yield to total seed yield in 2001 was less among cultivars than in the previous year, ranging from 45 to 56% for the mid-MG VII and late MG VI cultivars, respectively (Table 5). This response was probably due to the more favorable rainfall amounts and distribution that occurred during the 2001 growing season.
Cultivar x Seeding Rate Interaction Effects
Total seed yield per unit area differed between seeding rates only for the early MG V cultivar in 2000 and the late MG VII cultivar in 2001 (Table 6). Lower seed yields of the early MG V cultivar in 2000 were probably due to the lack of rainfall following planting, whereas drought stress during the later stages of reproductive development may have had a deleterious effect on the mid-MG VII cultivar in 2001. In both of these cases, seed yields from the main-stem fraction were less with the lower seeding rate than with the recommended seeding rate, and seed yields of the branch fraction failed to compensate for this reduction, probably because of the limited rainfall. In 2000, seed yields from the main-stem fraction at the low seeding rate were less than those of the recommended seeding rate for the early and late MG VI cultivars. However, total seed yields were not reduced because plants compensated for the reductions in seed yield on the main stems by increasing seed yield on the branches. For the early MG V and early and late MG VI cultivars in 2001, neither total, main-stem, nor branch seed yields varied between seeding rates, indicating that both the main stems and branches compensated for the fewer plant numbers at the low seeding rate by producing a higher seed yield.
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Table 6. The effect of cultivar and seeding rate on total, main-stem, and branch seed yields per unit area in 2000 and 2001.
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The late MG VI cultivar compensated the most for the lower plant numbers at the low seeding rate, as evident by the 72% average increase in total seed yield per plant over both years (Table 7). The greater seed yield per plant at the lower seeding rate was due to a 174% greater branch seed yield per plant compared with the recommended seeding rate. In contrast, in both years, the three other cultivars showed less potential to produce higher branch yields at the low seeding rate, with the average increase in seed yield per plant ranging from 32 to 38%. Thus, when reducing seeding rates, it is important to select a cultivar with the greatest compensatory ability, mainly in terms of branch yield, especially in environments where soybean is less prone to drought stress. In contrast, cultivars with superior main-stem seed yields would perform best at recommended or even higher plant populations. Based on the four cultivars evaluated in this research, the early MG VI cultivar had the highest main-stem yield at the recommended seeding rate while the main-stem and branch compensatory ability of the late MG VI cultivar make it a suitable cultivar for seeding at lower-than-recommended rates.
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Table 7. The effect of cultivar and seeding rate on total, main-stem, and branch seed weight per plant in 2000 and 2001.
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SUMMARY AND CONCLUSIONS
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This research shows that the compensatory ability of soybean total, main-stem, and branch seed yields at lower-than-recommended seeding rates varies with cultivar selection and is also linked to the environmental conditions under which the crop is produced. Rainfall pattern within a growing season appeared to be more important in determining seed yields than did seeding rate or cultivar. The contribution of the branch seed yield to total seed yield was generally greater in 2001 than in 2000, which was probably due to increased rainfall amount and seasonal distribution in 2001. Frederick et al. (2001) found a majority of total seed yield was produced on branches under conditions that optimize seed yield, such as irrigation, whereas in low-yielding environments, the main stems produced the greatest percentage of total seed yield. Others have also reported a greater percentage of total seed yield coming from the main stems than from branches in dryland environments (Board, 1987; Frederick et al., 1998). The effects of the drought stress also differed between years. In 2000, drought stress during vegetative development had more of an effect on the early maturing cultivar due to its shorter period of vegetative growth. However, as evident in 2001, the early maturing cultivar can yield as well as or better than later-maturing cultivars at recommended and lower-than-recommended seeding rates when plants are not subjected to periods of drought stress during vegetative development.
Aboveground biomass differed among cultivars at 9 wk after emergence in both years; however, all cultivar and seeding-rate combinations intercepted 93 to 97% photosynthetically active radiation before pod set (data not shown), an amount sufficient to maximize seed yields (Ball et al., 2000). Seed yield differences were found among cultivars in both years, but yields were often similar between seeding rates for each cultivar. Aboveground biomass production of the two seeding rates was similar by 37 and 53 d after emergence in 2001 and 2000, respectively (data not shown), which would account for the lack of seed yield differences between seeding rates. Others have also found that soybean seed yields depend on the amount of vegetative biomass produced. In the absence of drought stress, Norsworthy and Oliver (2001) found yields over a range of seeding rates to be similar in a year when greater than 95% photosynthetically active radiation was intercepted, but yields differed among seeding rates in years when soybean failed to achieve 95% photosynthetically active radiation interception. In addition to soybean yields being directly affected by the quantity of light intercepted during vegetative and reproductive development (Ball et al., 2000; Board and Harville, 1993), seed yields depend on aboveground biomass production (Shibles and Weber, 1965).
Because only two or fewer cultivars were evaluated within each maturity group, we were unable to examine if yield responses among cultivars within a maturity group are similar. Due to inherent differences in growth habit of cultivars within a maturity group, it is likely that some cultivars may be better suited for seeding at lower-than-recommended seeding rates than others, as evident by differences in the compensatory ability of main-stem and branch yields of the early and late MG VI cultivars.
Seed cost, including a technology fee, is an important consideration for selecting the most economical cropping system to use when weed control and herbicide costs for conventional and glyphosate-resistant soybean are similar (Reddy and Whiting, 2000). While multiple glyphosate applications are needed when seeding rates of drill-seeded, glyphosate-resistant soybean are lowered, the savings in seed costs more than offset the added expense of an additional glyphosate application (Norsworthy and Oliver, 2001). Furthermore, the recommended seeding rate of drill-seeded, glyphosate-resistant soybean can be lowered, without negatively affecting seed yields. Because the technology fee of glyphosate-resistant soybean is based on the quantity of planted seed rather than unit area planted, lowering seeding rates will reduce the amount of money spent on both seed and technology fees. At a glyphosate-resistant soybean cost of $1.10 kg-1 seed, and assuming an average seed size of 6600 seed kg-1, seeding at the recommended rate of 620 000 seeds ha-1 would cost $103.33 ha-1, whereas seed costs would be reduced to $61.67 ha-1 at the low rate, a seed cost savings of $41.66 ha-1.
When grown with row widths of 19 cm, soybean branch growth has been reported to occur well after canopy closure on the Coastal Plain (Frederick et al., 1998). Therefore, both vegetative and reproductive branch development occur under less-than-optimal light conditions, especially for branches developing from lower main-stem nodes. Reducing the plant population may result in more light penetration into the canopy, allowing for greater branch development, as we observed. Some producers may be reluctant to lower seeding rates for fear that thin stands may not be as aesthetically pleasing, increase the risk of poor crop emergence, and result in the need for multiple herbicide applications (Norsworthy and Oliver, 2001). To minimize the risks associated with a lower-than-recommended seeding rate, one must be sure to plant high quality seed having excellent germination and seedling vigor in addition to optimizing emergence through standard practices such as ensuring adequate seed–soil contact, planting at an optimum depth, and having sufficient soil moisture to assure seeding emergence. If planting conditions are similar to those in this study and soybean does not suffer from extended periods of drought, seeding rates for glyphosate-resistant soybean (MG V through VII) can be reduced if planted at an optimal time, thereby lowering seed costs without sacrificing seed yields.
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ACKNOWLEDGMENTS
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The authors sincerely thank the South Carolina Soybean Board for partial support of this research and William Bonnette for field assistance with data collection.
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NOTES
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South Carolina Agric. Exp. Stn. Tech. Contrib. no. 4756. Mention of trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by Clemson University and does not imply its approval to the exclusion of other products or vendors that may also be suitable.
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TOP
ABSTRACT
INTRODUCTION
