Meeting the nutrient requirements of a high-producing dairy cow without disrupting the rumen environment can be challenging. Starch is a key energy source but can also lead to acidosis if it is overfed.
However, not all starch is created equal; the source of starch and how it is processed can greatly impact how it is broken down in the cow’s digestive tract. For example, different forms of corn grain can range from 30 percent to 80 percent digestion of starch in the rumen.
Although many dairy nutritionists consider ruminal versus post-ruminal protein supply when formulating diets, this approach is only beginning to be applied to the carbohydrates in the diet. Let’s consider the implications of changes in site of digestion for lactating dairy cows and whether this can help enhance the productivity of cows.
Ruminal starch digestion
Like dietary protein, digestible carbohydrates are needed to support both the cow and the microbes in the cow’s rumen. However, there is often a fine line between sufficient and excessive dietary carbohydrate levels. On one hand, fermentable carbohydrates are key to driving microbial growth, which increases the production of microbial protein, including fiber-digesting enzymes.
On the other hand, too much fermentation can decrease ruminal pH, ultimately hindering microbial growth and fiber digestibility.
Microbial protein yield is generally dependent on two factors: rumen-degradable protein supply and ruminally fermentable organic matter supply. If either of these factors is limiting, microbial protein production can be hindered. In the U.S., it is rare we would underfeed rumen-degradable protein in lactation diets, so ruminally fermentable organic matter supply is the factor most likely to constrain production of microbial protein.
However, research has not demonstrated a strong relationship between microbial protein yield and ruminal organic matter digestion. The most significant predictor of metabolizable protein yield is feed intake. In a situation where ruminally fermentable organic matter supply is low, like when feeding low-quality forages, low neutral detergent fiber digestibility and poor intakes can certainly hinder metabolizable protein production.
Excessive starch digestion in the rumen can lead to a reduction in ruminal pH and milkfat production by altering the biohydrogenation of fatty acids in the rumen. Although other factors like physically effective neutral detergent fiber concentration and animal characteristics can contribute to milkfat depression, dietary starch concentration and fatty acid load are undoubtedly the two most critical factors.
Intestinal starch digestion
How much starch can a cow digest in the small intestine? Some support the view intestinal starch degradation is a limiting factor in cattle, but intestinal starch digestion rates in excess of 6.5 pounds per day have been measured. So let’s assume a cow’s small intestine is reasonably adept at digesting starch. From an energy-efficiency standpoint, there are some reasons to believe small intestinal starch digestion has an advantage over ruminal starch digestion.
First, shifting starch digestion to the small intestine has potential to decrease methane emissions, since methane is a byproduct of ruminal fermentation. Second, it is possible increased intestinal starch digestion may decrease energy losses since the microbial breakdown of starch produces a great deal of heat. Although there is currently little direct evidence to support these ideas, they are worth considering.
It is worth noting a shift in starch digestion post-ruminally generally increases the amount of starch reaching the large intestine. Although some volatile fatty acids are lost in the feces, and microbial protein produced in the hindgut is often left uncaptured, the energy yield from hindgut fermentation is usually considered to be similar to that resulting from ruminal fermentation.
Rapid volatile fatty acid production in the hindgut, however, increases the risk for hindgut acidosis, which may be a risk factor for hemorrhagic bowel syndrome.
Post-absorptive impacts
Yields of energy and metabolizable protein are not the end of the story. Digestion of starch via ruminal fermentation versus small intestinal digestion results in very different end products available for absorption. Ruminal starch fermentation leads to the production of volatile fatty acids, primarily propionate. Propionate produced in the rumen is critical for gluconeogenesis.
Small intestinal digestion leads to the production of glucose for absorption by intestinal epithelial cells. Much of the absorbed glucose is metabolized by intestinal tissue, either oxidized to CO2 or converted to lactate. Infusion studies suggest intestinal glucose absorption is a more potent means of supplying glucose to the cow compared to ruminal propionate supply.
In addition to the differences in absorbed end products, shifting the site of starch digestion can have substantial impacts on feed intake. For example, replacing high-moisture corn, which is highly digestible in the rumen, with dry-rolled corn, which is primarily digested intestinally, increased dry matter intake and solids-corrected milk yield in several studies.
Rapid ruminal starch fermentation leads to increased production and absorption of propionate, which can suppress feed intake.
Considering site of digestion in diet formulation
How should our understanding of site of starch digestion impact how we formulate diets? It is clear the carbohydrate fraction of the diet should be formulated to support the production of microbial protein and to supply sufficient energy for high milk protein and lactose production without impairing rumen function or milkfat synthesis.
However, the appropriate nutrient profile to achieve these goals likely depends on the characteristics of the cows being fed.
There is evidence cows in peak lactation can handle high ruminal starch loads, resulting in increased fat-corrected milk production, whereas the same diet can lead to ruminal acidosis, milkfat depression and bodyweight gain in late-lactation cows. Utilizing starch sources of varying ruminal fermentability, starch content and site of digestion can be varied by stage of lactation to help optimize productivity and body condition.
We shouldn’t forget fine-tuning the site of starch digestion is of little value if we are ignoring neutral detergent fiber digestibility. The ideal lactation diet continues to pivot on high-quality forages. With this in mind, it is possible managing the site of starch digestion may make the difference between average and highly productive dairy cows. 
PHOTO: In several studies, replacing high-moisture corn with dry-rolled corn increased dry matter intake and solids-corrected milk yield. Photo by Ray Merritt.
Ben Saylor is a graduate assistant, dairy cattle nutrition, with the University of Florida. Email Ben Saylor.
References omitted but are available upon request. Click here to email an editor.
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Barry Bradford
- Professor - Department of Animal Science
- Kansas State University
- Email Barry Bradford
