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Published in Crop Sci 39:1380-1384 (1999)
© 1999 Crop Science Society of America
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Crop Science 39:1380-1384 (1999)
© 1999 Crop Science Society of America

CROP ECOLOGY, PRODUCTION & MANAGEMENT

Sorghum Intercropping Effects on Yield, Morphology, and Quality of Forage Soybean

Daren D. Redfearna, Dwayne R. Buxtonb and Tom E. Devinec

a LSU Agric. Center, Southeast Res. Stn., Franklinton, LA 70438 USA
b USDA-ARS-NPS, Beltsville, MD 20705 USA
c USDA-ARS, Weed Science Lab., Beltsville, MD 20705 USA

dredfearn{at}agctr.lsu.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Shading patterns when two forage species are intercropped may be different than in a monocrop environment. Our objectives were to quantify yield and forage quality response of forage soybean [Glycine max (L.) Merr.] intercropped with forage sorghum [Sorghum bicolor (L.) Moench] and compare to the measurements of monocrop soybean. Soybean plants were harvested from the middle portion of individual rows in plots containing only soybean and from plots having alternating soybean and sorghum rows spaced 76 cm apart. Morphological and forage quality measurements were determined on leaf and stem fractions. Morphological measurements included main stem length, node number, leaf area ratio (LAR), specific leaf weight (SLW), and stem diameter. Forage quality constituents included in vitro dry matter disappearance (IVDMD), neutral detergent fiber (NDF), hemicellulose, and cellulose, and crude protein (CP) concentrations. Intercropped soybean had 6 more plants m-1 of row, less advanced morphological development, and 2.3 Mg ha-1 less dry matter than monocrop soybean. Leaf IVDMD, NDF, hemicellulose, and cellulose did not differ between intercrop and monocrop soybean. However, stem IVDMD was 33 g kg-1 greater for intercropped than monocrop soybean, reflecting the 36 g kg-1 decrease in NDF concentration. Intercropped soybean was lodged both years more than monocrop soybean, which may have been due to the significant decrease in stem NDF. Leaf CP concentration was 25 g kg-1 greater for monocrop soybean than intercropped soybean; however, stems from intercropped soybean had 12 g kg-1 greater CP than monocrop soybean stems. Soybean exhibited a high degree of morphological plasticity, presumably in response to increased competition for solar radiation. Although forage quality of intercrop soybean was greater than monocrop soybean, intercropping forage-type soybean with another tall-growing forage does not appear to be practical because of the decrease in dry matter accumulation.

Abbreviations: ADF, acid detergent fiber • ADL, acid detergent lignin • CP, crude protein • DM, dry matter • IVDMD, in vitro dry matter disappearance • LAR, leaf area ratio • NDF, neutral detergent fiber • SLW, specific leaf weight


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
INTERPLANT COMPETITION is usually mediated through competition for soil water, available nutrients, and solar radiation, although other factors such as temperature fluctuation and pest infestations may be equally important (Buxton and Fales, 1993). Plants grown together frequently compete primarily for solar radiation. Thus, forage plants typically experience shaded growing conditions resulting from both intra- and interspecies shading. Intraspecies shading can occur in several ways. In monoculture cropping systems, one plant may shade another or a plant component may cast a shadow on a plant component on the same plant. Interspecies shaded growing conditions, such as would occur in agroforestry or multispecies cropping systems, may be more common since two or more plant types with vastly different growing habits often compete for the same space.

Direct and indirect effects of shading on forage quality, morphological development, and forage yield have been reported. These differences may have resulted from species variation, length of the shading period, changes in the leaf-to-stem ratio, or environmental conditions (Buxton and Fales, 1993). Cell-wall concentrations have been shown to decrease under shaded conditions (Wong and Wilson, 1982; Kephart and Buxton, 1993). Therefore, shading may reduce available energy necessary for secondary cell-wall development (Kephart and Buxton, 1993). These decreases in cell-wall concentration have been related to increases in in vitro dry matter disappearance (IVDMD) in some studies. However, reported effects of shading on forage digestibility have been small. Kephart and Buxton (1993) observed only a 50 g kg-1 increase in IVDMD with heavy shading of both C3 and C4 grasses. Conversely, Wong and Wilson (1982) reported that although cell-wall concentration decreased, IVDMD also decreased with increased shading for a C4 grass and a tropical legume. Henderson and Robinson (1982) observed that digestibility was affected more by temperature than irradiance flux density for four C4 grasses. In this same study, species variation for digestibility was substantial and positively correlated with temperature effects.

Although growth rates and subsequent dry-matter accumulations generally decreased with increased shading for alfalfa (Medicago sativa L.) (Walgenbach and Marten, 1981; Kephart et al., 1992), some studies have reported higher yields under moderate shading conditions than in full sunlight (Eriksen and Whitney, 1981). Kephart et al. (1992) reported that leaf area ratio and specific leaf weight for three C3 grasses and two C4 grasses decreased with increasing shade. Although leaf blade mass decreased 210 g kg-1 with increased shading in the C4 species, leaf area ratio increased with shading. Leaf blade mass per shoot for the C3 species did not respond to shading. Shading did not affect leaf-to-stem ratio, shoot length, or rate of development for either the C3 or C4 species.

Soybean is desirable as a forage crop because of higher protein concentration than is found in many other forages and its capability for dinitrogen fixation. Soybean was first introduced into the USA for use as a high yielding forage crop (Willard, 1925). There is little information on yield and forage quality of forage-type soybean. Available data is from studies where grain-type soybean was evaluated for its forage potential. Some of the most definitive work on the value of soybean as a forage crop was conducted in southern Wisconsin (Hintz and Albrecht, 1994). Among grain-type soybean varieties, dry matter yield and forage quality were closely related to maturity. Today, most soybean cultivars presently used for forage are the shorter grain types (Hintz et al., 1992). Tall-growing forage-type soybean developed specifically for forage production are approximately twice as tall as conventional grain types, if harvested late in the growing season (T.E. Devine, 1990, unpublished data).

Adaptive responses in forage yield, developmental morphology, and forage quality under shaded conditions of potentially high-yielding and high-quality forage-type soybean are not well understood. A tall growing forage-type soybean may compete better than the conventional, short-statured grain types when intercropped with another tall-growing forage species. Our objectives in this experiment were to evaluate the yield and quality responses of morphological components of tall-statured forage soybean intercropped with forage sorghum compared with monocrop forage soybean.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Field plots were established on 25 May 1995 and 21 May 1996 at the Iowa State University Agronomy and Agricultural Engineering Research Center located approximately 13 km west of Ames, IA (42° N lat., 93° W long.). Plots were established on a Nicollet loam (fine-loamy, mixed, mesic Aquic Hapludoll) in a randomized complete block design with four replicates. Experimental lines and cultivars used in the study were `OR 19-12-2' forage soybean and `Kansas Orange' forage sorghum. Experimental units consisted of 8-row plots, 7.6 m long with 0.76 m between rows. Soybean was planted at 19.7 seeds m-1 of row. Sorghum was planted at 29.6 seeds m-1 of row and thinned after emergence to 19.7 plants m-1 of row. The intercropped plots were planted on alternating rows at the same seeding rates as the monocrop plots. Sorghum received N (hand-applied along the row) as ammonium nitrate (34-0-0) at 168 kg ha-1 approximately 1 wk after emergence.

Evaluation of intercropping effects on forage sorghum was not part of this study. Soybean plants were harvested when monocrop soybean reached full-flower (Ritchie et al., 1982). This occurred 83 d (16 August) and 98 d (27 August) after planting in 1995 and 1996, respectively. Plants were hand-harvested at ground level from a 25.5-cm section of the middle portion of an individual row within plots. Node and plant developmental stages were noted according to the descriptions of Ritchie et al. (1982). Determination of main stem length, number of nodes, and plant developmental stages were made after harvest. Extended plant height was measured from the base of the plant to the tip of the last fully expanded leaf cluster. All harvested plants were enclosed in plastic bags and placed on ice for approximately 2 h until remaining morphological parameters were determined.

Stems were separated into upper and lower stem segments immediately below the eighth node from ground level. Stem diameter determinations were made immediately above the eighth node for the upper stems and immediately below the cotyledonary node for the lower stems with a sliding micrometer. Leaflets were removed from the upper and lower stem sections and leaf area determinations made using an LI-3100 Area Meter (LI-COR Inc., Lincoln, NE). Dry weight contributions from the individual morphological components from the upper and lower canopy were determined after drying to a constant weight at 55°C. Data from leaf area determinations and dry matter contribution were used to calculate leaf area ratio (LAR) and specific leaf weight (SLW).

Dried leaf and stem samples were ground in a Wiley mill (Thomas Manufacturing, Philadelphia, PA) to pass a 1-mm screen. Leaf and stem samples were analyzed sequentially for neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) concentrations by the procedures of Goering and Van Soest (1970), which were modified by excluding decalin and sodium sulfite. Additionally, 2 mL of a 2% (w/v) {alpha}-amylase (Sigma Chemical Co., St. Louis, MO, no. A-3051) solution was added at the mid-point of refluxing during the NDF procedure (Van Soest and Robertson, 1980). Hemicellulose concentration was calculated as the difference between NDF and ADF and cellulose concentration was calculated as the difference between ADF and ADL concentrations. Crude protein (CP) was measured colorimetrically (AOAC, 1990). In vitro dry matter disappearance was determined following anaerobic fermentation in rumen fluid for 48 h by the direct acidification method (Marten and Barnes, 1980). Rumen fluid was collected from a ruminally fistulated steer (Bos taurus) fed an alfalfa hay diet.

The experiment was designed as a randomized complete block with a split-plot arrangement of treatments. Data were analyzed as a split-plot with cropping system (monocrop vs. intercrop) as the whole plot and canopy position (upper vs. lower) as the sub-plot. All analyses were performed using the GLM procedure of SAS (SAS, 1988). Treatment means of significant main effects and interactions were compared by Fisher's protected least significant difference at (Steele and Torrie, 1980).


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Yield and Yield Components
Total dry matter yield and dry matter contribution for leaf and stem yield components were not different across years, so all yield data is presented as the mean of 2 yr. Total dry-matter yield of forage soybean was different between the two cropping systems (Table 1) . Monocrop forage soybean produced 2.3 Mg ha-1 more total dry matter compared with intercropped forage soybean.


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  		Table 1 Total dry-matter and morphological-component yields from upper and lower canopy positions of a forage-type soybean grown as a monocrop and intercropped with forage sorghum

 
For the leaf and stem yield components, there were significant cropping system x canopy position and year x canopy position interactions. For both yield components, the interactions were due to differences in magnitude only. The largest portion of error was partitioned into the effects of canopy position and cropping system; therefore only these two significant main effects will be discussed. Leaf and stem components differed in their contribution to overall dry-matter yield of monocrop and intercropped soybean. Dry-matter contribution from the yield components were similar in the lower canopy, although stem weight for monocrop soybean was greater than intercropped soybean. Actual contribution of yield components in the upper canopy was significantly greater for both the leaves and stems for monocrop soybean than intercropped soybean (Table 1).

Averaged across canopies, leaf and stem yield was approximately 56% greater in monocrop soybean than intercropped soybean. Averaged across cropping systems, stem yield was 40% greater in the upper canopy than lower canopy, but dry-matter contribution from the leaf fraction was more than four-fold greater in the upper canopy than lower canopy. The results of this study are consistent with those of Walgenbach and Marten (1981) and Kephart et al. (1992) conducted on different species, who observed decreased dry matter accumulation under shaded conditions.

Plant Morphology
Canopy height of soybean intercropped with sorghum was approximately 35 cm in height because of lodging of the canopy (data not presented). Conversely, the canopy of monocrop soybean was not lodged and approximately 50 cm high. In contrast, the height of the sorghum plants was approximately 240 cm. Corré (1983) observed enhanced stem elongation under moderately decreased irradiance. However, main stems were longer for the intercropped soybean compared with monocrop soybean (Table 2) . Plant density was 6 plants m-1 greater for the intercropped soybean compared with the monocrop soybean. Since both systems were seeded at 19.7 seed m-1 of row, this suggests that little, if any, mortality occurred within intercropped soybean. More than 98% of the plants survived in the intercropped system, but only 69% survived in the monocrop system.


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  		Table 2 Main stem length, plant density, node number, and growth stage of a forage-type soybean grown as a monocrop and intercropped with forage sorghum

 
Number of nodes was greater for monocrop soybean compared with intercropped soybean (Table 2). This may be directly related to the increased yield contribution for both leaves and stems, particularly in the upper canopy. This coupled with the observation that main stem length was different for the two cropping systems suggests that internode length was greater for the intercropped soybean. The increased length between nodes may have had a profound effect on the canopy architecture of soybean in the intercropped plots since shading often reduces node number (Ballare et al., 1991; Sánchez et al., 1993).

Morphological development was more advanced for monocrop soybean compared with intercropped soybean (Table 2). At harvest, monocrop soybean was in full-flower (R2), whereas the intercropped soybean had no flowers on the main stem and were still vegetative (V17). Apparently, critical levels of irradiance necessary for induction of reproductive development were not reached in intercropped soybean.

Stem diameter was 3.2 mm greater for monocrop than intercropped soybean in the lower canopy and 3.3 mm greater in the upper canopy of monocrop soybean compared with intercropped soybean (Table 3) . These differences in stem diameter support the conclusions of Corré (1983) and Jones (1985) on other plant species who observed that altered photosynthate partitioning maintains stem length at the expense of stem diameter when plants are shaded.


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  		Table 3 Leaf area ratio (LAR), specific leaf weight (SLW), and mean stem diameter of intercropped and monocrop forage-type soybean taken from upper and lower canopy positions

 
There was a year x cropping system x canopy position interaction for LAR. Monocrop soybean grown during 1996 had the lowest LAR in the lower canopy and the greatest LAR in the upper canopy. This difference was 10-fold, whereas the differences between monocrop and intercropped soybean LAR were usually two- to three-fold. There were significant cropping system x canopy position and year x canopy interactions for both LAR and SLW (weight per unit area). These differences were in magnitude only. The largest portion of error was partitioned into main effect of canopy position.

Soybean LAR was not different for the lower canopy between the two cropping systems. However, the upper canopy LAR was 1.6 m2 kg-1 DM greater for intercropped soybean compared with monocrop soybean (Table 3). Soybean SLW was not different for the lower canopy between the two cropping systems. However, upper canopy SLW for monocrop soybean was 20.3 g DM m-2 greater than upper canopy intercropped soybean. There was a significant year effect for soybean SLW. Soybean leaves were thinner in 1995 than 1996. However, the reason for this not clear. Greater LAR associated with lower SLW suggested that leaves in the upper canopy of intercropped soybean were larger and thinner than leaves of monocrop soybean. These responses were consistent with those of Corré (1983) and Pearce and Lee (1969) who observed similar responses of other plants grown under low irradiance.

Forage Quality
There were several significant interactions for fiber concentration and fiber composition. There was a significant cropping system x canopy position x morphological component interaction for cellulose concentration. This difference was in magnitude only. There were significant year x cropping system interactions for NDF, ADF, and cellulose concentrations. Neither NDF, ADF, nor cellulose were different in 1996 for either cropping system, whereas, 1995 monocrop NDF, ADF, and cellulose were 28, 37, and 31 g kg-1 greater, respectively than 1996 intercropped soybean. Likewise, a year x morphological component interaction existed for NDF concentration. This occurred as a result of leaves from 1995 having greater NDF concentration than leaves from 1996, even though NDF concentration for stems was similar for both years. Only the cropping system x canopy position and cropping system x morphological component interactions were determined to be nutritionally important in explaining fiber concentration and fiber composition differences.

Fiber concentration and fiber composition of leaves were not different between monocrop and intercropped soybean (Table 4) . Monocrop soybean stems had 36 g kg-1 greater NDF and 24 g kg-1 greater ADF concentration than intercropped soybean stems (Table 4). Similarly, hemicellulose and ADL concentrations were 12 and 7 g kg-1 greater, respectively, for stems of monocrop soybean compared with intercropped soybean. However, cellulose concentration was 18 g kg-1 greater for intercropped soybean compared with monocrop soybean (Table 4). Because intercropped soybean was less mature than monocrop soybean (Table 2), these differences in forage quality may be mostly related with developmental characteristics.


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  		Table 4 Neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose (HCEL), cellulose (CELL), acid detergent lignin (ADL), and crude protein (CP) concentrations of the morphological components from sole-cropped and intercropped forage-type soybean

 
Crude protein concentration was 58 and 35 g kg-1 greater for upper leaves and stems, respectively, than lower leaves and stems (Table 5) . Averaged across upper and lower canopies, leaves of monocrop soybean had 25 g kg-1 greater CP than leaves of intercropped soybean (Table 4). Conversely, CP concentration for intercropped soybean stems was 12 g kg-1 greater than monocrop soybean stems (Table 4). Concentrations of NDF, ADF, cellulose, and ADL were greater for both the leaves and stems of the lower canopy compared with leaves and stems in the upper canopy (Table 5). Conversely, hemicellulose differed for only the stems. Neutral detergent fiber was 30 and 163 g kg-1 greater for lower leaves and stems, respectively, than upper leaves and stems. Likewise, ADF was 29 and 145 g kg-1 greater for lower leaves and stems, respectively than upper leaves and stems. Hemicellulose concentration was identical for leaves, but 18 g kg-1 greater for stems in the lower canopy. Cellulose concentration was 16 and 102 g kg-1 greater for lower leaves and lower stems, respectively, than upper leaves and stems. Acid detergent lignin concentration was 13 and 45 g kg-1 greater for lower leaves and stems, respectively, than upper leaves and stems. These forage quality responses may have resulted only from less mature morphological components within the upper canopy compared with the lower canopy. However, changes in plant metabolism could have occurred because of altered irradiance within the intercropped plots. Hintz and Albrecht (1994) noted that NDF and lignin concentrations of leaves and stems generally increased with maturity; however, CP of leaves generally decreased. In stems, CP was relatively unchanged between R1 and R5 and then decreased by R7. Ocumpaugh et al. (1981) reported that forage yield and quality was optimized when soybean was harvested between R5 and R7.


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  		Table 5 Neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose (HCEL), cellulose (CELL), acid detergent lignin (ADL), and crude protein (CP) concentrations of the morphological components selected from upper and lower forage-type soybean canopies when averaged across cropping systems

 
Differences in IVDMD of the morphological components were less straightforward than the observed morphological differences (Fig. 1) . Similar to the differences observed for fiber composition and CP, several sources of variation were significant for IVDMD. There was a significant cropping system x canopy position x morphological component interaction. Generally, greater differences in IVDMD occurred for the stems than leaves. Lower leaves and stems of intercropped soybean had 33 and 42 g kg-1 greater IVDMD, respectively, than lower leaves and stems of monocrop soybean. Upper stems of intercropped soybean had 33 g kg-1 greater IVDMD than upper stems of monocrop soybean. In contrast, upper leaves of intercropped soybean had 25 g kg-1 lower IVDMD than upper leaves of monocrop soybean. Munoz et al. (1983) observed that IVDMD of soybean leaf blades was near 700 g kg-1 for all maturity stages. Stems ranged between 400 and 500 g kg-1 with the lowest values in both the most immature and mature stages. Digestibility of whole-plant soybean forage was nearly constant during the growing season because of the increased contribution from the pod component late in the season.



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  		Fig. 1 In vitro dry matter disappearance (IVDMD) of intercropped and monocrop forage-type soybean morphological components from upper and lower canopy positions;

 
Although the canopy height of the forage-type soybean typically exceeded 50 cm when planted as a monocrop, its competitiveness when intercropped with forage sorghum was not as great since sorghum plants were approximately four-fold taller. Intercropped soybean exhibited a high degree of morphological plasticity, presumably in response to increased competition for light. This was primarily manifested in the differences in main stem length, plant density, and growth stage of the intercropped soybean. In spite of the morphological changes that followed in response to shade, dry matter accumulation was reduced. The severity of lodging for intercropped soybean likely resulted from a combination of several factors, namely thinner stems with longer internodes and decreased NDF concentration.

Most likely, the 36 g kg-1 decrease in stem NDF in the intercropped soybean in combination with longer internodes had some effect on the tendency of these soybean canopies to lodge. Furthermore, this same decrease in NDF probably had a major influence on the 33 g kg-1 increase in stem IVDMD in the intercropped soybean. Although forage quality of intercropped forage-type soybean did increase, intercropping forage-type soybean with another tall-growing forage crop does not appear to be viable because of the significant decrease in dry matter accumulation. It is likely that forage soybean yield may not be reduced to the extent observed in this experiment if it is grown with another forage of similar stature, such as a dwarf pearl millet [Pennisetum americanum (L.) Leeke] variety or another short or intermediate sorghum variety.SAS Institute Inc 1988


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Joint contribution of the Corn Insects and Crop Genetics Unit Ames, IA, and U.S. Dairy Forage Res. Center, Madison, WI. Names are necessary to report accuracy of available data; however, the USDA neither guarantees nor warrants the standard or the product, and the use of the name by the USDA implies no approval of the product to the exclusion of other suitable products.

Received for publication September 7, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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