IMPROVING THE AEROBIC STABILITY OF SILAGES

Limin Kung, Jr., Ph.D.
Ruminant Nutrition & Microbiology Laboratory

Introduction 

Microorganisms involved in the ensiling process include beneficial bacteria (homolactic acid bacteria), detrimental bacteria (for example, clostridia) and unwanted yeasts and molds.  In a good silo, air (oxygen) is eliminated by respiration from the plant and packing, thus preventing the growth of detrimental aerobic bacteria, yeasts and molds. When air is eliminated from the silo, production of acid begins and a low pH eventually curtails the growth of all bacteria.  A limited amount of heating during the early days of ensiling is good because it is an indication that bacteria are starting to ferment the silage.  Heating for prolonged periods of time is an indication that there was too much air in the silage mass. Excessive heat can result in heat-damaged proteins, low energy values, and low dry matter recoveries.  Preventing silage from spoiling due to excessive air can improve the efficiency of a farm by preserving forage as high quality silage that is palatable to cows.

Aerobic Stability 

When fermentation is completed, and silage is exposed to air during feedout or during storage (e.g., leaky silos, holes in bag silos, poorly packed silage), heating in the silo and feed bunk is usually caused by yeasts (and to a lesser extent molds and some bacteria). Aerobic stability is a term that nutritionists have used to define the length of time that silage remains cool and does not spoil after it is exposed to air.  Silages that are aerobically stable are good.  Other than air, we know of only a few things that control the aerobic stability.  In general silages that spoil rapidly when exposed to air have large amounts of yeasts (more than 100,000 yeast per gram of wet silage); those with low levels of yeasts (less than 10,000 yeast) remain stable. Silages that have undergone clostridial fermentation and have high concentrations of butyric acid are, ironically, very stable when exposed to air.  However, clostridial silage is undesirable because loss of dry matter and nutrients are extremely large when this type of fermentation occurs.  Silages that have a strong vinegar smell (acetic acid) are also usually very stable because acetic acid is very toxic to yeasts.  However, high levels of acetate are indicative of a heterolactic fermentation that is less desirable than a homolactic fermentation.  

Because air fuels the growth of yeast, minimizing air in silage is an important goal.  Harvesting forages at optimum moisture levels (not too dry), correct particle size (not too long or short), filling and sealing quickly and feeding out adequate amounts of silage from the silo each day will help to minimize silage’s exposure to air.  Clean and efficient removal of silage from the face of bunkers and bag silos will also help to minimize spoilage. 

Usually, the problem of spoiling silage is worse in high moisture corn, corn silage, and cereal silages than in alfalfa silage. If heat is detected in silage months after silo filling, this is usually an indication of spoilage and reduced nutritive value.  Depending on the degree of spoilage, cows may eat less feed and/or produce less milk. 

Preventing Spoiling with Additives 

There are many products available to producers that claim to improve silage quality.  Bacterial inoculants, based on homofermentative lactic acid bacteria, should be added to silages to improve fermentation and increase dry matter and energy recovery.  However, most of these inoculants do little to inhibit growth of yeasts and molds.  Some inoculants contain bacteria that can make propionic acid (Propionibacteria).  However, in most studies, improvement in aerobic stability has not been better because these organisms are generally acid-intolerant and slow growing.  In some recent studies, newer bacterial inoculants that produce large amounts of acetic acid (Lactobacillus buchneri) have improved aerobic stability.  However, these products are not currently approved for use in the U.S. and more research is needed in this area.  Enzymes are commonly found in many silage additives but they do not inhibit yeasts or molds.  In fact, reports in the literature have shown that, sometimes, silages treated with enzymes have very poor aerobic stability. 

There are several types of silage additives that can help to control yeasts and molds. Most of these products contain buffered propionic acid as the primary active ingredient.  These preservatives also contain other antifungal compounds such as sorbic acid, benzoic acid, and acetic acid.  Buffered propionic acid-based products are generally non-corrosive and safe to handle. Unbuffered propionic acid is less expensive to use but most advise against its use because it is highly corrosive to equipment and people.  My lab has conducted research with several of the newer buffered propionic acid products. Label recommendations range from 1 to 4 lbs./ton of forage.  As expected, our initial research suggests that higher application levels are better than lower.  An optimum dose level cannot be recommended at this time because we cannot predict how many yeasts are naturally occurring on a forage in a particular field.  In addition, packing, moisture, fill rate, and feedout rate markedly affect growth of yeasts in the silo.  To date, there is insufficient data to show that any one product is consistently better than another. 

A less commonly used additive to control yeasts and molds is anhydrous ammonia (5-7 lbs./ton).  The major drawback with ammonia is operator safety during application.  Ammonia is quite dangerous to work with and safety precautions should always be taken when it is used (protective eye-ware and respirator if needed).  Rations with ammonia treated silage must also be carefully balanced for proper amounts of rumen-degradable and -undegradable protein. 

Both ammonia and propionic acid are most effective in controlling yeasts when applied at the time of ensiling.  Table 1 shows some results from tests conducted in my lab with ammonia and propionic acid based preservatives.  Comparisons cannot be made between the two preservatives as each was tested separately.  In addition, these numbers should not be used as absolute values for aerobic stability because they were generated under controlled laboratory conditions.  What is important when viewing this data is the relative difference among treatments.  

Table 1.  Effect of preservatives on the aerobic stability¹ of corn silage under controlled laboratory conditions. ²
 

Preservative	Untreated Silage	Treated Silage
	----------------Aerobic Stability---------------
Ammonia3	< 1.5 days	> 6 days
Buffer propionic acid4	2.4 days	3.4 days (2 lb./ton)  4.0 days (4 lb./ton)

¹Aerobic stability = days before silage heats after exposure to air.
²Data from the University of Delaware.  
³6 lb. of anhydrous ammonia equivalent/ton of 35% DM corn silage
⁴A commercial product containing buffered propionic acid as the primary active ingredient.

  

Diagnosing  Problems With Aerobic Stability 

All silages contain some yeasts and molds.  Yeasts make alcohol, so if silage smells like alcohol, many yeast are present.  Silage with over 100,000 to 1,000,000 (also denoted as 10⁵ to 10⁶) yeast per gram are usually very unstable and will heat quickly.  This is especially true when silage is exposed to air during hot weather.  Cold weather usually retards the growth of yeasts and molds so producers tend to see less of a problem with heating silages during the winter. Silage can be sent to a laboratory for analyses of yeasts and molds.  However, caution should be taken to ensure that the sample is frozen, because yeasts and molds can grow rapidly while in transit and thus give a false reading. 

A question that is often asked is when should propionic acid type products be used?  Here are some scenarios.  As an additive used at ensiling, propionic acid can be used to treat historic problems of silages heating in the silo or bunk (oversizing, slow feedout rate, poor packing and filling).  Corn silage or high moisture corn that has been stored for prolonged periods of time (more than 6-9 months) or silage fed during hot weather are other good candidates for treatment. Treating an entire silo or all of your silage may not be justifiable if the problem occurs for only a few weeks out of the year.  However, it is extremely difficult to predict in advance whether silage will remain cool.  Silage moved from one silo structure to another and  purchased silage that is moved and exposed to air for several days before feeding should be considered for treatment with buffered propionic acid.  However, ammonia and bacterial inoculants should not be used to treat silages that have completed the fermentation process.  Spraying the face of a bunk with a propionic acid-based preservative is probably not useful because only silage on the immediate face is protected as air can penetrate deeply into the silage mass.  In an attempt to reduce costs, some suggest that only the top several loads of silage in a tower silo or bunk should be treated with a propionic acid-based product.  However, this will not help to improve the aerobic stability of the remainder of the silage.  Because of their relatively high cost, preservatives designed to be added directly to the TMR should be used only in instances where they are to be used for short periods of time.  In a heating TMR, be sure to evaluate all sources of yeasts.  For example, a silage may remain cool and stable by itself,  but when mixed with other feeds, the mixture may spoil rapidly.  The primary source of heating may have come from the other dietary components such as other silages or high moisture corn.  If given a choice, preliminary data from our lab suggests that it is better to control yeasts at that time of ensiling rather than after the fact in a TMR. 

Molds and Mycotoxins 

            Mycotoxins can cause major health problems for cows.   Unfortunately, our understanding of their control and knowledge is very poor.  Molds produce most mycotoxins and for the most part, mycotoxins are a separate problem from spoiling feed in the bunk.  The majority of mycotoxins are produced in the field while a much smaller amount may be produced in the silo.  None of the additives discussed in this article are effective in reducing mycotoxin concentration in the silo. 

Justification for Using Silage Preservatives 

The cost of a preservative may be justifiable if silage is constantly spoiling and leading to poor animal performance.  I have created a table projecting theoretical costs of treating a ton of 35% DM corn silage ranging from $1 to 4/ton (Table 2).  The cost per cow per day was calculated depending on how much of this silage a cow was eating (20, 40, 60, or 80 lb./day).  Based on this simple calculation, if a cow lost 1 lb. of milk per day (worth $0.14/day) because of consuming spoiled silage, it would be justifiable to treat this silage in all situations with the exception of where 80 lbs. of silage treated at a cost of $4.00/ton were fed.  It has been difficult to measure the amount of milk production lost when cows are fed spoiled silage but in a recent study by Wisconsin researchers, cows fed spoiled high moisture corn produced about 7 lb. less milk per day than those fed unspoiled silage. 

 

Table 2.  The cost treating corn silage with a preservative.

 

	Cost the preservative
Corn silage, lb. per cow/day	$1.00/ton                     $2.00/ton           $3.00/ton                   $4.00/ton ---------------------------Cost (cents) per cow per day------------------------
20	1	2	3	4
40	2	4	6	8
60	3	6	8	10
80	4	8	12	16

            Preservatives also often help to reduce loss of nutrients in the silo.  Calculations can be made for potential savings of dry matter and used to justify the use of the preservative. 

Conclusions 

Heating and spoiling silage is undesirable because of losses in nutrients and lowered animal performance.  Producers should work with their nutritionists and feed consultants to evaluate their specific problems in this area.  Proper sizing and maintenance of silos and proper harvesting, filling and sealing of silos should be emphasized.  Bacterial inoculation can help with silage fermentation quality.  In instances where spoilage is still a major factor, preservatives such as buffered propionic acid and ammonia can be used.   Research suggests that treating silage at the time of ensiling with these preservatives is more efficacious than trying to treat a TMR.  Conditions on each farm should be carefully evaluated.