Nitrogen Management on Dairy Farms
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Evaluating and Improving Diets to Maximize N Efficiency

Nitrogen excreted by dairy cattle is reduced as rations are formulated to more accurately meet the requirements of the animal. Computer programs developed from equations in the CNCPS and NRC nutrient requirements are used to evaluate and improve the current diets being fed to improve performance, reduce costs, and to change the proportion of feeds in the ration, based on their availability and cost. The previous pages within the N Utilization and Excretion section provides information on the nutrient requirements of dairy cattle.

Accurate prediction of animal requirements of each group of dairy cattle requires information that can be obtained on the farm to be used by a computer model to predict their requirements for maintenance, lactation, pregnancy, growth, and replenishment of body reserves. Accurate prediction of energy, rumen degradable and un-degradable protein, and amino acid supply available to meet requirements on each farm depends on feed analysis information that can be obtained from feed testing laboratories. Thus, animal, environment, and feed composition must be described as accurately and completely as possible. However, because many of the factors (body size, environmental conditions, feed digestion rates, particle size, etc.) depend on field observation, the input factors must be adjusted in a logical way until the model predicts the performance that is being observed before a more accurate diet can be formulated. This approach allows requirements to be computed for the specific animal, environmental, dry matter intake and feed composition conditions on each farm.

We have developed the following sequence of steps in using the CNCPS or NRC models to determine the first limiting nutrient (energy, absorbed protein, amino acids) for specific conditions, based on our use of the CNCPS on dairy farms. This hierarchy is necessary, because of the "ripple effect" of all of the interactions in the model. When one factor is altered, several others will likely be affected. The hierarchy inherent in the CNCPS and NRC models assumes that energy is first limiting, and that amino acid requirements are designed to complement the energy supplied through intake for daily gain and/or milk production over that needed for maintenance and pregnancy.

Consider the following steps for developing accurate, farm specific rations.

  1. Describe the animals and their environment as accurately as possible. The inputs that the model needs to compute requirements are as follows.
  1. Describe feed composition as accurately as possible. Samples of each forage, grain, food processing co-product, and protein supplement source should be analyzed for the carbohydrate and protein fractions required by the computer model being used. These values are used to compute the energy, microbial protein and amino acid production, degraded protein available to meet microbial growth requirements, undegraded protein and animo acids available to meet animal requirements. The CNCPS and NRC models require NDF, lignin, CP, protein solubility, NDF protein (slowly digested), ADF protein (bound and poorly digested), fat, and ash. Feed should be analyzed as often as needed to represent the feeds being fed to each group of animals.
  2. Accurately determine dry matter intake. The actual dry matter intake must be accurately determined, taking into account bunk clean out, moisture content of feeds and scale accuracy. The accuracy of any model prediction is highly dependent on the DMI used. Intake of each feed must be as uniform as possible over the day, because as far as we know all field application models assume a total mixed ration with steady state conditions.
  3. Compare energy allowable to actual milk production or daily gain with the current diet. The daily gain or milk production being obtained should agree with those predicted from the diet considering animal type, environmental conditions, feed intake and diet feedstuff carbohydrate composition. If not, the user should evaluate the following.
  1. Make adjustments to insure effective fiber requirements are being met. In high producing cows or feedlot cattle on high energy diets, it is difficult to balance fiber requirements because of the increase in energy density needed to meet energy requirements for maximum production. We make adjustments to insure that diet eNDF is a minimum of 20% in lactating cows and replacement heifers. As much as 25% eNDF may be required to maintain an adequate rumen pH, depending on feeding management.
  2. Balance the rumen for nitrogen. Feeds such as soybean meal that are high in degradable true protein are added until ruminal peptide needs are met if amino acids are expected to be deficient; they are required for optimal fermentation of nonstructural carbohydrates. Then adjust remaining ruminal nitrogen requirements with feeds high in NPN or soluble protein until ammonia needs are met. In addition to maximizing microbial amino acids supplied, the total tract digestion of both fiber and starch are dependent on the extent of ruminal fermentation.
  3. Balance the animal's metabolizable protein (MP) requirements. This component represents an aggregate of nonessential amino acids and essential amino acids. The MP requirement is determined by the animal type and the energy allowable gain or milk production. The adequacy of the diet to meet these requirements will depend on microbial protein produced from structural and nonstructural carbohydrate fermentation and feed protein escaping fermentation. If MP balance appears to be unreasonable, we check first the starch (Carbohydrate B1) digestion rates, using the ranges and descriptions in the CNCPS help system or users manual. Altering the amount of degradable starch will also alter the peptide and total rumen N balance, because of altered microbial growth. Often the most economical way to increase MP supply is to increase microbial protein production by adding highly degradable sources of starch, such as processed grains. Further adjustments are made with feeds high in slowly degraded or rumen escape (bypass) protein (low protein B2 digestion rates; see the CNCPS or NRC feed libraries).
  4. Compare essential amino acids supplied to requirements. This is last to be adjusted, because the amino acid balance is affected by changes made in all of the above. The NRC and CNCPS models give recommended ratios for methionine and lysine in the intestines, because they are most often the first limiting in milk production. Essential amino acid balances can be estimated within the structure of the CNCPS because the effects of the interactions of intake, digestion and passage rates on microbial yield, available undegraded feed protein and estimates of their amino acid composition can be predicted along with microbial, body tissue and milk amino acid composition. The development of more accurate feed composition and digestion rates, and more mechanistic approaches to predicting utilization of absorbed amino acids will result in improved predictability of diet amino acid adequacy for cattle. Sources of first limiting essential amino acids are adjusted where practical to improve the amino acid profile. Efficiency of nitrogen use will improve as the suite of essential amino acids supplied better matches that of requirements.