CORN, ALFALFA AND GRASS SILAGE PRESERVATION PRINCIPLES R. E. Muck 1 Abstract Ensiling is the primary means of preserving moist forages for feeding livestock. In ensiling, the crop is stored anaerobically, and lactic acid bacteria found naturally on the crop ferment sugars in the crop. The crop is preserved by the combination of the acids produced by the lactic acid bacteria, the resulting low ph and the anaerobic environment. Silage management strives to create conditions so that the lactic acid bacteria dominate other microorganisms and oxygen exposure is minimized. Key management steps include harvesting at the right moisture content, chopping at the right length, packing to a high density, sealing the crop well, holding plastic film tightly to the crop on bunker or pile silos, feeding off the face of horizontal silos at least 1 ft./day, and keeping a smooth feed out face with no loose silage left overnight. Introduction: There are two principal means of preserving forages. One is to dry the forage to a sufficiently low moisture content so that growth of spoilage microorganisms is prevented. The other is to ensile the crop whereby the crop is placed in an anaerobic or oxygen-free environment and lactic acid bacteria ferment sugars in the crop to primarily lactic acid that reduces the ph of the crop. With hay, quality can be maintained for a long period provided the hay is kept dry. Silage, however, is at a moisture content where quality can change during storage and feeding. As a consequence, good silage management is critical to deliver a feed that is similar in quality to the forage at harvest. This paper will discuss how ensiling preserves the crop and the key management principles to ensuring low losses and high quality from a silage. Silage Preservation: Preservation of a crop in the silo is dependent on three things - the fermentation products, a low ph and an anaerobic environment. Each of these elements preserves the silage from the activity of different groups of microorganisms. Lactic acid bacteria growing on sugars in the forage produce mainly lactic acid, but depending on the sugar and species of lactic acid bacteria may produce acetic acid, ethanol and carbon dioxide. Other minor compounds are also produced. Lactic acid inhibits listeria (a pathogen commonly on the crop at ensiling) and acetic acid bacteria (a group of bacteria that can initiate the aerobic spoilage process in corn silage after silo opening). Acetic acid is a good inhibitor of yeasts and molds, both of which are involved in spoiling silage in the presence of oxygen. Ethanol can inhibit various microorganisms but is rarely at sufficient concentrations to enhance the preservation of silage. Low ph is a general inhibitor of microbial growth. Most microorganisms internally have a neutral ph and want to maintain that ph. In an acid environment, the greater concentration of hydrogen ions in the silage causes microorganisms to spend more energy to keep a neutral ph internally and less energy can be expended for growth. Most of the anaerobic bacteria that can compete with lactic acid bacteria for sugar, primarily enterobacteria and bacilli, are stopped from 1 Supervisory Research Agricultural Engineer, Dairy Forage and Aquaculture Research Unit, U.S. Dairy Forage Research Center, USDA, Agricultural Research Service, Madison, WI
growing by reducing ph below 5.0. That is not necessarily true for clostridia, the anaerobic bacteria that produce butyric acid and have a negative effect on palatability. The critical ph to prevent their growth is dependent on the moisture content and crop type (Fig. 1). The wetter the crop is the lower the ph needs to be in order to prevent clostridial growth. The lowest normal silage ph values (~3.7), however, will not prevent the growth of most yeasts and molds present in silage nor acetic acid bacteria. The key to preventing the growth of molds, acetic acid bacteria and many yeasts is an anaerobic environment. These microorganisms require oxygen to grow. Fermentation and low ph may slow their growth, but only keeping oxygen out of the silo will prevent their growth. So the combination of fermentation products, low ph and anaerobic conditions are all necessary for good preservation. When all three are present, fermentative yeasts, which ferment sugars to ethanol, are the only microorganisms beside lactic acid bacteria that may substantially alter silage quality. Harvesting Principles: While silage additives can have a positive effect on silage quality, it is still wise to assume that silage quality from a livestock perspective will be no better than the quality of the crop placed in the silo. If you want a highly digestible silage for lactating dairy cows, the crop must be harvested at optimum conditions. For alfalfa, this means bud or late bud stage; for corn, half to three-quarters milk line; for grass, vegetative to boot stage. For livestock with lower energy demands, cutting forages at later maturities is fine. The second harvesting issue is the dry matter (DM) content of the crop at ensiling. A crop ensiled too wet may produce effluent or leachate and will be more susceptible to a clostridial fermentation. A crop ensiled too dry will be more porous (more volume occupied by gas) and thus subject to greater spoilage and heating when oxygen is present such as at feed out. Effluent should be avoided because of its high nutrient content. Effluent is high in sugars, fermentation products and soluble nitrogen so that it is a loss of digestible nutrients from the silage. On the environmental side, effluent is as strong as manure slurry and can cause fish kills if it gets into surface waters. Optimum dry matter contents for different silo types are as follows: bunker, pile and bag silos - 30 to 40% DM; concrete stave silos - 40 to 50% DM; oxygen-limiting tower silos - 45 to 55% DM; wrapped bales - 40 to 60% DM. The low value will avoid effluent and a clostridial fermentation for all silo types with one exception. Under Wisconsin conditions, alfalfa silage at 30% DM often turns clostridial with increasing concentrations of butyric acid with storage time. So it is recommended to begin ensiling alfalfa at 35% DM in horizontal silo types. In locations where one can reliably make alfalfa silage with ph values at less than 4.5, then ensiling would be safe at 30% DM. The third harvesting issue is the set up of the forage harvester. From an engineering perspective, cutting the forage into small particles is ideal for achieving a high density in the silo. However, silage is being fed generally to ruminants that benefit from long forage particles. So a compromise must be reached. Recommended theoretical lengths of cut range from 3/8 to 3/4 inch or approximately 10 to 20 mm. Where silage is the only source of physically effective fiber, long particle sizes may be desirable to promote rumination. If livestock diets include long hay, then silage can be chopped at shorter lengths to improve silage density. With corn silage, there is an additional consideration - breaking of the kernel. It is desirable that all kernels entering the
silo be cracked or broken to ensure maximum digestion of starch in the silage. Kernel processors have become common accessories on forage harvesters to crack the kernels. The kernel processor should be set between 1 to 3 mm to be effective in kernel breakage. Loads coming to the silo should be checked periodically to see if there is sufficient kernel damage. If not, the gap on the kernel processor should be reduced. When a kernel processor is used, it will reduce particle size. Consequently theoretical length of cut on the harvester is generally set to 3/4 in. or 20 mm. Filling Principles: In silo filling, there are two goals - fill rapidly but achieve a high density. These goals can conflict with one another. On one hand, one wants to minimize the loss of sugar from the crop by respiration that will occur until the crop becomes anaerobic in the silo; thus the recommendation to fill rapidly. On the other hand, one of the principal factors in packing a bunker or pile silo is packing time. As harvest rate increases, packing time per ton will decrease, and silage density as a result will be lower unless other packing practices are adjusted. A lower density will increase spoilage losses during storage and feed out. In practice today, filling rate is not a substantial issue because new forage harvesters provide high harvesting capacity. The primary filling concern is packing forage adequately. Density in tower silos is dependent on self-compaction so that little can be done to affect density. Use of a distributor ensures a more uniform filling of a tower, helping to maximize density. In bag silos, density in most baggers is controlled by tension on cables running between the backstop and the bagger. Increasing tension on the cable increases density. However, if the tension is too high a lumpy bag will result. This should be avoided because lumpy bags provide pathways for air to enter the full length of the bag when opened for feeding, increasing spoilage losses. With experience, the operator needs to learn the art of making a smooth but dense bag. This is easier in corn silage than in alfalfa or grass. Density increases as particle size decreases (Muck and Holmes, 2006), but as mentioned earlier the setting of cut length on the forage harvester is a compromise between the needs of livestock and the desire for a high density that minimizes losses. Density in bunker or pile silos is affected by the packing practices as loads are spread and packed by a tractor or tractors. In a survey of densities in corn and alfalfa silage in bunker silos, the most important factors correlated with high DM density were tractor weight, how thinly each load was spread, packing time per ton, depth of the silage and DM content (Muck and Holmes, 2000). Particle size also affects density. Tractors used for packing should be fully weighted within manufacturer's specifications. On piles or large bunkers, more than one tractor may be used for packing. This is especially beneficial with high harvest rates such as when a custom harvester is used. However, to gain the greatest benefit from multiple tractors, they should be of similar weight. If one tractor is less than half the weight of the other, the lighter tractor may be useful for spreading the load but will contribute little to the density. Use of duals adds extra weight to packing tractors and increases safety. The interrelation of the major packing factors on density in bunkers and piles can be explored in spreadsheets available on the University of Wisconsin Team Forage website (http://www.uwex.edu/ces/crops/uwforage/storage.htm). These spreadsheets are useful to both producers and consultants to explore ways to improve bunker and pile densities. When we began this research, we emphasized improving DM densities so that one would maximize the amount of feed in a bunker. Today, we emphasize bulk or as-fed density
because that emphasizes the porosity of the silage, which is the key factor influencing how fast and far oxygen moves back from the face of a silo or a hole in the plastic. Bulk density also is easier to estimate on the fly while filling a bunker or pile if you know the weights of the loads coming into silos. Sealing Principles: Low losses can only be achieved if the crop is sealed to prevent oxygen coming into the silage. For horizontal silos and wrapped bales, that means sealing with a plastic film. While producers have tried various alternatives on bunkers and piles, no other sealing technique currently available keeps oxygen out of the silo or reduces surface losses like plastic film. Thicker polyethylene films (8.5 mil) have reduced losses in the top 6 in. by 5 percentage points compared with 6 mil films. New 'oxygen barrier' films reduce the amount of oxygen that diffuses through the film to the silage beneath but do not always make significant differences in DM losses compared to an 8 mil polyethylene film that is well secured. Where multiple sheets are needed to cover a surface, the sheets should be overlapped by at least 3 ft. On bunker silos, shoulder spoilage at the walls is common even with a high quality cover. Placing plastic down the walls and lapping the wall sheets on to the top at least 3 ft. before installing the top sheet can greatly reduce losses at the wall and the amount of precipitation that seeps into the silage. The wall film will also keep silage acids from corroding the concrete surface prolonging bunker life. A high quality plastic film alone is not sufficient to keep oxygen out of a silo. That film needs to be held tightly to the crop. A film that billows in the wind acts as a bellows drawing air under the cover. Tires touching tires has been the traditional means of properly securing a film. However, woven tarps or plastic mesh secured with gravel bags are equally effective as tires. These products need to be reused because of cost and are meant to last 5 or more years. This is not convenient in climates where snow cover may last for weeks and longer. Under such conditions, narrow tarps spread parallel to the face will help with removal during the winter. Feed Out Principles: When a silo is opened for feeding, oxygen is free to move back from the face allowing spoilage microorganisms to grow. Studies have shown that oxygen can move back 3 ft. from the face of a well-compacted bunker silo. Consequently, silage is normally exposed to oxygen for days prior to feeding. To minimize oxygen exposure, there are three goals: remove only what you are going to feed that day, leave a smooth face, and have bunkers, bags and piles designed so that you can remove at least 1 ft. from the whole face each day. Loose silage sitting at the face overnight is most likely to heat due to greater access to oxygen. A smooth face also minimizes exposure to oxygen. Devices that make a smooth face increase DM recovery by at least one percentage point compared with a bucket mounted on a tractor or skid steer (Muck and Rotz, 1996). The amount of improvement is greater at low feed out rates and/or low densities. Today the availability of low cost attachments to a skid steer to mill the face make it easy and profitable to do. The most important factor for reducing losses at feed out is removing sufficient silage from the face. Losses increase geometrically as feed out rate decreases. A feed out rate of at least 6 in./day is necessary to keep losses at feed out below 3%. However, it is prudent to design
bunker and piles to remove 1 ft./day so that there is flexibility if numbers of livestock are reduced or a silage is used at a lower fraction of the ration than expected. Spreadsheets to assist in silo design are available at the UW Team Forage website listed above. Conclusions: Silage management is aimed at ensuring the proper conditions for lactic acid bacteria to dominate fermentation and minimize the exposure of the crop to oxygen both during storage and at feeding. The combination of a good fermentation and the lack of oxygen allow the producer to deliver a silage to livestock that is close in quality to that of the crop at harvest. Literature Cited: Leibensperger, R. Y., and R. E. Pitt. 1987. A model of clostridial dominance in ensilage. Grass Forage Sci. 42: 297-317. Muck, R. E., and B. J. Holmes. 2000. Factors affecting bunker silo densities. Appl. Eng. Agric. 16: 613-619. Muck, R. E., and B. J. Holmes. 2006. Bag silo densities and losses. Trans. ASABE 49: 1277-1284. Muck, R. E., and C. A. Rotz. 1996. Bunker silo unloaders: an economic comparison. Appl. Eng. Agric. 12: 273-280. 6.5 6.0 Final ph 5.5 5.0 Legume 4.5 Corn, Grass 4.0 15 20 25 30 35 40 45 50 Dry Matter Concentration, % Fig. 1. The ph below which the growth of Clostridium tyrobutyricum ceases as affected by dry matter concentration and crop based on Leibensperger and Pitt (1987).
CORN, ALFALFA AND GRASS SILAGE PRESERVATION PRINCIPLES R. E. Muck 2 Ensiling is the primary means of preserving moist forages for feeding livestock. In ensiling, the crop is stored anaerobically, and lactic acid bacteria found naturally on the crop ferment sugars in the crop. The crop is preserved by the combination of the acids produced by the lactic acid bacteria, the resulting low ph and the anaerobic environment. Silage management strives to create conditions so that the lactic acid bacteria dominate other microorganisms and oxygen exposure is minimized. Key management steps include 1) harvesting at the right dry matter (DM) content (bunker, pile and bag silos - 30 to 40% DM; concrete stave silos - 40 to 50% DM; oxygen-limiting tower silos - 45 to 55% DM; wrapped bales - 40 to 60% DM), 2) chopping at the right length (3/8 to 3/4 in. depending on circumstances), 3) packing horizontal silos to a high density, 4) sealing the crop well to keep out oxygen, 5) holding plastic film tightly to the crop on bunker or pile silos, 6) feeding off the face of horizontal silos at least 1 ft./day, and 7) keeping a smooth feed out face with no loose silage left overnight. 2 Supervisory Research Agricultural Engineer, Dairy Forage and Aquaculture Research Unit, U.S. Dairy Forage Research Center, USDA, Agricultural Research Service, Madison, WI