Silage Density Determinations Evolving

Thursday, November 19, 2009 12:54 PM CST

Producers have been told for years to maximize efficiency with bigger equipment and harvest their forages faster.

“Watch out what you pray for,” remarks UW-Madison Ag Engineer Brian Holmes who adds that many operations have found it tough to keep up with the necessary packing back at the bunker.

“As new harvesters with high harvest rates are introduced, the harvest/storage team often is not prepared to deliver and pack the forage in the storage unit,” Holmes says. “Frequently, filling and packing machines for the bunker silos or drive-over piles are insufficient in numbers and weight to get the forage packed to the desired density.”

Holmes adds that the bunker must also have sufficient width to accommodate the numbers of tractors operating safely, or more than one bunker must be filled simultaneously to allow the tractors to operate.

While producers looking to save cost on the front end are often tempted to build their bunkers too wide (i.e. fewer walls per volume of stored silage), Holmes explains that the wider the bunker, the greater the feed-off rate needs to be to maintain quality. He says going taller isn’t the answer either, because that makes for dangerous overhangs when the height of the material exceeds the unloading capacity of your equipment.

Holmes says “tunneling” is evidence that not enough feed is being removed per day. Some producers recognize this and feed off only one-half or one-third of the bunker at a time. However, this process exposes feed to the air n and quality loss.

Worse yet, Holmes notes that some producers — looking at a lower-yet initial investment — build a pad and decide to pile feed to save the cost of walls altogether. The problem, Holmes says, is that piling inherently increases surface area per unit volume. The top of the pile (or bunker) is where losses are occurring (as well as the feed-out face).

“Management being the same, a pile has higher dry matter (DM) loss,” Holmes states. “Even with good management, DM loss is your biggest cost component on an annual basis.”

The best system is the “one you can manage according to recommendations.” If you combine a low-cost feed storage system and mismanagement, “the DM losses are going to kill you,” he states. “The stuff (silage) is so valuable.”

This is why management is extremely important when it comes to your bunker or pile.

“It’s how you manage the forage that’s critical in terms of value n and annual cost,” Holmes says. He adds that he sized a pile and bunker for 200 cows and calculated surface area, and the pile had more than twice the surface area as the bunker, with the same volume of material.

Bottom line, Holmes says, is that producers shouldn’t skimp on walls and retrofit bunkers that may be too wide with an additional wall. He advises that producers build a bunker right to begin with as they are going to use it a lot of years, and any savings can quickly be overcome by DM loss of material on a yearly basis.

Holmes stresses the importance of packing.

While previous recommendations were based on DM density, he says the new emphasis is on “porosity,” a measure of the voids between the solid particles of feed.

Holmes explains that pore space can be filled with gas and/or water in the silage and is most influenced by bulk density (fresh weight density) over the range of DM contents recommended for ensiling bunkers, piles and bags (30 to 40 percent).

Bulk density is affected by the same packing practices as DM density: tractor weight, packing time, spreading layer thickness and depth of silage. But producers should be aware that the same packing practices result in a lower bulk density as DM content increases.

This trend is the opposite of what occurs with DM density. As forage becomes drier, porosity increases for the same DM density. Higher porosity allows for an increased oxygen infiltration rate with consequent increased DM loss due to aerobic deterioration.

Holmes recommends bulk density of 44 pounds per cubic foot or more on an as-fed basis. He says this bulk density will keep porosity at the recommended 40 percent or less goal when packing a bunker and drive-over pile.

To reach these goals, forage moisture of harvested material needs to be in the recommended range of 60 to 70 percent (60 to 65 percent for haylage and 65 to 70 percent for corn silage).

“And, the packing effort needs to be adequate,” Holmes says.

If the forage is drier than 60 percent moisture, Holmes says packing should be much greater to produce high enough bulk density and to keep porosity at or below the 40 percent target.

Holmes says he has seen a general increase in lower bulk density and higher porosity in bunkers. Factors include increased harvest rates without increased packing effort, as well as a trend for field-wilted forages to be harvested drier.

“Arguments for drier forage at harvest include a desire to avoid clostridia fermentation, and a desire to increase the DM content of the total mixed ration when other feed ingredients are high in moisture,” he states.

When field mowing and raking rates exceed harvest rate, Holmes points out that drying occurs too fast (i.e. drying gets ahead of harvest rate).

Whatever the reasons for ensiling forage too dry, nutrient losses will increase but won’t be recognized until samples document reduced feed quality. Some nutritionists are now using infrared photography to show differences in silage temperature on the feed-out face, with the differences showing up as different colors on the images.

“This might be a good troubleshooting tool for producers who experience excessive DM loss or aerobically unstable silage,” Holmes remarks.

The UW ag engineer says it all comes back to packing, with the goal of 3 percent DM loss at feed-out. Two factors influence achieving this goal: density and feed-out rate.

Holmes notes that, within a range of densities, a producer can meet the 3 percent goal so long as feed-out is at least 12 inches a day. He adds that packing density can make up for lower removal rates on the backside.

Silage density can be determined in several ways. One is taking core samples at the face. Although this practice is widely publicized and practiced on some dairies, Holmes says the coring devices may be hard to come by and walking up to the face of a bunker is risky in terms of farm safety. Thus, Holmes says UW is “discouraging” using the coring method to determine silage density and the adequacy of bunker packing.

An alternative is a spreadsheet on the UW’s “Team Forage” website—www.uwex.edu/ces/crops/uwforage/storage.htm—that can help a producer “estimate what you’re going to achieve or might have achieved based on how you packed it.” Producers can change all sorts of variables such as number of packing tractors, percent of the time the packing tractor is working, packing tractor weight, layer thickness, moisture and harvest rate. The latter is very handy in making a switch from a pull-type to a self-propelled harvester, Holmes adds.

Holmes maintains that too many operations have significantly increased their harvest capacity but haven’t been prepared for it back at the bunker and “run into problems.”

The other way to monitor packing and bunker density management is the feed-out method. With this method, the TMR scale is used to monitor the amount of feed being removed. Concurrently, a producer should mark off on the bunker wall the volume of feed that was removed. In other words, producers are tracking how far back the feed-face recedes.

A spreadsheet on the Team Forage web site can help with the math. Holmes cautions, however, that producers should not undertake feed-out measurements until they get past the filling wedge.

To learn more about packing and other aspects of bunker management and additional spreadsheet tools; contact Holmes at mailto:bjholmes@facstaff.wisc.edu or 608-262-0096.

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