Silage Additives Profit Makers or Profit Takers?

Limin Kung, Jr.

Dept. of Animal Science & Agricultural Biochemistry,
University of Delaware,
Newark, DE, U.S.A. 19717- 1303 E-mail: lkung@chopin.udel.edu

Take Home Messages

Silage additives are not substitutes for good management.

Microbial inoculants can improve the nutritive value of silage, but not all inoculants are of equal value. Use of enzymes as silage additives has not resulted in consistent improvements in silage quality.

Anhydrous ammonia and propionic acid can improve the aerobic stability of silages.

Introduction

In the process of making silage, forage material is chopped which releases proteolytic (protein degrading) enzymes and nutrients from plant cells. During this time, plant material is still alive and released enzymes are active but, in general, the reactions that occur do not aid in the preservation of forage. Packing forage quickly and tightly helps to eliminate residual oxygen. Then, if conditions are ideal, lactic acid bacteria (LAB) utilize water soluble carbohydrates (WSC) to produce lactic acid which causes a decrease in pH. The low pH stops plant enzymatic activity and further microbial metabolism which preserves the forage as silage assuming that oxygen is not allowed to penetrate the mass. Many factors can affect the fermentation process just described. Some of the more important factors include types and numbers of naturally occurring microorganisms (also known as epiphytic organisms), amount of soluble (fermentable) sugars, rapid elimination of oxygen, moisture content, and buffering capacity of the crop.

Because fermentation is based on microbial metabolism it is important to understand the role of the microorganisms involved in the process. Lactic acid producing bacteria, enterobacteria, clostridia, and yeast are the primary organisms of interest during ensiling. Naturally occurring LAB are responsible for converting soluble sugars to acids which lowers the pH and preserves the silage. Enterobacteria can compete with LAB for sugars and can produce detrimental endotoxins that could reduce animal performance. Eliminating oxygen and acidification will inhibit their growth. Yeasts and molds grow well in the presence of oxygen. Yeast can also compete for fermentable substrates and can use acids. It is well established that yeasts are primarily responsible for aerobic spoilage of silages. Molds can be detrimental because they can produce mycotoxins which have negative effects on animal health and performance. Clostridia are organisms that thrive under moist conditions and degrade sugars and proteins. They can be controlled by wilting forage above 30% DM prior to ensiling. Fermentation pathways from yeast, molds, and clostridia are extremely inefficient and can result in large losses of DM and energy.

Microbial Inoculation

Although LAB are primarily responsible for preserving silage, the numbers of these organisms may be too low and competition from undesirable organisms may dominate the ensiling process. In addition, many of the naturally occurring LAB are classified as heterofermentative bacteria which means that when they metabolize sugars to acids, there are multiple end products other than lactic acid. This process is inefficient. Thus, the idea behind microbial inoculation is to add homofermentative lactic acid bacteria (those that produce primarily lactic acid) to stimulate the rate of fermentation and decrease nutrient loss through more efficient fermentation pathways.

Common LAB used in silage inoculants are:

Lactobacillus plantarum,

L. acidophilus,

Pediococcus cerevisiae,

P. acidilactici, and

Streptococcus faecium.

Inoculants may contain one or more of these bacterial strains which have been selected from the natural environment for their ability to rapidly lower the pH in silage. Multiple organisms are used for various reasons. For example, pediococci are sometimes used because some are more active during the early stages of ensiling than lactobacilli. In addition, some pediococci are more active than some lactobacilli in cool conditions. Although many inoculants appear to have the same organism, users should be aware that products from different suppliers may vary tremendously because of strain differences (e.g., there are many different strains of Lactobacillus plantarum). Recently, Propionibacteria have been included in some silage additives in hopes of improving aerobic stability of silages. These bacteria convert lactate and glucose to acetate and propionate. Higher levels of propionate (and to some extent acetate) would theoretically inhibit spoilage fungi. Effects of adding Propionibacteria to silage from our lab (Kreck and Kung, University of Delaware unpublished data) and from the USDA Forage Research Center (R. E. Muck, personal communication) has not been encouraging. Data from our lab suggests that most strains of Propionibacteria are acid intolerant and exhibit poor growth when the pH is below 4.4. Thus, more acid tolerant strains of Propionibacteria must be identified for use in cereal silages where pH is often below 4.0.

In North America, some have suggested that inoculants supply 100,000 (105) organisms per gm of forage for maximum effectiveness and it is probably uneconomical to add 106 organisms per gram of forage as is done in Europe where silages are significantly wetter. However, users should understand that the efficacious organism(s) are needed.

Microbial inoculants are sold in a dry (powder or granular) or liquid form. Use of chlorinated water may be detrimental to the bacteria. If chlorine content is in question, the addition of 1 cup of milk solids to about 200 liters of water will neutralize the chlorine. Application can be with a simple watering can by weighing the incoming forage load and adjusting the application based on the average unloading time. A better method is to use a metered liquid sprayer to evenly disperse the inoculant on the forage. Most manufacturers suggest discarding unused liquids after a 24 to 48 h period. If forage is hauled long distances prior to ensiling (as in some places in southern California) application would be better if it were done at harvest (rather than several hours later at the point of ensiling). Proper distribution cannot be overlooked and is important for the inoculant to be effective. Throwing a cup of dry inoculant into a wagon load of forage and hoping for even distribution is not acceptable.

Bacterial inoculants have improved fermentation and animal performance in cereal silage (5), grass silages (3) and alfalfa silage (4). When compared to untreated silages, inoculated silages usually (but not always) are lower in pH (due to higher lactic acid), acetic acid, butyric acid and ammonia-N (Table 1). However, changes in fermentation end-products, digestibility, or intake are meaningless unless accompanied by one or more of the following:

increased DM (nutrient) recovery;

increased animal performance (milk [quantity and/or composition], gain, body condition, reproduction); or

decreased heating and molding during storage and feedout.

Recently, Muck (6) summarized studies with inoculants from 1985 to 1992. Recovery of DM was improved in about 60% of the studies, intake and gain were improved in about 25% of the studies, and milk production and feed efficiency were improved in about 40% of the studies.

Because the cost of inoculation is fairly low (typically $.50 to 1.50 (US dollars) to treat a ton of forage) small benefits can yield good pay backs. However, the producer should not be fooled by the low cost and only use an inoculant when deemed necessary and when proven effective. Inoculants are not a substitute for good management and cannot overcome poor forage quality. In addition, if fermentable substrates are limiting, they may not be effective. One way to provide adequate fermentable substrates may be to use cell wall degrading enzymes with an inoculant.

Enzymes as Silage Additives

Various fiber degrading enzymes (cellulases, pectinases, hemicellulases, xylanases) and starch degrading enzymes (amylase) have been used in silage additives (2, 7). There are two primary reasons for adding fiber degrading enzymes to silage. First degradation of fiber will yield soluble sugars which can be used by LAB to make lactic acid and lower the silage pH. Second, partial digestion of fiber may improve digestibility. Research in North America with cellulase enzymes is conflicting. Fiber content of grasses has been consistently decreased by enzymes, but this is not the case in alfalfa. In fact, use of enzymes in North America has not resulted in consistent effects on silage fermentation and animal performance. This is unfortunate because many companies are selling silage additives that contain both bacteria and enzymes.

Use of NPN Compounds

Non protein nitrogen (NPN) compounds such as anhydrous ammonia have been added to whole plant corn and corn stalks at the time of ensiling (1). Ammonia results in several beneficial effects on fermentation:

addition of an economical source of CP,

prolonged bunk life during feeding(aerobic stability),

less molding and heating during ensiling, and

decreased protein degradation in the silo.

Addition of anhydrous ammonia or water-ammonia mixes initially buffers the plant material. For example, corn forage may have a pH of 5.9 (slightly acidic), but treated corn forage will have a pH of about 8.5 to 9.0 (very alkaline). We have observed that ammonia treatment causes an initial delay followed by stimulation in growth of bacteria which make lactic acid and pickle the forage. When fermentation in the silo is complete, corn silage treated with anhydrous ammonia usually is .1 to .2 units higher in pH, contains .5 to 1.5% (DMB) more lactic acid, .5 to 1.5% more acetic acid, and less residual water soluble carbohydrates. Forages treated with ammonia have also been shown to be higher in insoluble N and true protein (both of which are beneficial) primarily because ammonia reduces plant proteolysis. Ammonia has been suggested to be anti-fungal in nature and results in improved aerobic stability (reduced molding and heating) during storage and feedout. However, recent data from our lab would suggest that improved aerobic stability is due to an increase in acetic acid and decrease in residual water soluble carbohydrates after ensiling, not due to a direct fungicidal effect from ammonia. Regardless of what the mechanism is, bunk life is improved.

Ammonia can be added at the chopper, blower, bagger, or bunk. Mixed ammonia solutions are bulkier than anhydrous ammonia, but retention of ammonia is usually greater. In addition, molasses (to improve palatability and fermentation) and minerals can be added in these solutions. Some ammonia will be lost (between 10 and 30%) and losses will be greater if ammonia is not applied properly and if forage becomes too dry. Ammonia should be applied to the forage before it contacts the blower to minimize losses. Ammonia should be added at the end nearest the cutter in a chopper with an auger system. If no auger is used, ammonia can be added behind the cutter prior to entering the blower. Ammonia can also be spiked into bunks between loads and it will disperse into the mass.

Application of anhydrous ammonia should be at approximately 7.9 kg of N per 1,000 kg of forage DM. Excess ammonia may result in poor fermentation (because of a prolonged buffering effect) and animal performance. Adding 3.2 kg of ammonia per tonne of 35% DM corn silage will increase the CP from about 8% to 12.5% (DM basis). Using the Cold- flo method is the simplest way to add ammonia to silage. Gaseous ammonia is super cooled in a converter box and about 80 to 85% becomes liquid. Anhydrous ammonia should not be added to corn forage if the DM content is above 40 to 42% because fermentation is restricted in drier material and binding of ammonia will be less; thus normal fermentation may be disrupted. In instances where forage DM is above 40 to 42%, water-ammonia mixes or molasses-ammonia mixes should be used. Application for molasses-ammonia mixes should be as recommended by the manufacturer.

Ammonia is a hazardous gas and should be handled with care. Eye protection should be worn when making connections to pressurized tanks. Water should be available at all times. Ammonia is also corrosive to zinc, copper, and brass; therefore, storage of ammonia-treated forage in zinc coated steel silos is not recommended. Problems with hyper-irritability (bovine bonkers syndrome) in cattle fed ammoniated forages has not been observed in cattle fed ammoniated corn forages. Addition of ammonia to corn silage has no effect on nitrate levels in corn silage.

There has been recent interest in adding anhydrous ammonia to alfalfa silage, primarily to improve aerobic stability. Certain precautions must be considered for this application. First, alfalfa silage already contains excess amounts of rumen degradable protein and added ammonia will compound this problem. Secondly, there is some research that shows that when alfalfa is on the wet side (less than 30 to 33% DM) ammonia can cause an undesirable clostridial fermentation leading to high levels of butyric acid and protein degradation in the silo. If you do treat alfalfa silage with ammonia, be sure to discount the added nitrogen when balancing the diet. At the time of writing this article, this author would not recommend widespread use of ammonia on alfalfa silage until more research is conducted.

Propionic Acid as a Silage Additive

Propionic acid-based preservatives have been used for many years. Unlike bacteria inoculants, propionic acid does not stimulate fermentation. Propionic acid is most effective in inhibiting growth of molds > yeast > bacteria and thus has been used to improve bunk life and prevent molding and heating. In recent years there has been much interest in "buffered" propionic acid mixes which have a pH of about 5.5 to 6. These products are based on ammonium or sodium salts of propionic acid. Propionic acid is not widely used as a silage preservative because its application (5 to 10 lbs/ton) is costly. However, under certain circumstances where rate of feed out and bunk life are not easily managed, it is an effective preservative.

Summary

Silage additives should not be thought of as substitutes for good management. Silage additives can be beneficial and economical to use. Producers should use silage additives that are supported with published data. Microbial inoculants have improved the nutritive value of silages, but enzymes have been inconsistent. Effects of microbial inoculants on aerobic stability have also been inconsistent. Anhydrous ammonia and propionic acid can improve the aerobic stability of silages, but their use is not widespread due to difficult handling and cost, respectively.

References

1. Britt, D.G., and J.T. Huber. 1975. Fungal growth during fermentation and refermentation of nonprotein nitrogen treated corn silage. J. Dairy Sci. 58:1666.
2. Chen, J., M.R. Stokes, and C.R. Wallace. 1994. Effects of enzyme-inoculant systems on preservation and nutritive value of haycrop and corn silages. J. Dairy Sci. 77:501.
3. Harrison, J.H., R. Blauwiekel, and M.R. Stokes. 1994. Fermentation and utilization of grass silage. J. Dairy Sci. 77:3209.
4. Kung, L. Jr., L.D. Satter, B.A. Jones, K.W. Genin, A.L. Sudoma, G.L. Enders. Jr., and H.S. Kim. 1987. Microbial inoculation of low moisture alfalfa. J. Dairy Sci. 70:2069.
5. Kung, L., Jr., J.H. Chen, E.M. Kreck, and K. Knutsen. 1993. Effect of microbial inoculants on the nutritive value of corn silage for lactating dairy cows. J. Dairy Sci. 76:3763.
6. Muck , R.E. 1993. The role of silage additives in making high quality silage. In: Silage Production. From Seed to Animal. Proc. Natl. Silage Prod. Conf., page 57, Syracuse NY.
7. Sheperd, A.C. 1994. Effect of an additive containing plant cell wall degrading enzymes on the nutritive value of corn silage for ruminants. M.S. Thesis. University of Delaware.