Introduction
Silage fermentation can be roughly divided into four phases. The first phase of fermentation is characterized by the presence of oxygen in the silo. Plant respiration continues for several hours (and perhaps days if silage is poorly packed) and plant enzymes (e.g., proteases) are active until oxygen in the silo is used up. During this phase, excess oxygen can lead to unwanted protein breakdown, excessive heating, and the growth of undesirable yeasts and molds. Oxygen can be eliminated by quick packing, even distribution of forage in the storage structure, chopping to a correct length, and ensiling at recommended dry matters (DM) for specific storage structures and type of silage/haylage. Oxygen must be eliminated before optimal fermentation can take place.

Under anaerobic conditions (lack of oxygen), the second phase of silage fermentation is dominated by microbial activity. Fermentation is controlled primarily by:

• Type of microorganisms that dominate the fermentation.

• Available substrate (water soluble carbohydrates) for microbial growth.

• Moisture content of the crop.

During this phase, lactic acid-producing bacteria (LAB) should utilize watersoluble carbohydrates to produce lactic acid, the primary acid responsible for decreasing the pH in silage. Undesirable fermentations from microorganisms such as Enterobacteria and Clostridia can dominate if the pH does not drop rapidly.

When weather permits, wilting forages to obtain dry matter greater than 30-35% prior to ensiling can help eliminate Clostridia.

Lack of oxygen prevents the growth of yeast and molds, and low pH prevents the growth of most bacteria during the third phase of fermentation. Silage can be kept for prolonged periods of time if these conditions prevail. The last, and fourth, stage of silage fermentation is during feed out and the subsequent exposure to oxygen. Good silage will remain stable and not change in composition or heat during the third and fourth stages of fermentation. Airtight silos and removal of sufficient silage during feed out can prevent aerobic spoilage. Some good silage management practices are listed in Table 1.

The end products of silage fermentation are often monitored to assess silage quality. The composition of "normal silages" is presented in Table 2. Several commercial laboratories now offer analytical services for silage end products. Readers should be aware that numerous factors can affect silage composition.

Of the many factors that can affect silage fermentation, the type of microorganisms that dominate the process often dictate the final quality of the silage. As shown in Table 3, homolactic fermentation is more desirable than other types of fermentations because the theoretical recoveries of DM and energy are greatest. In contrast, other fermentations are less efficient.

What Good-Quality Silage Smells Like
Theoretically, high-quality silage which has undergone a fermentation that was predominately homolactic in nature should have very little smell. The reason is that lactic acid has relatively no smell when compared to the other major fermentation acids. In fact, you may be able to tell what type of fermentation dominated your ensiling process by smelling your silage. Silage that has a strong vinegar smell has most likely undergone a heterolactic acid fermentation. Such silages are high in acetic acid concentration and are usually very stable when exposed to air. In contrast, silages that smell very sweet, like alcohol, have undergone a yeast fermentation and are very unstable when exposed to air. A burnt, but sweet smelling, tobacco-like odor is often commonly found in very dry alfalfa silages and is a result of heat damaged protein. Finally, very wet silages that are slimy and smell fishy and putrid, have undergone clostridial fermentations and are low in energy and usually have excessive amounts of soluble nitrogen.

Microbial Inoculation
Natural populations of lactic acid bacteria (LAB) on plant material are often low in number and heterofermentative (fermentation yields various end products). Thus, the concept of adding a microbial inoculant to silage was to add fast growing homofermentative (fermentation yields one consistent end product) lactic acid bacteria (hoLAB) to dominate the fermentation, producing a higher quality silage.

Some of the more common h0LAB used in silage inoculants include Lactobacillus plantarum, L. acidophilus, and Pediococcus acidilactici. Commercially available microbial inoculants contain one or more of these bacteria that have been selected for their ability to dominate the fermentation. Some companies rationalize the need for multiple organisms for potential synergistic actions. For example, growth rate is faster in Streptococcus > Pediococcus > Lactobacillus. Some Pediococcus strains are more tolerant of high DM conditions than are  Lactobacillus and have a wider range of optimal temperature and pH for growth (they grow better in cool conditions found in late fall and early spring). However, organisms from different companies often have the same name, but are different. Thus, multiple strain inoculants are not necessarily better than an inoculant with a single strain of homolactic acid bacteria.

When compared to untreated silages, those silages treated with adequate numbers of a viable h0LAB should be lower in pH, acetic acid, butyric acid, and ammonia-N, but higher in lactic acid content. In a review of the literature between 1990-95, Dr. Richard Muck (U.S. Dairy Forage Research Center) and I reported that microbial inoculation lowered pH, improved the lactic:acetic ratio, and lowered ammonia nitrogen content in more than 60% of studies. Dry matter recovery was improved in more than 35% of the studies and bunk life improved in only about 30% of the studies. Dry matter digestibility was also improved in about one-third of the cases.

Microbial inoculation usually has little or no effect on the fiber content of silages because most lactic acid bacteria contain little or no ability to degrade plant cell walls. Decreases in fiber content may be due to partial acid hydrolysis of hemicellulose. Bunk life or aerobic stability has not been consistently improved by inoculation and, in some instances, inoculation has made aerobic stability worse. This was probably due to a lower acetic acid content.

Relative to animal responses, Dr. Muck and I also found positive responses to microbial inoculants on intake, gain, and milk production (see Table 4). The average response in milk production was a +3.1 lb per day in studies where milk production was statistically improved. Although literature summaries are encouraging, caution should be used when interpreting such data because all inoculants are not equal and the conditions (e.g. rate of application, inoculant viability, species of bacteria, crop, moisture levels) varied markedly among the studies. As many have pointed out in the past, products with organisms with the same name are not necessarily the same organism and may not have the same effectiveness.

An impressive number of animal experiments have been conducted using a single silage inoculant containing Lactobacillus plantarum MTD/1. A summary of 14 lactation studies conducted in university and government research institutes in North America and Europe using MTD/1 is shown in Table 5. Statistical analyses revealed DM intake was numerically increased by 4.8% and milk production was significantly increased by 4.6%. Improvements in milk yield were obtained with a variety of crops (grass, corn, and alfalfa) across a wide spectrum of DM contents (15 to 46% DM). Body weight gain also tended to be better in cows fed silage treated with MTD/1. Similarly, 19 comparisons among untreated silages and silages treated with MTD/1 were summarized by U.K. researchers for beef cattle. Across all studies and types of forage, cattle fed inoculated silage ate 7.5% more DM and gained 11.1 % more weight.

The organisms) from microbial inoculants must be present in sufficient numbers to effectively dominate the fermentation. Most research suggests that an inoculation rate of 100,000 (or 1 x 105) homolactic acid producing bacteria per gram of wet forage is a good compromise between adequate numbers and cost effectiveness of the additive. Purchasing an inoculant from a reputable company with good manufacturing and quality controls is a key to obtaining a product with viable organisms.

Most microbial inoculants are available in powder or granular form. Inoculants applied in the dry form are often mixed with calcium carbonate (limestone), dried skim milk, sucrose, or other carriers. These products can be applied by hand or by solid metering devices as per manufacturer's recommendations. Inoculants to be applied in the liquid form come as dried powders and are mixed with water just prior to use. (Caution: use of chlorinated water is not recommended because it may be detrimental to the inoculant.)

Microbial inoculants can be applied to the forage at a variety of locations. However, inoculant application to forage at the chopper is highly recommended to maximize the time microorganisms have in contact with fermentable substrates. Inoculants can also be applied at the blower of an upright silo or sprinkled over the forage mass between loads in a bunk silo. Application can be with a simple watering can by weighing the incoming forage load and adjusting application based on the average unloading time.

A better method is to use a metered liquid sprayer to evenly disperse the inoculant onto the forage. Unused liquids should be discarded after a period of 24 to 48 hours because bacterial numbers begin to decline and there is potential for growth of contaminating organisms. Proper distribution cannot be overlooked and is important for the inoculant to be effective. Throwing dry inoculant onto a wagonload of forage and hoping for even distribution is not an acceptable practice!

Storage is an important aspect of a high-quality inoculant that contains live microorganisms. Inoculants should be kept in cool, dry areas away from direct  sunlight. Moisture, oxygen, and sunlight will decrease stability of inoculants.
Opened bags of inoculants should be used as soon as possible.

Conclusion
Silage inoculants can be useful tools to improve silage quality and animal performance; however, they are not replacements for good management practices. Care should be taken when choosing a silage inoculant. Producers should ask for proof of claims that are usually in the form of published scientific articles that have undergone peer review.