Understanding Silage Preservation Characteristics
DR. SHANE GADBERRY
LITTLE ROCK, ARK.
Corn silage or grass haylage is often analyzed to determine moisture and nutrient composition for accurate diet formulation. With silage or haylage, there are additional tests that can be valuable for evaluating the preservation attributes. Creating a link between conditions at harvest storage and preservation attributes is beneficial for modifying future crop management, harvest and storage practices. The following sampling recommendations and tests to evaluate silage and haylage preservation characteristics are available through commercial laboratories.
Time
The first consideration in evaluating silage preservation is waiting a sufficient amount of time between storage and sampling. Silage is generally considered stabilized once pH has dropped to a level to prevent further bacterial activity until air exposure. Silage will generally stabilize in two to three weeks. Therefore, wait at least 28 days before evaluating preservation attributes of stabilized silage. If there is concern over changes in forage quality due preservation management, an initial sample prior to storage can be collected and analyzed for nutrient composition comparison to the preserved sample.
Sampling Handling
The second consideration in evaluating silage for preservation attributes or nutrient composition for ration balancing is handling the sample in a manner to minimize change between sampling and laboratory analysis. For sampling, place samples in a zip seal plastic-type bag and press out as much air as possible before sealing (vacuum seal if possible). With round bale silage, use a core-type sampler. Store samples in the freezer to minimize microbial action due to sample exposure to the air during collection. Ship samples, preferably overnight, with cold packs at the beginning of the week. Consider taking a subsample, and use the microwave test procedure with the sub-sample for immediate dry matter determination.
Dry Matter Test Results
Dry matter at the point of harvest and storage is a key component of preservation. Too much moisture can result in seepage losses and clostridial fermentation, whereas too little moisture can result in incomplete fermentation, heating and aerobic fermentation, as dryer material will not pack and exclude air to the extent of wetter material. Moisture targets are 60 to 65 percent for corn silage and 50 to 60 percent for grass silage.
pH Test Results
pH is a measure of acidity. It is the reduction in pH that is responsible for stabilizing silage by ceasing anaerobic activity. Corn silage will typically have a pH no greater than 4.2, whereas grass silage will have a pH of 4.2 to 5.0. Greater pH is generally associated with dryer silages, silages with less fermentable sugars and silages that have a high buffering capacity due to high protein or ash (mineral) content. Legume crops are known for their ensiling difficulty due to the high buffering capacity associated with high protein content.
VFA Profile Test Results
When aerobic bacteria digest plant material, oxygen is consumed and carbon dioxide is produced. Bacteria that thrive in an anaerobic environment generally produce acids instead of carbon dioxide as a product of digestion. These acids are responsible for the drop in pH and stabilization of silage. A VFA (volatile fatty acid) profile measures the concentration of lactic acid (most responsible for stabilization), acetic acid, propionic acid and butyric acid. The ratio of lactic acid to acetic acid may also be reported.
Acetic, propionic and butyric acid are end products of bacterial digestion in the anaerobic environment of the rumen, thus allowing silages to be a suitable method of preserving high moisture feedstuffs for ruminants. Ideally, lactic acid should represent 60 percent or more of the total acid content, and the ratio of lactic acid to acetic acid should be greater than 2:1. Some grass silage inoculants may elevate acetic acid to above normal (noninoculated) levels. This is considered normal.
The higher actetate due to inoculant is not considered detrimental to preservation and feedout. Silages that are high in butyric acid (> 0.5 percent DM) indicates a clostridial fermentation and is often characterized by higher fiber levels, high silage moisture and consequently reduced animal performance. Carcasses of small game and other wildlife unknowingly collected during crop harvest and accumulation for storage may be the cause of clostridial fermentation.
Ammonia Test Results
Clostridial fermentation or conditions that reduce the rate of pH decline and prolong the time to reach silage stability can increase silage ammonia concentration to an excessive level (> 12 percent DM). A high ammonia concentration may also be indicative of inadequate packing. High ammonia in the diet may not necessarily be detrimental to performance when rumen degradable nitrogen is balanced with rumen degradable carbohydrates. Milk test for urea nitrogen can help determine if rumen degradable nitrogen is excessive compared to the amount of rumen degradable carbohydrates provided. Keep in mind that much of the rumen degradable carbohydrate content of the diet is from structural plant sugars (fiber).
Silage fermentation profiles can be a useful tool for troubleshooting harvest and storage management issues that affect silage quality and shelf life. Silage quality, independent of nutrient composition, can affect animal performance through reduced feed intake. One common misconception about harvest options for grass is the assumption that putting grass up as silage results in better forage quality. There are times the hay equivalent will be better because excessive plant maturity at harvest results in too great of dry matter, reduced fermentable sugars, poor air exclusion, prolonged pH drop, heating and greater yeast growth when stored as haylage.
A silage fermentation test result will often accompany an interpretative summary. For assistance with interpreting results or troubleshooting harvest and storage issues, visit your local Extension office. ∆
DR. SHANE GADBERRY: Associate Professor, University of Arkansas