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© 2007 Plant Management Network.
Accepted for publication 6 September 2007. Published 1 November 2007.

Planting Date, Cultivar, and Seeding Rate Effects on Soybean Production along the Texas Gulf Coast

W. James Grichar, 3507 Hwy 59E, Texas Agricultural Experiment Station, Beeville 78102

Corresponding author: W. James Grichar. w-grichar@tamu.edu

Grichar, W. J. 2007. Planting date, cultivar, and seeding rate effects on soybean production along the Texas Gulf Coast. Online. Crop Management doi:10.1094/CM-2007-1101-01-RS.

Abstract

Soybean [Glycine max (L.) Merr.] along the upper Gulf Coast region of Texas may be planted from early March through the middle of May. However, soybean yields may vary considerably depending on planting date and cultivar. Field studies were conducted from 2001 through 2003 in Wharton Co. to determine the effect of planting date, soybean cultivar, and seeding rate on yield and pod set. Soybean was planted three times each year at approximately two- to three-week intervals beginning around 10 March. Three cultivars were compared at seeding rates of 145,000 and 220,000 seed/acre. Soybean plant populations varied from year-to-year but generally populations increased with later planting dates. Soybean pod height set varied with the cultivar but was usually taller with the later planting date and higher seeding rate. Below average rainfall was observed during the 2001 growing season while near average rainfall was received during the April to June 2002 growing season. Extremely dry conditions were prevalent during March through June of 2003 with above average rainfall in July of that year. In a near normal rainfall growing season (2002), the later planting date resulted in the highest yield while under drought-stressed conditions, planting date was not an important factor. Overall, soybean planted late-March to mid-April produced higher yields than those planted mid-March. Generally, the higher seeding rate did not improve net returns.

Introduction

Production components such as planting date, cultivar, and seeding rate can be manipulated to counter the effects of various environmental factors on soybean development and yield. Also, early planting dates help to avoid soil moisture deficits that may arise later in the growing season during the critical stages of plant growth (4,9,15). Although early-maturing cultivars usually produce lower yields when compared with full-season cultivars at normal planting dates, early-planted, early-maturing cultivars may produce superior yields by avoiding July and August summer drought conditions (4,9,15). These cultivars would have passed critical growth stages such as flowering, seed set, and seed filling before stored soil water is exhausted (11,13). Stress can reduce soybean yield by reducing number of pods, number of seeds, and seed mass (6). Both determinate and indeterminate soybean cultivars have reduced growth rates under drought stress and resume normal growth rates when such stress is removed (3,15). This may be an important growth attribute to consider if producers expect considerable soil moisture deficits due to several short, intermittent droughts during the growing season (15).

The planting of soybean at the lowest population for best yield reduces seeding costs, avoids some diseases, and minimizes lodging (17). Other research has shown that optimal plant populations vary from 120,000 to 200,000 plant/acre (7,10,12,19). The optimal soybean plant population can vary by as much as 100% across years, even when the same cultivar is grown in the same location (19). This variability can be explained by environmental conditions, with optimal plant populations increasing under adverse conditions (19).

The objectives of this research was to characterize the agronomic response of soybean cultivars commonly grown along the upper Texas Gulf Coast to natural rainfall conditions using two different seeding rates over a range of three planting dates beginning around 10 March and spaced at approximately two-week intervals. This is actually the planting window for the Early Season Production System (ESPS). All of the planting dates are early, i.e., prior to the conventional optimal planting period of early May to mid-June which once prevailed in the southern United States (4,13). Also, most producers in the area plant soybeans at the rate of 10 seed/ft; however, questions are raised about increasing seeding rates and effects on yield. This information will aid producers in adapting planting timings that will improve soybean yield and reduce the chance of reductions due to weather conditions.

General Methodology

Field experiments were conducted in 2001 through 2003 in Wharton Co. ,TX (29°14.406’N) with the soil type of Lake Charles clay (fine, montmorillonitic, thermic Typic Pelluderts) (Table 1). Fertilizer was applied as needed according to Texas Cooperative Extension recommendations for soybean. Boundary (S-metolachlor plus metribuzin) was applied preemergence at 1.0 qt/acre for weed control. Select (clethodim) was used postemergence at 10 oz/acre to control any annual grass escapes while Blazer (acifluorfen) was used at 1.5 pt/acre to control any broadleaf weed escapes. Weeds were treated when less than 6 inches tall and all postemergence herbicide treatments included Agridex added at the rate of 0.25% v/v.

Table 1. Soil description, planting dates, cultivars, and maturity groups of soybeans used at the Wharton Co. location.

 * Each cultivar noted for the respective year was planted at each date.

Soybean seed was planted with a vacuum planter (Monosem ATI Inc., Lenoxa, KS) to provide a uniform planting rate using seeding rates of 145,000 (10 seed/ft) and 220,000 (15 seed/ft) seeds/acre. Three planting dates, approximately two weeks apart, were used each year with the first date around 10 March (Table 1). Three soybean cultivars (maturity group 4.6 to 5.2) were selected which were representative of the cultivars planted in the area. Some cultivars did change over the years due to lack of availability but replacement cultivars were selected that were representative of the maturity group for the cultivars that was no longer available (Table 1). Plots were not irrigated and monthly rainfall data were recorded from planting to harvest (Table 2).

Table 2. Total monthly rainfall (inches) from soybean planting through harvest.

Plant populations were determined by quantifying the number of plants emerged 4 weeks post- planting for 3 ft of the two adjacent middle rows. Plant populations were then converted to plants/acre. The lowest pod height were measured prior to harvest with measurements taken from ground level to the tip of the first pod attached at the lowest node. Five plants/plot were measured and an average recorded. Harvesting was accomplished mechanically with a small plot combine and plot yields adjusted to 12% moisture.

Net returns were based on seed costs only and did not include land preparation, herbicide, or insecticide costs since these would remain constant over seeding rate. Seed costs were calculated based on calls to local seed representatives. Seed costs were based on a cost of $35.00/bag at 150,000 seed count per bag while the soybean price was calculated at $7.28/bu based on the close of the market on 29 April 2007.

Experimental Design and Data Analysis

Each year’s study was set up using a randomized complete block design with a split-split plot arrangement of treatments. Treatment factors included the main plots of planting date (3), the split plots of cultivar (3), and the split-split plots of seeding rate (2). Main plots were 24 rows wide by 30 ft long, split plots were 8 rows wide by 30 ft long and split-split-plots were 4 rows wide by 30 ft long. Treatments were replicated three times. An analysis of variance was performed using the ANOVA procedure for SAS (SAS Institute Inc., Cary, NC) to evaluate the significance of effects on soybean stand counts, pod height, yield, and net returns. The Fisher protected LSD at the 0.05 level of probability was used for separation of mean differences.

Plant Populations

Plant populations varied from year to year but populations were generally lower in 2002 (Table 3). This was possibly the result of lower rainfall received during the months of January through February (data not shown) and March (Table 2). Rainfall during March of 2002 was 50% of the rainfall received for March of 2001 and 2003. April and May of 2003 were extremely dry (Table 2) but the months of December 2002 and January and February of 2003 had above normal rainfall (data not shown).

Table 3. Influence of planting date, cultivar, and seeding rate on soybean stand counts.

In 2001, plant populations of all cultivars at the lower seeding rate did not differ as planting date was delayed with the exception of Pioneer 9482 planted mid-April. At the high seeding rate of 220,000 seed/acre, plant populations decreased as planting date was delayed (Table 3). In 2002, at the high seeding rate, plant populations increased between the first and second planting date for Pioneer 9482 and between the first and third planting date for DP 4690 and NK 452. No differences in plant populations were observed between the later two planting dates for any cultivar or seeding rate. Rainfall was near normal for April (Table 2) and the improved moisture conditions contributed to improved emergence of plants at the later two planting dates. The response seen in this study to moisture conditions for soybean emergence was similar to that reported by Heatherly (9).

In 2003, emerged plant populations varied considerably for planting dates. For DP 4690, at the low seeding rate, the late-March planting date had the highest number of emerged plants. For DP 4690, at the high seeding rate, both March plantings had a higher plant population than the mid-April planting (Table 3). With HBK 4920 or NK 452, no differences in plant populations were noted across planting dates for each seeding rate.

Pod Height

In 2001, with SG 468 at both seeding rates, the mid-April planting date produced the highest pod set (Table 4). With Pioneer 9482, no difference in pod height were noted at any planting date with the low seeding rate but the late-March planting date resulted in the highest pod height for the high seeding rate. With NK 452, results were also variable. At the lower seeding rate, the mid-April planting date resulted in the highest pod set while at the high seeding rate the late-March planting date produced the highest pod set.

Table 4. Influence of cultivar, seeding rate, and planting date on soybean pod set height.

In 2002, similar trends were seen as in 2001 with the mid-March planting date producing lower pod heights in most instances. Pod height with DP 4690 increased as planting date was delayed for both seeding rates (Table 4). Pioneer 9482 at 145,000 seed/acre produced the highest pod set with the mid-March planting date while at 220,000 seed/acre, the late-March and mid-April planting dates resulted in the highest pod set. With NK 452, at either seeding rate, pod set height increased as planting date was delayed (mid-April > late-March > mid-March).

In 2003, the initiation of pod set height was less affected by planting date as in the previous two years. With all cultivars at 145,000 seed/acre, the lowest pod set was initiated with the mid-March planting date (Table 4). At 220,000 seed/acre, the late-March planting date resulted in the lowest pod set initiation height with DP 4690 and NK 452 while with HBK 4920 the mid-April planting date resulted in the lowest pod set initiation. Parvez et al. (14) reported a positive correlation between plant height and main stem node number. Also, as pod set height decreases, harvesting equipment can not harvest the soybeans produced closed to the ground and pod yield losses can be noted (author's personal observation).

Soybean Yield

Soybean yield varied according to cultivar maturity group, plant populations, and planting date. There was a slight positive correlation for plant populations (r = 0.41, n = 162, P = 0.0001) and planting date (r = 0.44, n = 162, P = 0.0001) with soybean yield. Generally, lower mean yields were found with the mid-March planting date (Table 5).

Table 5. Influence of cultivar, seeding rate, and planting date on soybean yield..

In 2001, the late March or mid-April planting date generally produced higher soybean yields than the mid-March planting date across all cultivars. Rainfall amounts were low in June (Table 2). This had an effect on soybean yield for the mid-April planting date. Ashley and Ethridge (1) reported that lack of moisture had more of an effect on soybean yield when drought occurred from flowering to physiological maturity compared with the emergence to flowering period. For SG 468 and NK 452 at 220,00 seed/acre, the late-March planting date yielded better than the mid-March planting. Pioneer 9482 at a seeding rate of 145,000 seed/acre produced the highest yield when planted mid-April while at the seeding rate of 220,000 seed/acre, the late-March and mid-April planting dates resulted in higher yield than the mid-March planting date.

In 2002, similar trends were noted as in 2001, with the late-March or mid-April planting date producing a higher yield than the mid-March date regardless of soybean cultivar or seeding rate (Table 5). Rainfall during 2002 was below average for the winter months and all months during the growing season. A soybean yield increase was also noted when planting date was delayed from late-March to mid-April for DP 4690 and Pioneer 9482 at both seeding rates. A yield increase was noted with NK 452 at the low seeding rate when planting date was delayed. Increasing the seeding rate resulted in a decrease in yield with the DP 4690 when planted mid- and late-March but did result in a yield increase when planted mid-April. An increase in soybean yield was noted when the plant population was increased with the Pioneer 9482 planted late-March or mid-April. A yield decrease was noted with the NK 452 when seeding rate was increased with the mid-April planting.

In 2003, soybean yields were low compared to the other two years due to lack of sufficient rainfall during the April through June growing season (Table 2). The benefits of the Early Season Production System (ESPS) and the use of ESPS as a late-season drought avoidance system (4,9) were negated by early-season drought. Therefore, a delay in planting date did not result in as consistent increase in soybean yield with all cultivars as noted in previous years (Table 5). A yield increase occurred when planting date was delayed from mid-March to late-March with DP 4690 at 145,000 seed/acre. However, a decrease in soybean yield as seeding rate increased was noted when this cultivar was planted mid-April. With HBK 4920 or NK 452 seeded at 145,000 seed/acre, a yield increase was noted when planting date was delayed from mid- or late-March to mid-April but no differences in soybean yield was noted at 220,000 seed/acre. Drought stress during seed formation and development in soybean is mainly responsible for reduced seed yield (16). Reports of success with early-planted, early-maturing soybean systems in the southern US have been widespread. Boote (5) found MG II to IV to be well adapted to March planting in Florida. Savoy et al. (18) observed a good yield performance for a MG III cultivar planted in April in south-central Texas; however, late planting dates were not considered since the intent was to circumvent late season drought stress.

Partial Net Dollar Returns

Generally, the increased seeding rate did not result in a corresponding increase in net returns (Table 6). In 2001, the seeding rate did not affect net returns of any cultivar at the mid-March planting date. At the late-March planting date, Pioneer 9482 at the high seeding rate increased net returns over the lower seeding rate by 52%. No other differences in net returns were seen at this planting date. With the mid-April planting date, NK 452 at 145,000 seed/acre resulted in higher net return than the 220,000/acre seeding rate.

Table 6. Influence of cultivar, seeding rate, and planting date on partial net returns based on seed costs only.

 ^x Seed costs were calculated based on a cost of $35.00/bag at 150,000 seed/bag count while soybean price was calculated at $7.28/bu.

In 2002, at the mid-March planting date, DP 4690 at the lower seeding rate increased net return by over 300%. At the late-March planting date, DP 4690 at the lower seeding rate increased net return over the higher seeding rate; however, just the opposite occurred with Pioneer 9482 as the higher seeding rate increased net returns over the lower seeding rate. A similar trend was noted with Pioneer 9482 at the mid-April planting while NK 452 showed a greater net return with the lower seeding rate.

In 2003, no difference in net returns were noted with any cultivar or planting date with the exception of DP 4690 planted in mid-April which resulted in a net increase with the lower seeding rate over the higher rate.

Popp et al. (15) reported that cultivar selection, once the selection of planting date has been made, can affect returns by as little as $5/acre but also by as much as $66/acre. Furthermore, seed cost differences across cultivars tend to be minimal; therefore, paying attention to cultivar differences may net as much as $66/acre.

Conclusions

The range of planting dates from mid-March through mid-April during the three-year period resulted in a wide variation in rainfall (Table 2) during soybean growth and development. Even with variances in rainfall, the late-March to mid-April planting date produced the most consistent and highest yields. In the year of adequate moisture during the April through June growing season (2002), soybean yields were greatly enhanced with the mid-April planting date compared with the other two planting dates. In years of lower rainfall during the growing season (2003), the effect of planting date was not as great. However, above average rainfall in July due to a hurricane, probably negated some of the effects of late-season moisture stress. In previous research across the southern US, the planting of early-maturing cultivars was shown to be less important under conditions of more plentiful rainfall (5,11,18).

Increasing the seeding rate by 50% did not always result in a significant increase in soybean yield or net returns. Ennin and Clegg (8) reported that plant populations beyond 52,000 plants/acre might not be necessary to maximize yield with MG 3 determinate soybean cultivars. Bello et al. (2) also did not find an increase in yield of determinate, semi-determinate, and indeterminate soybean by doubling plant populations from 154,000 to 300,000 plants/acre. This suggests that maximum yields and net profits could still be obtained using fewer seeds, thus reducing seed costs.

In summary, soybean cultivar and planting date appear to be primarily responsible for the observed yield responses. The range of planting dates commonly used along the upper Texas Gulf Coast resulted in a wide variation in environmental conditions during soybean growth and development. Early planting resulted in the apparently unfavorable combination of the coolest vegetative-stage temperatures and lack of rainfall during the pod fill stage. Therefore a late March to mid-April planting date was most conducive for optimum soybean growth and development and subsequently higher yields.

Literature Cited

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10. Heatherly, L. G. 1988. Planting date, row spacing, and irrigation effects on soybean grown on clay soil. Agron. J. 80:227-231.

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13. Miller, T. D. 1994. Why early soybeans? A summary of the Texas experience. Pages 103-105 in: Proc. of the 1994 Southern Soybean Conf., Memphis, TN. 14-16 Feb. 1994. Res. Dep., Am. Soybean Assoc., St. Louis, MO.

14. Parvez, A. Q., Gardner, F. P., and Boote, K. J. 1989. Determinate- and indeterminate-type soybean cultivar responses to pattern, density, and planting date. Crop Sci. 29:150-157.

15. Popp, M. P., Keisling, T. C., McNew, R. W., Oliver, L. R., Dillon, C. R., and Wallace, D. M. 2002. Planting date, cultivar, and tillage system effects on dryland soybean production. Agron. J. 94:81-88.

16. Reicosky, D. C., and Heatherly, L. G. 1990. Soybean. Pages 639-674 in: Irrigation of Agricultural Crops, Agron. Monogr. 30. B. A. Stewart and D. R. Nelson, ed. University of Wisconsin, Madison, WI.

17. Rigsby, B., and Board, J. E. 2003. Identification of soybean cultivars that yield well at low plant populations. Crop Sci. J. 43:234-239.

18. Savoy, B. R., Cothren, J. T., and Shumway, C. R. 1992. Early-season production systems utilizing indeterminate soybean. Agron. J. 84:294-398.

19. Wells, R. 1991. Soybean growth response to plant density: relationship among canopy photosynthesis, leaf area, and light interception. Crop Sci. 31:755-761