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Hold the starch, pass the digestible forage

Kathleen Emery Published on 21 May 2010

Ration starch content is a hot topic among dairy nutritionists these days, as we look for ways to create more economical rations. Dairy cows need to consume adequate starch for proper rumen fermentation. Traditionally, corn provided this source of fermentable energy because it was cheap, available and high in starch. However, concerns about high corn prices and potential cow health problems have many producers looking for grain alternatives.

Too much of a good thing

Yes, starch promotes the growth of bacteria and protozoa necessary for rumen fermentation. However, feeding too much starch can result in an imbalance of volatile fatty acids (VFAs) in the rumen and reduced chewing, saliva production and rumen buffering. Combined, these consequences can reduce the rumen pH and lead to subacute ruminal acidosis, causing digestive upsets, reduced milk production, laminitis and other health issues.

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Replacing corn grain with forage

Current recommendations for dietary starch content range from 23 percent to 30 percent on a dry matter (DM) basis. High-quality forage can replace some of this corn grain to supply rumen-fermentable carbohydrates to dairy cows. Corn silage is an excellent choice as the primary forage in reduced-starch rations.

High-quality forage fiber provides a source of energy to maximize milk production. Diets high in effective fiber, combined with the correct particle length, reduce the risk of acidosis and help cows form a proper rumen mat. Diets high in effective fiber with long particles correctly sized minimize sorting and promote chewing and saliva production, maintaining a healthy rumen pH.

Benefits of highly digestible forages

Research has shown that high-producing cows benefit most from forages that have a high neutral detergent fiber digestibility (NDFD), or fiber digestibility. One such highly digestible forage is brown mid-rib (BMR) corn silage.

When a research study compared BMR to conventional corn silage, BMR boosted dry matter intake (DMI) and milk production.

  • Cows that produced approximately 120 pounds of milk per day ate 8 pounds more per day and increased production by 17 pounds per day when fed BMR corn silage.
  • Cows producing less than 70 pounds of milk per day had a similar intake and milk production for both BMR and conventional silages.
  • High-producing herds and high-producing cows within those herds will benefit most from the inclusion of highly digestible forage, since these cows are limited by the filling effect of fiber.

Another study compared solids-corrected milk production and bodyweight of cows fed BMR corn silage and conventional corn silage at low and high concentrations of fiber.

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  • Cows fed diets containing highly digestible BMR corn silage with no added grain increased milk without gaining bodyweight, compared with those fed conventional corn silage.
  • Cows fed diets with less digestible corn silage and 29 percent DM in added corn grain did not increase milk production and instead increased bodyweight gain by 0.79 kilograms per day.
  • Cows fed grain along with lower- quality forage converted less energy to milk and more energy to body condition.

These results show that highly digestible forages can be used in diets with less corn to prevent excess body condition without sacrificing milk.

Recommendations for reducing starch

It is not enough to reduce the amount of starch in a diet, without factoring in the amount that is degradable in the rumen. When working with your nutritionist to replace corn grain with forage, be sure to take into consideration the primary factors that affect starch digestibility – dry matter of the grain, particle size and endosperm type.

When planning to replace a portion of your corn grain with highly digestible forage, consider the following guidelines.

  • Select your corn silage varieties based on NDFD. Wet chemistry lab tests measure the neutral detergent fiber digestibility (NDFD) of a forage sample and provide information about the intake potential of the fiber and its rate of passage. High NDFD corn silage allows for greater DM intake potential and allows you to feed more forage. Reduce feed costs by replacing corn grain or soybean meal with digestible fiber.
  • Feed forages with highest NDFD to your highest-producing cows. High-producing cows, between 50 and 150 days in milk (DIM), are most limited by rumen fill. Forages with high NDFD will digest quickly, leaving the cow ready to eat again sooner. Low NDFD corn silage limits milk production because the cow fills her rumen with fiber and is not ready to eat again until this fiber passes. As DMI decreases, less energy is available to support milk production. When lactating groups exist, feed high NDFD forages and reduced grain to lower-producing cows to prevent excess body condition and maintain production.
  • Increase the physically effective neutral detergent fiber (peNDF) of your diet by including more forage and increasing the chop length. Reducing the starch content and the rate of fermentation will reduce the risk of acidosis. The peNDF of the diet DM should be between 20 percent and 22 percent. Measure this as particles retained on the 19-mm and the 8-mm sieves of the Penn State Particle Separator. For the forages, 60 percent or more of the material should be retained on the top two sieves. For example, using a corn silage of which 10 percent of DM was retained on the 19-mm screen and 60 percent of DM retained on the 8-mm screen, the peNDF is .70 (.10 +.60 = .70). If the corn silage contained 45 NDF, its peNDF is .70 x 45 = 31.5 percent.

During these challenging times, it is important to seek out opportunities to improve efficiency and lower costs. When balancing a ration, focusing on forage fiber instead of starch is one way to lower ration costs, boost milk production and decrease the incidence of acidosis – all of which will help to improve a dairy’s bottom line. PD

References omitted due to space but are available upon request by sending an e-mail to .

Kathleen Emery
  • Kathleen Emery

  • Dairy Nutritionist
  • Mycogen Seeds
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