by Richard Goings, Ph.D., Dairy Specialist

Recently there was a report of a dairy producer who added bakery by-product to his ration and got a production response. Could it have really happened? Yes. Have we discovered a "magic ingredient? No. We are just beginning to understand how to manipulate carbohydrate nutrition.

Carbohydrates compose about 70% of a dairy cow's dry matter intake and are the predominant energy source to make milk. There are two primary types of carbohydrates based upon the rate of digestion. Neutral Detergent Fiber (NDF) consists of lignin, cellulose and hemicellulose and is slowly digested in the rumen. NDF from forage is vital to form a "mat" in the rumen, stimulate cud chewing and controlling the rate of passage through the gut (thus intake potential). Most nutritionists have a good handle on meeting the fiber requirements of dairy cows.

The second type of carbohydrates is less well-defined and usually labeled as non-structural carbohydrates (NSC's). Table 1 shows the carbohydrate content of several feedstuffs. NSC values in feeds are calculated by subtracting the NDF, fat, protein and ash content from the total dry matter. NSC is also sometimes referred to as non-fiber carbohydrates. These are the more rapidly digested carbohydrates but consist of several different classes.

Organic acids - present in small amounts in all plants. The main contribution comes from ensiled feeds and the principal organic acid is lactic acid. Organic acids are water soluble and generally move out of the rumen quickly and do not provide energy to the rumen microbes.

Sugars - a rapidly available source of energy to the rumen microbes. Main sources in dairy rations are molasses, beet or citrus pulps and bakery or candy by-products.

Starch - a chain of glucose molecules and the primary energy source stored in seeds. Availability varies with source: oats>wheat>barley>corn>sorghum. Ensiling and processing can alter availability.

Fructans - a chain of fructose molecules and is the main storage form of carbohydrates in grasses. Animals do not have the enzymes to digest fructans but rumen microbes can digest them.

Pectins - chains of various sugars found mainly in legumes, beet or citrus pulps or soy hulls.

Beta Glucans - chains of glucose hooked together differently than starch. Found in grasses, oats, barley and wheat.

Sugars, starch and fructans are generally the source of propionic acid from rumen fermentation. The digestion of these NSC's can also affect rumen pH and lead to acidosis. Pectins and Beta Glucans are generally fermented to acetic acid and have less affect on rumen pH.

Corn Grain:

Corn grain presents a special case. It is a common ration ingredient and is quite high in starch. But the starch is present in two forms, which vary in availability in the rumen. Amylopectin is rapidly available in the rumen while amylose is much more slowly available. The amylose form is more predominant as the corn becomes drier or more mature. Processing to smaller particles is critical to allow the amylose to be available in the rumen. See Table 2 for processing guidelines.

Ration Balancing:

Generally the faster a carbohydrate ferments in the rumen, the more of its energy goes to producing microbes when enough rumen degradable protein is present and the pH does not become too acidic. The NSC's are critical to provide the energy needed to produce large amounts of milk and it becomes a balancing act to provide enough NSC's without driving rumen pH too low. Unfortunately there is no simple laboratory test to distinguish between the types of NSC's or the availability in the rumen. Best recommendations at this point in our knowledge are:

1. The lbs of NSC's should be about 3X the lbs of rumen degradable protein.
2. Starch and sugars should not exceed about 75% of the total NSC's.
3. Sugar should compose about 5% of the ration or processed starch grain sources should be fed especially in rations with high soluble proteins.

Table 1. Carbohydrate Content of Feeds (100% Dry Basis)

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Table 2. How Corn Should Be Processed.

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Dr. M. Hutjens 2000 4-State Professional Dairy Management Seminar