New in the Lab: Starch digestion is the next frontier

Typically composing anywhere from 15%-35% of the dairy cattle diet, starch is an important nutrient the cattle industry continues to strive to understand. Starch is one of the most energy-dense nutrients, yet it varies in rumen availability, which continues to challenge nutritionists and the producers with which they work.

Starch granules in grain are enclosed in a protein matrix that is not very digestible, preventing rumen microbes from accessing the starch it encases. So while starch may be present in the diet to offer energy, the energy may be tied up due to limited microbial access.

Adding difficulty to understanding this nutrient"s availability, starch digestion occurs in both the rumen and the hindgut. While rumen starch digestion has been proven to increase microbial protein and milk production, starch digested in the hindgut has not proven to have any effect in production and, in some extreme cases, may add to hindgut digestive upset.

So, the aim should be to best characterize rumen starch digestion and opportunities. Typically, assessing starch digestion requires the use of rumen fistulated cows. Nutritionists and laboratories use two rumen techniques: in vitro (simulated) or in situ (direct) digestion, to measure rumen starch digestion.


The variability factor

Variations in growing conditions, harvest environment, genetics and fermentation lead to variable rumen starch digestion. Some genetics offer favorable rumen starch availability, while others do not. Mother Nature can also change the starch-prolamin matrix to a great degree. Regardless of genetics or growing conditions, starch digestion shortly after harvest is generally below average until grain has adequate time to ferment. As high-moisture corn or corn silage ferments through the year, starch digestion increases as the prolamin proteins that bind the starch granules are broken down.

Failure to account for these starch digestion variability factors can lead to a ration bottleneck – with lower milk production or weight gain the result. Dairy producers and their nutritionists should allow forages and feeds to ferment for a few months, but with tight inventories this process can prove challenging.


Measure to assess and better digest

Assessing starch digestibility in feeds can provide valuable information for nutritionists to make decisions:

1)   If starch availability is limiting, add in more highly digestible starch, such as corn starch, fine ground corn, wheat or barley, to alleviate production losses; or

2)   Pull out an expensive energy-dense ingredient if sufficient or excessively rapid starch energy is provided by current feedstuffs.

When encountering a farm with poor silage or grain, several options exist to improve digestibility so producers are not missing out on performance. Steam-flaking, reducing grind size or re-grinding grain increases the surface area for rumen microbes to attach to, and improves rumen starch breakdown. If silage starch is challenged, work to build inventory this upcoming year when silage prices may be lower, and consider letting 2015 harvested silage ferment longer.

On top of the additional processing, nutritionist should consider starch digestion analysis options. By testing rumen starch digestion, nutritionists can better locate where the starch is being digested, ultimately gaining insights that can, in turn, improve the ration and production. New options to measure and better track starch digestion have developed as more research has been conducted and the importance of starch has been realized:

• Fecal Starch (TTSD): Provides the total starch digestion (rumen + hindgut). This can be useful in determining whether feed is digested well or not, but does not determine whether the starch is digested in the rumen or the hindgut.

• UW Grain Eval 2.0: Utilizes nutrient analysis, particle size, ammonia-N and prolamin to predict rumen starch digestion, but is only available for analysis of corn grain (not available for corn silage).

• Rumen Starch Digestion: Determines the amount of starch digested in the rumen and helps find hidden opportunities. These tests are becoming more popular, but make sure your chosen lab"s assay has been benchmarked and validated against real cattle data. Not all compare well against in vivo cattle data.


Each of these options has positive opportunities to offer insights into starch digestion, but the industry has much to learn. Testing starch digestion comes with additional costs, but can offer great return. One pound of performance gain (or loss) with 100 cows equates to $15 to $20 per day.

As mentioned previously, if highly digestible feeds offering adequate available energy are an option, feed costs could be dropped by only feeding the necessary amount of grain. If the feeds are found less digestible, explore options to improve performance by increasing the starch digestion of those feeds, or supplement with highly digestible starch sources.

As Class III milk futures suggest substantially lower milk prices in 2015, nutritionists should focus on keeping costs down while also optimizing performance.



Courtney Heuer

Technical Nutrition Analytic Consultant, Rock River Laboratories



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