Changes in the 2001 Nutrient Requirements of Dairy Cattle

The long-awaited "National Research Council's Nutrient Requirements of Dairy Cattle, 7th Revised Edition" was released in January 2001. The previous edition was released in 1989. One of the main differences is the size of the latest publication. The 2001 edition has 224 more pages than the 1989 publication! The authors have done an excellent job of reviewing and incorporating the latest dairy research. Another difference is the inclusion of a CD, which predicts animal requirements and can be used as a ration evaluator. Unlike some of the CDs published for other species, this CD is in a very user-friendly format. There are significant changes in some of the nutrient areas and these are summarized below.

Net Energy of Lactation (NEL) Values for Feedstuffs

In previous editions, the NEL of feeds was a constant value derived from TDN (total digestible nutrients). TDN is calculated at a maintenance level of intake and a constant 8% discount is then applied for cows eating at a 3X maintenance level to arrive at the NEL values. There are several problems associated with this approach. TDN values were based on old experimental data and were not updated to reflect current nutrient analyses. Although some dairy cows in a herd may, in fact, be eating at 3X maintenance, some may be eating at over 4X maintenance and others may be eating at 2X maintenance. Although a constant discount was applied, it is more likely that the discount should not be constant as dry matter intake increases.

The NEL of feedstuffs is no longer constant but changes to reflect changes in intake and ration digestibility. As intake increases or ration digestibility increases, the NEL value of a feedstuff decreases. The publication includes NEL values for 3X and 4X maintenance intake. The CD allows an individual user to calculate NEL at a specific level of intake. The TDN values have also been recalculated based on nutrient composition not experimental data.

The resulting changes in NEL are generally small but do improve the accuracy of feed formulation. For example, in 1989, the NEL of barley was 1.94 mcal/kg. In 2001, the NEL of barley at 3X and 4X maintenance intake is 1.86 and 1.76 mcal/kg, respectively. This assumes a ration digestibility of 74%.

Feed Composition Tables

Each nutrient analysis includes the standard deviation and number of samples used in its calculation. This allows users to make decisions concerning the validity of the data. Grasses and legumes have each been combined into two single classifications. They are subclassified based on NDF not on maturity designations. This is a less subjective method of categorizing forages.

Protein

There are changes in terminology from the 1989 edition. Absorbed protein has been replaced with metabolizable protein. This makes the terminology consistent with that used in the 1996 Beef NRC and avoids the implication that protein is absorbed. Degradable Intake Protein has been replaced with Rumen Degradable Protein (RDP) and Undegradable Intake Protein has been replaced with Rumen Undegradable Protein (RUP) to be consistent with terminology used in the Journal of Dairy Science.

The main differences between the 1989 and 2001 editions relate to predicting the amino acid supply to the small intestine. In 1989, a constant digestibility of 80% was assigned to protein leaving the rumen. No consideration was given to the amino acid composition of the RUP or the relative amounts of microbial versus dietary protein. In 2001, the RUP portion of each feed has been assigned an estimate of intestinal digestibility.

In 1989 microbial protein synthesis was predicted from energy intake. It is now predicted from total tract digestible organic matter and takes into account some of the factors that affect rates of passage and therefore RUP. For the first time, the 2001 NRC includes information on amino acid nutrition. It includes prediction equations for calculating amino acid flow and amino acid composition of duodenal protein. These changes help to further refine the feeding value of different feed sources.

Minerals

Calcium and phosphorus requirements are reported both as a % of total diet and grams of absorbable mineral. Absorbable mineral is defined as the sum of the maintenance, lactation, pregnancy and growth requirements and is the amount, which must be supplied to the tissues. Digestibility coefficients for inorganic mineral sources are included in the publication and digestibility coefficients for forages and grains are available in the CD.

Phosphorus requirements have decreased slightly reflecting an increase in the absorption coefficient assigned to feedstuffs. The zinc requirement has increased to 55 mg/kg from 40 mg/kg. New calculations showed that the zinc required for milk production had been underestimated in the past. The manganese requirement decreased from 40 ppm to 20 ppm. These mineral concentrations will vary somewhat depending on dry matter intake. Other mineral levels remain unchanged.

Vitamins

Vitamin A requirements for all classes of dairy have increased significantly. The increase reflects both the current level of high milk production compared to that achieved in the past as well as new data showing a decrease in vitamin A availability from concentrates. Milking cow requirements have increased from 52,000 IU/day to 75,000 IU/day. Dry cow requirements have increased to 85,000 IU/d during the far-away dry period and to 100,000 IU/d during the last 3 weeks prior to calving. The increase in dry cow requirements recognizes the role of vitamin A in mammary health and milk production. Vitamin A requirements for the young calf have increased from 42 IU/kg to 110 IU/kg BW.

Vitamin E requirements have been increased to 545 IU/day for lactating cows and 1200 IU/day for dry cows.

Other Changes

The "2001 Nutrient Requirements of Dairy Cattle" includes new sections on feeding the young calf, transistion and fresh cow requirements, dairy cattle nutrition and the environment and carbohydrate chemistry and feed processing. The most significant addition is the inclusion of software to determine specific animal requirements based on individual herd data. The previous sections on metabolic disorders and performance modifiers have been expanded.

For further information contact:

Karen Dupchak
Farm Production Extension, Animal Nutritionist
Manitoba Agriculture Food, and Rural Initiatives
204-545 University Crescent
Winnipeg, MB R3T 5S6
Phone: 204-945-7668
Fax: 204-945-4327