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.
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.
Factors Affecting DMI
Implants tend to increase feed intake while Rumensin tends to decrease feed intake. Meanwhile, Bovatec has little effect.
A protein deficiency (less than 6-8%) also may decrease feed intake. This often occurs when cattle are on corn stalks, straw or other low-quality roughage.
This explains why supplemental protein increases DMI, as well as the digestibility of the roughage. Protein is needed by the rumen bugs to multiply and digest fiber.
Palatability also may affect DMI. Highly palatable feeds encourage consumption, while bitter-tasting feeds, no matter the value, will limit consumption. Molasses-based supplements encourage dry matter consumption. They're a good example of how we can manipulate DMI and, inherently dry matter digestibility, which increases rate of passage.
Not An Exact Science
Predicting DMI isn't an exact science. But by understanding the factors that affect DMI and the tables to estimate levels, as well as some practical experience, we can become comfortable in using estimates.
One problem in estimating DMI from tables or published data using rumen fill or energy demand is the phenomena of large increases in DMI in certain situations. These would include during lactation, cold stress and shortly before an impending storm or low-pressure front advancing to an area.
In addition, decreases in DMI have been observed with advancing pregnancy just when there are increased nutrient demands on the cow.
When calculating diets for cows for the middle trimester, we may use low-quality roughage to meet both DMI and nutrient requirements. Nearer to calving, we should increase the nutrient density of the feed to meet the increased requirements with less rumen fill. In any event, we need to keep the appetite of the cow satisfied in addition to meeting her nutrient requirements.
By the way, you can find feed composition values for 300 commonly used feeds for sheep and cattle at www.beef-mag.com. Click on “Feed Composition Guide.”
David Wieland, Shepherd, MT, is a nutrition consultant specializing in cow/calf, feedlot and horses. He also publishes a subscription newsletter and runs a cowherd. Contact him at 406/373-5512 or firstname.lastname@example.org.