Nutritive Value and Silage Fermentation Characteristics of Forage Sorghum (Sorghum bicolor L.) Genotypes and Lablab (Lablab purpureus L.) Mixture ()
1. Introduction
Ensiling is an ancient method used to preserve the nutritive value of forages by packing and storing forage in airtight conditions. Silage fermentation occurs naturally under anaerobic conditions. The combination of forage legumes with non-legumes under intercropping has shown benefits in terms of reduced N fertilizers, improved yield, and nutritive value in silage [1] [2]. Maize and sorghum are the predominant cereal forages for silage. The nutritional quality of sorghum has poor quality, particularly in protein concentration [2] [3] [4]. In forage-based production systems, no single species can provide high-quality forage as well as tonnage for the dairy industry across the world. Legumes are grown individually as green cover or forage, generally have lower yielding than cereals but supply herbage with higher N concentration and fiber digestibility [5]. Further, research efforts have been made to improve the adaptability of these forage legumes and identify optimal management practices with cereals [6]. In crop combinations, maize and sorghum with lablab/cowpea [7], and sorghum with several annual legumes [8] have shown benefits in improving dry matter yield and forage nutritive value. Begna et al. [9] reported that high forage yield with quality in terms of RFV and TDN) for animal nutrition can be achieved from the mixed cropping of canola and pea at 50:50 and 75:25 seeding ratios. Cultivated legumes are introduced into the cropping systems as a component crop with sorghum. Further, the CP concentration was increased by greater legume contribution in the mixture. When maize was ensiled with common bean pH, lactic acid, and total volatile fatty acids were greater in the mixtures than in monoculture maize [10]. It was shown that legumes alone cannot be ensiled due to their poor nutritive value and high pH and butyric acid potentially reduce their use. It was evident that mixing lablab with sorghum silage should increase CP concentration, but it would also increase other constituents, such as NDF, lactic acid, and total acids concentrations. Therefore, it is important to determine the proper combination of lablab and cereals that will result in an optimum mixture for nutritive value and fermentation. This research has been conducted on sorghum genotypes for silage along with lablab to assess the nutritive value and fermentation characteristics suitable for sustainable livestock production.
2. Methodology
2.1. Experimental Site and Crop Management
Field and laboratory experiments were conducted at New Mexico State University’s Agricultural Science Center at Clovis (34˚N, 103˚12'W, and 1348 m elevation), with annual precipitation of 445 mm and Olton clay loam soil. Forage sorghum brown midrib cv. Dairy Master BMR (BMR-LB) and brachytic dwarf sorghum cv. 26,837 (BDS-LB) types were intercropped with lablab cv. Rio Verde. Each sorghum variety was mixed with the legume using six population density ratios. Treatment combinations were BMR-LB seeding rates at 150 - 0, 112.5 - 37.5, 75 - 75, 37.5 - 112.5, 0 - 150 and 150 - 150 (×103 plants·ha−1) similarly for BDS-LB combinations. Crops were fertilized according to soil test recommendations. The total planting density of sorghum and lablab was at 150,000 plants·ha−1 except in one treatment having equal density of 150,000 plants·ha−1 (Table 1). Forage sorghum was harvested at late-dough maturity stage of the grain development and lablab at 20% bloom stage. Field trial was irrigated to prevent water stress during the growing season. There was no serious problem with pests and disease incidence during the season. Both sorghum and lablab were combined and harvested at ground level from random locations within each field approximately 114 and 110 days after sowing, respectively in 2010 and 2011. Ensiling process for all the crops and crop mixtures were ensiled at the same time. Lablab was wilted for 18 h before chopping. All the crops were chopped simultaneously to a theoretical particle size of 15 mm with a two-row pull-type forage harvester (John Deere, Moline, IL). After chopping, approximately 20 kg of fresh material of sorghum and LB together were collected in separate plastic buckets and taken to the laboratory for ensilage.
2.2. Ensilage of Forage Mixture
Individual 500 g fresh mixtures were made for each treatment and placed in a 0.5-L glass jar with four jars per treatment. Quantity of lablab mixed in each treatment was depends on its contribution to forage mixture details of each treatment (Table 1). Mini-silos were fermented for 56 days at room temperature (25˚C). Before ensiling, two 250 g subsamples for each treatment were placed in a paper bag and oven dried at 60˚C for 48 h for dry matter determination. Subsamples were powdered and analysed for pre-ensiling chemical composition. At opening, each mini-silo was dumped into an ethanol-disinfected plastic container and mixed uniformly. A 250 g subsample was placed in a plastic vacuum pouch of 20 × 30 cm (Doug Care equipment, Springville, CA), immediately vacuum sealed using a fast vacuum machine (Doug Care equipment, Springville, CA) and frozen to −18˚C for later analysis for fibre and fermentation characteristics. The remaining 250 g sample was frozen as a backup sample.
2.3. Forage Quality Analysis
The biomass samples from the final harvest were used to analyze forage quality served as pre silage sample. Crude protein, NDF, ADF, IVTD, and NDFD were determined using a near-infrared spectroscopy system with equation calibrations used for sorghum types. The pH, acetic, butyric, propionic, and iso-butyric acids, lactic acid were determined using standard procedure [10]. All these analyses were conducted at Dairy One Laboratory (Ithaca, NY).
3. Statistical Analysis
The data were analysed by using SAS PROC MIXED software programme version 9.4 (SAS Institute, 2020). For significant mixtures and mixture x, crop effects were used to assess the order of the response trend as well as the nature of the interaction with the crop. Significance was defined at P < 0.05.
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Table 1. Quantity of dry matter contribution (%) to total mixture and mixed with sorghum during ensilage.
4. Results and Discussion
4.1. Nutritive Value
The Dry matter concentration at the time of ensiling was below 300 g·kg−1 for sorghum and lablab. The sorghum was harvested at the maturity stage recommended for ensiling and lablab was wilted 18 h before ensilage. Dry matter concentration was in a lower range between 270 and 277 g·kg−1. Even at these high moisture conditions, both sorghum genotype silage mixtures fermented well. Adding lablab to sorghum affected the nutritive value of silage mixture (Table 2). Lablab was intercropped with sorghum to increase the crude protein content of the silage. Addition of crude protein rich lablab was increased in the mixture. The effect of lablab on crude protein was similar in both the sorghum genotypes. Earlier studies also reported that maize and legume mixture increased crude protein concentration from 12.9% to 29.0%. The NDF content of the silage was varied from 29.8% to 40.2%. The presence of lablab in the ensiled mass increased ADF (377 - 383 g·kg−1 DM) and but decreased NDF (553 - 529 g·kg−1 DM) levels (Table 2). The digestibility of NDF was also decreased by increased concentration of lablab in association with sorghum (643 - 565 g·kg−1 DM). Similar results were also with total digestible nitrogen (Table 2). The calcium concentration was increased by 3.1% with increased concentration of lablab in the mixture. It was consistent with the sorghum genotypes. The relative feed quality of intercropping forage was not significantly influenced by lablab inclusion. Htet, et al. [11] reported that the maize intercropped silages increased pH, and CP contents (P < 0.05), whereas decreased NDF, ADF, and ash contents. No difference (P > 0.05) was found in K contents of nutrient composition of silage among the four treatments. The Ca contents in the intercrop silages were higher than the SM silage.
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Table 2. Pre-silage forage content of crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), NDF digestibility (NDFD), total digestible nutrients (TDN), calcium and relative feed quality (RFQ) of legumes with sorghum genotypes at different seeding ratios.
In general, lower concentration of fibres in the DM of legumes in relation to grasses. However, Dawo et al. [1] found that a greater increase in crude protein would be expected when the legume was at 50% of the normal plant density. Sorghum species variation and maturity at harvest could have caused lower crude protein values in this study. Contreas-Govea et al. [10] reported that cell wall structural carbohydrates were not affected during fermentation and lactic acid bacteria consumed the pool of water-soluble carbohydrates. Dry matter concentration of the pre-ensiling mixtures was below 300 g·kg−1 for sorghum and lablab mixtures.
The pH across the mixtures was ranged from 3.79 to 4.58, which were below those reported by Jones et al. [12]. While lactic acid concentration increased as the quantum of lablab increased in the mixture indicating acidic conditions were conducive to cell wall hydrolysis, which decreased NDF and to some degree ADF. Apparently, ADF fraction was also susceptible to hydrolysis, but at a lower degree than NDF fraction. The ADF of ensiled mixtures with greater proportion of sorghum in 150:0, 112.5:37.5 planting density was lower (Table 3). The ADF concentration increased as the quantum of lablab increased in the mixture. In contrast, NDFD was reduced as the concentration of lablab increased (0:150) in the mixture while it was in sorghum alone (Table 1). These results in agreement with those reported by Contreras-Govea et al. [10] in which NDFD declined by adding beans to maize. In addition, our IVTDMD values were similar in magnitude to those reported by Contreras-Govea et al. [13]. Hence, positive impact of adding lablab in mixture with sorghum could be greater in low-quality sorghum than in high-quality sorghum cultivars. A summative approach that considers the true digestibility of CP, NDF, non-fiber carbohydrates and fatty acids was used to determine TDN concentration. Across treatment TDN (630 to 755 g·kg−1) was greater when lablab increased from 0 to 75 but then declined when lablab increased from 75 to 150 seeding ratios (Table 2). The IVTDMD ranged from 747 to 840 g·kg−1 across forage mixtures. These results may be attribute high crude protein concentration in legumes, which is conducive to greater proteolysis than sorghum types. In addition, legumes have greater buffering capacity than grasses.
4.2. Fermentation Profile
The most predominant acids in the silage mixtures were lactic and acetic acids and whereas propionic, butyric and iso-butyric acids were very low. Lactic acid was higher in sorghum mixtures then increased linearly with increased lablab proportion. The concentration of lactic acid was highest in sorghum genotype “Brown mid rib” and it increased as the lablab proportion increased from 0 to 1,12,500 plants·ha−1 (56 to 83 g·kg−1). Similar trends were also observed for acetic acid and total acids. However, the lactic acid: acetic acid ratio showed a reverse trend and it decreased as acetic acid increased. Legumes have larger organic acid concentrations than grasses; therefore, in general legume silages have higher pH
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Table 3. Fermentation products in terms of crude protein (CP), lactic acid (LA), acetic acid (AA), total acids (TA), in-vitro dry matter digestibility (IVTD) of sorghum mixed with different proportion of lablab bean mixtures planned at different population levels (Mean of two years).
because of the higher buffering capacity caused by the organic acids. Earlier studies on fermentation analysis of legume silages like kura clover, lucerne, faba bean and soybean, indicated that these crops having higher values of lactic acid and acetic acid concentration [13] [14]. This study showed that the concentration of lactic acid, acetic acid and total acid increased with the increase in the proportion of lablab bean in the mixture (Table 4). Earlier studies by Contreas-Govea et al. [13] on fermentation analysis of legume silages also showed higher values of lactic acid and acetic acid concentration due to high buffering capacity. Adding legumes to grasses was found to extend the fermentation time resulting in increased accumulation of fermentation end products. The results of the current study were agreed with previous studies [1] [13]. Lactic acid is a stronger acid as compared to other acids (acetic, propionic, butyric) and thus contributes most to the drop in silage pH. It increased linearly in both Brown mid rib-lablab (3.8 to 4.63) and Brachytic dwarf sorghum-lablab (4.0 to 4.95) mixtures as lablab increased from 0 to 150 seeding ratios (Table 4).
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Table 4. Post silage content of crude protein (CP), acid detergent fibre (ADF), neutral detergent fiber (NDF), NDF digestibility (NDFD), total digestible nutrients (TDN), relative feed quality (RFQ) of sorghum and lablab seeded at different population ratios.
A higher (65% - 70% of total acids) lactic acid fraction is indicative of a good fermentation, and it results in lowest dry matter and energy losses from the crop. However, high (>30 - 40 g·kg−1 DM) acetic acid values will reduce the overall dry matter and energy recovery from the silage mixture [15]. Therefore, lactic acid formation is preferred over acetic acid production. However, lactic acid production is governed by the nature of fermentation and the balance between homolactic or heterolactic fermentation processes [16] with heterolactic fermentation producing a weaker (lactic) acid than the homolactic process. The ratio of lactic: acetic acid decreased with the increased lablab proportion in the silage. There was no significant difference in lactic: acetic acid ratio between Brown mid rib and Brachytic dwarf sorghum mixtures.
Bolsen [17] reported that forage sorghum had normally higher acetic acid concentration, which was further increased by the addition of lablab to the mixture. It was suggested that a 3:1 lactic: acetic acid ratio is a good indicator of fermentation. Results from this study showed that lactic: acetic acid ratio was decreased as lablab content increased in the mixture, and it was 3.6 to 1.9 in Brown mid-rib-lablab mixture and 3.9 to 1.1 in Brachytic dwarf sorghum-lablab. This could be attributed to sorghum which is normally low in acetic acid (12.8 to 16.6 g·kg−1 DM). Adding lablab to sorghum would potentially increase acetic acid concentration to a level (31.5 g·kg−1 DM) that could affect palatability and dry matter intake. This will result in a decrease in dry matter intake by cattle [14] and present a disadvantage of adding lablab to the silage mixture. Anil et al. [18] showed that silage made from the maize-soybean mixtures contained greater lactic and acetic acid concentrations than that of sole maize. Increase in lactic acid concentration when cereal was ensiled in mixture with other legumes. Therefore, options, methods, and possibilities for mixing forage sorghum with lablab bean in a silage mixture should be explored further during subsequent studies.
5. Conclusion
The findings of this study concluded that nutritive and fermentation profile was significantly influenced by the addition of lablab in sorghum silage mixture. Lablab can be ensiled with Brown mid rib and Brachytic dwarf sorghum genotypes, and they produce greater crude protein and acceptable fermentation in all mixtures. However, a greater benefit in nutritive value was observed when lablab was between 75 and 112.5 seeding ratios of the mixture. In addition, increased crude protein concentration in the silage could potentially reduce crude protein supplementation to cattle and possibly reduce N excretion to the environment.
Acknowledgements
The authors thank Aaron Scott, Maria Teresa Nunez, and Miguel Nunez, for their support in field experimentation and during the ensiling process. The authors are also thankful to Contreras-Govea, F.E. and Leonard L.M. for their technical help. Salaries and research support were provided by state and federal funds appropriated to the New Mexico Agricultural Experiment Station. This research was also partially supported by USDA-National Research Initiative, Grant Number 2007-35102-18120.