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TRAC Recommended Read: Understanding Silage Testing & Analysis

From Forage Lab Australia

This season we've had amazing conditions that have been favourable in most areas for fodder conservation, this has resulted in some great quality silage with very high energy content and great protein values. With the early shut off to the season we had some earlier cut forages that we're ensiled a little too wet and then with the quick dry off we've had some forages ensiled a little too dry, both of these moisture level scenarios will present their own challenges, however, in general quality looks great.

While last season's conditions weren't perfect for fodder conservation and the feed presented some challenges for producers, thorough testing gave us the information required to amend feeding plans and expectations mitigating some of the challenges.

The team at Forage Lab Australia recently released an article detailing the different phases of silage fermentation, when is the most suitable time to test your silage and how to gather a sample as well as providing their recommendation for which test will be the Number One choice for all forages this year.

We think it's worth a read...


'Last season we had some very poor forage conserved due to unpredictable weather and continuous rain from September through to December. We found it highly valuable to run A1 Plus on all forages to gain a better understanding of how that forage will perform in a production system. While it wasn’t possible to change the quality of the forage, feeding plans and expectations could be appropriately amended, mitigating some challenges. This year, the A1 Plus will still be the Number One choice for all forages, as this provides NDF digestibility on hay and silage, and silage acids on fermented feeds. For silages, that tells us how fermentation has gone for the silage sample and even indications of spoilage risk (think butyric acid). The A1 also shows valuable information regarding time point NDF digestibility for hay and silage. When purchased forage costs are increasing, having those extra checks on quality can make quite the difference to daily gains or production outputs. As always, a mould and yeast test should be done on any feed that has been on the ground for long periods of time or that has received any rain.


To get an accurate and reliable forage assay for your silage, it is best to take your sample after the silage has completed the fermentation process. To reach stable state is generally a minimum 6 weeks after being ensiled.

There are three main phases of the fermentation process.

Phase One:

Phase one is the aerobic phase, this begins immediately after cutting and early on after being ensiled. This is where respiration and proteolysis occur. During respiration, sugars and O2 get converted to carbon dioxide, water and energy for the microbes. This will continue as long as substrate and oxygen are available. When looking at time on the ground it is estimated there will be 5% nutrient loss per day - so it is a high priority to shorten this phase.

In this phase, the sugars are used for aerobic bacteria causing oxygen to drop in a closed stack. The other main factor in this phase is that we will see a rapid increase in temperature. If the temperature goes too high there will be more nutrients used and will result in an increase in NDF, ADF and a drop in energy for lactation – the increase in fibre as a percentage is because we are losing other components. Proteolysis is the second portion of phase one, where we can see protein breakdown resulting in high levels of ammonia (also from excess O2) or high temperature. The longer the silage is exposed to air, the greater protein loss will be observed.

Phase Two:

Phase two will start when all oxygen has been used and we begin fermentation. Initially Acetic Acid, and later Lactic Acid will begin to be formed and pH will start to decline, however there is only weak acids at this point and high nutrient losses can be observed. This phase should typically only take 1-3 days. An important time to consider whether it is necessary to relocate baled silage, by moving the silage we will be adding oxygen again and the entire process will restart, losing more nutrients.

Toward the end of phase two, Lactic Acid will begin to be produced. It should come to dominate the acid profile and pH will continue to drop under 5 and towards 4. The higher the sugar content to begin with, the stronger the fermentation that can occur as this is food for the microorganisms to create the preserving acids.

Phase Three:

The silage is now at a stable state with a constant temperature and no more fermentation taking place. Generally, we allow 45 days minimum to reach this phase, however for better results allow 90 days. It is only when oxygen is introduced back to the silage that fermentation can begin again - think of moving baled silage, accidental tears to the plastic and opening the face of the stack to feed out, all scenarios that can restart the process. However, if the oxygen is not eliminated such as taping the holes, it will never reach phase two and continue to lose nutrients and grow mould.

Using a thermometer to check for stable state by monitoring the temperature change daily until there is no change is an excellent way to check – remember to tape punctures. If the feed analysis comes back with 100% lactic acid, this suggests the silage hasn’t fully completed fermentation and quality may change slightly in the end product.


Wait a minimum of 6 weeks before testing silage, using a forage sampling corer is the best way to acquire a sample of silage in bales and pits. Giving the opportunity for multiple samples with less O2 entering the silage, with the benefit of a further reach into the bale or silage pit before being opened, 5-15 samples is recommended. When sampling stacks/pits, it is recommended to take multiple samples of 5-15 from the face after removing the days feed, additionally it is recommended to retest as you work your way through the stack. Place the sample in an A4 size Ziploc bag (50-80% full) and post express with your submission form.

Feed sampling technique is the greatest source of variation in tests, test randomly to achieve a good representation of the entire feed.


  • 60-70 % Lactic Acids of total acids (3-8 % DM grass or 4 – 8 % DM Maize)

  • Lactic acid : acetic acid – 3:1

  • Butyric acid < 0.5 % (less is better)

  • Propionic acid < 0.5 %


Total VFA is the measure of all volatile fatty acids (VFA) present in silage from fermentation. We like to see above 5% VFA in silages to indicate good fermentation for perseveration.

Lactic Acid is generally the preferred silage acid, however 100% Lactic Acid shows incomplete fermentation (e.g. tested too early). Lactic Acid produces a sweet tangy aroma and indicates low spoilage organisms and sound anaerobic fermentation (no oxygen) which is ideal for good silage outcomes.

Acetic Acid in small amounts is good, 1-2% is OK but any more is not ideal. Acetic Acid produces a vinegar aroma and can have palatability issues when dominating the silage acids.

Butyric Acid is the chief VFA produced by spoilage organisms that proliferate when air is not excluded during ensiling. Butryic acid grows detrimental microbes that convert nutrients to spoilage compounds. Burtyric acid smells like vomit/spew. It is unpalatable and can indicate further toxin issues. No Butyrate is best, but no more than 0.2% DM is wanted.

Last year we saw silage and hay made in all weather conditions, and from this we can learn how that impacts quality.

  • Too Wet: For some early cut silages where there has been less drying time, in an attempt to beat oncoming rain, moisture levels are higher from putting feed into bales or pits a little too wet. Where this has occurred, bales may have slouched, and some effluent may be seen coming from silage stacks. Many of these feeds will look around 17-25% Dry Matter (DM). These feeds will have higher acid content and likely lower pH.

With wetter silages, there is also a risk of butyric acid. The high acid content, low pH and the presence of butyric acid can all influence dry mater intake. For these silages it is best to introduce it to animals slowly and dilute with other feeds such as hay. Take a close look at ammonia, sugar content, acid balance and pH and determine if this feed could also pose a clostridial risk. This will also likely be accompanied by a very pungent smell. Cumberland Valley Analytical Services (CVAS) data shows that all silages below 32% DM, will have less than 55% successful fermentation. Above 32% DM the success rate is 74%. These are factors that cannot be changed once it is ensiled, however it can help aid decision making on how and in which animal groups this feed is best used.

  • Too Mature: Other silages which reached a very mature plant stage by the time it was able to be cut, which may be what we see more of this year due to the early season dry spell, silage will have had seed or grain head emergence with varying levels of filling. The plants that have reached maturity will find that NDF typically is higher than 50%, with many results of later cut forages returning results of 55-65% NDF. The biggest challenge of mature fodder into silage is the ability to get the oxygen out of the plant biomass, and to establish anaerobic ensiling. Ongoing presence of oxygen will inevitably see spoilage organisms as a greater risk, the silage starts to compost, not ensile.

Mature silage also has a lower starting sugar level, and with low sugar content we have less capacity to convert sugar to organic acids to allow preservation by acids. Cutting mature plants to make silage or hay, will also yield lower energy values.

  • Rain events: Silages that were cut and received rain will have lower protein, sugars and energy, depending on when the rain fell after cutting and how long the forage was on the ground. Depending on the time of rain and amount, proteins may drop to 6-7% Crude Protein, NDF will remain high around 55-65%, sugars may be under 5% and energy 7.5-8.5 ME. These results will be very similar for hay that received rain during the drying time. All hay and silage that was rained on should have a mould and yeast count done.

By sending a feed analysis to Forage Lab Australia, you will be provided with an analysis of your food to arm you with the understanding of the quality in both the good and poorer quality forages available to you. This information enables you to put together a strategy to feed the appropriate feeds to animal groups with different objectives of either weight gain, milk production, strong fertility outcomes or maintenance. Don’t be discouraged by thinking all feeds harvested on farm are the same quality or low quality, there is always something that can be done to ensure animals are maintained or go forward as required. That is the joy of animal nutrition.

Even those feeds with higher mould counts can be managed by talking to your local nutritionist and animal specialist about what toxin management tools to use and strategic feeding.'

For more information or to discuss this article further,

please get in touch with your local TRAC Expert In Ruminant Productivity

on 08 8733 1888 or email us at


Our Consultants


Tom Thorn

0427 243 319

Owen Rees

0429 437 823

Mikaela Baker

0457 243 319


To download a copy of this article, please click the link below...

November 2023 - Understanding Silage Testing and Analysis
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