Silage additives include feedstuffs, urea, ammonia, inoculants and acids. Their main functions are to either increase nutritional value of silage or improve fermentation so that storage losses are reduced. Response to additives depend on what forage is being treated. Corn silage does not need additives that improve fermentation since it ferments quite readily. Nutritional additives such as urea and ammonia, however, are beneficial. Hay crop silages generally are more difficult to ferment and may respond to many silage additives, but animal response when fed treated silages is usually not different from animals fed untreated silage. Silage additives are not magic bullets and will not replace good silage management.
Corn and hay crops (legumes, grasses, and legume-grass mixtures) are ensiled frequently in the midwest. The chemical composition and therefore the fermentation characteristics of corn and hay crops differ greatly. Corn has low concentrations of protein and some minerals, but high concentrations of fermentable carbohydrates. Hay crops vary in protein but when harvested at the proper stage of maturity usually have a high concentration of protein, and are good sources of minerals. Hay crops, however, have low concentrations of fermentable carbohydrates. The differences in nutrient composition and fermentation characteristics between corn and hay crop silage must be considered when making silage additive decisions.
The main goal of silage making is to preserve as much of the nutritional value of the original crop as possible. Preservation is achieved by acidity and by maintaining an oxygen-free (anaerobic) environment. Acids are produced by bacteria that convert fermentable carbohydrates into organic acids, predominantly lactic and acetic acids. As fermentation progresses, more acids are produced, pH drops, and eventually the acidity level is adequate to inhibit or kill most bacteria and other microorganisms. Factors involved in the rate of pH decline and final pH of the silage include fermentable carbohydrate concentration of the original forage, its buffering capacity (related to the amount of acid needed to change the pH), dry matter (DM) content, and the type and amount of bacteria present on the forage. Corn has relatively low buffering capacity and high concentrations of fermentable carbohydrates; therefore, pH decline is rapid and final pH is usually low (approximately 3.5). Hay crops, on the other hand, have a high buffering capacity (due to high protein and mineral content), and relatively low concentrations of fermentable carbohydrates. Therefore, pH drop in hay crop silage is slow and final pH is comparatively high (approximately 4-4.5). Hay crops that are extensively wilted prior to ensiling (high DM silage) undergo very limited fermentation and final pH values are usually between 4.5 and 5. Lactic acid producing bacteria present on fresh forage, and on silage making equipment are responsible for most of the acid production during fermentation. There is a positive correlation between number of bacteria present at the time of ensiling and the rate of pH decline. The relationship between number of bacteria and final pH is much weaker. In summary, for a rapid and extensive fermentation to occur, the forage must have high concentrations of fermentable carbohydrates, low buffering capacity, relatively low dry matter content (20-40%), and adequate lactic acid bacteria present prior to ensiling.
Good silage making techniques must be followed for maximum preservation of nutrients. Forage should be ensiled at the correct DM content (30-45%) and chopped to the correct length. The silo should be filled rapidly, a plastic cover should be placed on the top of the silage between cuttings (concrete stave silo), and the silo must be structurally sound. Likewise with bunker silos, tight packing during filling and prompt effective covering to exclude outside air are necessary. FOLLOWING THESE GUIDELINES USUALLY WILL HAVE A GREATER IMPACT ON SILAGE QUALITY THAN USE OF A SILAGE ADDITIVE! Silage additives cannot replace good silage management.
Many different silage additives are available and are used for different reasons. Additives are used to improve nutrient composition of silage, to reduce storage losses by promoting rapid fermentation, to reduce fermentation losses by limiting extent of fermentation, and to improve bunk life of silage (increase aerobic stability).
NON-PROTEIN NITROGEN (NPN). Urea and anhydrous ammonia can be added to silages to increase their crude protein (CP) content. Urea has 280% CP and anhydrous ammonia contains 512% CP. Therefore, relatively small additions of these compounds can markedly increase the CP content of silage. Corn silage is extremely low in CP (average 8%, DM basis); whereas hay crop silage may contain more than 20% CP. The recommended application rates for corn silage of normal DM content (approximately 35% ) is 7-9 lbs./ton of wet silage for anhydrous ammonia and 10-14 lbs./ton wet silage for urea. Anhydrous ammonia should be added using a Cold-flo system to reduce losses during application. Do not exceed the maximum recommended rates because both urea and ammonia buffer silage and will raise the final pH. Silage with high final pH is not well-preserved. Recently, research has shown that adding 2-4 lbs anhydrous ammonia/wet ton of hay crop silage reduces fermentation losses. The amount of ammonia added to hay crop silage must not exceed 4 lbs/ton or fermentation will be inhibited severely. Ammoniating corn and hay crop silage at the recommended rates should reduce fermentation losses, increase bunk life, and may enhance consumption of the silage by animals.
A benefit of adding NPN to corn silage includes increasing the CP content by 2-6 percentage units. Ammoniated or urea-treated corn silage usually has CP concentrations of 12-13% (DM basis). This can result in substantial savings by reducing the need for protein supplementation. Ammoniating hay crop silage does not increase its CP content greatly because hay crop silages generally have relatively high concentrations of CP and the amount of ammonia added is low (2-4 lbs/ton). Another benefit of adding NPN to silage crops is that it reduces plant protein destruction during fermentation. During normal fermentation, much of the plant protein is broken down into amino acids or small peptides. These compounds are degraded rapidly in the rumen when cattle or sheep are fed the silage. Saving plant proteins from breakdown during ensiling increases the amount of undegradable or bypass protein in the silage. This could be very advantageous with high quality grass or alfalfa silage.
FEEDSTUFFS. Feeds such as corn, small grains, and molasses can be added to forage at the time of ensiling. Addition of grain to corn silage is not useful, but adding it to hay crop silage has two benefits. First, adding grain to hay crop silage increases the energy content of the silage. This will reduce the amount of supplemental grain that has to be fed. If silage will be the main or only feed offered, then adding some grain to the forage at ensiling will make it a more complete feed. However, grain mixed with silage prior to ensiling or at feeding is nutritionally equal; therefore, if supplemental grain must be fed anyway, no true benefit is realized. Secondly, adding grain to forage will increase the dry matter content of the silage. Hay crops that are not wilted sufficiently prior to ensiling can cause seepage and result in an undesirable fermentation. Added grain may also make wet silage easier to unload from the silo.
Recommended application rate for grain is 100-200 lbs./wet ton. This rate will increase the dry matter content of silage by about 5 percentage units. The grain should be cracked or rolled prior to ensiling for maximum benefit. In general, adding grain does not improve fermentation because starch (the main carbohydrate in grain) is not readily fermented in the silo.
Molasses has been used as a silage additive for many years. Molasses, unlike grain, provides fermentable carbohydrate; therefore, molasses addition can improve the fermentation of some hay crop forages. Adding molasses at 200-400 lbs./wet ton of hay crop silage (approximately 45% dry matter) usually will increase fermentation noticeably and result in good quality silage. Adding 50-100 lbs. molasses/wet ton will increase fermentation and usually increase bunk life of silage. Molasses should not be added to wet silage (<35% dry matter) because of increased seepage loses. There is no benefit of adding molasses to corn silage.
MINERALS. Minerals such as calcium, phosphorous, sulfur and magnesium have been added to forage at the time of ensiling. Usually these either have no effect on fermentation or act as buffers resulting in higher pH silage. The only reason for adding minerals at the time of ensiling is if the silage will be the only feed offered to the animals. Addition of minerals will make the silage more nutritionally complete. If concentrates are going to be supplemented, it is better to add the minerals to the concentrate mix.
ACIDS. Acids are added to forages at ensiling to cause an immediate drop in pH or to increase bunk life. Formic acid and mineral acids (sulfuric and hydrochloric) added at 10-30 lbs./wet ton will reduce pH quickly and greatly limit fermentation losses of protein and carbohydrates. These acids are extremely caustic and hazardous to use. Very few producers in the U.S. use these compounds, but they are frequently used in Europe.
Propionic acid is weaker than formic and mineral acids, but can be a useful additive for silages. Adding 30-40 lbs. propionic acid/wet ton of hay crop silage increases its bunk life and reduces surface molding. Low DM silage needs more propionic acid than does drier silage. On wilted silage, 30 lbs. propionic acid/ton significantly decreased aerobic spoilage, but 100 lbs./ton was needed when the silage had 30% dry matter. The cost of adding sufficient propionic acid is relatively high and usually is not cost-effective.
Another acid-type preservative is sodium diacetate. This is mixture of acetic acid and its sodium salt. Recommended rate of application is 1-2 lbs. active ingredient/ton of wet silage. This compound produces results similar to propionic acid and is quite effective in reducing top spoilage. Adding sodium diacetate at the recommended rate to just the last few loads of forage going into the silage is effective in reducing top spoilage. The same scheme will probably work for propionic acid products. In both cases, the top of the silage mass must be covered with plastic immediately after filling for the compound to work. By just treating the last few loads, treatment costs are reduced considerably.
MICROBIAL INOCULANTS. Inoculants are added to forage to increase the number of desirable bacteria present at the time of ensiling. A host of different inoculants are currently available and many research trials have been conducted testing their efficacy. Research results have been variable. In general, no benefits with respect to silage quality, and animal performance have been found for inoculating corn silage. Corn plants at harvest have from 500,000 to 2 million lactic acid bacteria (LAB)/g. Most commercial inoculants when applied at recommended rates add about 100,000 LAB/g. Therefore, the relative increase in LAB due to inoculant range from 5 to 15%.
Generally fewer LAB are on hay crops than on corn plants prior to ensiling. Hay crops can have less than 10,000 LAB/g to more than 1,000,000 LAB/g wet forage. Factors affecting the number of indigenous LAB include forage species, environmental temperature, and wilting period. Forages that are cut during cool temperatures and have a short wilting period generally have low indigenous LAB populations. The US Dairy Forage Research Center has developed equations that can be used to estimate the amount of LAB present on alfalfa prior to ensiling. If the environmental temperature during the wilt period averaged 66-72 degrees F, use equation (1); if the temperature averaged 72-77 degrees F, use equation (2):
(1) LAB (log) = 6.94 - .0917 x % DM (2) LAB (log) = 6.9 - .0765 x % DM
For example, if the average temperature was 68 degrees F and the DM of the forage was 35% when you were going to start chopping, the number of LAB would be approximately 5400 LAB/g (6.9 - (.0765 x 35) = 3.72; using a calculator, the antilog of 3.72 = 5376). There are some data that suggest when inoculation increases the indigenous population by a factor of 10, fermentation and animal performance may be enhanced. One can predict the number of LAB present by using the above equation and then add that number to 100,000 (the number of LAB most inoculants add) and then divide that sum by the original number of LAB present. If that value is greater than 10, positive responses might be found when the silage is fed. If the number is less than 10, it is less likely benefits will be observed. In the above example the indigenous population was 5400. Adding 100,000 to that yields 105,400 and dividing that by 5400 equals 19.5 In this situation, inoculation might be warranted. However if the temperature was 75 and using equation (2), then the number of indigenous LAB equals about 16,700 and adding 100,000 LAB only increases it by about 7. In this case inoculation may not be needed.
Inoculation may decrease storage losses because in most experiments adding an inoculant to hay crop silages increased rate of pH decline and increased lactic acid concentration in the silage. Final pH of the silage was usually similar between untreated and inoculated. Even though fermentation of hay crop silage often is effected by inoculation, milk production of dairy cows fed the silages seldom was different. Inoculating corn silage, however, often results in significant improvements (about .2 lb/day) in growth rates of beef cattle. Dairy cattle often gain more weight when fed inoculated silage.
An economic analysis revealed that if all forage (alfalfa silage) was inoculated prior to ensiling, a producer would receive a 1.3:1 return on investment. It cost approximately $1/ton to treat silage. A 100 cow dairy herd fed only alfalfa silage plus concentrate would need about 1000 tons of silage (45% DM). This means that it would cost the farmer $1000 to treat his silage and he would realize a total return of about $1300 and a net return of about $300. There are no data showing any detrimental effects of silage inoculants, but potential benefits are not great. If you decide to use silage inoculants, make sure the product is from a reputable firm and adds at least 100,000 VIABLE lactic acid bacteria/g.
The decision to use an additive should be based on type and DM of the forage, and type of animals that are being fed. Urea and ammonia are usually beneficial to corn silage by economically increasing CP content. Adding grain to wet hay crop silage will reduce seepage and aid in unloading. No benefits are observed when adding grain to wilted silage. Molasses can improve fermentation of hay crop silage under some conditions if enough is added. Silage inoculants have very little effect on corn silage, but often increase rate of fermentation for hay crop silage. Milk production of cows fed inoculated silages (corn or hay crop) is usually not different from cows fed untreated silages. Cattle (beef and dairy) often gain more weight when fed inoculated vs. untreated silage. The bottom line is no additive can replace good silage making techniques, but may improve fermentation and nutritional value of some silages. The response is usually not large.
Prepared by:
Bill Weiss
Assistant Professor, Dairy Science
John Underwood
District Specialist Emeritus, Agronomy
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