MANY farmers and contractors still think of silage in terms of being 'cooked', referring to the amount of heat generated in the stack or bunker. The hotter the stack, the better the cooking - they say. Wrong! The hotter the stack, the more the farmer has stuffed the nutritive value of that stack.
Silage undergoes a fermentation process and while this process does result in some heat being generated, it should be warm at most.
In a well-compacted stack that is immediately sealed well, the average temperature should not rise more than about 5 degrees Celsius to 12 degrees C above the ambient temperature at filling. However, temperatures can reach 43 degrees C to 54 degrees C in the upper layers of the stack during filling as a result of excessive air (oxygen) trapped in the surface layers of the forage. These temperatures promote unfavourable fermentation bacteria such as Clostridia and Enterobacteria.
Storage temperatures at about 27 degrees C favour the desirable lactic-acid-producing bacteria. Stack temperatures will decrease rapidly with further packing of new material or once the stack is sealed.
What causes this increase in temperature? The presence of air is the culprit. Air, actually the oxygen in the air, is the enemy of silage and becomes an issue from the moment the crop is mown.
Silage plus air (oxygen) results in the production of water plus carbon dioxide plus heat.
Examining the ensiling process at each stage of harvest reveals why it heats and what can be done to minimise heating, thereby reducing substantial loss of quality and dry matter. Stages to be discussed are: Mown crop. Harvesting. Compaction. Sealing. Maintenance and repair.
Mown crop The mown forage continues to be metabolically active and plant enzymes systems continue to function, i.e. the plants continue to live until the forage reaches a dry matter (DM) content of about 70 per cent, well above stack and baled silage ideal DM contents for harvesting.
However, this activity is greatly reduced once plants have wilted to about 50 per cent DM, as long as there is still food (substrate) available to the microorganisms. The longer the wilting period, the greater the losses.
Action: Increase the rate of wilting. Use the latest forecast off the web and mow in fine weather. Mowing during rain results in moisture being trapped between the grass leaves and is similar to trying to cure moisture from between horizontal sheets of plastic.
Use a tedder immediately after mowing and possibly ted twice early in the season. Mow with a mower-conditioner leaving wide windrows. Mow once the dew has lifted and avoid mowing late in the day.
Harvesting Aim to harvest within 24-48 hours of mowing. Plant sugar losses due to plant respiration and aerobic microbial activity increase extremely rapidly about two days after cutting. Proteolysis, the breakdown of protein to soluble non-protein nitrogen (NPN) such as peptides, free amino acids and amides, is also substantially reduced if the forage is, ideally, wilted to the correct DM content for stacks and bales within 24 hours or 48 hours at the latest.
Action: Harvest quickly. Ensure the contractor is on time. The forage harvester and baler must be of reasonable capacity for the job's size and must be well maintained and routinely serviced. Any barriers to speed and efficiency such as rough tracks, narrow gateways, long distances, should be minimised. Chopping the material as short as possible will usually result in higher weights being carted to the stack or being baled per unit of time.
Compaction Once the forage is in the stack or bale, oxygen is trapped among the ensiled forage and continues to allow respiratory activity of the plants and aerobic microorganisms. Short chop length allows denser compaction and less air being trapped in the stack or bale.
If sealed soon after harvest, this trapped oxygen is 'used' quickly. In the right conditions, this then allows the anaerobic (no air) bacteria to increase in number and start to ferment the plants' water soluble carbohydrates (WSC) to produce lactic acid. This does result in a slight drop in nutritive value from that of the original forage, but is unavoidable. This fermentation leads to an increase in acidity of the forage, which pickles the crop and maintains the nutritive value near the new level as long as air and water is prevented from entering the sealed storage.
If the forage is too long and/or too dry, compaction is difficult. This results in excess air being trapped in the storage causing extended respiration activity and storage temperatures to quickly increase. Also, an unseen exponential growth of aerobic microorganisms (moulds, yeasts, aerobic bacteria) will occur and they will cause a rapid deterioration of the silage when opened or holed.
Action: Densely compact the stacks and bales. The drier the forage, the shorter the chop length. Maintain sharp knives and chopping mechanisms to reduce needless fuel usage and more efficient cutting rather than 'smashing' the forage.
Spread forage thinly on the stack (no more than about 15-20 centimetres depth). Roll the stack slowly with heavy, wheeled equipment. Bale at a slightly slower speed than often used to produce denser bales. Net-wrap bales rather than using string to give a more even finish. Ensile at the correct DM content for each silage form.
Sealing Until the ensiled forage is sealed, the oxygen in the storage will be used by aerobic bacteria and respiration activity to produce carbon dioxide, water and heat. The heated gas, or steam in extremely poor ensiling practices, escapes the storage but is immediately replaced by cooler air, setting up a convection cycle. This cycle is best stopped by immediately sealing the stack or bales as airtight as possible after harvesting is finished.
The rolling of the stack should keep up with the harvesting, so a final short roll at the end should be all that is required before final sealing. Rolling the next day really just pushes more air into the stack. If the stack sinks from the next day's rolling, why is this? If rolling was done well during harvest, it should not be needed the next day. Obviously, if harvest is completed during the late hours of the night, practicality may dictate a quick morning roll and then sealing.
Bales should be wrapped no later than a couple of hours after baling.
Unfortunately, many stacks are well covered but are poorly sealed. Hence air slowly leaks into the edges and top of many stacks.
Table 1 shows the DM losses of stacks left uncovered or that were poorly to well covered.
The perimeter of the plastic should be sufficiently weighted or even buried to prevent any air from entering the stack. Adding weight such as tyres touching all over the stack or placing gravel-filled 'sausages' every few metres and along all plastic joins greatly minimise air movement into or under the plastic sheets.
Action: Seal the storage airtight as soon as practical after harvesting is completed. Use ultra-violet light-treated plastic film, placing the white side up. Consider using newer plastic films and sealing systems that incorporate an oxygen barrier, which is 20 times more effective than existing plastic sheets. Consider temporarily covering stacks overnight if harvesting over several days.
Apply four to six layers of cling wrap film to bales ensuring no underlapping and 55 per cent stretch. Be aware that some new films require 70 per cent stretch and possibly a change of gearing on the pre-stretcher or wrapper.
Maintenance and repair Holes or deteriorating film will allow air entry resulting in aerobic microbial deterioration beginning anew resulting in temperature increases. Holes can be caused by many means and each should be prevented or avoided. The bigger the hole or longer the silage is exposed to air, or the drier the ensiled forage, the greater the amount of air entry. This means greater heat production (cooking) and substantially greater losses in nutritive value and DM itself.
Action: Maintain an airtight seal. Regularly inspect storage for new holes and immediately repair them with plastic repair tape specifically made for silage film. Lay vermin bait around bales, especially cereals harvested at the milk to soft dough stage. Avoid storage where tree twigs and branches may fall and control prickly weeds such as blackberries, thistles, which start to grow among the bales. D
Senior dairy extension officer, Agriculture Victoria, Victoria
Want to read more stories like this?
Sign up to receive our e-newsletter delivered fresh to your email in-box twice a week.