Silage is preserved forage produced through lactic acid fermentation under anaerobic conditions, and it is one of the core roughage sources in modern cattle feeding. Well-made silage provides consistent nutrient value throughout the year, whereas poor silage making leads to nutrient losses, mold growth, mycotoxin risk, reduced dry matter intake, and lower animal performance. This review covers fermentation biochemistry, harvest timing, packing, additives, quality assessment, and prevention of aerobic spoilage.
Economic importance
Poor silage management can raise dry matter losses to 15-40%. Aerobic spoilage at the feedout face alone may cause 10-15% dry matter loss. Silage with high butyric acid can contribute to butyric ketosis in dairy cows and may reduce dry matter intake by 10-20%. High-quality silage therefore affects not only feed cost, but also intake, milk yield, and herd health.
1. Biochemistry of silage fermentation
Silage fermentation proceeds through four phases. The objective is to allow lactic acid bacteria (LAB) to dominate quickly, reduce pH, and stabilize the plant material before undesirable microorganisms become established (McDonald et al., 1991).
| Phase | Duration | Conditions | Microorganisms | Outcome |
|---|
| Aerobic phase | Hours | Residual oxygen is still present | Plant enzymes, aerobic bacteria, yeasts | Respiration and sugar loss continue until oxygen is depleted |
| Initial fermentation | 1-3 days | Oxygen falls, soluble sugars remain available | LAB begin to dominate | Lactic acid starts accumulating and pH declines |
| Stable fermentation | 2-3 weeks | Strict anaerobic environment | LAB dominate, undesirable flora suppressed | pH reaches a stable low range and forage is preserved |
| Feedout phase | After opening | Oxygen re-enters at the face | Yeasts and molds regain activity | Aerobic spoilage risk begins immediately after exposure |
2. Harvest timing and dry matter
Harvest timing is the most critical determinant of silage quality. Early harvest causes low dry matter, seepage loss, and butyric fermentation risk. Late harvest leads to higher NDF, lower digestibility, and more difficult packing.
| Silage type | Optimal DM (%) | Harvest timing | Chop length | Critical note |
|---|
| Corn silage | 30-35% | Half milk line to 2/3 milk line | About 1.0-1.5 cm, adjusted to kernel processing | Too wet increases seepage and clostridial risk; too dry impairs packing |
| Alfalfa silage | 35-40% | Late bud to early bloom | About 1.5-2.0 cm | Too wet raises butyric fermentation risk, too dry increases leaf loss |
| Grass silage | 30-35% | Early heading stage | About 1.5-2.0 cm | Fiber digestibility falls rapidly if cutting is delayed |
- Harvest at the right dry matter: target about 30-35% DM in most practical systems.
- Adjust theoretical length of cut: use chop length appropriate to crop type and processing needs.
- Fill rapidly: finish filling as quickly as possible, ideally in less than 3 days.
- Pack tightly: target at least 240 kg DM/m³ with continuous tractor packing.
- Seal immediately: cover at once with plastic plus weights such as tires or sandbags.
- Use double covering when possible: oxygen-barrier film plus standard polyethylene sheet.
- Protect edges and corners: these are the areas with the greatest losses.
- Wait at least 21 days before opening: avoid feedout before fermentation is sufficiently established.
3. Silo management
3.1 Packing density
Packing density determines how much air remains in the silage mass. Insufficient packing creates air pockets, promotes aerobic deterioration, and increases mold and heating risk.
- Target density: ≥240 kg DM/m³
Packing rules
- Tractor weight: roughly 25-30% of hourly filling rate (tons/hour)
- Layer thickness: ≤15 cm before packing
- Continuous packing: tractor movement should continue throughout filling
4. Silage additives
| Additive type | Active component | Effect | Indication |
|---|
| Homolactic inoculants | L. plantarum, P. pentosaceus | Rapid pH drop and improved fermentation efficiency | General use where fast acidification is desired |
| Heterolactic inoculants | L. buchneri | Improves aerobic stability by increasing acetic acid production | Useful where heating at feedout is a major concern |
| Organic acids | Propionic, formic acids | Suppress spoilage organisms and limit heating | High-risk materials or face-spoilage problems |
| Enzymes | Cellulase, hemicellulase | Increase fermentable substrate availability | Forages with limited soluble carbohydrate |
5. Silage quality assessment
5.1 Fermentation profile
| Parameter | Good silage | Intermediate | Poor |
|---|
| pH | Low and stable for crop type | Borderline reduction | Too high, unstable |
| Lactic acid | Dominant acid | Moderate | Low |
| Acetic acid | Moderate | Variable | Excessive or too low depending on stability issues |
| Butyric acid | Absent or minimal | Detectable | High, indicating clostridial fermentation |
5.2 Nutrient value analysis
| Parameter | Corn silage (good) | Alfalfa silage (good) |
|---|
| Dry matter | 30-35% | 35-40% |
| Crude protein | 7-9% | 18-22% |
| NDF | 35-45% | 38-48% |
| Digestibility | High starch and fiber digestibility | Good leaf retention and fiber digestibility |
6. Aerobic spoilage and face management
Signs of aerobic spoilage
- Heating: silage face temperature more than 5°C above ambient
- Mold: visible white, green, or black mold colonies
- Odor: alcoholic, vinegary, or moldy smell instead of a clean acidic aroma
- Color: dark brown or black areas
- Intake effect: spoiled silage can depress DMI by 10-30%
- Daily progression: at least 30 cm/day in summer and 15 cm/day in winter
- Smooth face: keep the face flat and avoid tearing forage loose with a bucket
- Silage facer: best tool for maintaining a dense, smooth face
- Peel the plastic back gradually: expose only 1-2 days of feed at a time
- Separate spoiled feed: never feed moldy or heated silage to cattle
7. Mycotoxins and silage
| Mycotoxin | Source mold | Effect in cattle | Tolerance limit |
|---|
| Aflatoxin | Aspergillus spp. | Liver stress, reduced performance, milk residues | Very low tolerance; regulatory concern is high |
| Zearalenone | Fusarium spp. | Reproductive disturbances | Keep exposure minimal in breeding herds |
| DON | Fusarium spp. | Lower intake and rumen disturbance | Moderate tolerance but chronic exposure is harmful |
| T-2 toxin | Fusarium spp. | Oral lesions, immune suppression, poor performance | Low tolerance |
8. Herd-level silage monitoring
| Parameter | Target | Alarm | Measurement |
|---|
| Dry matter | Within crop-specific target range | Too wet or too dry | Oven, Koster, or NIR |
| pH and fermentation acids | Stable, crop-appropriate profile | High pH or elevated butyric acid | Lab fermentation analysis |
| Heating at face | No marked temperature rise | >5°C above ambient | Infrared thermometer or probe |
| Mold and visual damage | No visible growth | Visible mold colonies or blackened areas | Daily face inspection |
Silage quality should be monitored systematically, not only when obvious problems appear. Dry matter, fermentation profile, heating, and visual quality all affect intake, ration consistency, and animal response. Good silage management is therefore part of herd health management as much as feed conservation.
9. References
- McDonald, P., et al. (1991). The Biochemistry of Silage (2nd ed.). Marlow, UK: Chalcombe Publications.
- Muck, R. E. (2010). Silage microbiology and its control through additives. Revista Brasileira de Zootecnia, 39(suppl.), 183-191.
- Wilkinson, J. M., & Davies, D. R. (2013). The aerobic stability of silage: Key findings and recent developments. Grass and Forage Science, 68(1), 1-19.
- Kung, L., et al. (2018). Silage review: Interpretation of chemical, microbial, and organoleptic components of silages. Journal of Dairy Science, 101(5), 4020-4033.