Published in Agron. J. 96:966-970 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
PRODUCTION PAPER
Forage Soybean Yield and Quality Responses to Plant Density and Row Distance
Stefan Seitera,
Craig E. Altemoseb,* and
Michael H. Davisc
a Dep. of Agric. Sci., Linn Benton Community College, Albany, OR 97321
b Penn State Coop. Ext., 420 Holmes Street, Bellefonte, PA 16823
c Cornell Univ. Willsboro Res. Farm, Willsboro, NY 12996
* Corresponding author (cea10{at}psu.edu).
Received for publication May 6, 2003.
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ABSTRACT
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We conducted a study to determine the effect of row distance and plant density on yield and quality components of ‘Donegal’, a soybean [Glycine max (L.) Merr.] cultivar specifically developed for forage production in the northeastern United States. Dry matter yield ranged from 4.5 to 6.3 Mg ha–1 and 8.7 to 13.9 Mg ha–1 in 2000 and 2001, respectively. Yield in rows spaced 18 cm apart was significantly higher than in rows spaced 76 cm apart. Soybean plants started to lodge between 56 and 84 d after planting. At most sampling dates, the canopy was lower, and individual plants were shorter in 18- compared with 76-cm rows. Soybean attained the beginning seed (R5.5) stage before a killing frost in both years. At that stage, soybean averaged 155 g kg–1 crude protein (CP), 362 g kg–1 acid detergent fiber (ADF), and 469 g kg–1 neutral detergent fiber (NDF). Acid detergent fiber, NDF, and CP increased between beginning pod (R3) and R5.5 growth stages in 2000 and 2001. Forage quality in 2001 was lower at 76- compared with 18-cm rows. Plant densities between 234650 and 555750 plants ha–1 had no consistent effect on forage yield or quality. Donegal showed good potential as a forage crop in the northeastern United States.
Abbreviations: ADF, acid detergent fiber • CP, crude protein • DAP, days after planting • NDF, neutral detergent fiber
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INTRODUCTION
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HISTORICALLY, soybean was planted in the United States as a forage and hay crop (Smith and Huyser, 1987). However, the use of soybean shifted over time, and by 1941, the acreage of soybean grown for grain exceeded that grown for forage. Hay and forage soybean production since then has been of minor importance and is practiced most often when crop damage limits grain harvest (Sheaffer et al., 2001). Recently, interest in growing soybean specifically as a forage crop has increased, in part due to the availability of soybean cultivars that were bred for forage (Devine and Hatley, 1998; Devine et al., 1998a, 1998b). Donegal was developed for the northeastern United States, and it is estimated that more than 3700 ha were planted there in 2001. The acreage of forage soybean production was expected to double in 2002 (USDA-ARS, 2001).
Livestock producers in north-central and northeastern North America are interested in growing and ensiling forage soybean as an alternative to perennial legumes such as alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.). Despite substantial improvement of winter-hardy cultivars, stands of these perennials are often difficult to maintain long enough to justify the high expense of establishment. Hintz et al. (1992) noted that on farms that have difficulty maintaining alfalfa stands, forage soybean may be a viable alternative with comparable forage quality. Similar to perennial legumes, forage soybean provides benefits when grown in rotation with corn (Zea mays L.) or other nonleguminous field crops. However, as an annual crop, forage soybean may be more attractive to include in crop rotations than perennial legumes because land is not committed for multiple years. This is especially important in the northeastern United States where corn is grown without rotation on a large percentage of the scarce productive land to supply continually increasing dairy herds with sufficient high-energy feed. Dairy farmers also may have greater opportunity to apply manure if an annual legume is used in the rotation with grain and corn, rather than alfalfa. In addition, the same equipment may be used to plant and harvest corn and soybean.
Few studies have examined the effect of plant density and row distance on yield and quality of new forage soybean cultivars. Without available recommendations, farmers currently use a wide range of production practices. Row distance and plant densities are often based on grain soybean production. This practice may not provide optimum forage yield and quality because of differences in vegetative and reproductive development between grain and forage cultivars. Grain soybean in the United States is commonly planted between 18- and 89-cm row distance, depending on planting equipment available on the farm. Parvez et al. (1989) noted that grain yield of both determinate and indeterminate soybean grown in the northern United States is often higher because a more equidistant spacing between the plants in the narrow-row culture increases radiation use efficiency, particularly before soybean canopy closure (Savoy and Cothren, 1992; Wells, 1991). Typically, grain soybean in the northern United States is planted at a density of 300000 to 600000 seeds ha–1. While the optimum density may vary with location and specific field conditions, soybean exhibits considerable plasticity and may not respond to changes in density within this range (Hicks et al., 1990). A yield response can be expected at lower densities. For example, Munoz et al. (1983) observed increasing soybean hay dry matter production with plant density ranging from about 100000 to 300000 plants ha–1.
Sheaffer et al. (2001) tested new forage varieties at two row distances and observed significantly higher yields in narrow-row culture. Hintz et al. (1992), using grain cultivars, also reported that narrow row distance produced higher forage yield but lower CP concentrations. They further found that plant density did not affect forage yield or quality. In addition to plant density and row distance, soybean developmental stage at harvest is a significant factor in determining forage yield and quality. Counteracting protein and fiber concentrations of different tissues during the reproductive growth stages result in changing forage quality. For example, fiber concentration of stem tissues continually increases during the reproductive growth stages while increasing amounts of highly digestible pods counteract this effect (Munoz et al., 1983). Protein levels also increase as pods develop. Several studies suggest full seed (R6) to beginning maturity (R7) as the optimal growth stages for harvest (Hintz et al., 1992; Munoz et al., 1983; Willard, 1925) when seeds are fully developed and dry weight and nutrient accumulation begin to slow (Fehr and Caviness, 1977).
This study was conducted to investigate agronomic practices for growing Donegal, currently the only forage soybean cultivar adapted specifically for the northeastern United States. Our objective was to determine the effect of plant density and row distance on Donegal yield and forage quality. We also observed development of plant length, canopy height, and forage quality over time to establish optimum harvest dates and further refine agronomic recommendation for this cultivar.
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MATERIALS AND METHODS
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A replicated small-plot study was conducted in New Hampshire in 2000 and 2001 involving Donegal, a Maturity Group V forage-type soybean cultivar. The trial was conducted on a Charlton loam soil (Inceptisol, coarse-loamy, mixed, active, mesic Typic Dystrudepts) at the University of New Hampshire Kingman Research Farm in Durham. The site is at 43° N latitude, with an average of 216 frost-free days and 2264 growing degree days (base 50).
Experimental treatments included two intrarow distances (18 and 76 cm) and three plant densities (low, medium, and high). The medium target plant density was the same in 2000 and 2001 while the low and high densities were changed in 2001 to expand the density range tested in this study (Table 1). A split-plot design with four replications was employed using row distance as the main-plot treatment and population as the subplot treatment. Plot sizes were 3.05 by 5 m (four rows per plot) and 1.42 by 5 m (eight rows per plot) for 76- and 18-cm row distance treatments, respectively.
We applied 200 kg ha–1 10–10–10 fertilizer and 3.4 Mg ha–1 lime according to soil test recommendations before planting in both growing seasons. Weed control was achieved manually. In 2000, the experiment was planted by hand on 2 June and not thinned due to poor seedling emergence. The actual densities in the field were determined by counting plants in three randomly selected, 1-m row sections within each plot. The plots were harvested 125 d after planting (DAP) with a mechanical forage harvester on 5 October, the day after the first fall frost. In 2001, soybean was planted on 29 May with a four-row corn planter and a custom-made eight-row cone drill for 76- and 18-cm row distance treatments, respectively. Plots were thinned by hand to the desired densities 35 DAP and harvested 127 DAP on 3 October, the day before anticipated frost.
We measured canopy height throughout the growing season by randomly selecting three locations in each plot and measuring from the soil surface to the highest plant component, which was usually a soybean leaf. At the same locations, we collected plant length data by stretching out individual plants and measuring from the soil surface to the top soybean leaf. Lodged plants were untangled from neighboring plants when necessary. To determine dry matter yield, we harvested 3-m sections of the center two rows and the center five rows from the 76- and the 18-cm row distance plots, respectively. We weighed the fresh-cut forage using a digital bench scale and took 500-g subsamples from each plot, which were oven-dried for 72 h at 60°C to determine moisture content.
Forage quality data were collected three times. The sampling dates were 100 and 115 DAP and harvest (125 and 127 DAP in years 2000 and 2001, respectively). These harvest dates corresponded to reproductive growth stages R3, full pod (R4), and R5.5 (Fehr and Caviness, 1977). At each sampling date, 10 plants from rows outside the area that was used for final yield determination were cut at 5 cm above ground and analyzed for forage quality. Crude protein was determined by Kjeldahl procedure (% CP = % N x 6.25) according to semiautomated method of AOAC Official Method 976.6 (AOAC, 1990). Acid detergent fiber content was determined according to AOAC Official Method 973.18 (AOAC, 1990). Neutral detergent fiber content was determined by method of van Soest et al. (1991). Data were analyzed separately for each year using a split-plot design model in the GLM procedure in SYSTAT (SPSS, 2000).
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RESULTS AND DISCUSSION
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Yield
Dry matter yield ranged from 4.5 to 6.3 Mg ha–1 in 2000 and 8.7 to 13.9 Mg ha–1 in 2001 (Fig. 1)
. The lower yield during the first year was likely the result of adverse climate conditions. Unseasonably low temperatures and frequent rain over a prolonged period delayed planting, reduced seedling emergence (Table 1), and slowed seedling growth. Cooler-than-normal weather and late planting date strongly affect soybean yield in the northern United States (Beaver and Johnson, 1981; Parker et al., 1981). Yield of a Group V cultivar, which would not be planted as grain at this latitude, may be especially sensitive to cool temperatures and delayed planting date. Yields observed in the current experiment are comparable to previously published studies, which reported yield between 2.6 and 14.3 Mg ha–1, depending on environmental conditions and management (Koivisto et al., 2002b; Nayigihugu et al., 2000; Sheaffer et al., 2001). Similar to our study, Devine et al. (1999) reported year- to-year differences of over 100% in Donegal dry matter yield.
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Fig. 1. Dry matter (DM) yield at final harvest of forage soybean at two row distances and three plant densities in 2000 and 2001 at Durham, NH. Graphs show mean values; n = 4, vertical bars = standard errors of means.
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No interactions between row distance and plant density were observed in regards to dry matter yield (Fig. 1). Row distance had a significant effect on forage yield while plant density had no effect in this study. Similarly, Hintz et al. (1992) found no response in forage yield to plant densities ranging from 280000 and 890000 plants ha–1. These results correspond to soybean grain yield, which usually shows little response to changes in plant density within a wide range (Johnson, 1987). While all soybean plants have a general lack of apical dominance and potential for prolific branching (Raper and Kramer, 1987), these characteristics are likely stronger in cultivars bred for forage production. Compared with grain cultivars, one could expect a greater ability of forage cultivars to compensate for low plant density when branching and leaf production continues vigorously throughout the growing season.
Higher yields were achieved at the 18- compared with the 76-cm row distance in both years (Fig. 1). Averaged over population treatments, dry matter yields in narrow rows were 32% and 49% higher in 2000 and 2001, respectively. Hintz et al. (1992) and Sheaffer et al. (2001) also found higher forage yield in narrow rows although differences were not a great. These findings are consistent with the effect of row width on soybean grain yield (Parvez et al., 1989).
Plant and Canopy Height
Forage soybean cultivars were bred for exceptionally tall height (Devine et al., 1999). Canopy height is important when considering harvest equipment and the potential of forage soybean for intercropping with other crops. Canopy heights of 155 cm have been observed (Nayigihugu et al., 2000). Canopy height in the current trial was lower with a mean of 74 and 90 cm in 2000 and 2001, respectively (Table 2). Canopy height was higher in wide-row compared with narrow-row treatments throughout the growing season in 2000 and from 56 to 84 DAP in 2001. Length of individual plants exceeds canopy height as soon as soybean plants start to lodge. Plants were longer in wide compared with narrow rows in both years (Table 1). Longer plants may develop in wide-row culture because closer spacing within the row results in partial shading at earlier growth stages. Partial shading can change the light quality in the canopy (Raper and Kramer, 1987), triggering plants to develop longer stem internodes (Kasperbauer, 1971). Despite large yearto-year differences in yield, individual plant length at harvest was similar in both years, ranging from 143 to 179 cm. Thus, dry matter yield increase in 2001 did not reflect increased plant length but likely was the result of increased branching, and leaf production.
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Table 2. Row distance and plant density effects on canopy height and plant length of forage soybean between 28 d after planting (DAP) and final harvest at Durham, NH, in 2000 and 2001.
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Soybean plant height and lodging tend to increase with higher seeding rates (Johnson, 1987). We observed a trend of longer plants with increased plant density in 2000 but did not observe an effect in 2001. Plants in all treatments lodged considerably, and we did not observe a particular effect of plant density on the degree of lodging. Canopy height in all population treatments was only slightly lower than plant length between 0 and 56 DAP, indicating that plants were upright. Lodging of plants started to occur between 56 and 84 DAP as we observed increasing differences between canopy height and plant length (Table 2). No measurements were conducted between 56 and 84 DAP. However, we observed that within that period, lodging occurred later in narrow-row treatments where plants initially were not as tall and appeared to provide physical support to each other for a longer period of time because of the more equidistant planting pattern.
Quality Components
Forage soybean in both years matured to the R5.5 stage before a killing frost. At that growth stage, soybean averaged 155 g kg–1 CP, 362 g kg–1 ADF, and 469 g kg–1 NDF (Table 3). Crude protein, ADF, and NDF were lower in 2001 compared with 2000. Forage quality was comparable to previous studies with Donegal (Devine et al., 1999; Koivisto et al., 2002a; Nayigihugu et al., 2000) while Sheaffer et al. (2001) observed generally higher ADF and NDF values for other forage soybean cultivars.
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Table 3. Row distance and plant density effect on crude protein (CP), acid detergent fiber (ADF), and neutral detergent fiber (NDF) of forage soybean at reproductive growth stages R3 (beginning pod), R4 (full pod), and R5.5 (beginning seed) at Durham, NH, in 2000 and 2001.
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We observed an increase in CP, ADF, and NDF concentrations between R3 and R5.5 (Table 3). Hintz et al. (1992) found a constant CP concentration from R3 to R5. Whole-plant CP concentration of grain soybean cultivars can increase significantly beyond R5 as protein becomes concentrated in the pods while CP in other tissues such as stems and leaves remains constant or decreases only slightly (Munoz et al., 1983). We do not know if Donegal CP concentration would increase similarly at R6 or R7. Increasing ADF and NDF concentrations in our study were likely the result of increasing fiber concentrations of stem tissues. Increasing amounts of highly digestible pods beyond R5 would counteract the overall increase in fiber concentration (Munoz et al., 1983). However, pod and bean tissues in forage soybean are of minor importance before R5 (Koivisto et al., 2002b).
We did not observe an effect of plant density on CP, ADF, or NDF. Only minor effects of plant density on forage quality were found in previous studies. For example, Munoz et al. (1983) found a slight increase of in vitro dry matter digestibility (IVDMD) of the stem and petiole tissues at lower densities while observing no effects on leaf and pod tissues. Hintz et al. (1992) found higher CP and lower NDF and ADF at lower plant density but concluded that the observed effects were not biologically meaningful.
Forage soybeans had higher forage quality in 18- compared with 36-cm rows in 2001. Crude protein concentration was higher while ADF and NDF were lower from R3 through R5.5. The effect of row distance on forage quality was less consistent in 2000. Hintz et al. (1992) and Sheaffer et al. (2001) found increased stem diameter with decreasing row distance but no consistent effect of row distance on CP, ADF, and NDF. We also observed increased stem diameter with decreasing row distance in the early growth stages as plants grew taller and thinner in wider rows while competing with closely spaced neighbors in the row. However, by the time we sampled for yield and quality at R3 to R5.5, the trend was reversed and stem diameter in the wide rows was larger than stem diameter in the narrow rows.
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CONCLUSIONS
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The soybean cultivar Donegal showed good potential as a forage crop in the northeastern United States. High dry matter yield and resulting high total CP production per unit area may attract potential adopters. However, significant year-to-year yield variation and relatively high fiber levels of this cultivar may limit the appeal. Under the local climate conditions, Donegal did not mature beyond R5.5 before a killing frost. There is a potential for higher forage quality if forage soybean could be grown to R6 or R7 when CP concentrations increase and ADF and NDF concentrations decrease due to nutrient accumulation in pods and beans. Higher CP and lower fiber levels would be desirable to achieve a nutritional value comparable to alfalfa cut at bud or early-bloom growth stage (Frame et al., 1998). Soybean cultivars of maturity groups more closely adapted to the region may attain the R6 or R7 stages and provide the desired quality. However, higher quality most likely will mean lower forage yield as available short-season grain cultivars have been bred for high grain yield at the expense of leaf area (Morrison et al., 2000).
Forage soybean should be planted in narrow rows. Wide rows planted with standard corn-planting equipment produced lower yield in both years and lower forage quality in one year. Our study showed that forage soybean has a remarkable plasticity in yield response to a wide range of plant densities. Standard densities for grain soybean (400 000 to 500 000 plants ha–1 in the northeastern United States) should provide adequate forage yield and quality.
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ACKNOWLEDGMENTS
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We thank Seedway Inc. for providing seed used in this study. This investigation was supported in part by Hatch grant NH00419 and by the college of Life Science and Agriculture, The University of New Hampshire–Durham. This is scientific contribution no. 2181 from the New Hampshire Agricultural Experiment Station.
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ABSTRACT
INTRODUCTION
