Silage Additives - What’s the Secret?

The purpose of ensiling grass is to preserve and minimise the loss of nutrients thus improving silage feeding value. Silage additives are used to enhance the ensiling process, which can result in improvements in animal performance, milk quantity and quality, body condition and fertility. Altering the fermentation process without quantifiable improvements in one or more of these categories is of minimal benefit. Studies at AFBI, Hillsborough have shown that, averaged over 11 studies, the milk yield benefit from using an inoculant was 0.54 litres/cow/day with improved milk quality. Allowing for additional silage intake and the cost of additive this would give a benefit of £13.50/cow over a 150-day feeding period.

Silage fermentation can be divided into four phases. In the first phase oxygen (from grass respiration) is present after forage is chopped and ensiled. During this phase excess oxygen can lead to unwanted protein breakdown, excessive heating and the growth of undesirable yeasts and moulds. Filling the silo quickly, distributing forage evenly in the silo, chopping grass to the correct length and ensiling at recommended dry matters, can eliminate oxygen at this stage.

The second phase of silage fermentation takes place under anaerobic (lack of oxygen) conditions. Micro-organisms in the grass produce lactic acid which is the primary acid responsible for lowering the pH in grass, producing silage and making it stable. Undesirable micro-organisms can dominate if the pH does not drop rapidly. Where weather permits, wilting grass to 25-30% DM prior to ensiling can eliminate these problems.

Lack of oxygen prevents the growth of yeast and moulds and the low pH inhibits the growth of most bacteria during the third phase of fermentation. Silage can be kept for long periods of time if these conditions prevail. The last, and fourth, stage of silage fermentation is during feed out and exposure to air. Good silage will remain stable and not change in composition or heat during the third and fourth stages of fermentation. Airtight silos and correct silo management during feed-out can prevent aerobic spoilage.

Effective silage fermentation must therefore promote high levels of lactic acid production and create a low pH (3.8-4.2). One strategy available to assist this is the use of silage additives. A wide variety of additives are available with virtually all commercial products falling into one or more of the following categories: bacterial inoculants, enzymes, non-protein nitrogen sources, acids and sugar sources. It is important to emphasise that none of these products are a substitute for good silage making techniques and management but they should assist in making a good situation better.

This is the most common category of silage additive. These products contain lactic acid producing bacteria, which supplement those naturally occurring in the grass crop and help guarantee a fast and efficient fermentation in the silo. Guidelines for their application should be strictly adhered to for maximum benefit.

Enzymes aim to reduce the fibre content of the grass and release sugars that can be used for fermentation by lactic acid bacteria. However enzymes can produce excess unfermented sugars, or ferment sugar to alcohol, and this may decrease aerobic stability.

Ammonia and urea are non-protein nitrogen (NPN) sources sometimes used on low protein silage to increase crude protein of the silage and reduce the potential for heating after the silo is opened. Addition of NPN to the crop during silo filling has the undesirable effect off immediately raising the pH of the material entering the silo.

This is added to increase the amount of sugars in the fermenting material. However, to be effective high rates are required in poor conditions, and achieving an even application throughout the silo is difficult.

Acids (propionic, formic, acetic) are used to guarantee preservation of crops low in sugar through direct acidification. Propionic acid is used primarily to improve feedout stability and to inhibit the growth of yeasts and moulds. Acid products are popular where wilting of crops is difficult but correctly wilted grass has adequate sugar for a proper fermentation.

Although additives may aid the preservation of forage, they cannot compensate for poor ensiling practices. Use of an additive should always be associated with good ensiling management - cut grass at the correct stage of maturity, dry matter and chop length; fill the silo rapidly and consolidate well. Following these practices will improve ensiling and increase the effectiveness of an additive or reduce the need for one. Evaluate the use of an additive based on its effect on ensiling and livestock performance, as well as cost and return on investment.