Good silage: a matter of fermentation and stability
|By Luiz Ferraretto|
The author is an assistant professor of livestock nutrition in the University of Florida Department of Animal Sciences.
The ensiling process is a well-established tool to store forages while maintaining their nutrient composition and availability. Thus, ensiled forages are predominant feed ingredients used in diets to supply adequate levels of energy, protein, and physically effective fiber to high-producing dairy cows.
Attention to detail is required in order to achieve a successful ensiling process. The objective of this article is to provide an overview of the fermentation process, aerobic stability, and to briefly discuss strategies to optimize each of them.
Silage fermentation occurs naturally under anaerobic conditions. Thus, it’s crucial to fill and seal silos rapidly in order to limit the amount of oxygen introduced into the silage. After the silo is sealed, the remaining oxygen is used up by respiration of plant material and microorganisms.
Fermentation by anaerobic epiphytic (native) bacteria begins as soon as oxygen is absent. These bacteria are present in the fresh forage mass and ferment water-soluble carbohydrates (sugars) into lactate and acetate. As lactate and acetate accumulate, pH drops until silage reaches a stable phase. Fermentation is still undergoing during the stable phase, but changes are not as pronounced as during the initial ensiling period.
Many factors influence the silo fermentation process. Some items are related to forage characteristics whereas others are management issues. Excess oxygen allows for the growth of undesired microorganisms such as yeasts and molds and prolongs plant respiration, allowing for enhanced utilization of sugars prior to the anaerobic fermentation process. Some harvesting practices such as coarse chop length and a late harvest maturity may impair oxygen removal due to greater silo porosity.
Simply stated, porosity is the gap space between the forage mass that is filled with air, including but not limited to oxygen. Packing density is likely the best indicator of porosity. Greater packing density reduces porosity, improves fermentation, and reduces storage losses. Inadequate fermentation, however, may occur even in dense silage material.
Some crop-related factors that influence fermentation include moisture content, buffering capacity, availability of water-soluble carbohydrates, and the amount and type of epiphytic bacteria. These considerations are thought to restrict bacterial growth and reduce the accumulation of acids that could prevent the growth of yeasts and molds.
Overall, the fermentation process will determine silage stability or instability. Aerobic stability is the length of time that silage lasts before heating or being spoiled after air exposure. Impaired fermentation and reduced acid accumulation exacerbate this issue, but other factors also contribute.
Although bacteria in the silo ferment sugars, yeasts can ferment not only sugars but also starch. Thus, forages with greater starch concentration such as corn tend to be more prone to spoilage after air exposure. Practices that allow for air infiltration into the silage material also reduce aerobic stability.
Some common observations include an uneven silo face, a slow rate of silage removal, and plastic removed from a given section of the silo too far in advance. Weather also plays a major role in aerobic stability. Silage fed during the summer or in warmer climates is more prone to reduced aerobic stability.
Can inoculants help?
Epiphytic bacterial populations vary within and across crops and fields. Microbial inoculants are additives containing bacteria selected to improve silage fermentation efficiency and may aid in scenarios where the epiphytic bacterial population is unable to dominate the fermentation process. Thus, inoculants may preserve more nutrients.
Alternatively, greater aerobic stability may be achieved depending on the type of inoculant selected. Bacteria commonly found in microbial inoculants are divided into two main groups, homofermentative and heterofermentative. These groups differ in fermentation end product produced and are also known as homolactic and heterolactic bacteria, respectively.
Homofermentative bacteria have lactate as their main fermentation end product. Microbial inoculants based on this type of bacteria accelerates the pH drop as lactate is a stronger acid than acetate. A fast decline in pH prevents growth of undesired microorganisms and reduces protein degradation.
A recent review evaluating homofermentative inoculants is in Table 1. Briefly, it compared noninoculated (control) silage with silage inoculated with lactate-producing bacteria.
Some of the benefits observed include: lower pH, reduced acetate and butyrate concentrations, lower mold and clostridia counts, greater lactate concentration, and improved dry matter recovery. Furthermore, this study also revealed greater milk production (1 pound per cow per day) when cows were fed inoculated silage.
Homofermentative-based inoculants may be a perfect fit in scenarios where the epiphytic bacterial population is scarce or the crop buffering capacity is too high. For example, in legume silages such as alfalfa, the high crude protein levels lead to greater ammonia-nitrogen concentrations. Ammonia-nitrogen is a natural buffer and inhibits a rapid pH drop. It is important to remember, however, that silage inoculated with homofermentative inoculants is more susceptible to lower aerobic stability.
Heterofermentative bacteria produce lactate, acetate, and ethanol. The most used heterofermentative bacteria in silage as microbial inoculants is Lactobacillus buchneri. This type of inoculant has a slower fermentation rate than lactate-producing bacteria but has the ability to convert lactate to acetate. A review evaluating the use of L. buchneri in corn silage is summarized in Table 2.
Greater aerobic stability is often observed with this type of inoculant and it is likely related to greater acetate concentration. Acetate has antifungal activity and inhibits yeast and mold growth. Microbial inoculants containing heterofermentative bacteria are suggested when ensiling crops that are rich in starch and will be fed during the summer; these silages may have lower aerobic stability.
In addition, farms that experienced poor aerobic stability could also benefit from this technology. However, it is critical to remember that poor management practices cause many issues related to aerobic stability, and priority must be given to adjusting your silage-making techniques before selecting a microbial inoculant.
Ensuring adequate fermentation and aerobic stability is crucial to provide high-quality silage to dairy cows. Many factors alter fermentation and aerobic stability, and the sum of these factors determines silage quality. Using a microbial inoculant is a good technology to consider in order to protect your silage investment. However, inoculants cannot replace recommended filling, packing, and storage practices. If using microbial inoculants, relying on research trials and not price to make your decision is the best option.
This article appeared in the April/May 2018 issue of Hay & Forage Grower on pages 34 and 35.
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