Álvaro García
Corn silage is an essential feed source for the U.S. dairy and beef cattle industries. According to USDA data, approximately 7.6 million acres of corn were harvested for silage in 2023, yielding around 7.60 million tons. With such significant production, the associated gaseous emissions raise environmental concerns. Carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) are released during both the fermentation and feeding phases. Silage inoculants, particularly those containing homofermentative bacteria, offer potential in reducing these emissions by increasing fermentation efficiency. By enhancing the fermentation process, these inoculants reduce CO₂ emissions during ensiling by minimizing residual fermentable material. Additionally, inoculants with fiber-degrading enzymes like cellulases and xylanases aid in breaking down plant fibers, which enhances fiber utilization in the rumen and may reduce methane emissions from enteric fermentation. Given the scale of corn silage production in the U.S. and its environmental footprint, microbial inoculants represent a promising strategy to mitigate greenhouse gas (GHG) emissions associated with livestock production.
Corn silage significantly contributes to GHG emissions, particularly CO₂ and N₂O, through the silage fermentation and feed-out stages. For context, agricultural emissions from cropland alone in the U.S. total approximately 233 million metric tons of CO₂ equivalent annually, with corn and soybean fields representing a major portion (USDA ERS, 2023). From this total, N₂O emissions from fertilized corn for silage can contribute up to 16 million metric tons of CO₂ equivalent due to nitrogen application and soil microbial activity. Ensiling corn, which produces volatile organic compounds (VOCs) such as ethanol, also indirectly contributes to emissions. The anaerobic conditions during fermentation further promote methane production, which varies in intensity but adds to the overall emissions impact of corn silage in livestock diets. By incorporating homofermentative silage inoculants with enzymes like cellulases, amylases, and xylanases, producers may mitigate some of these emissions. These enzymes facilitate a faster and more efficient fiber breakdown, potentially reducing CO₂ and N₂O emissions by improving silage stability and enhancing fiber digestibility, which may also reduce emissions from manure.
Justification for enzyme-enhanced homofermentative inoculants
Corn silage contributes to greenhouse gas (GHG) emissions both directly and indirectly. Directly, it produces GHGs during the fermentation process through the following reactions:
- With inoculants containing homofermentative bacteria (e.g. Plantilactibacillus plantarum), bacteria primarily convert carbohydrates to lactic acid, producing fewer by-products:
C6H12O6 (glucose) à 2C3H6O3 (2 lactic ac. molecules)
- With heterofermentative bacteria (e.g. Lentilactibacillus buchneri) inoculants, bacteria convert carbohydrates into multiple products, including lactic acid, ethanol (or acetic acid), and carbon dioxide:
C6H12O6 (glucose) à C3H6O3 (lactic ac.) + C2H5OH (ethanol) + CO2 (carbon dioxide)
Or, with acetic acid production:
C6H12O6 (glucose) à C3H6O3 (lactic ac.) + CH3COOH (acetic acid) + CO2 (carbon dioxide)
The main difference between homofermentative and heterofermentative bacteria lies in their fermentation pathways. Both types rely on glucose as their primary substrate, but they use it differently. In corn silage, although small amounts of simple sugars like glucose (a monosaccharide) and sucrose (a disaccharide) are present, most carbohydrates exist as complex polysaccharides such as cellulose and hemicellulose. Enzymes like cellulase and xylanase are essential for breaking down these polysaccharides into simpler, fermentable sugars accessible for bacterial fermentation. Additionally, alpha-amylase can assist in breaking down disaccharides like sucrose, which neither homofermentative nor heterofermentative bacteria can naturally ferment in their intact forms. The inclusion of these enzymes increases glucose availability, optimizing fermentation efficiency and enhancing the breakdown of silage carbohydrates.
Volatile organic compounds and air quality
Volatile Organic Compounds (VOCs) are organic chemicals that easily vaporize, contributing to air pollution and potential health risks due to their reactivity and role in ground-level ozone formation. Research by Mitloehner et al. (2014) highlighted the impact of VOCs, including ethanol, ethyl acetate, and methyl acetate from certain silage inoculants, on air quality. Reducing these VOCs could improve air quality and lower emissions, as ethanol and acetate esters notably contribute to GHG emissions associated with silage. Although acetate enhances aerobic stability by inhibiting yeast and mold during feedout, it increases acetate concentration, which, along with byproducts like ethanol from heterofermentative fermentation, can lead to the formation of VOCs such as methyl acetate. While these VOCs support silage preservation by suppressing spoilage organisms, they also present air quality concerns and may impact on human health and the environment.
The production of volatile organic compounds (VOCs) during silage fermentation can begin with glucose (C₆H₁₂O₆) as a primary substrate. When heterofermentative bacteria ferment glucose, they produce a variety of byproducts, including ethanol, acetic acid, and carbon dioxide. These can contribute to VOC emissions.
The general pathway for VOC production through glucose fermentation in a heterofermentative process is as follows:
Homofermentative and heterofermentative bacteria
Homofermentative bacteria, like certain lactobacilli (LAB), offer an advantage due to their specific fermentation pathway, which exclusively converts glucose to lactic acid, avoiding the production of acetate or ethanol. This reduces VOCs production and minimizes emissions that could impair air quality or pose health risks. By focusing solely on lactic acid production, homofermentative bacteria improve silage quality without generating the VOCs typical of heterofermentative fermentation. Consequently, homofermentative inoculants, particularly those combined with carbohydrate-degrading enzymes, provide a more environmentally friendly option, enhancing silage stability, nutrient preservation, and greenhouse gas reduction.
Heterofermentative lactic acid bacteria (LAB), like Lentilactobacillus buchneri, are frequently used to improve aerobic stability in silage due to their acetic acid production. However, this process often results in higher VOCs emissions, including ethanol and acetic acid, limiting the potential for GHG mitigation. Hafner et al. (2014) reported that L. buchneri inoculants may even increase 1-propanol levels, adding to the VOCs emission profile. By contrast, homofermentative LAB combined with carbohydrate-degrading enzymes avoid producing these VOCs, fostering a stable lactic acid profile without the additional emissions associated with ethanol or acetate.
Enzyme-enhanced homofermentative inoculants play a key role in silage fermentation. They accelerate lactic acid production, causing a rapid pH to drop that limits the growth of spoilage organisms, stabilizing the silage earlier in the fermentation process. Additionally, enzymes that break down cellulose and hemicellulose increase nutrient bioavailability, improving fiber utilization in the rumen. Enhanced fiber digestibility not only supports higher production efficiency but also lowers methane emissions associated with fiber breakdown in the rumen.
Field-based-research is needed
Although initial findings are promising, factors like moisture content, ensiling duration, and microbial populations can influence the effectiveness of silage additives in reducing emissions. Mitloehner et al. (2014) demonstrated that VOCs production varied with different silage additives and was affected by storage conditions, underscoring the need for further research to optimize these additives across varied field applications. Future studies should focus on the interactions between silage inoculants, enzymes, and chemical additives to offer a comprehensive approach to reducing greenhouse gas emissions in silage management.
In conclusion, the use of homofermentative silage inoculants fortified with enzymes such as cellulases, amylases, and xylanases presents a practical strategy for reducing CO₂ emissions from corn silage. By minimizing ethanol and acetic acid production and enhancing fiber utilization in the rumen, these inoculants offer substantial advantages over heterofermentative inoculants, contributing to more sustainable livestock production.
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