Dry matter intake (DMI) is a factor that must be estimated before an animal's diet can be properly calculated. Yet, DMI is a concept that's commonly misunderstood.

The National Research Council (NRC) requirement tables include a column for DMI. The amount listed, however, is often mistaken to be the amount of dry matter that a cow must consume regardless of the quality of the feed. In reality, this is the level of intake that a cow must consume of a ration that contains the energy concentration recommended for her by nutrient tables.

Consumption controls

It's well known that consumption of less-digestible, low-energy, high-fiber diets is controlled by rumen fill and the feed passage rate through the animal. Meanwhile, consumption of highly digestible, high-energy, low-fiber feeds is controlled by the animal's energy needs and by metabolic factors.

These concepts may seem quite simple, but the factors that regulate DMI are complex and aren't fully understood.

How much dry matter will a cow eat? There's no simple and exact answer, but we can predict DMI reasonably well from tables and practical experience.

Depending on the quality of the diet, a mature cow will usually consume 1-3% of her body weight (BW). Consumption of low-quality feeds may be 1-2% of BW, while green pasture may be 2-3%.

The factors that influence the amount a cow will eat include her size, body condition, stage and level of production. Other factors include the quality and availability of forage, amount and type of supplements and her environment.

With diets high in fiber, the rate and level of digestibility — and rate of passage of the feed — will have a large effect on intake. The faster the feed is digested, the faster it passes through the digestive tract and the more it allows for an increase in consumption. Poor-quality roughage such as straw, on the other hand, will have a slower rate of digestion than a high-quality feed such as alfalfa.

Feeding more dry matter than is required to meet an animal's needs is a waste of feed. With high-quality alfalfa hay, the energy and protein requirements can be met with about 17.5 lbs. of hay. Since the rate of passage will be relatively fast, the cow may appear hungry before the next feeding, even though her nutrient requirements have been met.

With straw, the cow would have to eat about 44 lbs. (NRC) to meet her energy and protein requirements. This she can't do. We can predict that she would eat only 20 lbs. [1,000 lb. X 2% DMI (max.)].

The cow will appear full, but she won't be meeting her nutrient requirements. It's a scenario I see quite often in cattle on corn stalks or other poor quality roughage.