Understanding spoilage and aerobic stability in forages

By Cole Diepersloot, Ph.D., Vita Plus forage specialist and dairy nutritionist

When it comes to silage, most people think of spoilage in one of two ways: the crusty, moldy silage on the top of a pile, or part of the percentage of shrink they track in spreadsheets. However, spoilage occurs in a more complex process that can best be described as a downhill slide. To make high-quality silage, it is important to understand how spoilage occurs and minimize it. Likewise, understanding the concept of aerobic stability, a proxy for time to spoilage, can be important to prevent silage spoilage.

Spoilage in silage

Spoilage begins when silage is exposed to oxygen, which “awakens” the yeasts still present after fermentation. Since yeasts cannot be seen with the naked eye, their presence can go unnoticed aside from the silage heating. In addition to increasing losses, research has shown that yeasts can decrease NDF digestibility of TMR under laboratory conditions without spoiling the feed. This highlights the negative impact of spoilage, even before it becomes noticeable.

Once established, yeasts consume lactic acid, increasing silage pH and allowing molds and bacteria to grow. This increases DM losses and energy available from silage, as these microbes consume the most digestible parts of silage (sugar, starch and digestible fiber). So, spoilage leaves farmers with less of a lower quality feed, missing opportunities to support higher milk production with cheaper, home-grown feeds.

Aerobic stability

Aerobic stability is defined as the “shelf life” of silage and is a test to evaluate how long it takes for silage to begin spoiling. The aerobic stability assay tracks how long it takes for silage temperature to increase — a sign of spoilage causing microbes. However, aerobic stability is measured under lab conditions and doesn’t always translate to silage “shelf life” on farm. For example, aerobic stability is measured at room temperature, while moisture and high temperatures outside can speed up spoilage. Silage plastic on farms is also prone to holes created by animals or machinery, while laboratory silos are protected from air exposure before opening. This means spoilage on farms may begin before the silo is opened, increasing the extent of spoilage in silage.

The best way to improve aerobic stability and reduce spoilage in silage is to use a research-proven microbial inoculant that contains Lactobacillus buchneri. These inoculants may have up to 10 days of aerobic stability, which often prompts the question, “Do I really need that long?” As mentioned before, spoilage can happen more quickly on farms than in a lab, especially during hot and wet weather. This means that 10 days of aerobic stability may only mean a few days before silage begins heating on farm — and that 10 days of aerobic stability may be beneficial after all.

Ultimately, signs of spoilage should be addressed in future harvests with a microbial inoculant containing L. buchneri to improve nutrient preservation in silage and avoid negatively impacting dairy cow productivity. Additionally, it can be helpful for farmers or nutritionists to focus more on how much aerobic stability increases when selecting inoculants, rather than the hours of aerobic stability itself.