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© 2005 Plant Management Network.
Accepted for publication 23 August 2005. Published 26 September 2005.

Soybean and Clay Cowpea Grown for Forage Production in the Subtropics

Paul Mislevy, Range Cattle Research and Education Center, University of Florida, Ona 33865-9706; Ann R. Blount, North Florida Research and Education Center, University of Florida, Marianna 32446; Frank G. Martin, Statistics Department, University of Florida, Gainesville 32611-0840; and Brian T. Scully, Indian River Research and Education Center, University of Florida, Fort Pierce 34945-3138

Corresponding author: Paul Mislevy. pmislevy@ifas.ufl.edu

Mislevy, P., Blount, A. R., Martin, F. G., and Scully, B. T. 2005. Soybean and clay cowpea grown for forage production in the subtropics. Online. Crop Management doi:10.1094/CM-2005-0926-01-RS.

Abstract

In the southeastern U.S., few warm-season legumes grow rapidly, have high nutritional value, and tolerate high rainfall and temporary waterlogging. Soybean [Glycine max (L.) Merr.] and cowpea [Vigna unguiculata (L.) Walp.] are two warm-season legumes that are fairly well adapted to growing conditions in the southeastern U.S., however little is known about their production under high moisture and high temperature conditions. This study examined the feasibility of growing soybean and cowpea and their nutritional value under specific stress conditions common in subtropical environments. Significant (P < 0.05) dry biomass (DB) differences were found among soybean averaging 2.8 ton/acre (1999) to 3.0 ton/acre (2000) from a single harvest. Cowpea appeared to be more sensitive to saturated soil conditions; ‘Florida Clay’ cowpea yielded 0.4 ton/acre and ‘Iron Clay’ cowpea died from saturated soil. Mean differences in crude protein (CP) and in vitro organic matter digestion (IVOMD) among legumes were significant in 2000 (P < 0.012) and 2001 (P < 0.019), but not in 1999. Highest IVOMD yields were noted for ‘F94-2290' Long Juvenile (LJ) (3554 lb/acre in 1999) and ‘Biloxi’ (3925 lb/acre in 2000; 3074 lb/acre in 2001). These data indicate that soybean with high DB yields are best suited for forage production under subtropical environmental conditions. Long Juvenile soybean and Biloxi performed best overall, while cowpea tended to be more sensitive to excessive moisture and wet soil conditions.

Introduction

Options for warm-season legumes that tolerate high temperature and moist soil conditions for silage and hay production in the southeastern U.S. are limited. A warm-season annual legume would be desirable in livestock production systems in the southeast to help reduce soil-borne diseases and reduce nematode populations that occur when annual grass crops, such as corn (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench] are grown continuously (10,12). Legumes not only provide a good rotation scheme when alternated with grass crops, but also provide a high protein and high energy forage crop for livestock. Particularly in south Florida, a legume component in the cropping system would be desirable for dairy producers currently using a grass rotation system with corn-sorghum-ryegrass (Lolium multiflorum Lam.). While this cropping sequence produces high dry biomass yields and high energy for dairy cattle, it is low in protein, except for the ryegrass component. Dairy farmers could benefit from substituting all or part of their summer sorghum crop with a legume of high nutritional value, which will grow under the stressful climatic conditions in this region. While few annual legumes fit into this rotation system, forage soybeans might work.

The nutritive value of forage soybean varies with maturity (Fig. 1) due in part to the shift from vegetative to reproductive growth or seed production (50% flowering, 17% CP; 90% pod fill, 21% CP) (18). Differences in digestibility of forage soybean based on maturity are small [50% flowering, 59% in vitro dry matter disappearance (IVDMD); 90% pod fill, 61% IVDMD] (11,18). Cultivars selected for late maturity produced 6000 lb/acre in central Florida with a nutritive value of 15 to 18% CP and 59 to 64% IVOMD (18). Late maturity varieties specific for forage produce greater yield and have thinner stems than earlier maturing varieties (18). Ideally, harvest or grazing occurs at stage R6 (full seed) or R7 (beginning of physiological maturity) when quality is highest (pods are high in quality) but leaf loss is a minimum for maximum forage production (17). Ensiled soybean has yielded 4.3 ton/acre (11).

Fig. 1. Jose Moreno checking plant height of (left to right) Biloxi, F94-2119 LJ, Tyrone, and Benning soybean, 60 days after seeding.

Recently, ‘Hinson LJ’ and F94-2290 LJ soybean were developed and released by the Florida Agricultural Experiment Station (FAES). These soybeans can be harvested 90 to 100 days after seeding and provide forage of good nutritional value (1). The term "long-juvenile" refers to delayed flowering under short-day conditions. The LJ trait is believed to be under genetic control (5,16). The number of days to first bloom in LJ soybean is similar to conventional types when planted at normal planting time. The goal of utilizing this genetic material in southern soybean variety development was to widen the window of opportunity over a longer planting season. Long juvenile soybeans were selected for improved forage and seed yield compared to conventional soybeans.

Cowpea historically has been used as a cover crop for soil conservation and soil fertility improvement (11,18), however it may be feasible as a warm-season legume forage (14) (Fig. 2). Cowpea is not very drought tolerant but may be grown as forage if moisture is adequate (6). Under wet tropical conditions cowpea has yielded 1.6 to 3.8 ton/acre annually, while under drier conditions with irrigation, 1.9 to 2.5 ton forage per acre (11,14). Crude protein content averages 17 to 21% during the growing season and 14 to 22% during autumn in Texas (14). Cowpea has been inter-seeded into bermudagrass pasture and used for grazing goats (3).

Fig. 2. Iron clay pea (Vigna unguiculata) is a warm season legume grown as a cover crop or with corn (Zea mays) as a forage in southeastern U.S.

The purpose of this study was to determine the feasibility of growing soybean and cowpea under high seasonal temperatures and temporary water logging in central Florida and to quantify forage yield, whole plant fodder, and pod nutritional value of selected soybean and cowpea entries.

Site Description and Field Growing Practices

The experiment was conducted from 1999 through 2001 and was seeded each year during the last week of June at the University of Florida, Range Cattle Research and Education Center, Ona, FL. The soil was an Ona fine sand (sandy, siliceous, hyperthermic typic Alaquod). The experimental design was a randomized complete block with four replications in 1999 and 2000 and six replications in 2001. Soybean and cowpea entries used, seed source information, and years that entries were grown are shown in Table 1. Soybean and cowpea were inoculated with soybean and cowpea-type rhizobium to obtain adequate nitrogen fixing bacteria. Both soybean and cowpea were seeded to a final population of 156,000 plants per acre in a 30 inch-row spacing.

Table 1. Soybean entries, seed source information, and years grown at Range Cattle Research and Education Center, Ona, FL.

Fertilizer was applied pre-plant at 0-30-60 lb/acre (N-P₂O₅-K₂O) and 30 days post-seeding at 0-20-40 lb/acre (N-P₂O₅-K₂O). Total micronutrients applied pre and post seeding were Cu, Zn, Mn, and Fe (sulfate form) at 2.5 lb/acre; B at 0.25 lb/acre; and S at 5.0 lb/acre. In 2001, N at 50 lb/acre was applied to the experimental site 6 weeks after seeding.

A 10-foot row of forage was harvested to a one-inch stubble, when evidence of first leaf drop was observed (90 to 100 days after seeding). Plots were sampled for DB yield, whole plant nutritional value (CP and IVOMD), fodder-pod ratio (percentage on a dry biomass basis), CP and IVOMD of both fodder and pod, and CP and IVOMD yield per acre. Crude protein and IVOMD yield was calculated by percentage whole plant CP or IVOMD × DB yield per acre. Nine random plants were selected from each plot and separated into fodder and pods. Fodder consisted of stem, leaflets, and petioles without the pods, harvested at first leaf drop. The pods included the immature seed.

Harvested forage subsamples were dried at 60°C, ground, and analyzed for total N concentration (2,4) and IVOMD. Crude protein concentration was calculated as 6.25 × N. The IVOMD was determined by two-stage procedure of Tilley and Terry (19) modified by Moore and Mott (13).

Data were analyzed using PROC MIXED (SAS Institute Inc., Cary NC) and a model statement appropriate for a randomized complete block design. Since there was a significant difference between years, data was analyzed for each year separately. Differences were investigated (P > 0.05) using the Waller-Duncan k-ratio procedure with k = 100.

Annual Dry Biomass Yield

Differences for forage yield were significant in 1999 (P = 0.005) and 2001 (P = 0.019), but not for 2000 (P = 0.27). Forage yield of soybean from a single harvest in 1999 averaged 2.8 ton/acre (Table 2). Experimental line F94-2290 LJ produced the highest yield averaging 3.1 ton/acre, however this yield was not higher (P > 0.05) than entry F94-2119 LJ, which averaged 2.8 ton/acre.

Table 2. Dry biomass yield of soybean and clay cowpea grown during 1999, 2000, and 2001 at Ona, FL.

 * Means within the column followed by the same letter(s) are not different at the 0.05 level, Waller-Duncan k-ratio.

 † Entry died due to wet edaphic conditions.

Soybean DB yields for 2000 increased slightly to an average of 3.0 ton/acre. Biloxi and entry F94-2119 LJ produced yields averaging 3.4 and 3.1 ton/acre, respectively. Dry biomass yields for 2001 averaged 2.2 ton/acre, with Biloxi producing the highest (P < 0.05) DB yield averaging 2.9 ton/acre.

In general, average soybean DB yields were related to summer (July, August, and September) soil moisture conditions. Subtropical environments tend to receive excessive moisture during the summer period. Soybean prefers adequate soil moisture but not saturated soil conditions. However, soybean will tolerate wet soil with no standing surface water, by producing massive fibrous roots at the soil surface allowing plants to obtain oxygen and sustain nodulation.

Rainfall totals were 15 and 18 inches during the summer of 1999 and 2000 (7,8). Dry biomass production under these conditions averaged 2.8 and 3.0 ton/acre, respectively. These rainfall values were 30% lower than the long term average of 24 inches. However in 2001 total rainfall was 42 inches (78% higher) during the three summer months (9) resulting in a reduced DB yield to 2.2 ton/acre (Table 2).

The cowpea (Florida Clay and Iron Clay) is far more sensitive to saturated soils than soybean. In 2000 Iron Clay cowpea produced similar DB yields to soybean (Table 2) because summer rainfall was about 30% below average. However in 2001 when summer rainfall exceeded the average by 78% both cowpea entries performed poorly with Florida Clay yielding 0.4 ton/acre and Iron Clay cowpea dying in all replications.

Forage Nutritive Value

Mean differences in CP and IVOMD were significant in 2000 (P < 0.012) and 2001 (P < 0.019) but not for CP (P = 0.631) and IVOMD (P = 0.739) in 1999. Whole plant CP and IVOMD averaged 17.8 and 58.6% in 1999, respectively (Table 3). No differences were found in CP among cultivars, which ranged from 16.6% for F94-2119 LJ up to 18.8% for Hinson LJ. Whole plant digestibility ranged from a low of 57.7% for F94-2290 LJ to 59.8% for F94-2119 LJ. In 2000, the CP concentration averaged 17.7% for soybeans and 14.7% for cowpea with ‘Benning’ soybean containing the highest concentration (P > 0.05) averaging 19.3%. All other entries were not different (P > 0.05) except F94-2119 LJ, and Iron Clay cowpea (Table 3). Digestibility of F94-2290 LJ averaged 63.9% which was higher (P < 0.05) than F94-2119 LJ (59.1%), Biloxi (58.2%), and Iron Clay cowpea (56.8%). The higher whole-plant digestibility of F94-2290 LJ may be due to higher pod to fodder ratio and the increased digestibility of both pod and fodder when compared with the other entries (Table 4).

Table 3. Nutritional value [Crude Protein (CP) and in vitro organic matter digestion (IVOMD)] of whole plant soybean and cowpea harvested as forage during 1999, 2000, and 2001 at Ona, FL.

 * Means within the column followed by the same letter(s) are not different at the 0.05 level, Waller-Duncan k-ratio.

Table 4. Fodder-pod ratio, crude protein (CP), and in vitro organic matter digestion (IVOMD) of both fodder and pod of soybeans grown during 2000 and 2001 at Ona, FL.

 * Means within the column for each year followed by the same letter(s) are not different at the 0.05 level, Waller-Duncan k-ratio.

In 2001, average CP concentration for soybean dropped considerably to 12.7% when compared with 1999 (17.8%) and 2000 (17.7%) (Table 3). Experimental line F94-2290 LJ averaged 16.7% CP which was significantly higher than all entries except Benning. The low CP concentration in 2001 may be due to the excessive soil moisture from the above average rainfall received during the summer period (9). In fact, during one 36-day period the experimental site received 18.1 inches of rainfall including 4.0 inches within an hour. All entries turned yellow from saturated soil conditions. To encourage continued growth the experimental area was fertilized with N at 50 lb/acre. Within one week, all soybean entries turned dark green and were growing rapidly. Because of wet soil the soybean in this study developed a massive root system cluster at the soil surface with functioning nodules to about a 2-inch depth. Similar results were observed by Nathanson et al. (15). Unless extremely wet soil conditions (standing water) develop, this massive root cluster will support good soybean growth under wet edaphic conditions. However, cowpea did not develop the enlarged root system, consequently plants were severely injured from excessive soil moisture conditions. Iron Clay cowpea died in the study that year and only a sparse population of Florida Clay survived.

Whole plant soybean digestibility for 2001 averaged 56.6% and 70.1% for cowpea (Table 3). This value was similar to that obtained in 1999 and 2000, indicating soil moisture conditions had little effect on plant digestibility. Florida Clay cowpea had the highest digestibility averaging 70.1%, with all soybean cultivars being lower (P < 0.05).

To help determine the reason for differences in whole plant nutritional value, plants were separated into fodder and pods to determine the fodder-pod ratio and nutritive value of each plant part. Average fodder-pod ratio in 2000 was 57:43 with Benning having the highest pod ratio (Table 4). Since soybean seed are high in carbohydrates and Benning had a high pod ratio, this would partially explain why whole plant IVOMD was high. Significant differences were found in CP and IVOMD for both fodder and pod in 2000. Experimental line F94-2290 LJ had the highest CP concentration of fodder averaging 12.9%. This value was 4.8 and 4.5 percentage units higher than CP concentration in fodder of Benning and Tyrone, respectively. The digestibility of fodder for Tyrone was also very low averaging 41.9% when compared with Biloxi and the LJ soybean entries. The CP concentration and IVOMD content found in the pod was high for all entries averaging 30.0 and 70.4%, respectively. The cultivar Benning contained the highest CP concentration (33.0%) and among the highest pod IVOMD (72.0%) content.

The fodder-pod ratio in 2001 was 70:30 when soil conditions throughout the summer were extremely wet (Table 4). These conditions may be responsible for increased percentage fodder and decreased percentage pod. Comparing the fodder-pod ratio between individual cultivars for 2000 and 2001 revealed a similar relationship between the two years, even though the percentage fodder increased in 2001. Crude protein content during the wet year was also lower in both the fodder by 2.5 and the pod by 6.7 percentage units when compared with 2000. Digestibility also decreased in the fodder and pod portion of the plant during the wet year of 2001 compared with below normal rainfall of 2000.

To make an informed decision regarding the better cultivars for haylage, nutrient yield per acre is expressed as a single value. Average CP yield for 1999, 2000, and 2001 was 994, 1034, and 552 lb/acre, respectively (Table 5). The CP yield for 2001 was about half the values for 1999 and 2000. This decrease in CP yield is basically due to excessive moisture in 2001 causing plants to increase fodder content in the fodder-pod ratio, which is normally lower in CP percentage compared with pod. Over the 2- to 3-year period average CP yield was highest for F94-2290 LJ, Hinson LJ and Biloxi. The IVOMD yield followed a similar pattern to protein yield. Average yield of digestible forage for 1999, 2000, and 2001 was 3275, 3581, and 2474 lb/acre, respectively (Table 5). Highest IVOMD yields for 1999 were from F94-2290 LJ (3554 lb/acre), and Biloxi for 2000 (3925 lb/acre) and 2001 (3074 lb/acre).

Table 5. Crude Protein (CP) and in vitro organic matter digestion (IVOMD) yield as influenced by soybean and cowpea entry during 1999, 2000, and 2001 at Ona, FL.

Summary

These data indicate it would be best to select soybean entries with high DB yields for forage production. These entries tend to produce high CP and IVOMD yield. Long Juvenile soybean and Biloxi have performed very well. Soybean forage averaged about 16.1 and 58.8% CP and IVOMD, respectively. Plants tend to compensate in nutritional value between fodder and pods. Plants with high fodder nutritional value were lower in pod nutritional value. Therefore maximum nutritional yield per acre is obtained from soybean with high DB yield. Cowpea tend to be more sensitive to excessive moisture and wet edaphic conditions than soybean.

Acknowledgments

This research was supported by the Florida Agricultural Experiment Station.

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