Early-Maturing Double-Crop Soybean Requires Higher Plant Population to Meet Leaf Area Requirements

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David L. Holshouser, Virginia Polytechnic Institute and State University, Tidewater Agricultural Research and Extension Center, 6321 Holland Road, Suffolk VA 23437; and Brian P. Jones, Department of Crop and Soil Science, The Penn State University, University Park, PA 16802

Abstract

Information on the plant population-yield response of early-maturing soybean (Glycine max L.) varieties grown in a double-crop system is limiting. This study compares double-crop soybean yield and leaf area index (LAI) response to plant population of maturity group III and V varieties at populations ranging from 50,000 to 330,000 plants per acre. The maturity group III variety yielded up to 12 bushels/acre less at lower population, but equal to or up to 4 bushels/acre more than the maturity group V variety at higher populations. The ability to maximize yield depended on the ability of soybean to obtain a LAI of 3.5 to 4.0 by flowering. Yield did not continue to increase with higher LAI levels. During years of adequate moisture, the maturity group V variety was able to obtain this LAI level with 150,000 plants per acre, but 300,000 plants were required for the maturity group III variety. In a drought-stresses year, neither variety obtained an LAI of 3.5 to 4.0 and yields continued to increase with increasing population.

Introduction

New cropping systems must be investigated to ensure continued efficient and profitable agriculture. A continuous, no-till system of four crops in two years is being researched in the Mid-Atlantic USA (1). The crop rotation for this system is winter wheat (Triticum aestivum L.) double-cropped with soybean and winter barley (Hordeum vulgare L.) double-cropped with corn (Zea mays L.). This cropping system maximizes crop production per unit land area, offering the potential for twice the production that would be expected with a single crop per year. However, the shorter growing season for double-crop corn and soybean usually lowers yields of those crops (1,5). Double-crop soybean yield reduction can be attributed to the inability of the crop to acquire adequate leaf area (4) and maximize light interception (2). Narrow row spacing and higher plant populations have been used to increase leaf area and minimize the yield reductions due to late planting. Most states have developed specific row spacing and seeding rate recommendations for double-crop soybean that differ from its full-season counterpart.

In addition to narrowing rows and increasing plant population, using the appropriate soybean maturity group may help minimize yield reductions from late soybean planting. Using the latest maturity group that will complete its development before frost should help ensure maximum leaf area development. However, the 4-crops-in-2-years system requires an earlier-than-recommended soybean variety to ensure timely barley planting in October, which could further restrict leaf area production in a system which is already leaf-area limited (4). Assuming a 10-day difference in maturity between maturity groups and a harvest that needs to be completed two to three weeks earlier, one would need to select a variety that is 1.5 to 2.0 maturity groups earlier to fit a 4-crops-in-2-years cropping system.

In most double-crop systems, soybean is either drilled in 7.5- to 10-inch rows or planted in 15- to 20-inch rows. Current plant population recommendations are based on using adapted varieties. Reducing row spacing or increasing plant population beyond that of current recommendations could increase leaf area and yield of an early-maturing variety planted after small grain. Limited information on the plant population-yield relationship for early-maturing varieties planted late is available; therefore the objective of this research is to compare the plant population-yield response of an early-maturing soybean variety to that of a later-maturing variety, when planted in a double-crop system.

Field Studies With Two Maturity Groups

Field studies were conducted in 2000 and 2001 on a Eunola (fine-loamy, silicious, thermic, Aquic Hapludults) loamy fine sand, and in 2002 on a Lynchburg (fine-loamy, silicious, thermic, Aeric Paleaquults) fine sandy loam. Southern States (Southern States Cooperative, Richmond, VA) brand varieties RT-3975 (indeterminate, maturity group III) and RT-557N (determinate, maturity group V) were planted into wheat stubble on 5 July in 2000, 26 June in 2001, and 3 July in 2002. The maturity group V variety would allow maximum leaf area development and is recommended for double-crop plantings in Virginia. The maturity group III cultivar is considered early-maturing in this region, but would ensure timely small grain planting. Sufficient seeds were planted to obtain a final plant population of 50, 100, 150, 200, or 250 thousand plants per acre in 2000 and 60, 90, 120, 150, 180, 210, 240, 270, 310, or 330 thousand plants per acre in 2001 for both varieties. In 2002, emergence was low due to relatively dry soil conditions at planting, with final populations for RT-3975 of 50, 85, 120, 155, or 190 thousand plants per acre, and final populations for RT-557N of 60, 105, 150, 195, or 240 plants per acre. The discrepancy in percent emergence between the two varieties could not be explained.

The experimental design was a randomized complete block in a split-plot arrangement with twelve, three, and four replicates for 2000, 2001, and 2002, respectively. Main plots were soybean cultivar and sub-plots were soybean population. Plot size was ten 15-inch-wide rows by 24 feet long. Plant population density was measured by counting the number of plants in 3 feet of row at two locations within the plot at harvest. Leaf area index (LAI) measurements were obtained at the full flower stage with the LAI-2000 Plant Canopy Analyzer following sampling methods described by LI-COR (6). Leaf area index is defined as the ratio of unit leaf area of the crop to unit soil surface area (e.g., for an LAI of 3.5, 3.5 ft² of leaves per ft² of soil surface) and is related to crop yield (3,4,9). Soybean was harvested with a small-plot combine equipped with moisture tester and data logger. Seed yield, adjusted to 13% moisture content, was measured by harvesting the six interior rows of each plot that had been end-trimmed to 17 feet. The variety RT-3975 was harvested on 26 October, 18 October, and 18 October in 2000, 2001, and 2002, respectively. The variety RT-557N was harvested on 22 November, 1 November, and 9 December in 2000, 2001, and 2002, respectively.

The MIXED procedure of SAS (7) was used to test for significance of main effects and their interactions. The LSMEANS statement was used to compute the least-squares means of the fixed effects. The PDIFF option of the LSMEANS statement was used to request that the differences in LS-means be displayed for comparison. Mean separations were considered significant if P-values were < 0.05. Linear regression procedures were used to describe the relationship between plant population and LAI or yield.

Leaf Area Index and Maximizing Yield

Results varied with year, but similarities existed between the data sets. In 2000, yield of both varieties increased with increasing plant population, but the slopes of the quadratic response were different (Fig. 1). Yield between the three highest populations of the maturity group V variety did not differ. Therefore, yield was maximized at 150 thousand plants per acre. In contrast, the predicted population-yield response for the maturity group III variety was steeper in slope and continued to increase as population increased. Yield of maturity group III did not differ between the two highest populations, maximizing at 200 thousand plants per acre. In 2000, at the lower populations of 50 and 100 thousand plants per acre, the maturity group III variety under-performed the later maturity variety, but yielded higher at 200 and 250 thousand plants per acre.

Fig. 1. Yield response of two soybean varieties to plant population. The maturity group (MG) III variety is represented by Southern States brand RT-3975 and the MG V variety is represented by Southern States brand RT-557N. Vertical bars represent ą standard error of the mean.

Leaf area index measurements reveal possible reasons for the difference in yield response of maturity group to plant population. Other research has suggested that soybean need to obtain an LAI of 3.5 to 4.0 by flowering to maximize yield (3,9). For both maturity groups, soybean LAI increased with population (Fig. 2). But the maturity group III variety never obtained the required LAI to maximize yield except for the highest population tested in 2001. It should be noted that LAI was not different between the two highest populations of the early variety and is likely the reason for no significant yield difference at these populations. The maturity group V variety reached an LAI of 3.5 at 150 thousand plants per acre. Although LAI continued to increase, soybean had obtained sufficient leaf area to maximize yield at this population; therefore yield did not respond to higher populations.

Fig. 2. Leaf area index (LAI) response of two soybean varieties to plant population. The maturity group (MG) III variety is represented by Southern States brand RT-3975 and the MG V variety is represented by Southern States brand RT-557N. Horizontal broken lines represent an LAI of 3.5 and 4.0. Vertical bars represent ą standard error of the mean.

Results in 2001 were similar, but the ratio of yield increase to increasing population was less (Fig. 1). For the maturity group V variety, the predicted maximum yield (approximately 200 thousand plants per acre) was similar to the previous year, but much higher yields were realized at lower populations. This may be due to higher LAI at these populations (Fig. 2). Adequate LAI levels for the early maturity group were not reached until population exceeded 300 thousand plants (Fig. 2). Therefore, yield continued to increase with increasing population (Fig. 1). At the highest population tested, yield of the maturity group III variety was not significantly different from the maturity group V variety.

The growing seasons of 2000 and 2001 produced the two highest average soybean yields on record in Virginia (8). Very little drought stress occurred, resulting in good soybean growth. In contrast, soybean experienced very dry conditions in 2002. Yields and LAI were much lower, but results followed similar patterns to previous years (Figs. 1 and 2). Yield of the later maturity group was higher than the early maturity group at low populations. However, yields were equal at the highest-tested maturity group III population. Although the yield response was flatter with the late maturity group, a maximum yield was not obtained as in previous years. Leaf area measurements revealed that adequate LAI was not achieved with either maturity group in 2002. Therefore, the linear yield increase was reasonable.

These data validated previous research indicating that an LAI of 3.5 to 4.0 was needed by the flowering stage to maximize soybean yield. Regardless of maturity group, maximum yield was not reached unless adequate LAI was achieved. This research also indicated that plant population recommendations for double-crop systems might need adjusting to reflect soybean maturity group. Early-maturing soybean is less likely to develop adequate leaf area, and therefore may respond to higher than normally recommended populations.

Recent research revealed that LAI might not be limited in all cropping systems or on all soils, especially where soil with low capacity for holding plant-available water restricted LAI to yield-reducing levels (4). On better soils capable of producing more leaf area, increasing plant population may increase leaf area but not necessarily increase yield at the same rate as in these experiments. Thus, plant population adjustments for early-maturing varieties should be implemented on a site-specific basis to ensure the highest yield potential and lowest input cost. Plant population recommendations should be based on the ability of the crop to achieve adequate leaf area and not solely on plant population-yield experiments. By identifying fields or areas within fields of low leaf area, one could make a more informed decision on varying plant population.

Acknowledgements

The authors would like to acknowledge the Foundation for Agronomic Research, the United Soybean Board, the Virginia Soybean Board, and the Virginia Agricultural Council for their financial support.

Literature Cited

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