Fermentative profile, losses and chemical composition of silage soybean genotypes amended with sugarcane levels

Drought is a significant challenge in global agriculture. Brazil is a major soybean producer, and developing technologies to improve food production is crucial. Soybean plants are typically grown in temperate climates, but Brazilian hybridized genotypes are adapted to tropical and subtropical regions. These genotypes have a longer vegetative growth period, increasing biomass production. However, climatic variability can affect grain production and morphological characteristics. While grain yield may be reduced, biomass remains available for silage production. Soybean has high crude protein but low soluble carbohydrates and high buffering power, making silage production challenging. Adding sugarcane, rich in water-soluble carbohydrates, may improve lactic acid bacteria (LAB) activity during fermentation. However, high soluble carbohydrates in sugarcane can lead to ethanol production and dry matter losses. Mixing soybean and sugarcane silage may offer a solution by combining the beneficial characteristics of both plants. This study aimed to optimize the fermentative and nutritional profile of soybean silage by adding sugarcane.

Several studies highlight the challenges of producing high-quality soybean silage due to its high protein content, low soluble carbohydrate levels, and high buffering capacity. These factors can impede the efficient reduction of pH and lead to undesirable fermentative processes. Other research explores the use of additives or mixed silages with grass species to improve fermentation. Sugarcane, with its high water-soluble carbohydrate content, presents a potential additive to support LAB activity. Existing literature also shows that sugarcane silage, when ensiled alone or at different maturity stages, can lead to lower dry matter recovery and higher ethanol production compared to mixed silages or those with additives. The timing of sugarcane harvest is also linked to the concentration of water-soluble carbohydrates (WSC). Mixed silages, combining legumes and grasses, often show improved characteristics, offsetting the limitations of each individual plant.

The experiment used a completely randomized design (CRD) in a 4x5 factorial scheme (four soybean genotypes and five sugarcane inclusion levels), with four replicates. Soybean genotypes included Pampeanas C50, C60, C70, and BRS 333. Sugarcane was added at 0, 25, 50, 75, and 100% levels (based on natural matter). Soybean was sown in January 2017 and harvested at the R-5:3 phenological stage (75 days after planting). Sugarcane was harvested at 18 months. The harvested plants were chopped, mixed, and ensiled in 3.5 kg polypropylene buckets with a sand layer at the bottom to collect effluent. Silos were sealed with Bunsen valves and stored for 70 days at 25±2°C. Dry matter losses (gas and effluent) and dry matter recovery (DMR) were calculated. Samples were analyzed for dry matter (DM), mineral matter (MM), crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), water-soluble carbohydrates (WSC), and in vitro dry matter digestibility (IVDMD). pH and ammoniacal nitrogen (N-NH3/NT) were also measured. Organic acids (lactic, acetic, propionic, butyric) and ethanol were determined using high-performance liquid chromatography (HPLC). Data were analyzed using ANOVA and regression analysis.

Significant interactions were observed between sugarcane levels and soybean genotypes for various parameters. Lactic acid content showed a positive quadratic effect with sugarcane inclusion, with the highest levels observed at 50% sugarcane for most genotypes. Acetic acid content was highest in silages with 100% sugarcane, except for genotype C60. The LA:AA ratio also exhibited a positive quadratic effect. Propionic acid showed a negative quadratic effect. Butyric acid levels were highest in silages with 100% soybean, indicating poorer fermentation. Ethanol levels showed a positive quadratic effect in silages with BRS and C70 genotypes and a negative quadratic effect in others. Ammoniacal nitrogen (N-NH3/NT) showed a negative quadratic effect across all genotypes. Dry matter recovery (DMR) showed a positive quadratic effect, with maximum values around 25-50% sugarcane inclusion. Significant interactions were also found for pH, gas losses, DM, MM, and CP. The C50 genotype generally showed superior performance, particularly in terms of crude protein and IVDMD. Higher sugarcane inclusion levels led to increased WSC but also higher ethanol and gas losses. The addition of sugarcane generally improved fermentation, particularly reducing ammoniacal nitrogen, but excessive amounts negatively impacted silage quality.

The addition of sugarcane reduced silage pH, primarily due to increased WSC, stimulating LAB growth and lactic acid production. Silages with 100% sugarcane had pH values below the recommended range, leading to increased ethanol production and losses. Soybean silages alone had higher pH values due to the high buffering capacity of soybean, resulting in butyric fermentation and higher ammoniacal nitrogen. The optimal sugarcane inclusion (25-50%) balanced these effects, promoting lactic acid fermentation and reducing undesirable fermentation products. The interaction between sugarcane and soybean created a synergistic effect, modulating the pH drop and reducing ethanol production. The C50 genotype demonstrated greater capacity to preserve nutritional value during silage fermentation, likely due to factors other than initial CP content. Differences in WSC content among genotypes influenced the production of organic acids and ethanol. Excessive sugarcane led to increased WSC and ethanol, impacting dry matter recovery. Gas losses were generally within acceptable limits but increased with higher sugarcane proportions. Effluent losses were not affected by sugarcane inclusion. Higher butyric acid levels in 100% soybean silage point to clostridial fermentation, highlighting the positive role of sugarcane in improving fermentation.

Optimal silage quality and nutrient preservation were achieved with 25-50% sugarcane inclusion in soybean silages. The C50 soybean genotype displayed superior performance in terms of biomass production, crude protein, and in vitro digestibility. Further research could investigate other additives or combinations of forages to further optimize silage quality and explore the specific mechanisms underlying the observed interactions between sugarcane and soybean genotypes.

The study was conducted in a specific location and climate, which may limit the generalizability of findings to other regions. The use of small-scale silos may not perfectly reflect large-scale silage production practices. A more detailed analysis of microbial communities would further elucidate the fermentation dynamics.