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© 2009 Plant Management Network.
Accepted for publication 14 February 2009. Published 1 April 2009.

Performance of Forage Sorghum-Legume Mixtures in Southern High Plains, USA

Francisco E. Contreras-Govea, Assistant Professor, Agricultural Science Center, New Mexico State University, Artesia, NM 88210; Leonard M. Lauriault, College Associate Professor, Agricultural Science Center, New Mexico State University, Tucumcari, NM 88401; Mark Marsalis, Assistant Professor, Sangu Angadi, Assistant Professor, Naveen Puppala, College Assistant Professor, Agricultural Science Center, New Mexico State University, Clovis, NM 88101

Corresponding author: Francisco E. Contreras-Govea. fecontre@nmsu.edu

Contreras-Govea, F. E., Lauriault, L. M., Marsalis, M., Angadi, S., and Puppala, N. 2009. Performance of forage sorghum-legume mixtures in southern High Plains, USA. Online. Forage and Grazinglands doi:10.1094/FG-2009-0401-01-RS.

Abstract

Forage sorghum [Sorghum bicolor (L.) Moench] and sorghum × sudangrass (S. bicolor var Sudanese) hybrids may produce as much dry matter yield as corn (Zea mays L.) for silage but with less water. Planting sorghum forage with annual legumes could increase digestibility and crude protein (CP) concentration, making the mixture more suitable for dairy cow rations. The objective of this study was to assess dry matter (DM) yield and nutritive value of brown midrib (BMR) sorghum forage grown as a monoculture or in combination with selected annual legumes. BMR100 (a forage sorghum) and PS210BMR (a photoperiod sensitive sorghum × sudangrass) were planted with four annual legumes: cowpea [Vigna unguiculata (L.) Walp.], lablab (Lablab purpureus L.), soybean (Glycine max L.), and tepary bean (Phaseolus acutifolius A. Gray). Lablab was most complementary with sorghum for forage. The lablab-sorghum mixtures contained more CP with no consistent effect on neutral detergent fiber (NDF) and acid detergent fiber (ADF) compared to monoculture sorghums. This finding opens another possibility to produce good quality forage that could be used as an alternative forage crop to corn in the Southern High Plains.

Introduction

In the last 15 years the dairy industry has been growing in eastern New Mexico and the Texas Panhandle. As a consequence, high quality forage demand also has increased. Alfalfa hay and corn for silage are the main crops that provide forage for dairy producers (personal observation). These two crops have excellent yield and nutritive value, but produce 1.32 and 2.75 kg DM/mł of water for alfalfa (14) and corn (9), respectively. Forage sorghum can produce as much dry matter yield as corn for silage, but with 25% less water than corn (12). Similar to corn (7), forage sorghum CP concentration is low, ranging from 90 to 100 g/kg (15). However, one concern of dairy producers about replacing corn silage with sorghum in dairy cow rations is lower fiber digestibility than corn. The brown midrib (BMR) mutants in sorghum open the possibility that sorghum can replace corn silage without any negative effect on milk yield. The brown midrib mutation is a recessive gene characterized by brown color in the leaf midrib (8). Lignin concentration in BMR sorghum has been reported 9.0% lower and in vitro fiber digestibility 7.2% greater than normal sorghum (5).

Planting grasses or cereal crops with legumes also has an effect on nutritive value of the mixture. Winter pea [Pisum sativum subsp. arvense (L.)] or hairy vetch (Vicia villosa Roth.) grown with wheat (Triticum aestivum L.), triticale (× Triticosecale rimpaui Wittm.), or barley (Hordeum vulgare L.) increased increased the crude protein of the mixture compared with monoculture wheat, barley, and triticale (6,11). Corn mixed with black medic (Medicago lupulina L) or yellow sweetclover (Melilotus officinalis Lam.), produced higher forage quality than monoculture corn (1). In a recent study, corn was grown with lablab, velvet bean, and scarlet runner bean. Compared with monoculture corn, the corn-bean mixtures did not affect DM yield, but CP concentration increased from 13% to 16% (4). Kawamoto et al. (10) also found that sorghum-legume forage had greater DM yield and CP concentration in the mixture than monoculture sorghum. A second benefit of mixing forage species is more efficient utilization of the resources, water, light, and land than conventional systems (2). In addition, the potential nitrogen residue that the legume could leave in the soil for the next crop is another benefit that should be considered as an advantage of mixing legumes with grasses.

In eastern New Mexico and the Texas Panhandle, forage sorghum is an important crop, but there is no information on DM yield and nutritive value of mixtures that include annual legumes. Therefore, the objective of this study was to assess the effects of growing selected sorghum forages with warm-season annual legumes on DM yield and forage nutritive value compared with monoculture sorghum.

Assessing Yield and Quality of Sorghum-Legume Mixtures

This study was conducted at the New Mexico State University Agricultural Science Center at Tucumcari [35°12'0.5"N, 103°41'12.0"W; elevation 1247 m; Canez fine sandy loam (fine-loamy, mixed, thermic Ustollic Haplargid)], and NMSU’s Agricultural Science Center at Clovis [34°N, 103°W; elevation 1280 m; Olton clay loam (fine, mixed, superactive, thermic Aridic Paleustolls)]. Both of these soil types are calcareous and have high pH (7.5 to 8.5) and cation concentrations that generally negate the need for lime applications, even for alfalfa (unpublished data). Fertilizer (N-P-K) was applied based on historical soil tests recommendations for sorghum forages at each location. Two experiments were conducted from 2002 to 2004. Experiment 1 (2002 and 2004) was conducted in Tucumcari and Experiment 2 in Clovis (2004). Annual precipitation during the growing season for each location is presented in Table 1.

Table 1. Monthly average (97-year for Tucumcari, 43-year for Clovis) and actual precipitation (mm) at Tucumcari and Clovis, NM, USA.

Experiment 1. Two sorghum forages (forage sorghum cv ‘BMR100’ and sorghum × sudangrass hybrid cv. ‘PS210BMR’) and four legumes (cowpea cv ‘Chinese Red,’ lablab, soybean cv ‘Derry,’ and tepary bean cv ‘Aztec’) were selected and planted at NMSU’s Agricultural Science Center at Tucumcari in 2002 (10 May) and 2004 (14 May). A randomized complete block design in a split-plot arrangement was used, with forage sorghum as whole plot and legume as subplot with three and four replicates in 2002 and 2004, respectively. Plot size was 8.1 m˛. The seeding rate for sorghum in monoculture or mixture was 17.0 kg/ha and for legume in mixture was 33 kg/ha. Before planting, 109 kg N per ha and 117 kg P₂O₅ per ha were applied to each plot. Plots were sown using a disk drill fitted with a seed-metering cone and drills spaced 20 cm apart. Seed of mixture components was combined in the same packet and thoroughly mixed prior to sowing. In 2002, the experiment was irrigated only on 20 May with approximately 15 cm of water, and not at all in 2004 due to the unavailability of irrigation water. Each study was designed as a single-cut system with harvests taken when BMR100 was in the soft dough stage. In 2002, the study was harvested on 29 August at a maturity stage that ranged from vegetative (PS210BMR) to soft dough (BMR100). In 2004, the study was harvested on 26 October at a maturity stage that ranged from heading (PS210BMR) to soft dough (BMR100). Treatments were harvested with a self-propelled forage plot harvester equipped with a sickle-knife and electronic scale, leaving 7.6 cm stubble heights. Before harvest, each plot was visually rated for legume contribution to yield as a percentage by the same observer each year (11). While these observations were not empirical, they were valid for relative plot to plot comparisons.

Experiment 2. The same sorghum forages and legumes used in Experiment 1 were established at NMSU Agriculture Science Center at Clovis in 2004 (16 May). Two harvesting dates were assessed under a randomized complete block design with a strip-split-plot arrangement. Harvest date, Early (30 August, when BMR100 was in soft dough of the grain and PS210BMR was vegetative) and Late (22 October, when BMR100 was at black layer of the grain and PS210BMR was heading) was the main plot (strip plot), sorghum type was the subplot and legume was the sub-subplot, with four replicates. The experimental unit size was 7.2 m˛. Seeding rates were the same as those in Experiment 1. Before planting 26 kg N, 121 kg P₂O₅ and 68 kg K₂O per ha were applied to each plot. Plots were sown as described in Experiment 1. Plots were irrigated once on 18 May with approximately 7.5 cm of water, followed by good distribution of precipitation the following months (Table 1). Harvest technique and visual assessment of legume contribution to yield was the same as described for Experiment 1 and by the same observer.

Dry samples from Experiments 1 and 2 were ground to a 1-mm particle size for prediction of CP, NDF, and ADF concentrations by near infrared reflectance spectroscopy (NIRS), using an equation that included sorghum data [Ward Labs, Kearney, NE., member of the National Forage Testing Association (NFTA)] (11). Net energy for lactation (NEL) was calculated from ADF values generated by the NIRS analysis.

Statistical analysis was performed for DM yield, CP, NDF, ADF, and NEL concentration using the Mixed Procedure of SAS (SAS Institute Inc., Cary, NC). In Experiment 1 because of different harvest time, each year was analyzed individually where sorghum type, legume, and sorghum-legume interaction were fixed effects and replicates were considered as random effects. In Experiment 2 harvest (Early and Late), sorghum type, legume and the interactions were considered fixed effects and replicates as random effect. If significant difference was found among treatments, LSMEANS comparisons were made (α = 0.05) to separate means. All differences reported are significant at P ≤ 0.05.

Forage Yield and Nutritive Value: Experiment 1

In 2002, tepary bean was the legume with higher proportion at both sorghums (Table 2). Cowpea and lablab had better proportion than soybean but lower than tepary bean. Soybean was the legume with lowest proportion in both sorghum types. It is likely that soybean was more susceptible to competition with sorghum than the other legumes. Dry matter yield was not different among legumes, but it was between the two sorghum forages (P = 0.001). PS210BMR yielded more (6.1 Mg/ha) than BMR100 (4.6 Mg/ha) which agrees with previous results reported in Texas (13). Photoperiod sensitive sorghums delay flowering until day length is less than 12.5 h, therefore they tend to accumulate greater DM yield than conventional sorghums and are less affected by changes in maturity stages.

Table 2. Legume proportion and nutritive value of sorghum forages in monoculture or mixture with one of four warm-season annual legumes. Tucumcari, NM, USA 2002.

 * Means within a row followed by the same letter do not differ (P < 0.05).

Crude protein concentration was different among legumes, but there was not a sorghum-legume interaction. Lablab (119 g/kg DM) and soybean (114 g/kg DM) had greater CP concentration than cowpea (111 g/kg DM), tepary bean (108 g/kg DM), and monoculture sorghum (103 g/kg DM). This increase in CP concentration agrees with previous research when mixing legumes with corn (3,4) or forage sorghum (10). When in mixture with corn, lablab was also the legume that most increased CP concentration in the silage (4).

Neutral detergent fiber concentration was not different across sorghum-legume mixtures and sorghum monoculture. In contrast, sorghum forage type and the legume used in the mixture interacted to affect the concentration of ADF and NEL (Table 2). The ADF concentration was not different between monoculture BMR100 and BMR100-legume mixtures, but differences did occur between monoculture PS210BMR and forage sorghum mixed with either cowpea, soybean, or tepary bean, but not lablab (Table 2). When averaged over these three legume treatments, sorghum forage-legume mixtures were lower in ADF concentration than monoculture PS210BMR. Monoculture PS210BMR also had lower NEL concentration than PS210BMR-legume mixtures. As it was mentioned above, photoperiod sensitive sorghum accumulates more yield than conventional sorghum forages, but also more structural carbohydrates without producing grain (13). This could explain the greater ADF concentration and lower energy in the monoculture PS210BMR than in the mixture.

In 2004 at Tucumcari (Table 3), PS210BMR yielded more and had a higher CP concentration than BMR100. However, NDF and ADF concentrations were greater and NEL was lower in PS210BMR than BMR100 (Table 3). This was not surprising since harvest occurred in October when PS210BMR was in the boot stage and BMR100 was in late dough stage of the grain. These differing maturity levels likely resulted in greater CP, NDF, and ADF concentration and lower energy in PS210BMR than BMR100. Among legumes, lablab was the legume with greater proportion in the mixture in 2004 (Table 3), but it was lower than in 2002. In 2004 the experiment was not irrigated at planting, and precipitation was below normal in May (Table 1), which likely affected the stand of all legumes. Therefore, lack of differences in DM yield and nutritive value, except NDF concentration, are likely to be more related to sorghum forage type and maturity differences than legume effect.

Table 3. Dry matter yield and nutritive value of sorghum forages in monoculture and mixture with one of four warm-season annual legumes. Tucumcari, NM, USA, in 2004.

 * Sorghum-legume means within a column followed by the same letter do not differ (P < 0.05).

Forage Yield and Nutritive Value: Experiment 2

In 2004 at Clovis, NM, early and late harvest had a significant effect on yield and CP concentration of both forage sorghum types, but there was no effect on NDF, ADF, and NEL (Table 4). Harvesting in October increased DM yield, but decreased CP concentration (Table 4). Of the two sorghums, PS210BMR yielded more than BMR100 on both harvest dates. When harvested in August, PS210BMR yielded more than BMR100 with only a slight differences in CP concentration. In this early harvest BMR100 was at the optimum maturity stage for silage (soft dough stage), and PS210BMR was vegetative which explains its high CP concentration. When harvested in October, the greatest difference between the two sorghums was in DM yield. PS210BMR yielded substantially more than BMR100, and had higher CP concentration (Table 4). Because of its photoperiod sensitivity, PS210BMR accumulated more fiber but did not produce grain compared to BMR100. However, the more advanced maturity stage of BMR100 with mature grain, may explain its lower CP concentration than PS210BMR.

Table 4. Dry matter yield and CP concentration of two sorghum forage
types at two harvest dates at Clovis, NM, USA, in 2004.

Similar to the study at Tucumcari, lablab was the legume with greatest proportion in mixture with sorghum forage on both harvest dates (Table 5). Delaying harvest by about two months doubled the proportion of lablab, but not the other legumes. Moreover, even though the proportion of cowpea, soybean, and tepary bean was very low in the mixture, it had an effect on CP concentration, but not in NDF, ADF, and NEL. In the August harvest, monoculture sorghum had lower CP concentration than all four legumes, but lablab was the only legume significantly different than monoculture sorghum, which agrees with previous research (3,4,10). Apparently the greater proportion of lablab in the mixture increased CP concentration more than the other legumes, with no significant effect on NDF, ADF, and NEL (4). When harvested in October, the contribution of the legume to the mixture was minimal, even though lablab proportion was greater than when harvested in August. None of the sorghum-legume mixtures yielded more than monoculture sorghum forage (data not shown). In nutritive value, sorghum-lablab was the only mixture that had greater CP concentration than monoculture sorghum, which was explained by its greater proportion in the mixture. It was clear that late harvest favored DM yield, mainly because of the characteristic of PS210BMR, but overall nutritive value was better in the early harvest than in the late harvest.

Table 5. Legume proportion and CP concentration of sorghum forage in monoculture or mixture with warm-season annual legumes at two harvest dates at Clovis, NM, USA, in 2004.

 * Means within rows followed by the same letter do not differ (P < 0.05).

Lablab was also the legume with greatest proportion in the mixture with both sorghums, and it was greater within BMR100, than PS210BMR (Table 6). However, the contribution to DM yield of the legume to the mixture was minimal, occasionally being reflected in lower yield than monoculture. For example, in BMR100 lablab and soybean yielded more than the monoculture, while cowpea and tepary bean yielded less than the monoculture (Table 6). Additionally, within PS210BMR, cowpea yielded similarly to the monoculture sorghum, both of which were higher than the mixtures with lablab, soybean, and tepary bean, even though lablab was a substantial part of the mixture. Compared with monoculture sorghum forage, the mixtures did not have a significant impact on CP, NDF, and ADF concentration, and NEL. This is opposite to previous studies that a legume in the mixture increases CP concentration relative to monoculture sorghum (10) or corn (3,4). However, relative to NEL value and digestibility, some studies report lower digestibility for the mixture than for a monoculture crop (4), but others report the opposite (10). In this study, sorghum-legume mixtures most of the time increased CP concentration, with no clear impact on NDF concentration and NEL. Maturity differences at harvest may have been a factor in those cases where NEL was lower in the mixture than in the monoculture.

Table 6. Legume proportion and dry matter yield of two sorghum forage types in monoculture or mixture with warm-season annual legumes at Clovis, NM, USA, in 2004.

 * Means within rows followed by the same letter do not differ (P < 0.05).

Sorghum forage may benefit more from the legume addition than corn (3,4,10). In corn-lablab, a substantial increase in CP concentration was reported, but digestibility decreased only slightly compared with monoculture corn (4). In a study of sorghum-lablab versus monoculture sorghum, digestibility was shown to increase, in addition to an increase in CP concentration (10). In the current study, the two BMR varieties selected produced forage that was lower in NDF and ADF concentration than conventional sorghum or sorghum × sudangrass (13). Crude protein concentration increased with the addition of the legume and energy concentration tended to increase in these two high quality sorghum forages. A greater impact of the legume may be possible when mixed with conventional, non-BMR sorghums, which have greater NDF and ADF concentration and lower digestibility than BMR sorghums (5,8). Across the three site-years in this study, we consistently found improvement in nutritive value of the mixture compared with monoculture sorghum forage.

Conclusion

One of the main goals when sorghum forage is mixed with forage legumes is to increase the CP concentration of the mixture and reduce CP supplementation to dairy cows. Lablab consistently provided a better stand than cowpea, soybean, and tepary bean, and demonstrated reliable effect on increasing CP concentration, but no consistent effect on NDF and ADF concentration. When compared to monoculture BMR sorghums, mixing lablab with the sorghum could be beneficial for eastern New Mexico and the Texas Panhandle. Additional research is needed to determine if sorghum-lablab systems have the potential to replace corn silage in dairy cow rations based on feeding trials.

Acknowledgments

We gratefully acknowledge technical and field assistance of George Arguello, Eutimio Garcia, Brad Griggs, Calvin Henson, Gilbert Lucero, Martin Mead, Larry Perkins, Kenneth Phipps, Leslie Robbins, and Aaron Scott; and the secretarial assistance of Doris Hight, Patty Cooksey, and Valerie Pipkin. The authors thank Dr. Kenneth A. Albrecht for valuable comments and suggestions on the manuscript.

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