Minimize Dry Matter Loss by Minimizing Silage Porosity
Brian Holmes, University of Wisconsin Madison and Richard Muck, USDA, ARS, US Dairy Forage Research Center-Madison
A major dry matter (DM) loss mechanism in silage is deterioration of readily available carbohydrate by aerobic microorganisms. These microorganisms need a supply of oxygen to grow, and that growth causes silage heating. Evidence that aerobic activity has occurred for some time is increased fiber and crude protein contents and in extreme cases moldy gray to black colored silage. We preserve silage by taking measures (plastic covers, bags and tight concrete walls) to exclude oxygen during the fermentation and storage phases. The aerobic deterioration process begins as soon as oxygen is reintroduced to the silage, whether by a failure of the oxygen exclusion measures just mentioned or when the silo is opened for feed out. The rate of deterioration is influenced by the rate at which oxygen can infiltrate through the silage. The oxygen infiltration rate is a function of the silage gas filled porosity. Porosity is a measure of the voids between the solid particles of a material. Pore space can be filled with fluids including gas and/or water in silage. For gases to move throughout the material, the pores must be continuous. The preferred gas in stored silage is carbon dioxide. In high concentration, carbon dioxide displaces oxygen. To minimize silage porosity, forage should be packed to a bulk density which is as high as practical.
Silage bulk density (wet density) increases with degree of packing and moisture content. The recommended range of moisture content for hay crop silage is 60-65% and 65-70% for whole corn crop silage. When forage is in this range of moisture content, the porosity is in the range listed in Table 1. We currently recommend porosity be kept below 40%, which is achieved by maintaining bulk density at greater than 44 lbs AF/cu ft. The higher the bulk density, the lower the porosity and the slower the rate at which oxygen can penetrate through the silage. In the extreme case, silage removed from a silo and left on the floor between feedings has a very low bulk density (25-30 lbs AF/cu ft) with correspondingly high porosity (near 60%). This explains why heating of this silage often occurs before feeding.
The degree of aerobic deterioration at the feed out face is a function of oxygen infiltration rate and time of exposure to oxygen. Silage heating may be evident at the feed out face but more likely it is more evident at a distance behind the feed out face where active aerobic deterioration is occurring. Thus temperature measurements at the feed out face surface may be deceiving. Oxygen has been measured up to three feet behind feed out faces in well-packed bunkers (~50 lbs AF/cu ft). The time silage is exposed to oxygen is also a function of feed out rate. Table 2 lists time of exposure to oxygen for different feed out rates and three different aerobic zone depths. A 1-foot aerobic zone distance from the feed out face represents an exceptionally well-packed bunker or pile whereas the 5-foot distance, a poorly packed one. Conditions which lead to silage exposure to oxygen for more than 8 days can result in dry matter losses of more than the recommended minimum of 3% of total dry matter in the storage. This is why a minimum of 6 inch/day face removal rate has been recommended when designing bunker and pile silos. Dry matter losses in the 5-11% range are common when bulk density is low (porosity is high) and feed out rate is also low (Figure 1). Note from Figure 1 that it is essentially impossible to get a high enough bulk density to keep feed out loss under 3% if you are removing feed at 2 inches/day. Even at a 6 inch/day removal rate, bulk density needs to be at least 40 lbs AF/cu ft to be under that target loss.
For further reading on this subject refer to Density and Porosity in Bunker and Pile Silos and Packing Bunker and Pile Silos to Minimize Porosity found on the Harvest and Storage Page of the UW Extension Team Forage Web site at: http://www.uwex.edu/ces/crops/uwforage/storage.htm
