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Maximising Starch and Total Diet Digestibility in TMR Systems

By Mikaela Baker, (B. Ag Sci), TRAC Ruminant Productivity Consultant

Starch is the nutritional backbone for highly efficient dairy systems as it is the pre-curser for glucose synthesis. The primary pathway of glucose synthesis involves starch enabling the production of volatile fatty acid propionate after fermentation which is then synthesized into glucose in the liver. Glucose fuels milk synthesis, the immune system and fertility. Cereal grains such as wheat and barley follow this pathway, whereas a portion of starch from corn can bypass this process and allow more efficient glucose production, specifically valuable for the fresh cow.

As farms push the ceiling of high production expectations in Australia, while still maintaining excellent animal health and fertility, the use of multiple sources of starch becomes more important. There is a limit to how much wheat can be added to a diet while maintaining a healthy rumen environment and balanced fibre sources. When cows consume wheat or other highly fermentable feeds, it causes a drop in rumen pH, below some of the rumen microbes survival zones, importantly this is related to the duration the rumen is sitting at a low pH. The influence of rumen pH is why TMR feeding and moving a high amount of grain from the dairy to the wagon has been so widely adopted and benefits to milk volume, milk solids and health has been observed in well managed systems.

The trend towards using corn silage in mixer wagons follows the natural progression of improvements to feeding cows with the aim to smooth out rumen changes and provide a more consistent diet for maximum feed conversion. The drive for selecting feeds to go into milking cow diets is based on the ability to feed a high energy dense ration, with high digestibility. Often digestibility can be overlooked, especially as these numbers are only shown on the Forage Lab A1 Plus analysis, not the standard A1 or other standard tests, starch analyses require a separate test. The digestibility for NDF at both 24 hour and 30 hour provides the most relevant information to dairy producers. These numbers provide an insight into how much of the food can be accessed by the cow, rather than simply passing through. Energy is only created from feeds that are fermented and absorbed within the rumen, and a small amount within the intestines. If the food passage rate through the cow is too quick or whole fibre or grains are observed in the manure, that portion of the feed has been either un-utilized or under-utilized.

While we can use visual assessment on farm and NDF digestibility through the A1 Plus report, we can also use Faecal starch analysis to monitor our backbone ingredient. Faecal starch shows how much has passed through the cow and can be assessed via NIR. The preference is to have less than 5% starch remaining, and for best results less than 3% starch remaining. However, this isn’t new information, Faecal starch and starch availability was widely talked about in 2010. Therefore, Forage Lab (CVAS) developed an analysis that observed ‘Apparent Nutrient Digestibility’ through combining TMR and Faecal samples in one report to show a comparison and provide a truer number of starch digestibility, over the single Faecal analysis. On a faecal starch sample, the starch shows 5.7% while in the Apparent Starch Digestibility, 91.9% of starch was absorbed. The ideal range is 94-98%, whereas the expected ranges 88.5 - 99.6%. In this example dietary starch is 26.2%, meaning only 24.07% was used, and 2.13% was wasted. If we use a value of $490/T of processed wheat with a starch value of 71.3% (Forage Lab Australia data, Nov 2021-Nov 2022). This equates to $0.59 loss per cow, per day in under-utilised starch. Not that a target of 100% should be chased, but monitoring and managing this number could save money, not just in the short term but long term too.

There are multiple ways we can analyse this information, but more importantly what can we do to influence the outcome. Firstly, the basics that are essential in every scenario; an upset rumen will not be able to efficiently breakdown or digest feed. Consistency in the diet is a non-negotiable, changing the ration daily by estimating quantities or mix times will see the perfect ration perform poorly. Next is looking at management of feeds. Keeping bunkers clean from week old or mouldy feeds and mud will prevent unnecessary influences on intake and compromising the immune system. In addition, sound management and cow comfort are also essential.

Once the basics have been confidently achieved, selecting the feed ingredients is next. Selecting forages based on their 30 hour NDF digestibility will allow more access to the nutrients in the feed, especially in a year where digestibility is expected to be very low from the relentless rain resulting in plants maturing and excessive ground times before being baled or ensiled. Target over 60% NDF digestibility as a percentage of NDF (not DM) at all opportunities, or at a minimum aim for over 50 percent. Where forages are not able to be sourced at this quality due to the floods and crop damage, look at adding in alternative feed sources that provide very highly digestible nutrients.

Starch, our backbone ingredient, is also better by having higher digestibility. There are many influencing factors to starch digestibility. In cereal grains, identifying the ideal particle size to your system will have an influence on rumen pH and production outcomes. Too fine in a rapidly fermentable diet or slug feeding dairy grains will cause a very rapid drop in pH and risk of SARA (Sub-Acute Ruminal Acidosis) or Acidosis. Too large and the grain may not have time to fully breakdown in the cow by having a lower surface area. Inconsistent daily grain particle size will also have negative effects. When looking at corn, processing size is also critical. Corn has a waterproof seedcoat made out of prolamins. Prolamins take a long time to breakdown, and without degrading the capsule the starch is not accessible. Cracking the corn allows better access into the kernel, however it is still essential to breakdown the prolamin proteins for the greatest surface area. Both ensiling skills and time spent in the stack will influence the breakdown of the seed coat. Processing the grain thoroughly is the first step, followed by good ensiling skills. Ideally, we’re targeting a 34% DM in pit silage, if the silage is too dry, fermentation is slower and may not produce enough acids. Standard practices of thorough compaction and avoiding extended exposure to air will also have a major influence that will ensure a good fermentation pathway. The time in the stack is the final critical element. Good planning to produce enough feed for the herd to have carry over silage will allow the opportunity to have the feed truly ready by the time it needs to be opened. The minimum time recommended for silages is 6 weeks or 45 days. However, allowing the recommended time of 90 to 120 days will improve the quality of corn silage. This also holds true for cereal and pasture silages to reach stable state, although the time ranges from 90 days when ensiled in good conditions, and 120 days for poor conditions.

For those who have seen or attended TRAC’s workshop by Senior Ruminant Productivity Consultant, Owen Rees will be familiar with the corn silage table below. The RFS (Rapidly Fermentable Starch at 2 hour) value improves with time in the stack. Interestingly, soluble protein also increases. In dairy cow diets we look at minimizing excessive soluble protein and in grass silages, high values are often a negative. However, as the corn seed is encapsuled in a protein casing, the increase in soluble protein shows that the casing is being broken down, making more starch available.

The understanding of starch in different forms is a great tool to use in a similar way we use different fractions of protein, looking at rumen degradable, soluble, ammonia and bypass protein. While the starch information isn’t as widely available yet, it is still accessible and within time will become a part of normal ration balancing. Until then, focusing on good management practices on farm and having the confidence in making a beneficial change is good. Taking the time to take more forage samples as feeds change and as we work through the stack, or double checking the quality of commodities received regularly all aids in the ability to making decisions.

Looking to 2023, forages will likely have lower than average energy, protein and digestibility, with higher than average NDF. This means focus should be placed on topping up home grown forages with purchased feeds that have low NDF and good digestibility, while taking the extra steps to achieve great feed conversion by managing cows well through the transition period, change of season and through daily tasks.

To discuss how we can apply this information on your farm,

please speak with your local TRAC consultant today,

your Experts in Ruminant Productivity or

phone the TRAC Office on 08 8733 1888


Our Consultants


Tom Thorn

0427 243 319

Owen Rees

0429 437 823

Mikaela Baker

0457 243 319


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Maximising Starch and Total Diet Digestibility in TMR Systems
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