Nutrient Requirements of Dairy Cattle

 

 

The 7th Revised Edition (2001) of the Nutrient Requirements of Dairy Cattle published by the National Research Council (NRC) was released in mid-January. The new Dairy NRC includes expanded discussions on most of the topics contained in the previous editions as well as the addition of several new topics. The 7th Edition includes more comprehensive descriptions of management and environmental factors that affect nutrient requirements for various life stages and levels of production. The revised approach to predicting nutrient requirements is made easier with a computer model. This revised approach to predicting requirements also increases the user's capability for accurately defining animals, diet, and management conditions to estimate nutrient requirements.

Dry Matter
The dry matter intake (DMI) discussion includes factors that affect intake and methods of predicting it. The new NRC takes into account characteristics of the animal's diet, environment, and physiologic make- up as well as relevant management issues related to DMI. Prediction equations, tables, and graphs of intake across a lactation are provided. The prediction equation used for lactating cows requires information on 4% fat corrected milk yield, cow body weight, days in milk, and parity. The prediction equation used for growing Holstein heifers was taken from the 1996 NRC Nutrient Requirements for Beef publication and requires heifer body weight and diet NEM concentration information for predictions. Both prediction equations give good estimates of DMI, but are not as good as actual measurements for DMI.

Energy
Definition of energy units, calculations used in estimating energy values of feeds, and energy requirements for maintenance, lactation, activity, and pregnancy are given. Also included is a discussion on body condition scoring along with a scoring reference chart. Calculated energy values of feeds presented in tables in the new NRC were thoroughly reviewed. The energy values were derived from composition data. In general, some energy values for forages tend to be lower in energy and non-forage feeds tend to be higher in energy compared to the 6th Edition (1989). The energy value for whole cottonseed changed the most with a large reduction (14%) in calculated energy value due to a lower fat content (19 vs. 23%). There is also discussion about the use of NEL on a total diet basis instead of an individual feed basis to account for associative affects of feeds and diet intake level. The equations for calculating energy requirements for cows were updated with the biggest change occurring in the calculation of energy required for grazing. In the 6th Edition, energy required for grazing was calculated as +10% or +20% of maintenance depending on distance from parlor to pasture. The new NRC makes adjustments for distance as well as topography of the pasture.

Fat
Digestibility and energy values of fat are discussed as well as the affects of fat on rumen fermentation and the use of fat in lactation diets. This edition does not assume the energy values of all fats/oils are equal, as did the 6th Edition. Characteristics of fats that affect digestibility, such as chain length, degree of saturation, and structure (free fatty acid vs. triglyceride), were considered in calculating the new energy values for fats/oils; however, fats were not taken into account relative to rumen fermentation in the software. A table of fatty acid composition of fats and oils is also presented. The current edition indicates a maximum inclusion rate of 3-4% supplemental fat (6-7%) total dietary fat) in lactation rations.

 

Carbohydrates
A comprehensive review of carbo hydrates is a new addition. Structural and non-structural carbohydrates are discussed and requirements for neutral detergent fiber (NDF) and acid detergent fiber (ADF) were given special attention. The requirement for ADF is 19-21% and NDF is 25-28% with 75% of the NDF provided by forages. It is recommended that as forage NDF in the ration decreases, then total NDF and ADF should be increased in the ration and non-fiber carbohydrate (NFC) levels decreased.

Protein/Amino Acids
The greatest enhancement in the new Dairy NRC is evident in discus- sions of protein and amino acid nutrition. Table 1 shows some changes made in protein terminology. Extensive documentation of the literature base used for developing equations to predict metabolizable protein requirements are given. One major difference in this edition is that RUP is recognized as not being constant for a feed and is affected by rate of degradation and rate of passage. The amino acid section is greatly enhanced over the previous edition and discusses predicting amino acid passage to the small intestine. However, the authors did not believe there was sufficient evidence to include a requirement for specific amino acids at this time. The NRC committee did suggest that a lysine to methionine ratio of 3:1 in metabolizable protein should provide the best response for milk protein production. In addition, it was implied that amino acid needs could be addressed by balancing for lysine and methionine levels as a percentage of metabolizable protein in the model software.

 


Minerals
 Function, requirement, sources, bioavailability deficiency symptoms, and potential toxicity are discussed for macro and trace minerals. One major change in this edition is the lowering of phosphorus requirements, which is at least partially a result of environmental pressure to reduce phosphorus excretion by livestock.

Vitamins
Each vitamin is discussed in the context of the animals that will be ingesting it. Sources and bioavailability of vitamins are provided, followed by a discussion of functions, animal response, requirements, and factors that affect requirements. Requirements are only given for vitamins A, D, and E. These levels are based on supplementation. No credit is given to feedstuff vitamin levels. Vitamin A requirements increased from 19 to 36 IU/lb body weight for heifers and from 35 to 50 IU/lb body weight for lactating cows. Vitamin D requirements stayed essentially the same as in the 6th Edition. Vitamin E requirements were increased to 0.73 IU/lb body weight for heifers and dry cows and 0.36 IU/lb body weight for lactat- ing cows. These new requirements for vitamin E represent a five-fold increase in the requirement for heifers and dry cows and a 33% increase for the lactating cow as compared to the 6th Edition. The vitamin A requirement for calves also increased (19 to 501U/1b body weight) as did the vitamin E require- ment (18 to 23 IU/lb diet dry matter).

Water
Metabolism and requirements of water as well as factors in the environment and water itself that affect intake are discussed. Among the factors considered are nutrients in the water and the presence of bacteria and algae. The variables used in the prediction equations for water intake include milk yield, diet dry matter percent, and DMI. A rough rule of thumb is three lb of water intake for each one lb of milk produced. Approximately 83% of water intake is obtained by drinking.

Metabolic Disorders
The new NRC includes discussions on feeding the transition cow and a section on performance modifiers including buffers and direct-fed microbials, which were not included in the previous edition. As in the previous edition, there are discussions of metabolic disorders including fatty liver, ketosis, udder edema, milk fever, grass tetany, and retained placenta. Additionally, this edition includes discussions on displaced abomasum, rumen acidosis, and milk fat depression. Nutritional strategies that have been utilized to prevent metabolic diseases in dairy cattle are given. These recommendations often suggest an approach quite different from simply meeting the nutrient requirements of the cow. For example: to prepare the rumen environment for the high-energy ration the cow will consume after calving, a ration higher in energy than is required is fed to the cow in late gestation.

Calf/Heifer
Information pertaining to young calf and growing heifer nutrition is a new addition. Many topics related to the young calf (<220 1b), including the different phases of feeding (liquid, transition, ruminant) as well as a separate discussion on feeding veal calves, is included. Important recommendations given follows:

  • Within one hour of birth, a calf should receive three liters of colostrum.

  • Before weaning from liquid feed, a calf should be consuming 1.5 lb of a dry starter daily.

 Calculation of heifer nutrient requirements based on size-scaling rather than large/small breed is given. This enables nutrient requirements to be calculated for any size heifer at any growth rate with rate of gain predictions based on current size and age of heifer relative to the estimated mature size and age of the cow. Adjustments for environmental conditions that affect nutrient requirements should also be taken into account.

 

Environment
New to the 7th Edition is a chapter devoted to dairy nutrition and the environment. An overview of nutrients of concern, primarily nitrogen and phosphorus, and the challenges faced by managers in reducing nutrient excretion is given.

Processing
Analytic procedures for evaluating carbohydrates are described and the effects of processing on energy in feed are reviewed in the new NRC. The effect of processing on grain energy value is now better represented in the feed composition data.

Nutrient Requirement Tables
Nutrient requirement tables generated from the computer model for large and small breed cows at various stages of lactation are provided. These tables are not intended to describe the nutrient requirements for all situations. Nutrient requirements are not static and depend on animal factors, diet composition, DMI, and source of nutrients. Therefore, it is recommended the computer model be used to deter- mine the best estimate of nutrient requirements for a given situation.

Composition of Feeds
The average nutrient composition of common feeds fed to dairy cattle is presented in tables that appear different than those found in the 6th Edition. The updated tables provide more nutrient information including NEL values calculated at both 3X and 4X maintenance intake, protein fractions A, B and C, and amino acid content of feeds. Also, presented in the tables is the number of samples used in estimating each nutrient for each feed. A notable change in the new NRC is that all legume-type forages (alfalfa, clover, birdsfoot trefoil) are listed as legume forages and given the same nutrient analysis. Similarly, cool-season grass forages (bluegrass, brome, canary grass, fescue, orchard grass, ryegrass, timothy) are listed as cool-season grasses and are given the same nutrient analysis. Additionally, a section on grass-legume mixes is included.

Computer Model
Computer model evaluation, mathematical configuration, and use of prediction equations in the computer program, a glossary of terms, and a user's guide are given.

Conclusion
The 7th Revised Edition of the Dairy NRC is a tremendous improvement over the 6th Edition. The new NRC enables more accurate predictions of nutrient requirements of dairy cattle in all stages of growth, development, and production as well as more accurate estimates of the nutrient content of feeds and diets fed to dairy animals. Even though many advances have been made in dairy cattle nutrition since the 6th Edition was published, there are still many areas which need to be investigated if we are to continue improving lactating performance to meet the food needs of an increasing global human population.

 

 

 

ADM Alliance Nutrition, Inc. , a wholly owned subsidiary of the Archer Daniels Midland Company