This article will cover the major reasons that might explain why silage additives may not be effective all the time.
Challenges specific to microbial inoculants
In order for a microbial inoculant to have a chance to be effective, the organisms must be alive and applied in the correct suggested amount. Lack of meeting these criteria would certainly reduce the probable effectiveness of an inoculant. Care should be taken to ensure that microbial inoculants are stored in cool and dry environments (or frozen if directed by the manufacturer) to maintain their viability. Quality microbial inoculants should have an expiration date, after which time they should not be used because optimal viability decreases with prolonged time of storage.
Once mixed in water, the number of bacteria added to the applicator should remain relatively stable. However, there are several factors that may result in a reduction of viable bacteria. For example, unused dry inoculants sitting in applicator boxes probably begin to lose their viability after more than a few days. Likewise, liquid inoculants sitting in a tank for more than about 48 hours may also be compromised.
This is usually not an issue when harvesting large volumes of forage in a short time but could be more of a challenge on small farms. Water that has been treated with hydrogen peroxide or chlorine (greater than 1 part per million) can also be toxic to inoculant bacteria.
High-quality inoculants usually have moisture scavengers and components to nullify effects of chlorine in water. Another factor that has been found to affect the viability of bacterial inoculants is excessive heat in the inoculant tank. In a laboratory study, my group reported numbers of lactic acid bacteria in many inoculants were substantially reduced in less than six hours when the water they were in exceeded 95ºF. In a follow-up study, we sampled inoculant tanks in the Midwest and California and found a strong negative correlation between temperature of water and expected concentrations of inoculant bacteria in tanks.
The temperature at which inoculants would not be effective was almost identical to that found in the lab study and was as high as 131ºF on one farm. Placement of tanks near motors or exhausts are the probable causes of high heat in inoculant tanks. (Hot water should also not be used to make up inoculant mixes.)
The ambient temperature and temperature in the forage mass during ensiling may also affect the resulting silage fermentation and how well a silage inoculant works. Anecdotal reports from some areas in the U.S. (e.g., Texas, California and Florida) suggest it is difficult to make good silage under extremely hot and humid conditions, and research definitively shows silages stored under elevated temperatures undergo less-than-desirable fermentations. At the other end of the spectrum, reports of forages harvested under very cool ambient temperatures and apparently not fermenting are common from the Upper Midwestern section of the U.S.
In order for added microbial inoculants to be effective, they must be able to grow and compete in the silage environment against a host of undesirable micro-organisms. Unfortunately, there may be conditions that could preclude this from happening. For example, there are “bacterial viruses” that can attack lactic acid bacteria, resulting in their death. The exact role bacterial viruses play in the efficacy of added lactic acid bacteria in silage is unknown.
Lactic acid bacteria in inoculants are dependent on adequate amounts of fermentable water-soluble carbohydrates for growth and production of lactic acid, which lowers the pH of silage. Low concentrations of fermentable sugars can be a problem in forage crops that have undergone excessive respiration because of long wilting times, because of cloudy weather or because they were rained on. Adding even high levels of lactic acid bacteria via inoculation will not positively affect silage fermentation if there is a lack of fermentable substrate.
Another reason that might explain why microbial inoculation may be ineffective is related to the amount of moisture available for bacterial growth. In heavily wilted or extremely dry forages, silage fermentation is partially curtailed because of a lack of available moisture for growth of lactobacilli. Liquid- applied inoculants may be more advantageous than dry granular inoculants in forages with greater than 40 to 45 percent dry matter (DM) because bacteria in the former case are added with their own moisture to hasten their rehydration and growth. It may be wise to increase the volume of water used (several quarts per ton rather than a few ounces per ton) when applying inoculants to very dry forages.
Challenges facing other silage additives
Weak organic acids such as buffered propionic acid, potassium sorbate and sodium benzoate are common active ingredients of chemical additives used to improve the aerobic stability of silages because these acids have good antifungal attributes. A major factor affecting their efficacy is the pH of the environment to which they are added. For example, at a pH of 4.87, only 50 percent of propionic acid is in an “active” (undissociated) form. As the pH decreases below 4.87, more of the acid becomes “active” but, conversely, as the pH increases above 4.87, more of it becomes less active (dissociated form). Thus, as silages become drier, ferment less and thus have higher pH, more acid is needed to be effective. Instances where such chemical additives appear to be ineffective may be due to insufficient amounts of “active” ingredients being applied.
Silage additives may contain enzymes that have the potential to digest different components in forages. There are several factors that can affect how well these types of additives work. First, it has been questioned if sufficient amounts of these enzymes are being used in some formulations. Second, these enzymes all have temperature and pH optimums at which they are most active, and these conditions may not be met in the silo. Thirdly, enzymes are proteins, and thus they could be subject to degradation during the ensiling process.
Calibration of applicators and distribution of silage additives
Proper calibration of applicators and distribution of all additives onto the forage mass is essential if an additive is to be efficacious. This is especially true with the use of low water volume applications for inoculants that may result in only 1 to 2 ounces of liquid per ton of forage. Distribution of an additive is probably less than optimal when the “shower” method (e.g., a liquid solution is sprayed on the top of forage in a wagon) or “manual spread” method (e.g., a person dips a small can into the source of inoculant and attempts to spread this over a large load of forage or area of forage mass) is used.
Interactions with other areas of silage management
The success of any silage additive could certainly be affected by harvest, storage and feeding conditions. For example, at harvest, slow packing rates, poor packing densities, delayed fills and inadequate sealing of silos could all potentially affect the efficacy of a silage additive.
Challenges during silo filling could be detrimental to an inoculant because the upper layers of forage will remain poorly packed during the evening hours if filling is stopped. This can result in excessive respiration and high temperatures in the forage mass in those layers.
During storage there is an increased chance of damage to plastic coverings with prolonged storage from a variety of issues such as weather- and critter-related damage. Even the best silage additives would have a difficult challenge when used in a bunker or pile silo that was never covered with plastic or where plastic covering was compromised. During silage feedout, low removal rates of silage, disruption of the feeding faces that allows air to penetrate into the silage mass and extremely hot weather can challenge any silage additive.
Conclusions
When used correctly, various additives can help farmers maintain and sometimes improve the quality of their silages. However, a variety of factors can interact and affect the efficacy of a silage additive. Additives must be stored and managed properly to maximize their potential effectiveness. Producers must realize silage additives are not an alternative to good silage management practices. 
References omitted but are available upon request. Click here to email an editor.
Limin Kung Jr. is with the Department of Animal and Food Sciences at the University of Delaware. Email Limin Kung Jr.
