A corn silage bunker can lose dry matter and feed value before any mold or obvious waste appears. Crop sugars and other nutrients are used by plant cells and microorganisms, then released as carbon dioxide, water and heat.

This is often described as silage shrink: the difference between the dry matter placed in the bunker and the dry matter recovered for feeding. Some loss during fermentation is unavoidable. Higher losses usually mean that oxygen remained in the crop for too long, entered during storage or reached too much silage during feed-out.

The bunker may still look sound. However, heating, lower-than-expected stocks and inconsistent feed analysis can show that valuable nutrients have already been lost. Knowing when and where this happens makes the problem easier to control.

What are invisible corn silage bunker losses?

Physical waste is easy to see. It includes moldy material removed from the top, shoulders or face. Invisible loss is dry matter that has been converted into gases and heat. It also includes a drop in feed value that does not produce an obvious pile of waste.

Moisture can hide the true position. Fresh-weight tons include water, so they cannot show on their own how much feed has been preserved. Dry matter gives a more reliable basis for comparing what entered the bunker with what was eventually fed.

Where dry matter and feed value go

Losses can occur at four stages. What happens at each stage affects how stable the silage will be later.

1. Filling and the initial aerobic phase

Freshly harvested corn continues to respire while oxygen remains in the crop. Plant cells, yeasts, molds and aerobic bacteria use available sugars. They release carbon dioxide, water and heat, taking nutrients away from fermentation and feeding.

With good management, this aerobic phase should be brief. Slow filling, corn that is too dry, long chop length, inadequate rolling or delayed sealing can extend it. More sugar is lost, the crop heats and yeasts and molds increase. These organisms can become active again when oxygen returns at feed-out.

2. Fermentation

Once oxygen has been displaced, lactic acid bacteria use crop sugars to produce acids and lower the pH. This preserves the corn, but it also uses some dry matter. The aim is a fast, efficient fermentation that stabilizes the crop with limited nutrient loss.

A slow or inefficient fermentation uses more sugar and can leave the silage less stable. Crop dry matter, chop quality, consolidation and the speed of sealing all affect the result. Appearance alone cannot confirm fermentation quality. Representative forage analysis provides stronger evidence.

3. Sealed storage

Microbial activity should remain low once the silage is stable and the bunker stays airtight. Air can still enter through damaged film, loose overlaps, poorly sealed edges, wall junctions or areas where the cover has lifted away from the crop.

Yeasts and molds then use sugars, fermentation acids and other nutrients. They release carbon dioxide and heat, reducing the dry matter and energy left in the feed. A problem can develop beneath the sheet before any surface mold is visible.

4. Feed-out

Opening the bunker exposes the face to oxygen. Yeasts are usually the first organisms to become active. They use remaining sugars and fermentation acids, creating heat and raising the pH. Mold and obvious spoilage can follow.

Heating is an early warning. By the time mold is visible, dry matter and nutrients have already been lost. A rough face, loose silage at its base and slow progress across the bunker allow air to reach further into the silage for longer.

Where losses are most likely

Loss is rarely spread evenly through the bunker. The highest-risk areas are usually those that are hardest to consolidate or seal:

·      The top surface, particularly where density is lower or the sheet does not follow the crop closely.

·      Shoulders and wall junctions, where the crop can pull away and leave channels for air.

·      Sheet overlaps, perimeter edges and seams that are inadequately weighted.

·      Punctured or worn areas beneath damaged protective covers.

·      The open face, especially where daily use is low relative to the exposed area.

An AHDB assessment of 20 English farm bunkers found that the area within roughly 1.5 to 2 feet of the top sheet or side wall made up, on average, 27% of bunker volume and 21% of fresh weight. The study examined grass silage rather than corn, so the figures are not a corn benchmark. They do show how much forage can sit close to vulnerable surfaces.

Five signs your bunker may be losing feed value

1. The silage is heating

Silage that is warmer than expected after opening shows that microorganisms are active. Check the exposed face and freshly removed material. Repeated heating should lead to a review of the face condition, rate of advance, consolidation and sealing.

2. The bunker is emptying faster than planned

If the bunker is emptying sooner than planned, check the figures behind the stock estimate. Crop yield or dry matter may have been measured incorrectly. If those records are reliable, the difference may point to losses during fermentation, storage or feed-out, including discarded material.

3. Feed analysis is poorer or less consistent than expected

Lower energy values, changing dry matter or a poor fermentation profile can show that the crop has not been preserved evenly. Samples should be representative and reviewed with the farm nutritionist or forage advisor. One sample cannot measure total bunker loss, but repeated variation can reveal a wider problem.

4. Intake or palatability declines

Animals may sort or refuse unstable silage before extensive mold appears. Reduced intake can have several causes, so check the temperature, smell, appearance and forage analysis before linking it to the bunker.

5. You notice off smells, color changes or loose areas

Off smells, dark or discolored areas, loose material and condensation can indicate uneven fermentation or air entering the bunker. Record where the problem occurs. Repeated issues at the same edge, shoulder or sheet overlap can reveal a specific failure that needs correcting.

How to estimate corn silage shrink

Estimating bunker volume is not enough to measure shrink. The basic calculation compares the dry matter put into the bunker with the dry matter recovered:

·      Dry matter ensiled = fresh crop weight multiplied by its measured dry matter proportion.

·      Dry matter recovered = fresh silage fed multiplied by its measured dry matter proportion.

·      Dry matter recovery (%) = dry matter recovered divided by dry matter ensiled, multiplied by 100.

·      Estimated shrink (%) = 100 minus dry matter recovery (%).

The calculation depends on reliable records. Weigh loads where possible, take representative dry matter samples during harvest, record the silage fed and track discarded material separately. Different sampling methods, changing moisture levels or inaccurate weights can make the loss appear larger or smaller than it is.

Treat the result as a management indicator unless weights and samples have been recorded consistently. A year-on-year comparison using the same method is often more useful than one estimated percentage.

How to reduce invisible losses

Most avoidable losses can be reduced by keeping oxygen out and maintaining the same standard from harvest to feed-out.

·      Harvest corn at an appropriate dry matter and chop length so it can be consolidated effectively.

·      Match delivery rate with sufficient rolling capacity and spread the crop in controlled layers.

·      Complete filling efficiently and seal as soon as final consolidation is finished.

·      Use wall film and an oxygen barrier film in close contact with the crop.

·      Protect the sealing layer from birds, weather and mechanical damage with a suitable cover or net.

·      Secure the perimeter and sheet overlaps, then apply an appropriate weighting pattern across the surface.

·      Inspect the covering system during storage and repair damage promptly with compatible silage patch tape.

·      At feed-out, uncover only the area required, maintain a firm face and move across it at a rate suited to the bunker width and season.

Silostop Agri's recommended bunker-silo covering method combines wall film, an oxygen barrier top film, a protective cover and gravel-bag weighting. These components must work as one system. Even a high-performance film cannot make up for poor consolidation, an unsecured edge or unrepaired damage.

Make the loss visible

Visible mold is a late warning. Temperature checks, consistent forage analysis, accurate stock records and routine bunker inspections can identify problems earlier.

Keep one harvest-to-feed-out record for each bunker. Note the estimated dry matter stored, repairs completed, opening date, feed-out rate, rejected material and any heating. This provides traceability and makes repeated losses easier to locate before the next corn harvest.

Silostop Agri supplies oxygen barrier films, Secure Covers and bunker accessories for complete silage protection systems. Contact the team to check the film sizes, protective covers and weighting required for your bunker.