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McKinnon: It’s silage season again

For many readers, as this article comes to press, silage season is either just starting or about to start. Putting up high-quality silage is both an art and a science. With this article I will review some of the key aspects of silage fermentation management that relate to putting up high-quality silage.

Before we focus on management, let’s review the basics of fermentation. Ensiling forages such as whole crop corn or barley is basically the process of taking fresh cut, immature forage and allowing that forage to ferment under anaerobic (oxygen-free) conditions. Scientists who study the ensiling process typically break it down into a minimum of four phases. Phase 1 can last from as little as a few hours to as long as four or five days. During this phase, aerobic bacteria as well as plant enzymes inherent in the fresh forage use oxygen in the silage pack during the respiration process. It is important to keep this phase to a minimum as this aerobic respiration uses plant water soluble carbohydrates (WSC) that are critical for the next phase of the ensiling process. As well, excessive aerobic respiration generates heat which can lead to heat-damaged silage.

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Once the oxygen is used up, the process enters a lag phase where there is a transition in the type of bacteria present in the pack. Basically, aerobic bacteria die off and we see a rapid increase in the number of anaerobic bacteria or those bacteria that live without oxygen. These bacteria use WSC to produce silage acids such as acetic and lactic acid. As acid production increases, pH drops and we again see a shift in the bacterial population to one that produces primarily lactic acid. Production of lactic acid continues until all WSC are used up or until the pH of the silage pack drops to 4.0 or less. At such a low ph, all microbial activity ceases and the forage is ensiled and will be preserved as long as anaerobic conditions are maintained.

The key point is that fermentation must proceed as soon as possible under oxygen-free conditions. Ensuring such conditions is contingent on your silage management program, specifically cutting and harvesting at the optimal stage of maturity, efficient packing of the pile and in the case of bunker silos, covering the pit.

Let’s look at cutting and harvesting. Cereals such as corn and barley are ideally cut within the range of 63 to 67 per cent moisture, while alfalfa silage is typically wilted to 68 to 70 per cent moisture. Achieving the proper moisture content ensures optimal plant WSC levels that are necessary for the fermentation process. As well, excessive moisture (greater than 70 per cent) can lead to seepage issues while very dry silage (less than 60 per cent moisture) can lead to issues with packing and poor oxygen exclusion.

Another important consideration, particularly as it relates to efficient packing and oxygen exclusion, is the forage chop length. Forages that are cut too long are difficult to pack. Achieving optimal forage particle size is a function of your forage harvester, specifically selecting the optimal theoretical length of cut, sharp knives, as well as attention to speed of harvest. When harvesting corn, it is advisable to use a grain processor to ensure optimal fermentation conditions as well as efficient nutrient use.

In the case of less than desirable fermentation conditions (i.e. low WSC levels, less than optimal moisture content), a silage inoculant can supplement the native anaerobic bacteria and thus enhance the fermentation process.

Packing the pit is perhaps your best weapon against excessive oxygen being present in the silage pack. The rule of thumb is “to pack, pack and pack some more,” as the importance of excluding oxygen cannot be over-emphasized. Obviously, having the correct size of tractor(s) that matches the size of your pit is critical for efficient packing and keeping up with the forage harvester. Seams of off-colour silage are often an indicator of poorly packed silage. Not only is this silage unpalatable but is of poor nutritional quality.

Last, in the case of bunker silos or where the forage is piled on the ground, covering the silo is the last weapon in your arsenal to minimize oxygen levels in the silage pack. If one looks at an uncovered bunker silo, typically you will see a crust of black, burnt silage ringing the top of the pile that might extend to a depth of 12 to 36 inches. This burnt material is the result of oxygen penetration into the silage pack and the resulting undesirable aerobic fermentation. From a nutrition perspective, such silage has reduced WSC levels, higher fibre content, lower energy and higher levels of heat-damaged protein. While one might say this damage is restricted to only the top one to two feet of the pile, extended over a 20,000-tonne pit, the damage is excessive. Covering the pit with appropriate plastic is therefore critical to optimizing both the quantity and quality of ensiled forage.

As indicated above, attention to the basics of silage fermentation is key to high-quality silage. Just how good it gets, however, will depend on your management and experience and it is at this point that the “art of ensiling” takes over. c

About the author

Contributor

John McKinnon is a Professor Emeritus at the University of Saskatchewan and a consulting nutritionist who can be reached at [email protected].

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