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Understanding ensiling process can aid silage feeding goals

Jennifer Heguy and Noelia Silva-del-Rio Published on 20 November 2012

With high feed costs, now is the time to evaluate your current ensiling techniques in order to make improvements to the process.

To do this, let’s take a look at the different phases of the ensiling process, what pitfalls exist and what steps you can take to reach your silage feeding goals.



1. Initial aerobic phase
Oxygen is entrapped within the fresh forage delivered to the silage structure and this oxygen maintains the respiration of plants and micro-organisms. During this phase, heat, water and CO2 are produced and lost.

Best management practices to minimize losses in Phase 1

  • Exhaust oxygen rapidly through adequate compaction of the forage in order to limit time in Phase 1. Factors affecting compaction are:

1. Tractor time and weight – heavier tractors packing for longer amounts of time will do a better job of packing material and removing oxygen.

2. Packing layer thickness – the forage should be packed in layers no greater than six inches to allow for adequate packing of the material.

3. Forage particle length – shorter is easier to compact, but too short has rumen health consequences. Your nutritionist will be able to help you pinpoint the particle length that best meets your feeding needs.


4. Dry matter (DM) of forage – wetter is easier to pack but too wet leads to leachate and fermentation issues. Recommended DM at harvest is in the 30 to 35 percent range.

  • Cover silage as soon as possible with plastic to prevent oxygen exposure of the forage mass.

2. Main fermentation phase
Oxygen has been depleted but pH is still relatively high, allowing spoilage microorganisms to grow. As lactic acid bacteria proliferate and consume plant carbohydrates to produce lactic acid (strong acid), the forage mass pH decreases below the critical point, thus inhibiting or killing spoilage microorganisms.

During the fermentation phase we observe effluents, silage gas production and shrinkage of the forage mass. A rapid decline in pH will minimize dry matter (DM) losses.

Best management practices to minimize losses in Phase 2

  • Harvest forage at adequate DM. If forage is too wet (DM is too low), there will be important effluent losses in this phase. Furthermore, low-DM forages have lower carbohydrate content and there will be lower acid production, creating a longer period to reach the stable phase (Phase 3).

  • Bacterial inoculants can be utilized to aid in the rapid decrease in forage mass pH. Cost and efficacy of additives need to be determined as additives are not a solution to poor management in other phases of silage production (poor packing and harvesting at improper DM, for example).

3. Stable phase
Lactic acid bacteria are dominant and lactic acid becomes the predominant end product formed. Acid-tolerant enzymes are active but little microbial activity will take place during this phase.

However, if air ingresses into the silage mass, spoilage microorganisms will grow. Poor covering, inadequate weighting of covers, tears in the cover material, etc. are potential sources of oxygen exposure. Losses in Phase 3 should be minimal.


Best management practices to minimize losses in Phase 3

  • Cover the silage structure properly with plastic and place weight on the plastic cover to prevent oxygen exposure.

  • Periodically evaluate the condition of the cover and repair any flaws (holes, tears) that allow oxygen to enter the silage structure.

4. Feedout
At feedout, the ensiled forage face is exposed to oxygen, which supports yeast growth. Yeast metabolizes lactic acid to produce heat and volatile organic compounds (VOC).

Silage pH increases, allowing previously inhibited fungi and bacteria to grow and further reduce silage quality.

Best management practices to minimize losses in Phase 4

  • Size the silage structure according to feedout needs to minimize the surface area exposed to oxygen and to rapidly progress through the silage face (depth removed).

  • Minimize the time between removal of silage from the structure and feeding to animals in order to limit exposure to oxygen.

  • Maintain a straight, smooth face to reduce exposed surface area and to prevent oxygen from penetrating the forage mass.

Poor silage management practices in all phases have been associated with DM losses as high as 40 percent. Storing forage as silage results in dry matter losses to the environment, and these losses can be categorized as avoidable or unavoidable losses.

Unavoidable losses

  • Residual respiration: Exhaustion of oxygen in Phase 1 (1 to 4 percent losses).
  • Fermentation: The conversion of plant sugars to acids in the absence of oxygen (2 to 6 percent losses).

Avoidable losses (the target of best management practices)

  • Effluent from harvesting forages that are too wet (0 to 5 percent losses).
  • Secondary fermentation (0 to 5 percent losses).

Aerobic spoilage is categorized as both avoidable and unavoidable because some spoilage is expected, but the extent of spoilage is within the silage team’s control (proper uncovering and face management, for example).

  • Aerobic spoilage during storage (1 to 10 percent losses)
  • Aerobic spoilage during feedout (1 to 10 percent losses).

Silage is a high-value commodity with large potential losses. It is worth your time to sit down with your silage team (PCA, nutritionist, feeder, etc.) to ensure quantity and quality expectations are met.

In times of high feed costs, high-quality, home-grown forages are imperative to your bottom line.  PD

Noelia Silva-del-Rio is a dairy adviser with the University of California Cooperative Extension.

—Excerpts from University of California Dairy Newsletter, Vol. 4 No. 3

Jennifer Heguy
  • Jennifer Heguy
  • Dairy Farm Adviser
  • University of California Cooperative Extension