Grain and forage rumen starch digestibility has historically been a hot topic; however, more recently, interest and emphasis have escalated in the dairy and beef industries. This comes as no surprise as grain prices have increased as high as $8 per bushel in recent years and prices have violently fluctuated. The desire to maximize performance per bushel is on the rise, and the rumen is the focus.

Diet energy from grains, corn silage and other starch-rich forages largely comes from starch, which can make up 20 to 30 percent of total energy in your dairy cattle ration.

Limited rumen starch digestion creates challenges for dairy and beef cattle due to decreased rumen microbial protein production, milk production, limited milk components (due to lacking microbial protein and energy) and daily rate of gains.

Starch digestion values are increasingly being used in diet formulation for optimizing your herd’s performance. Animal nutrition models should capture rumen starch digestion levels ranging from 40 percent to more than 80 percent.

This difference in rumen starch digestibility alone corresponds to 5 to 7 pounds per cow in performance with a typical dairy diet.

The substantial range in rumen starch digestibility is due to several contributing factors. Rumen bacteria’s access to starch granules, due in part to available starch surface area (effective particle size), can limit capacity for digestion; the larger the surface area (smaller effective particle size), the more attachment sites are available for the rumen bacterial enzymes, leading to better digestion results.

Beyond grain particle size, different grain physical and chemical properties further affect starch digestion. These varying factors include kernel hardness, kernel processing, fermentation extent, and heat or chemical/enzymatic treatments.

These factors change the effective particle size for the grain. For example, a high-moisture corn ground to 1,000 microns but not adequately fermented may actually feed similar to corn ground to 1,500 microns and cause performance challenges. How can your dairy determine if this is a limiting factor for your performance?

Both starch digestion rate and ultimately digestion site (rumen versus hindgut) are critical in determining how well your corn and corn silage performs. Cows can digest starch similar to humans in the intestines; however, the rumen should be the main site for starch digestion, with the aim being 75 to 85 percent of total starch degraded here.

Starch digestion in the rumen not only provides volatile fatty acids for energy to the cow but also spurs other nutrient digestion and generates microbial protein, which is a building block for milk protein.

With many factors affecting grain potential, determining ruminal starch digestion in the laboratory can be difficult. Several analyses are available to assess rumen starch digestion: UW Feed Grain v2.0, in vitro rumen starch digestibility, and more recently, in situ rumen starch digestibility.

The UW Feed Grain v2.0 assay is based on published literature that includes ammonia nitrogen, prolamin, particle size and several other metrics in a model that in return calculates effective mean particle size, ruminal starch digestibility, total tract starch digestibility and a relative grain quality index.

The grain quality index is very similar to relative forage quality and can be interpreted as such. The UWFGE v2.0 analyses join the physical and nutritive composition of the grain to provide a sound system to evaluate your grains. However, this method is strictly available for grains, but what about forages?

Determining starch digestibility using an in vitro (lab bench) rumen system has gained popularity and can be a good tool when it comes to comparing within feeds, but it often overestimates true ruminal starch digestion.

Lab bench in vitro rumen 7-hour starch digestion measures average 80 to 90 percent of total starch for grains and corn silages and often do not drop below 70 percent, yet real rumen starch digestion averages closer to 60 percent and ranges from approximately 30 to 85 percent.

We have further shown that lab bench in vitro digestion measures are substantially greater than actual rumen starch digestion measured by an in situ rumen technique. This rumen in situ technique, more closely related to the cow, puts forages and grains within lactating cows’ rumens for digestion.

Although this approach is more expensive than the two techniques discussed above, it appears more closely related to actual rumen digestion, is precise and demonstrates wide ranges in forage and grain starch digestion systems because it incorporates the actual high-performing cow rumen environment.

In 2013, our team presented research at the ADSA joint annual meetings comparing lab bench in vitro and in situ rumen incubation digestion results.

We compared both techniques using five different starchy feed types (high-moisture corn, dry corn, corn silage, snaplage and TMR) and found lab bench in vitro rumen digestion results yielded substantially greater results than the actual rumen-digested samples ( Table 1 ).

This confirmed that lab bench in vitro measures are greater than in situ rumen incubation results. In vitro technique accuracy then came into question. Can we really measure what’s happening in the rumen with a lab bench in vitro approach? At this point, maybe not, although additional work should be done.

Moving forward with the rumen digestion (in situ) approach, performance opportunities can be better defined. The rumen incubation approach appears to also better differentiate exceptional from poor-quality starch sources.

Corn grain or silage with 40 to 50 percent rumen starch digestion (below average) has the opportunity to improve to 70 percent or better through letting the feed ferment longer, finer grinding or regrinding, or replacing with alternative highly digestible, starch-rich feeds.

By improving rumen starch digestion from 50 percent to 70 percent, rumen energy and diet TDN improve, and we should recognize performance gains of 5 pounds per cow or more.

The rumen carbohydrate metabolism focus for dairy and beef cattle is not a new area. Many great researchers have done exceptional work in this area over the past 50 years helping us to continue learning, yet we have substantial room to gain more knowledge.

Your dairy stands to benefit in performance and profitability by continuing to better understand how fiber and starch are used in the rumen. We encourage you to work with your consulting team to determine if starch digestion is an opportunity for your herd. PD

John Goeser earned a Ph.D. in animal nutrition from the University of Wisconsin – Madison where he currently serves as an adjunct professor in the dairy science department. He also directs animal nutrition, research and innovation efforts at Rock River Lab Inc. based in Watertown, Wisconsin. Courtney Heuer is also with Rock River Lab Inc.

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John Goeser
Director of Nutrition, Research and Innovation
Rock River Laboratory Inc.