Agronomy Journal 93:1028-1034 (2001)
© 2001 American Society of Agronomy
FORAGES
Growth, Development, and Yield of Soybean Lines Developed for Forage
Witjaksana Darmosarkoroa,
Matthew M. Harbura,
Dwayne R. Buxtonb,
Kenneth J. Moore*,a,
Thomas E. Devineb and
Irvin C. Andersona
a Dep. of Agronomy, 2101 Agronomy Hall, Iowa State Univ., Ames, IA 50011
b USDA-ARS, Beltsville, MD
* Corresponding author (kjmoore{at}iastate.edu)
Received for publication August 25, 2000.
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ABSTRACT
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Recently, soybean [Glycine max (L.) Merr.] cultivars have been developed specifically for use as a forage crop. The objective of this study was to determine the effect of maturity group on and compare the agronomic performance of forage and grain soybean cultivars in Iowa. In 1994, 13 forage- and five grain-type cultivars were studied. In 1995, one additional forage cultivar and grain cultivar were each evaluated. Node number, plant height, lodging, and dry matter accumulation were measured biweekly during the growing season. By 135 days after planting (DAP), forage cultivars yielded 5 to 19% more dry matter than ‘Sherman’, which had the greatest yield among grain cultivars. Forage cultivars were 37 to 69% taller than ‘Biloxi’, the tallest grain cultivar, which may partially explain the greater lodging of the forage cultivars compared with the grain cultivars. Forage cultivars initiated reproductive growth 60 to 88 DAP, whereas the locally adapted Sherman cultivar initiated reproductive growth 55 DAP. Forage cultivars produced more dry matter than grain cultivars, but had a lower leaf/stem ratio and leaf + pod/stem ratio in August and September, respectively, which may reduce forage quality. Forage cultivars developed in Pennsylvania generally accumulated more dry matter than forage cultivars developed in Virginia by late August, but initiated reproductive growth sooner and produced less dry matter by late September. The significant productivity differences observed between forage and grain cultivars suggest the potential of breeding to improve the forage potential of soybean.
Abbreviations: DAP, days after planting
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INTRODUCTION
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SOYBEAN was introduced into the USA from China as a potential forage crop (Arny, 1926; Good, 1942). The grain potential of soybean was soon realized and soybean has since been grown primarily as a grain crop rather than as a forage crop. In 1976, the USDA-ARS began a breeding program to increase the forage potential of soybean. Four soybean cultivars, chosen for their high leaf retention (Wilson 6) and pest and stress resistance (Forest, Perry, and L76-0253) were crossed, double-crossed, and segregated. Resulting cultivars (PA 4-11b and PA 4-11g-1) were crossed with Tracy M, Burlison, BSR 201, and Ripley cultivars to increase vigor and resistance to lodging and seed shatter (Bernard et al., 1988). The resulting forage cultivars, evaluated in our experiment, are listed in Table 1.
Forage quality generally increases with the leaf/stem ratio. The leaf mesophyll, which contain a high proportion of chlorenchyma, are more digestible than the sclerified cells contained by the stem, epidermis, and vascular tissue (Akin, 1989; Esau, 1977). In soybean, leaf dry matter is maximum at reproductive stage R5, according to Fehr and Caviness (1977), but the total protein content of the plant continues to increase with pod fill until stage R7 (Willard, 1925; Good, 1942; Munoz et al., 1983; Ritchie et al., 1982). For this reason, Good (1942) found that late-cut soybean hay contained a greater proportion of protein and oil than did early-cut hay.
Soybean leaf dry matter and total dry matter are also influenced by photoperiod. A soybean cultivar will generally produce greater vegetative growth and dry matter as the photoperiod is increased (Johnson and Major, 1979). Greater internode elongation and leaf expansion also occur as the photoperiod increases (Caffaro and Nakayama, 1988). Soybean cultivars are classified according to maturity groups, ranging from 00 (early maturity) to VIII (late maturity), reflecting the photoperiod that maximizes grain yield.
The objectives of this study were to compare the vegetative growth, development, and aboveground dry matter yield of grain-type and experimental forage-type soybean cultivars in Iowa and to determine whether differences between forage and grain cultivars were due to maturity group or other qualities.
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MATERIALS AND METHODS
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Location and Design
The experiment was conducted during 1994 and 1995 on the Bruner Farm of Iowa State University, Ames, IA (42°N, and 93°W) on a Clarion (Typic Hapludoll) soil and on a Nicollet (Aquic Hapludoll) soil in the Clarion–Nicollet–Webster (CNW) soil associations. The experimental design was a randomized complete block with four replications. Each plot consisted of four 76-cm wide rows, 7.6 m in length, arranged in a north–south direction. The experiment was planted on 13 May 1994 and 22 May 1995, with 20 seeds m–1 of row. In 1994, 13 forage and five grain cultivars were evaluated (Table 1). In 1995, one check cultivar, Flyer, and one forage cultivar, PA 15-12-5-2, were evaluated in addition to the other cultivars. Cultivars designated PA and OR were selected in State College, PA, and Orange, VA, respectively. ‘Derry’ was formerly the experimental line OR 14-13-2 and ‘Donegal’ was formerly PA Bu2-2 (Devine et al., 1998; Devine and Hatley, 1998). ‘Biloxi’ (PI 584444) was introduced into the USA from Zhejing Province, China, in 1908.
Plant Sampling and Measurements
The center two rows from each four-row plot were sampled. Dry matter production was determined by harvesting all the aboveground plant material from the southern 1.5 m of the 7.6 m center rows on 9 Sept. 1994 and 22 Sept. 1995. The remaining area was sampled eight times during the growing season to measure the progression of growth and development. Samples were collected biweekly from 27 June until 4 Oct. 1994, and from 27 June until 2 Oct. 1995. These samples included three or more plants taken from 30 cm of row length.
Plant height, node number, lodging score, reproductive stage, dry matter of stem, leaf, and pod, and aboveground dry matter production were determined on the plants that were sampled biweekly. Height was measured from the soil surface to the terminal bud. The number of nodes was noted according to the descriptions of Ritchie et al. (1982). Reproductive stage was determined according to the system described by Fehr and Caviness (1977). The first reproductive stage, R1, was the stage at which plants had one to two flowers on the main stem, and R8 was the stage at which plants had reached physiological maturity. Lodging in each plot was estimated visually on 22 Aug. 1994 and on 23 Aug. 1995 and scored on a scale of 1 (upright) to 5 (prostrate). Dry matter of stem, leaf, and pod were measured after drying at 55°C for 48 h.
Ames weather data were collected at the Iowa State University Agronomy and Agricultural Engineering Research Center, which was approximately 2.5 miles west-northwest of the research site.
Statistical Analysis
The data were analyzed using the general linear model (GLM) procedure of SAS (SAS Inst., 1985), with the model:
where Y = measured parameter, Y = effect of year, B = effect of blocks within years, E = effect of entry, D = effect of sampling date, i = ith year, j = jth replication, k = kth entry, l = lth sampling date. Tests of F were performed by using B(i)j for Yi, YEik for Ek, BE(i)jk for YEik, YDil for Dl, BD(i)jl for YDil, YEDikl for EDkl, and BED(i)jkl for YEDikl. Means were compared with a protected least significant difference (LSD) test at the 0.05 probability level (Steel and Torrie, 1980).
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RESULTS
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Differences for all measured traits were observed for year (except yield), entry, entry x year (except leaf dry matter, height, leaf area index, and node number), date of sampling, year x date, and entry x date (Table 2). Except for yield and lodging, effects were analyzed separately by year, because of the interaction of year with treatment effects and the unbalanced experimental design (due to the addition of the two cultivars to the experiment in 1995).
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Table 2. Mean squares from analysis of variance of leaf dry matter (DM), stem DM, pod DM, plant height, reproductive (R) stage, yield, lodging, leaf/stem ratio (LS), and leaf + pod/stem ratio (LPS).
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The 1995 growing season had a cooler, wetter spring and warmer, drier summer than that of 1994 (Table 3). The 1994 growing season was generally warmer and drier than normal during the spring and cooler and wetter than normal during the late summer. The 1995 growing season was cooler and wetter than normal during the spring but warmer and drier than normal during late summer The later planting date in 1995, and the resulting shorter growing period, may explain the generally lower values for height, node number, dry matter accumulation, and reproductive development stages observed at harvest in 1995.
Plant Height, Lodging Score, and Node Number
The plant height of the forage cultivars was greater than that of the grain cultivars and may have contributed to lodging of some forage cultivars (Table 4). Lodging generally was greater among forage cultivars, especially the PA cultivars, than the Kenwood, Pella 86, and Sherman cultivars (Table 4). However, forage cultivars OR 19-2-2 and OR 25-11-1 significantly differed in lodging, despite their similar heights. In addition, lodging of some forage cultivars was as low as that of the grain cultivars. Other characteristics, such as chemical composition of the stem, may have affected lodging (Mancuso and Caviness, 1991). Height and lodging did not differ among most grain cultivars, suggesting that maturity group alone did not explain differences in height and lodging between the grain and forage cultivars (Table 4).
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Table 4. Plant height; lodging score; leaf node number; leaf, stem, and pod dry weights; and reproductive stages of forage and grain cultivars on 22 Aug. 1994 (102 d after planting) and 22 Aug. 1995 (93 d after planting), and total dry matter yield on 9 Sept. 1994 (116 d after planting) and 22 Sept. 1995 (107 d after planting).
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On average, the forage cultivars produced three to four more leafed nodes than the grain cultivars, including the later-maturing Hutcheson and Biloxi cultivars, indicating the greater potential of the forage cultivars for vegetative growth (Table 4). Significant variation in node number occurred both within and between the two sets of forage cultivars. Node number tended to be greater in the PA cultivars than the OR cultivars, even though the PA cultivars were in a lower maturity group than the OR cultivars.
Dry Matter Accumulations
Leaf dry matter accumulation was similar among all entries until August, but thereafter differed (Fig. 1 and 2)
. Forage cultivars, in general, had greater leaf dry matter by late August than did locally adapted grain cultivars (Table 4). The leaf dry matter of locally adapted grain cultivars was generally found to decline sooner and more rapidly than forage cultivars, as demonstrated by the Sherman cultivar in both years (Fig. 1) and the Pella 86 cultivar in 1994 (Fig. 2). Grain cultivars, however, have the potential to produce comparable leaf dry matter to forage cultivars, as suggested by the high leaf dry matter of the later-maturing Hutcheson cultivar in both years (Table 4). The PA cultivars included those that produced the greatest leaf dry matter (PA 10-1-2 and Donegal) but also cultivars that were far less productive.
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Fig. 1. (A and B) Dry matter accumulation pattern of grain cultivar Sherman in 1994 and 1995, and (C and D) forage cultivar Donegal in 1994 and 1995. Data points are averages of four replications.
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Fig. 2. Dry matter accumulation pattern of forage cultivars (A) OR 14-12-2 and (C) Donegal, and grain cultivars (B) Pella-86 and (D) Biloxi in 1994. Data points are averages of four replications.
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Stem dry matter was generally higher among forage cultivars than among grain cultivars, although the difference was more pronounced in 1994 than 1995. The Hutcheson cultivar again demonstrated the potential of grain cultivars to perform similarly to forage cultivars. Appreciable variation was observed among forage cultivars (Table 4). The PA cultivars also included those with the greatest stem dry matter (PA 10-1-2 and Donegal).
Far less pod dry matter was produced by the forage cultivars compared with the grain cultivars. Pod dry matter was greatest in the Kenwood, Pella 86, and Sherman cultivars, which are in the maturity groups (II and III) that are most appropriate for Iowa (Table 4). All PA cultivars produced pod dry matter, while only one OR line [OR-5-12-1(T)] produced pods.
Reproductive Development and Dry Matter Allocation
Kenwood, Pella 86, and Sherman cultivars (maturity groups II and III) initiated reproductive growth sooner than either forage cultivars (maturity groups V and VI) or grain cultivars in later maturity groups (Fig. 3)
. The grain cultivars also reached a higher maturity stage (approximately R5 to R6) than did the forage cultivars (approximately R2 to R4) at the late harvest in each year. At this time, the PA cultivars were more mature than the OR cultivars. The delay in reproductive development in the forage cultivars likely explains the lower leaf + pod/stem ratio of the forage cultivars compared with the grain cultivars.
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Fig. 3. Reproductive stage of development (R index) of grain (Hutcheson, Biloxi, and Sherman) and forage (Donegal and Derry) cultivars according to staging descriptions of Fehr and Caviness (1977). Data are averages of 2 yr.
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Forage cultivars generally produced more dry matter at the late harvests than did the grain cultivars (Fig. 1 and 2; Table 4). The OR cultivars produced the greatest dry matter yields in September, even though the PA cultivars had greater leaf, stem, and pod dry matter when harvested in August. The dry matter of forage cultivars, however, was composed of a greater proportion of stem dry matter (Tables 4 and 5), which may decrease forage quality by decreasing palatability, animal dry matter intake, and digestibility (Buxton and Marten, 1989). The mature dry matter of grain-type soybean, in comparison, was primarily composed of reproductive tissue.
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Table 5. Leaf/stem ratio (LS) and leaf + pod/stem ratio (LPS) on 22 Aug. 1994 (102 d after planting), 20 Sept. 1994 (130 d after planting), 22 Aug. 1995 (93 d after planting), and 19 Sept. 1995 (121 d after planting).
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The leaf/stem ratio, which is positively correlated with forage quality, was generally higher in middle to late August in the locally adapted cultivars (Table 5). These same grain cultivars, however, had strikingly lower leaf/stem ratios than did the forage cultivars by middle to late September. This is likely due to the more rapid decline in leaf dry matter in the grain cultivars as compared to the forage cultivars. The leaf + pod/stem ratio, however, was greater in Pella 85, Sherman, and Kenwood than in the forage cultivars (Table 5). Cultivars with a greater leaf + pod/stem ratio may be used by producers who wish to include the energy-rich pods in their feed (Albro et al., 1993).
These results are similar to those of Sheaffer et al. (2001), who compared forage and grain type soybean in Minnesota and found that the forage cultivars had reached an average maturity of R3 when the grain cultivars had reached R6 and R7. The forage cultivars were found to produce little or no grain, depending on harvest date. The forage cultivars had more leaf and stem yield and less pod yield than the grain cultivars in that study.
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CONCLUSIONS
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Soybean cultivars developed for forage were later maturing than grain types adapted to Iowa and, accordingly, produced greater vegetative growth (height, leaf, and stem dry matter) and greater total dry matter. The forage cultivars also performed differently than grain type soybean within the same maturity group, suggesting that the greater productivity of the forage cultivars transcended the effect of maturity group. The PA forage cultivars produced greater dry matter production than the OR lines when harvested in August, but the OR lines produced greater dry matter when harvested in late September.
The forage quality produced by these cultivars remains to be considered. Forage cultivars had a lower leaf/stem ratio in August and lower leaf + pod/stem when compared with the grain cultivars. Sheaffer et al. (2001) reported a one-third decrease in crude protein in forage type compared with grain type soybean. The extent to which the greater yield of the forage cultivars is offset by this reduction in forage quality will be discussed in a future paper.
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NOTES
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Journal Paper no. J-17826 of the Iowa Agric. and Home Economics Exp. Stn., Ames, IA, Project no. 2899.
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ABSTRACT
INTRODUCTION
