Silage Lessons from a wet year- The FACTS not BELIEFS

Dr Dave Davies, an Independent Silage expert with 20 years’ experimental research AND practical on-farm experience, responds to the recent press articles claiming that lactic acid in silage is a bad thing, and that certain inoculants can improve silage because they contain no lactic acid bacteria.

Having read these articles, where claims based on belief, not science, have been made, I was horrified, and feel it is now time to put the case for scientific fact.

Yes, it is very possible to make high quality silage in a wet season, but this has to be done by following sound and well demonstrated scientific fact and not myths and beliefs based on little or no real evidence.  

So first, let us explode the myth that lactic acid is an undesirable acid in silage. Lactic acid is the ONLY desirable acid if the fermentation is based on a biological additive or a natural fermentation with no additive.  The only way to preserve wet silage with limited production of lactic acid and preserve the nutrients is to use a chemical additive to inhibit the fermentation.  

We need to consider the facts about lactic acid compared to the other acids produced in silage, either in a natural fermentation or in one where a biological additive has been added.  In this process we have the option of lactic acid (good), acetic acid and/or butyric acid (bad).  In reality little or no propionic acid is produced in silage! So we can ignore this.

If we do not promote a rapid lactic acid fermentation using homofermentative inoculants or food preservatives that eliminate harmful bacteria then more acetic acid will be produced. For every 1 g/kg DM of acetic acid produced 0.3 g/kg DM of water is produced and as a result the silage DM content is reduced. Because the silage gets wetter more acid is needed to be produced to control the fermentation.  This results in a vicious circle of more water and thus more acid.

I agree that we do not want to use too much sugar in the silo, but we must consider the silo fermentation, the rumen fermentation and the energy values to the cow of the different acids produced in the silo.  If we take the energy value of sugar as 100%, then converting this sugar to lactic acid in the silo preserves 94% of that energy.  This is highly favourable for the cow when compared to acetic acid - 45%, and butyric acid - 69%.  Lactic acid in the silage directs the rumen fermentation towards propionate which, in the rumen, reduces methane production.  Methane is not only a greenhouse gas but represents a loss of up to 8% of the gross energy to the cow, so in fact, lactic acid in the silo can have a benefit in the energy balance of the cow.

Yes, lactic acid is the strongest fermentation acid, and that’s GOOD because it results in quick acidification of the crop and thus the inhibition of processes that degrade protein and sugar. The result is higher true protein and more, not less, sugar in the silage.  If a slower fermentation is promoted, through the use of bacteria other than homofermentative lactic acid bacteria, more degradation of protein and sugar occurs, reducing the nutrient value of the silage.  Because these undesirable processes continue for longer MORE acid (both lactic and acetic) is produced in the silo, however the pH may be higher because acetic acid is a weaker acid.   This is a concept many salespeople struggle to comprehend, especially those promoting alternative products to the well accepted and researched inoculants based on L. plantarum.

‘Rumen acidosis is caused by the lactic acid in your silage,’ - the biggest load of bull spouted by misinformed visitors to your farm.  In a published review of many years’ data, the authors concluded that, whilst this was a common story from silage additive salesmen, it has no scientific basis.  Interestingly, if we consider lactic acid from silage in the rumen, it is very quickly converted to propionic acid, but acetic acid, being an end-product of rumen fermentation, will require absorption and has the slowest passive absorption rate (28%/h compared to butyric at 72%/h)  of all rumen acids. Acetic acid is also the acid produced in the highest proportion due to rumen fermentation, so it is likely that this acid contributes the greatest risk factor in sub acute rumen acidosis, not lactic acid. The promotion of an acetic fermentation is often advocated to improve aerobic stability in dryer silage, but, because of the reasons outlined above, this is counterproductive from the cow’s point of view. Where aerobic instability is likely to be a problem food preservative products have been clearly demonstrated to promote a lactic fermentation and to destroy the yeasts and moulds that cause silage to heat and go off.

Finally, does lactic acid reduce DM intake? The simple answer is no, there is much scientific literature on factors affecting silage intake and the biggest factors are protein breakdown products such as ammonia-N and probably amines (but these are rarely measured), and secondly total acid, not lactic acid.  In fact lactic acid tends to have a positive effect on silage intake with intake increasing as lactic acid content increases.  Thus it is butyric and acetic acids that have the negative effect on intake, and, as many UK silages are virtually devoid of butyric acid the most important acid in reducing intake is therefore acetic acid, not surprising as this is the dominant rumen acid and cows have a negative feedback control mechanism.  To get this across to the farming community I would use the analogy of eating a pot of yoghurt, a pleasant experience, compared to drinking a pot of vinegar. Yoghurt contains lactic acid and vinegar is acetic.  Maybe, if we bear this in mind, we can understand silage from the cow’s point of view.  Because a rapid lactic acid fermentation inhibits protein breakdown, silages produced by this process have lower levels of protein breakdown and thus ammonia-N, one of the biggest factors reducing intake, so a double win for this approach in increasing intake.  

This article is written with a background of scientifically proven fact and there are many other reasons why lactic acid in the silo is beneficial. These are all backed up by good independent scientific research over the past quarter of a century, and not based on ‘myths thought up as a good sales gimmick'.









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