Expert Answers - March 19, 2010

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The following answer was provided by Charles Schwab, professor emeritus of animal sciences at the University of New Hampshire and dairy nutrition consultant, and included in the proceedings of the Southwest Nutrition Conference, which was held Feb. 25-26 in Tempe, Ariz.


Q: How do you optimize amino acid formulations for lactating dairy cows?

A: We consider five steps as being important to maximizing milk components and metabolizable protein (MP) utilization through amino acid formulation.

1. Feed a blend of high-quality forages, processed grains, and byproduct feeds to provide a blend of fermentable carbohydrates and physically effective fiber that maximizes feed intake, milk production, and yield of microbial protein. Microbial protein, based on research to date, has an excellent AA composition for lactating dairy cows. The average reported concentrations of lysine and methionine in bacterial true protein approximate 7.9 percent and 2.6 percent, respectively; values that exceed the concentrations in nearly all feed proteins (NRC, 2001), and values that exceed the optimal concentrations in MP as estimated by the NRC (2001), CPM-Dairy (v.3.0.10) and AMTS. Cattle (v.2.1.1) models (see Table 1). Realizing maximal benefits of feeding a balanced supply of fermentable carbohydrates on feed intake, milk production, and yields of microbial protein requires use of high-quality feeds, adequate intakes of physically effective fiber, well-balanced and consistent diets, unlimited supplies of fresh water and superior bunk management.

2. Feed adequate but not excessive, levels of rumen-degradable protein to meet rumen bacterial requirements for amino acids and ammonia. Realizing the benefits of feeding a balanced supply of fermentable carbohydrates on maximizing yields of microbial protein also requires balancing diets for RDP. Rumen degraded feed protein is the second largest requirement for rumen microorganisms. It supplies the microorganisms with peptides, AA and ammonia that are needed for microbial protein synthesis. The amount of RDP required in the diet is determined by the amount of fermentable carbohydrates in the diet. Diet evaluation models differ in their estimates of RDP in feeds and animal requirements. The NRC (2001) model typically predicts RDP requirements of 10 to 11 percent of diet DM. Regardless of the model, use the predicted requirements as a guide and fine-tune according to animal responses. Monitor feed intake, fecal consistency, milk/feed ratios, milk fat concentrations, and MUN to make the final decision. A common target value for MUN is 10-12 mg/dl, but values lower than this are not uncommon in high-producing cows. Don't short-change the cows on RDP ... carbohydrate balancing can be negated with an inadequate supply of RDP. A deficiency of RDP will suppress the ability of the microorganisms to reproduce, but they can continue to ferment carbohydrates. This will often result in lower than expected milk/feed ratios because of lower than expected synthesis of microbial protein. Avoid over-feeding feeding RDP to the point that rumen ammonia concentrations markedly exceed bacterial requirements. Not only does it result in wastage of RDP, but research (e.g., Boucher et al., 2007), as well as a summary of N passage studies where rumen ammonia concentrations were also measured (Peter Robinson, personal communication), indicate that rumen ammonia concentrations in excess of bacterial requirements decreases flows of microbial protein to the small intestine.


3. Feed high-lysine protein supplements to achieve a level of lysine in metabolizable protein that comes as close as possible to meeting the optimal concentration. If protein supplementation is required, select high-quality, high-Lys protein supplements (e.g., soybean and canola meals, blood meal, and fishmeal). In this case, "high quality" refers to consistency in distribution of RDP and RUP and highly digestible RUP where one is certain that RUP-Lys digestibility is not compromised. Feeding low-Lys feeds such as corn gluten meal and distiller's grains as sources of additional protein is not consistent with balancing for AA. Purposely selecting high-Lys protein supplements has been the only option, at least until the recent release of the first rumen-protected Lys sources on the market, to at least partially compensate for the low content of Lys in the RUP fractions from forages, grains and distiller's grains. Achieving target formulation levels for Lys in MP will become easier, and the value of lower Lys protein supplements extended, if these rumen-protected Lys products can be demonstrated to be cost effective sources of MP-Lys.

4. Feed a "rumen-protected" methionine supplement in the amounts needed to achieve the optimal ratio of lysine and methionine in metabolizable protein. Feeding a rumen-protected Met supplement, in conjunction with one or more of the aforementioned high-Lys protein supplements, is almost always necessary to achieve the correct Lys/Met ratio in MP. We continue to be surprised with first time evaluation of diets how often we see Lys to Met ratios in MP of 3.3 or higher ... values as high as 3.5 and 3.6 are not uncommon. "Out of balance" Lys to Met ratios lowers the efficiency of use of MP for protein synthesis and the more "out of balance" the ratios, the less efficient the use. To achieve the desired predicted ratio of Lys to Met in MP, and to ensure full use of the available MP-Lys for protein synthesis, one MUST use a realistic estimate for the amount of metabolizable Met provided by the Met product that you are feeding. Over-estimating the bioavailability of some of the Met supplements has been way too common. This is unfortunate because it leads to disappointing production outcomes, and more often than not, leaves the nutritionist and dairy producer believing that balancing for Lys and Met has minimal impacts on animal performance.

5. Don't overfeed rumen undegradable protein. There are several disadvantages to overfeeding RUP. These include:

  • Lowered concentrations of Lys and Met in MP [because most sources of supplemental RUP are deficient in Lys, Met or both (fish meal is the only exception).
  • Lowered milk production (because surplus RUP usually replaces fermentable carbohydrates in the diet, the primary substrates for synthesis of milk components).
  • A more-expensive diet (because most sources of supplemental RUP are more expensive than most sources of NFC).
  • Increased urinary and fecal N (because of lowered conversions of feed protein to milk protein).

Identifying the optimum concentration of RUP in diet DM is challenging. As a first step, we believe it is critically important that one expresses RUP as a percentage of diet DM (as one does for RDP) and that one change it as dictated by animal performance. There is no logical basis for expressing RUP as a percentage of CP. RDP provides peptides, AA and ammonia for rumen microorganisms and RUP supplies intestinally digestible AA for the cow. Too often, when RUP is expressed as a percentage of CP, "more RUP" in a diet results in less RDP in the diet because there is a targeted level of ration CP that the nutritionist is trying to maintain. This approach is not consistent with balancing diets for RDP, RUP and AA. As a second step for identifying the optimum concentration of RUP in diet DM, we suggest that you let your cows tell you how much they need. The nutritional model that you use can be used as a guide for determining RUP requirements, but it should not be used to provide the final answer. There are two reasons for this recommendation. First, there are too many factors that determine the cows' requirement for RUP to allow the model to be very accurate. Three important factors affecting RUP requirements are:

  • Supply of microbial protein.
  • RUP digestibility.
  • AA composition of RUP.

Each of these factors can have a significant effect on how much RUP is needed. And second, current models do not adjust MP requirements, and thus RUP requirements, for changes in predicted concentrations of AA in MP. This is a serious deficiency and until models are designed to predict milk and milk protein yields from supplies of MP-Lys, MP-Met, or whichever AA is the most limiting, rather than MP per se, an effort is needed to modify models so that variable efficiencies of use of MP for milk protein synthesis are employed. Don't be surprised, as a result of balancing for Lys and Met in MP, how little RUP is actually needed in the diet. Field experience indicates that cows are more responsive to changes in diet RUP content when RUP has a good AA balance vs. when the balance is not good. This makes sense because the nutritional potency of the RUP is greater when it has a good AA balance vs. a poor AA balance.





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