Interpretive Summaries

 

 


METABOLIZABLE PROTEIN AND AMINO ACIDS FOR DRY COWS

Jim Aldrich, Ph.D., PAS Akey, Lewisburg, OH

 

Dry cows require protein (amino acids, AA) for maintenance, growth, gestation, mammogenesis, and perhaps growth of the dam. Amino acids are also important during transition for other metabolic functions such as VLDL synthesis. In addition, dry cows may deplete labile protein reserves to meet fetal needs if diets are inadequate in metabolize protein/AA. Several published studies have examined the effect of protein (protein level, RUP level, etc.) in transition with mixed results. Lack of response to protein/RUP level in some of these trials may have been caused by several factors, including treatment diets that did not increase metabolizable protein or limiting AA. Very few trials have looked at the effect of specific AA in transition.

 

There are several strategies for diet formulation for transition cows. The “traditional” close-up feeding strategy of feeding a higher energy diet has been challenged recently, and there has been a trend toward feeding higher forage/fiber diets pre-partum. An interesting exercise is to “model” how these two different feeding strategies (conventional higher energy vs. higher forage) might impact protein/AA balance. Using a factorial requirement system, with rumen microbial protein production predicted from carbohydrate digestion, shows that metabolizable protein supply (with similar dietary CP and protein sources) is lower on the high forage diet, mainly due to less microbial protein. This suggests that protein and AA supplementation may be more critical when transition cows are fed higher forage diets.

 

Furthermore, on either type of diet, other AA as well as lysine and methionine may be in short supply.  The AA composition of tissue, used to derive most of the requirement for dry cows, is different than milk. The efficiency of use of absorbed AA for protein synthesis is also generally lower for maintenance, growth, and gestation, than it is for milk synthesis. It is not surprising then, that the order of most limiting AA for dry cows may be different than for lactating cows.

 

Even though dry cow protein and AA requirements are low relative to lactating cows, more attention may need to be paid to metabolizable protein and AA in transition cow diets to optimize transition cow health and milk production in early lactation.

 

Jim Aldrich, Akey, PO Box 5002, Lewisburg, OH 45338-4031, PH: (937) 962-7038, FX: (937) 962-4031, EM: jaldrich@akey.com

 

Why Should We Be Interested in the Amino Acid Requirements of Dairy Cows?

Lou Armentano, 2005 Discover Conference

 

Dairy cows are an important source of protein for the human diet.  Dairy foods, beef and veal are proteins with high biological value for humans and provide foods with a wide diversity of desirable organoleptic properties.  The latter can be confirmed by anyone who has enjoyed a cheeseburger and shake for lunch or a fine dinner of veal parmigan with a desert of gelato or a cappuccino.  A machine that could process the raw ingredients in a dairy cows diet into the proteins found in milk, beef and new born calf would be considered amazing, even if the machine didn't manufacturer itself and its replacement.  What can we do to help this process happen in a way that is profitable and manageable for the farmer, efficient for the consumer, healthy for the cow, and kind to our shared environment?

            Like all living things, protein synthesized by the cow follows a genetic code that demands precise amount of the specific amino acids that make up these proteins.  The cow must assemble the precise amounts of each amino acid in order to conduct protein synthesis.  How can this happen so perfectly on such a wide variety of diets?  The fact is that animals have evolved to adapt their dietary proteins to their needs.  One major way they do this is to synthesize about half of the amino acids (the "non-essentials") present in proteins.  The rest of the amino acids are essential, and each is required in its own precise amount.  If supply of any one of these essential amino acids is inadequate, protein synthesis will be limited to the available supply of this amino acid because the cow cannot manufacture them in her body tissues.  In this case the cow, and her relatives, enjoy an advantage over other animals because they have a healthy population of microbes that can synthesize essential amino acids which are digested by the cow to supplement those present in the dietary rumen undegradable protein the cow eats.  Once these amino acids are absorbed the cow further tailors them to her needs.  What does a tailor do?  He cuts out ('wastes') excess material and sews the rest together.  Similarly the cow catabolizes excess essential and non-essential amino acids and combines the rest into milk and meat proteins.

            If all this sounds to good to be true, it is.  No biological process is perfect.  Feeds that we would like to use (available byproducts or crops that grow well in our climate) may be so low in certain essential amino acids (lysine, methionine and histidine primarily) that the microbes cannot add enough back to allow the cow to produce at her maximum. Even if we feed enough protein to meet the requirements set by the limiting essential amino acid, there will always be excess amounts of all the other amino acids to be catabolized because we don't (yet anyway!) control the amino acid content of the plants we grow to feed cows.  Also, in the process of the microbes doing their job, it is inevitable that a pool of ammonia will develop that the cow has to dispose of via her urine.  The end result is that just under a third of the nitrogen a cow eats every day ends up as food and the rest ends up as waste.  This waste (which bison, deer and antelope make just as well!) ends up in our air and marine ecosystems and can cause problems.  As practicing nutritionists we can help to minimize this waste by manipulating diets and diet constituents to do the following:

            maximize milk production, cow longevity, and efficiency of replacement raising

            meet the microbial needs for growth without creating excessive ammonia

            select protein sources and supplement that best complement the cows needs and the microbial contribution

            In the future, more control of the amino acid content of the feeds we grow for dairy cattle and understanding how to control excessive tissue catabolism of those amino acids that are most limiting in the diets may be possible.

Louis Armentano, University of Wisconsin Dept of Dairy Science, 1675 Observatory Drive, Room 266, Madison, WI 53706, PH: (608) 263-3490, FX: (608) 263-9412, EM: learment@facstaff.wisc.edu

 

Amino acid requirements for pregnancy and growth

Alan W. Bell & Michael E. Van Amburgh, Cornell University

 

Pregnancy and juvenile growth impose separate and, in the case of primiparous heifers, combined demands on amino acid supply to support protein synthesis and deposition in tissues of the conceptus and postnatal animal. During late pregnancy, amino acid requirements for mammogenesis and lactogenesis also must be considered. The notion that increased amino acid supply during the dry period can improve the capacity of the fresh cow to mobilize protein reserves during the early postpartum period has gained little experimental support and will not be discussed here.

 

Net requirements of individual amino acids for conceptus growth in multiparous Holstein cows increase through late pregnancy. The efficiency of utilization of absorbed amino acids (= metabolizable protein, MP) for conceptus growth has not been measured directly in cattle. Using a conservative factor of 33%, based on measurements in sheep and indirect evidence from cattle, the MP requirement for conceptus growth was estimated to be 350 g/d. Data for mammary growth and amino acid composition were used to estimate net amino acid requirements during late pregnancy. Application of an efficiency factor of 0.67 resulted in an estimated MP requirement of 120 g/d. Assuming the amino acid requirements for maintenance of a pregnant dry cow are the same as for a nonpregnant cow of the same body weight and dry matter intake, the total MP requirement of a multiparous Holstein cow during the close-up period is estimated to be ~1,100 g/d. This should be readily achieved with most conventional dry cow diets containing 12-14% CP.

 

The amino acid composition of ruminant tissues has been characterized and provides the basis for establishing net requirements for growth. Maintenance requirements and metabolism of absorbed amino acids provides a greater challenge for field application.

Previous estimates of amino acid requirements of the milk-fed calf, derived from several studies, were compared to our own recent estimates using original data for tissue amino acid composition and newly derived values for efficiency of use of absorbed protein. Estimates of requirements for all essential amino acids except phenylalanine were within 1 g/d of each other, giving confidence to these predictions. Application of Baker’s ideal amino acid ratio approach yielded ratios to lysine for the milk fed calf that were almost identical to those estimated for ruminant cattle. Data for ruminant cattle suggests that, under most feeding conditions, if metabolizable protein supply is adequate and cattle are fully fed, only methionine and cystine are limiting for growth. However, under conditions where metabolizable protein supply is not adequate or cattle are under limited intake conditions, other amino acids such as the branched-chain amino acids, glycine, and B-vitamin status will limit growth or negatively affect the metabolism and efficiency of use of absorbed amino acids.

 

Bell, Alan W., Department of Animal Science, Cornell University, Ithaca, NY 14853-4801, Tel: (607)255-2862, email: awb6@cornell.edu

 

Regulation of mammary amino acid utilization

Alan W. Bell & Dale E. Bauman, Cornell University

 

Protein has become the most valuable component of milk produced by dairy cows. Milk caseins, which account for about 80% of bovine milk protein, are especially valuable because of their high nutritional value and ideal properties for cheese manufacturing. Also, new procedures for separation and reconstruction of milk proteins offer the promise of unique and novel dairy products that will significantly add value to milk protein. Therefore, optimization of milk protein production has become a major objective of dairy producers and their advisors. This has led researchers to focus on nutritional approaches to optimize the supply and composition of amino acids to the mammary gland and, to a lesser extent, on factors regulating the uptake and intracellular utilization of amino acids by the gland.

 

Most amino acids supplied to the mammary gland in arterial blood are in free form and are taken up by membrane transport systems similar to those described in many other mammalian cell types. However, indirect evidence suggests that perhaps 10% of the total supply of mammary amino acids is derived from circulating small peptides that are hydrolyzed by peptidase enzymes at the cell membrane. Insufficiency of total amino acid supply or individual essential amino acids will cause a decrease in milk protein synthesis and secretion. However, in cows fed well-balanced rations adequate in rumen degradable and undegradable protein, attempts to increase milk protein synthesis by increasing arterial supply of amino acids have been largely unsuccessful. It is becoming clear that intracellular capacity for protein synthesis rather than arterial supply or membrane transport of amino acids is the major limiting factor for milk protein yield. Thus, variations in arterial supply can, within limits, be overcome by adaptations in mammary arteriovenous extraction of amino acids and blood flow. Milk protein synthesis is regulated directly or indirectly by several hormones. Most notably, it has been shown that in well-fed, high-yielding dairy cows and goats, milk protein yield can be increased 25-30% by treatment with insulin if the animal also is supplied with additional amino acids.

 

Demonstration that the capacity of the mammary gland for milk protein synthesis is far from maximized in well-managed dairy cows offers hope that identification of basic mechanisms for control of this process will lead to nutritional and genetic approaches to significantly increase milk protein yield. 

 

Bell, Alan W., Department of Animal Science, Cornell University, Ithaca, NY 14853-4801, Tel: (607)255-2862, email: awb6@cornell.edu

 

Mammary Gland Requirements: Amino acids for milk protein and much more!

Brian J. Bequette, University of Maryland, College Park

Milk protein is the most valuable component of fluid milk at the farm-gate whereas milk lactose is of lower value and of limited commercial use. Adding to the value of milk protein is casein’s high proportion of essential amino acids, in particular the branched chain amino acids. This gives casein a higher biological value than other protein sources (e.g. cereal and grain proteins). In this connection, consumption of milk and milk protein products have been shown to impart greater health benefits related to weight and fat loss, control of diabetes and hypercholesterolemia, and the building of muscle protein.

However, the US is a milk deficit nation, importing more dairy products than ever before in the form of milk protein concentrates. Strategies that increase milk protein and (or) reduce lactose contents will in consequence have economic benefits to dairy farmers and the industry. Over the past two decades, however, a survey of the concentrations of these milk components reveals little, if any, change. This despite advances in nutrition and genetic selection that have increased total milk yield. For example, there are numerous reports in the literature of small, or often no, increases in milk and protein yield (content) when supplemental dietary protein or “limiting” amino acids are given to dairy cows. Such observations serve to highlight that the knowledge is incomplete concerning how dietary nutrients influence or limit milk protein synthesis in the mammary gland.

Feeding ruminants to achieve desired milk component yields is complicated by two factors: 1) the high fermentative activity within the rumen that invariably changes the pattern of nutrients available for milk synthesis and 2) the scant knowledge of which and how much amino acids and energy the mammary gland requires to synthesis milk protein, fat and lactose. However, new findings have begun to reveal how the mammary gland orchestrates the metabolism of amino acids, glucose and fatty acids to achieve the fairly constant proportions of milk casein and lactose. In particular, the role of essential amino acids in general metabolism in the mammary gland, in addition to their use for milk protein synthesis. Although this process is highly interconnected and complex, at the least we now have a better description of the nutrients required for milk component synthesis and the boundaries for the potential to alter milk composition through nutrition and perhaps genetic selection.    

Brian J. Bequette, current address: Department of Animal and Avian Sciences, Room 4147 Animal Science Building, University of Maryland, College Park, MD  20742, Tel: (301) 405-8457, email: bbequett@umd.edu

Increasing Nitrogen Capture by Rumen Microorganisms

Glen A. Broderick and Santiago M. Reynal, US Dairy Forage Research Center, Madison, WI

 

Ruminants make efficient use of diets that are poor in protein content or quality because ruminal microbes synthesize good quality protein plus capture recycled urea N that would otherwise be excreted.  Lactating dairy cows use feed crude protein more efficiently than other ruminant livestock; however, dairy cows still excrete about 2-3 times more N in manure than they secrete in milk.  This happens in part because the rate of protein degradation in the rumen usually exceeds the rate of microbial protein synthesis.  Excess degraded protein is absorbed from the gastrointestinal tract as ammonia and excreted as urinary urea.  This is the form of excretory N most likely to cause environmental pollution.  Moreover, over feeding protein impairs energy status because protein sources typically have lower energy content and because of the metabolic cost of synthesizing excretory urea.  Feed proteins are also expensive.  So, economic and environmental returns accrue to maximizing microbial capture of degraded protein in the rumen.

 

Ruminal microbes can use of ammonia to meet part of their N needs, but only certain organisms, notably the cellulolytics, have absolute requirements for ammonia.  Urea or other sources of nonprotein N (NPN) that yield only ammonia in the rumen have been fed successfully under some circumstances; however, animal performance is usually poorer than on true proteins.  This is because proteins can partly escape the rumen, contributing amino acids directly to the animal, and because amino-N substantially stimulates microbial protein formation.  A full complement of protein amino acids appears to be required to maximize microbial protein yield in the rumen.  Research on NPN utilization has focused on the level of ammonia needed to optimize ruminal function.  Although rather high ammonia concentrations are necessary to maximize in situ digestion, in vitro evidence with pure and mixed cultures of ruminal organisms indicates that the ammonia “requirement” for maximal microbial protein yields is 5 mg/dl or less.  Protein degradation confounds effects attributed to ammonia concentration and amino acid supply in most experiments; degradation of true proteins contributes to both the amino-N and ammonia pools.  Replacing dietary urea with true proteins, even very degradable proteins, results in large improvements in animal performance as well as microbial efficiency and yield in the rumen.

 

Microbial protein formation was measured in a number of trials using 15N-ammonia to label the microbes plus omasal sampling to measure ruminal out-flow in lactating dairy cows.  Response of microbial protein flow to rumen-degraded protein (RDP) was quadratic, with maximum at about 20% RDP in dietary DM; however, microbial protein increased only about 20% as RDP increased from 10 to 20% in the diet.  This suggests that less than maximal microbial yields are obtained on typical diets and emphasizes the value of feeding rumen-undegraded protein (RUP) with complementary amino acid patterns.  Microbial intervention makes it difficult to determine precisely the amino acid requirements of the cow, either by direct amino acid supplementation or by feeding RUP with differing amino acid patterns.  A number of strategies can be used to increase microbial protein supply.  These include reducing grain particle size to increase ruminal starch digestion, feeding corn silage with hay-crop silages, adding sugars up to about 5% of dietary DM, and replacing hay-crop silage with hay.  Protozoal predation also appears to reduce microbial protein yields in the rumen.  New research is required to determine what accounts for the wide variations in observed microbial efficiencies and to identify RUP and protected-amino acid supplements that will complement microbial protein under normal feeding regimes.

 

Glen Broderick, USDA - ARS US Dairy Forage Res Center, 1925 Linden Drive West, Madison, WI 53706, PH: (608) 264-5356, FX: (608) 264-5147, EM: gbroderi@wisc.edu

 

REGULATION OF SPLANCHNIC AMINO ACID METABOLISM

Douglas Burrin, Ph.D.

USDA-ARS Children’s Nutrition Research Center, Department of Pediatrics

Baylor College of Medicine, Houston, Texas

 

The splanchnic tissues, namely liver and gut, play a major role in the regulation of whole body protein and amino acid metabolism. Given their anatomical design for assimilation of food by the animal, these tissues metabolize in first-pass a significant proportion of the dietary amino acids via protein synthesis and oxidation and thereby limit the quantity and alter the pattern of amino acids for systemic availability and productive tissue growth, muscle and milk. This substantial “metabolic cost” incurred by splanchnic tissues is generally attributed to maintenance amino acid requirement and largely related to their critical physiological functions, such as digestion, ureagenesis, gluconeogenesis, and acute-phase protein synthesis. Splanchnic tissues also play a key regulatory role by transmitting endocrine, immune and neural signals in response to the diet and environment, which in turn determine the rates of peripheral tissue protein metabolism and growth. Splanchnic tissue protein metabolism is regulated by dietary intake and hormones. However, amino acids also function as extracellular signals that influence cell metabolism, such as protein synthesis, proliferation, and cell survival. Establishing the metabolic fate of dietary amino acids used by splanchnic tissues is a key to understanding their requirement and availability for milk production in dairy cows. 

Douglas Burrin, Research Physiologist, USDA-ARS Children's Nutrition Research Center, 1100 Bates Street, Room 10068, Houston, TX 77030, PH: (713) 798-7049, FX: (713) 798-7057, EM: dburrin@bcm.tmc.edu

 

PRACTICAL AMINO ACID FORMULATION

 

Spence Driver

Vita Plus Corporation

 

Computer models have advanced to the point of predicting how well a diet supplies protein and amino acids for milking herds.  Although these models are far from perfect, they represent big steps forward in evaluating diets for rumen degradable protein RDP, rumen undegradable protein RUP, and amino acids and have been useful for improving the efficiency of conversion of feed crude protein to milk protein on many dairy farms.

 

We have been able to lower feed cost and/or improve productivity with the use of modeling.  I have shown a couple examples in the power point presentation. Most dairymen today have taken a cautious approach to modeling and amino acid technology due to the milk prices of late and the relatively low protein costs.  Nutrient management issues will most likely drive the movement to balance diets more efficiently.

 

The scientific community must continue to improve the model in order for it to be most effective in the industry.  Currently, the weakest link is the limited data used to generate the feed file and the inability to analyse feed commercially.

 

Spence Driver, Vita Plus Corporation, PO Box 259126, Madison, WI 53725-9126, PH: (608) 250-4227, FX: (715) 445-4299, EM: sdriver@vitaplus.com

 

Modeling Amino Acid Needs of the Dairy Cow

Mark D. Hanigan, Virginia Tech

 

Efficiencies of conversion of dietary nitrogen to milk nitrogen in the lactating cow are relatively poor as compared to other species.  To improve nitrogen efficiencies, a better understanding of nitrogen metabolism is needed, and this knowledge must be reflected in our requirement models.  Nitrogen requirement models currently in use in the dairy industry are aggregated at the protein level and generally do not reflect metabolism of individual amino acids.  There appear to be several deficiencies with the current prediction system that are likely biasing those predictions.

 

Significant blood urea nitrogen can be recycled to the rumen when RDP supply is low.  Such recycling apparently is adequate to maintain maximal microbial growth rates at RDP levels much less than current requirements.  Thus RDP requirements should be re-evaluated.  Accuracy of RUP flow predictions do not appear to benefit from use of tabular degradation rate data suggesting that tabular values in use may not reflect variation inherent in ingredients used. 

 

Models of amino acid metabolism at the tissue level are being developed, but additional work is needed.  Small intestinal amino acid flows can only be as accurate as the basal protein flow predictions given the current prediction scheme.  Splanchnic tissues appear to remove essential amino acids by mass action.  Thus the assumption of a fixed maintenance requirement for metabolizable protein or amino acids misrepresents the system resulting in prediction errors.  Provision of more protein in support of greater amino acid needs will result in greater losses of amino acid to splanchnic metabolism. 

 

Mammary amino acid transport activity appears to be highly regulated such that transport activity compensates for changes in blood amino acid concentrations suggesting significant flexibility in requirements.  Further improvements in nitrogen efficiency could be realized if regulatory mechanisms were leveraged to maintain milk protein output as systemic amino acid supply was reduced.  In such a scenario, mammary amino acid transport activity should increase to compensate for declining blood concentrations resulting in less recycling of amino acids to the splanchnic tissues and thereby less catabolic losses at that tissue bed.

 

Hanigan, Mark D., Address: Department of Dairy Science, Virginia Polytechnic and State Institute, Blacksburg, VA  24061, Tel: (540) 231-0967, email: mhanigan@vt.edu.

 

 

QUANTIFYING PASSAGE OF NITROGEN FRACTIONS TO THE SMALL INTESTINES – APPROACHES AND CHALLENGES

Ignacio R. Ipharraguerre, Cargill Animal Nutrition

Santiago M. Reynal, University of Wisconsin

 

The supply of metabolizable protein to dairy cows comprises proteins of microbial, feed, and endogenous origin that pass from the forestomach and are absorbed as amino acids from the small intestines.  Significant efforts have been conducted to understand and quantify the passage of microbial and feed protein to the intestines of dairy cows.  A salient characteristic of these estimates is their highly variable nature, which arises from the technical challenges encountered when measuring the flow of nitrogen though the digestive tract and the numerous factors that modulate this process.  In contrast, the intestinal flow of endogenous nitrogen has received much less attention and consequently published data is very scant. These limitations hamper the capacity of current protein systems to predict precisely and accurately the intestinal supply of amino acids in dairy cows.  However, precise and unbiased estimates are a prerequisite for improving the efficiency of nitrogen use for milk production and thereby reducing the potential contribution of dairy farming to nitrogen pollution.

The quantitative integration of nitrogen flow data (i.e., meta-analysis) allows identifying sources of variation and estimating their impact on the magnitude of reported responses.  However, neither meta-analysis nor modeling allows correcting for bias in the original data.  Therefore, the emphasis should be on (1) improving and/or developing more precise approaches for measuring the passage of nitrogen fractions to the intestines and (2) better understanding and quantifying the dynamics and kinetics of nitrogen transactions in the digestive tract that compromise the accuracy of current data.  Considerable research aiming to address these needs is currently being undertaken.

Results from the ultrafiltration of digesta flowing at the omasum indicate that the ruminal outflow of soluble amino acids of dietary origin may represent a significant proportion of the passage of total amino acids to the intestines.  This challenges the assumptions embedded in most protein systems that dietary soluble nitrogen and the portion of feed protein that is degraded in the rumen are entirely used for microbial protein synthesis or production of ammonia.  Before these findings can be broadly accepted, however, the techniques used in this experimentation (i.e., omasal sampling and ultrafiltration) need more validation.  Advancements on the identification of errors associated with digesta-sampling techniques and traditional microbial markers as well as the development of new microbial markers and isolation methodologies can be used to improve the quantification of microbial protein reaching the small intestines.  More important, some of these techniques can be combined to estimate the contribution of different microbial groups or even targeted microbial populations to the postruminal supply of nutrients (amino acids, fatty acids, and carbohydrates).  Likewise, emerging data show that the contribution of endogenous nitrogen to the delivery of amino acids to the absorption sites in the small intestines is substantial.  This research provides an application of the isotope dilution technique to quantify the input of different nitrogen pools to the intestinal flow of endogenous nitrogen and elucidate the impact of dietary factors on this relevant component of the protein supply to dairy cows.  

 

Ipharraguerre, Ignacio R. Cargill Animal Nutrition, Innovation Center, 10383 – 165th Avenue NW. Elk River, MN 55330. Tel: (763) 274-3066; email: Ignacio_Ipharraguerre@Cargill.com 


 Measuring UIP/DIP and Intestinal Digestibility
An Industry Perspective
(Wish List)

 

Phillip W. Jardon, West Central

                                                    

Producers of feeds for the dairy industry need better tools for describing and improving their products.  They need tools for accurate representative values for ration software, tools for research and product development, and tools for marketing.

 

There is a need for rapid accurate inexpensive methods for determining rumen undegradable protein (RUP) and intestinal digestibility (ID).  Tests currently available are slow, inaccurate, and/or expensive, making R & D efforts difficult.

              

Phillip W. Jardon, West Central, 406 First Street, PO Box 68, Ralston, IA 51459, PH: (712) 667-3200, FX: (712) 667-3215, EM: phillipj@westcentral.net

 

MAINTENANCE REQUIREMENTS

Hélène Lapierre1, Daniel R. Ouellet1, and Gerald E. Lobley2

1Agriculture and Agri-Food Canada; 2Rowett Research Institute, UK

 

Dairy farming has to be profitable and yet sensitive to environmental concerns. Both can be achieved through a lower input of dietary protein if feeds were better balanced to provide the correct mixture of amino acids (AA: the building blocks of proteins) needed to synthesize milk protein. That such a strategy can be very effective is shown by the pig and poultry industry who instigated such approaches several years ago and have benefited from improved cost effectiveness and a reduction in environmental pollution. Although recent dairy cow nutrition models (e.g. NRC, 2001 and CNCPS, 2000) now include AA nutrition, these models still under-predict milk protein yield at low protein supply and over-predict milk protein yield at high protein supply.  Prediction of milk protein yield is based on subtracting from the metabolizable protein (or AA) supply that required to support maintenance and then applying to the remainder a fixed factor of conversion. Therefore, there are two possible reasons for  the discrepancy between observed and predicted at low or high protein supply: (i) incorrect definitions and calculations of maintenance requirements especially when applied  to changes in nutritional status; (ii) inappropriate use of  a fixed factor of conversion to milk across a wide range of protein supply.  The first option will be examined in this presentation, while the second will be discussed in a second talk.

            Although the concept of maintenance fits a growing animal (i.e. supply that will provide no gain or loss of body mass), such a concept is “virtual” for a lactating animal as the cow will use body reserves before stopping milk production. Therefore, a pure “zero N balance” cannot be achieved but only derived by extrapolation. The current NRC (2001) defines maintenance requirements as a composite of scurf, endogenous urinary, metabolic fecal protein (MFP) and endogenous secretions in the duodenal flow. In this presentation, this latter component will not be integrated as it will neither be included into the supply of MP, as this is not a net supply but just recirculation of already absorbed AA. For the other components, and based on the studies analyzed  in Doepel et al. (2004), scurf represents 3% of total maintenance requirements, endogenous urinary  19% and MFP 79%. MFP could be replaced by a new model described in lactating dairy cows to estimate fecal endogenous losses (Ouellet et al., 2002 & 2005).

            In NRC (2001), while both scurf and maintenance urinary are based on body weight, MFP is based on dry matter intake (DMI). This means that the derived maintenance requirements include a large proportion related to the high DMI of a lactating animal. As the “maintenance” status is virtual for a lactating animal, there really is no need to spend time, effort and money to discriminate within each loss which fraction is attributable to maintenance and which relates to the high intake needed to support milk production. Instead, we could approach the problem differently and ask : “Why not also evaluate urinary N excretion for a lactating dairy cow fed a dairy ration and call this output  “metabolic losses”?”. Such an integrated approach would allow us to account for the decreasing marginal efficiency of transfer of AA supply to AA into milk with increased protein supply. If this approach is chosen, a major challenge will be to predict those urinary N losses derived from AA catabolism, excluding those from ammonia detoxification and purine derivatives. Whatever approach is chosen, other challenges will include developing knowledge of factors that affect MFP (fecal endogenous losses) and to more accurately define the AA composition of each fraction comprising the maintenance requirements.

 

Helene Lapierre, Agriculture & Agri-Food Canada, C.P. 90, 2000 route 108 est, Lennoxville, Québec, Canada  J1M 1Z3, PH: 819-565-9171 poste 234, FX: 819-564-5507, EM: lapierreh@agr.gc.ca

 

MILK PROTEIN SYNTHESIS AS A FUNCTION OF AMINO ACID SUPPLY

Hélène Lapierre1, Gastón Raggio2, Lorraine Doepel3 and David Pacheco4

1Agriculture and Agri-Food Canada; 2Université Laval, Qc, Canada;

3University of Alberta, AB, Canada; 4AgResearch, New-Zealand

 

To the continual challenge of making dairy farming more cost effective is now added the pressure to reduce environmental pollution from nitrogen derived from animal waste. Both targets could be reached through a lower input of dietary protein, provided productivity is not compromised. This could be achieved by use of diets better balanced in the supply of amino acids (AA) needed for both maintenance and milk protein (the maintenance issue has been addressed in a previous presentation). This approach is now being introduced for ruminants although the most recent NRC (2001) only expresses recommendations for two AA, lysine and methionine. Such recommendations were established on empirical observations between measured or predicted intestinal AA flow vs. milk protein output and have focussed on setting the amounts of these two AA relative to metabolizable protein (MP) supply (Rulquin et al. 1993; Schwab 1996). Other dairy ration programs estimate the supply of individual essential AA to the small intestine from intake of dietary protein and carbohydrate fractions and then applying an intestinal digestibility of the RUP and RDP fractions flowing to the small intestine to determine availability to the animal. They then base their recommendations for AA limiting milk production on the AA available for lactation (less maintenance needs) to that required for milk protein using a fixed conversion factor for each AA.  The CNCPS has assigned this conversion factor to milk protein from reported literature values of mammary gland uptake to output in milk, which range from 62% (valine) to 100% (methionine). Similarly, the NRC (2001) defines the requirements of MP based on a fixed conversion factor of 67% of MP available for milk into milk protein. This approach assumes a linear relationship between supply of AA and recovery into milk, independent of supply. However, it is recognized that the marginal recovery into milk declines as protein supply is increased. For example, a review of the recovery of infused casein into milk protein gave an average of 21% (Hanigan et al. 1998), far from the fixed 67% used by NRC (2001). It is also known that milk protein yield is usually underestimated at low protein supply and overestimated at high protein intake. This raises the important questions “What explains these changes in marginal recovery?” and “Can the responses be reliably predicted and incorporated into future ration formulation schemes?”

Two approaches were used to demonstrate that the return into milk of available AA declines as protein intake increases. First, based on measurement of AA absorbed into the portal vein and resulting milk protein yield, it was shown that the ratio of AA secreted into milk vs. AA absorbed decreased with increased supply and differed between AA (Raggio et al., 2004). The second approach was based on assembling published results of studies where protein or AA were infused. Similar to the first approach, it was shown mathematically that the incorporation of AA available for lactation into milk decreased as supply of AA increased, with conversion factors also differing between AA (Doepel et al., 2004).

Although we still need more research to fully understand the regulation of AA metabolism, there is now enough evidence to categorically state that the efficiency of AA utilization decreases as supply of protein increases.  There are also sufficient data to delineate these changes and integrate a variable coefficient of transfer in the models.  This approach, or the definition of “metabolic costs” as proposed in the presentation on maintenance, urgently needs to be embraced to offer to dairy producers a better assessment of the impact of a reduction in the protein content of the rations.

 

Helene Lapierre, Agriculture & Agri-Food Canada, C.P. 90, 2000 route 108 est, Lennoxville, Québec, Canada  J1M 1Z3, PH: 819-565-9171 poste 234, FX: 819-564-5507, EM: lapierreh@agr.gc.ca

 

Amino Acid Balancing for Dairy Rations—A Consultant’s Perspective

Robert A. Patton, PhD, PAS

Nittany Dairy Nutrition, Inc

Mifflinburg, PA, USA

 

Balancing the amino acids in the ration of dairy cattle has the potential to increase production of milk and milk protein, reduce protein costs for dairymen and decrease the amount of nitrogen spilled into the environment by dairy cattle.  However, before this can come to fruition, several gaps in our knowledge need to be filled and these need to be priorities of future research efforts.

 

Suggested research efforts include the following.  (1) Establish true amino acid and metabolizable protein requirements of lactating cattle.   (2) Define the role of amino acid ratios in describing the effects of amino acid requirements.  (3) Investigate other factors that potentiate the effects of amino acid balance so that milk and milk protein are increased or maintained on diets of lower protein content.

 

Additionally management practices must be changed so that rations can be balanced for the actual production of the group of cows.  This will include determination of actual group dry matter intake and abandonment of the one total mixed ration strategy in favor of feeding the cow or groups of cows to more specific requirements.

 

The widespread implementation of amino acid balance in dairy rations will depend on the profit that can be returned to the dairy in terms of greater production or reduced costs.

 

Robert Patton, Nittany Dairy Nutrition Inc., 9355 Buffalo Road, Mifflinburg, PA 17844, PH: , FX: , EM: nittnut@aol.com

 

Absorption and Delivery of Amino Acids by Splanchnic Tissues

C. K. Reynolds

 

An enlightened understanding of the metabolic use of amino acids within specific tissues of the body is needed to better define individual amino acid requirements of the dairy cow more precisely thus feed dairy cows more profitably with less environmental loss.

 

The splanchnic tissues are crucial to amino acid supply in mammals.  They are comprised of the portal-drained viscera and liver.  The portal-drained viscera includes the gut, as well as the pancreas, spleen and lots of adipose.  The anatomical position and function of these tissues makes them THE determinant of the supply of nutrients, including amino acids, for milk production and maintenance in lactating dairy cows.  As a consequence of their many critical functions, including digestion and nutrient absorption, they have a very high rate of metabolism and protein turnover, thus consume a disproportionate amount of oxygen on a mass basis relative to other body tissues.  The high rate of metabolism of the gastrointestinal tract requires fuel, and amino acids are catabolized to support the metabolism of gut tissues, to varying extents depending on the amino acid and the nutrition of the cow.  In addition, the liver, which sits astride the flow of nutrients from the gut, is an important site of amino acid metabolism and catabolism, integrating amino acid supply with the demands of other body tissues.  Because of this high rate of metabolism, and the anatomical location of the PDV and liver, it is generally assumed that the metabolism of the gut and the liver restricts the supply of amino acids to peripheral tissues, thus limiting milk protein synthesis.  This concept is supported by measurements of the net absorption and post-hepatic delivery of amino acids, which are generally much lower than amounts absorbed from the small intestine.  However, these measurements mask true rates of amino acid absorption to the extent that they are used during their absorption and extracted from arterial blood.  Measurements of amino acid metabolism by the portal-drained viscera, obtained using isotopic labeling, suggest that the true rates of absorption of most essential amino acids are masked by extensive uptake of amino acids from arterial blood by the rumen and other PDV tissues which do not have access to amino acids during their absorption.  Similarly, liver extraction of amino acids is much lower when expressed relative to total supply in blood, thus the majority of the amino acids removed are derived from the arterial pool.  Available data suggest that the fractional rate of amino acid catabolism within the gut and liver is determined to a large extent by the balance between amino acid supply and requirement for production. 

 

In addition to their roles as precursors for protein and metabolic fuels, most amino acids can also contribute carbon for glucose synthesis in the liver.  Therefore, it is often assumed that glucose synthesis limits milk protein synthesis.  However, increased supply of nonessential amino acids seldom increases liver glucose output, and in cows fed protein deficient diets mesenteric vein infusion of nonessential amino acids decreased milk protein concentration and yield, suggesting that amino acid use for glucose production in the liver does not limit milk protein production.

 

Chris Reynolds,  Department of Animal Sciences, The Ohio State University, OARDC, 1680 Madison Ave., Wooster, OH, 44676.  Reynolds.345@osu.edu.  330-263-3793 Fax: -3949

 

AMINO ACID REQUIREMENTS OF DAIRY COWS: GENERAL PERSPECTIVE

 

Charles G. Schwab

Department of Animal and Nutritional Sciences

University of New Hampshire

 

A goal in balancing diets for lactating dairy cows is to meet their N requirements for a desired level of milk or milk protein production with a minimum amount of dietary crude protein (CP).  This requires meeting but not exceeding two sets of N requirements: the N requirements of rumen microorganisms and the amino acid (AA) requirements of the cow.  The N requirements of rumen microorganisms are met by ammonia, AA, and peptides.  These are provided by microbial breakdown of proteins and recycled urea.  The AA required by the cow are supplied by ruminally synthesized microbial protein, rumen-undegraded feed protein (RUP), and endogenous protein.    

 

The Dairy NRC (2001) model predicts dietary requirements for RDP and RUP but makes no adjustments to RDP requirements for quality or source of RDP, or to RUP requirements for differences in AA composition.  The model predicts metabolic requirements for AA as MP (total absorbed AA) rather than on the basis of individual AA.  This was done because the AA requirements of the dairy cow had not been well- defined.  However, the NRC (2001) model was designed to predict the content of essential AA (EAA) in MP as well as to predict flows of EAA to the small intestine.  This was done so that the model could be used to define the optimal concentrations of EAA in MP for milk and milk protein production and to assist in formulating diets to achieve a desired profile of EAA in MP.  The model was used in conjunction with published experiments to generate requirements for lysine (Lys) and methionine (Met) in MP for maximal milk protein production.  Research and field experience with the model have shown that balancing diets for more adequate concentrations of Lys and Met in MP increases milk protein production, reduces the need for supplemental RUP, allows for more efficient use of MP for milk protein production and can increase herd profitability. 

 

NRC (2001) has provided the user some opportunity to balance diets for adequacy of limiting AA.  However, feeding dairy cows according to AA requirements requires a diet evaluation model that is accurate in its prediction of: 1) dietary supply of RDP and RUP, 2) N recycling and availability of N substrates from recycled N, 3) microbial cell yield from fermentable organic matter, 4) requirements of rumen microorganisms for ammonia, AA, and peptides, 5) microbial protein synthesis, 6) supply of absorbable AA from microbial protein and RUP, and 7) the AA requirements of the cow. 

 

Understanding and Improving

Practical Amino Acid Balancing of Rations for Dairy Cows

 

Brian K. Sloan, PhD

Adisseo USA Inc

 

In the dairy feed industry, when we contemplate making a ration change, we want to be able to predict the likely response. For many of us our reputation and livelihood depend on us being able to demonstrate within a relative short period of time that any change we propose is cost effective. This is a different mindset that simply trying to meet requirements for a given level of production.

 

We have a tendency in the feed industry and research to be over optimistic concerning the improvements in milk production and milk components that can be expected to enriching the levels of potentially limiting amino acids in dairy rations. This is because there is the simplistic assumption, that the efficiency of utilization of the extra quantities of limiting amino acids, we add to a ration will be similar to the overall utilization factor used for MP in the currently used models (CNCPS – 0.65, NRC – 0.67).

 

In fact the marginal efficiency of amino acid utilization appears to be closer to 0.2 for methionine and 0.4 for LYS. Thus when MET is limiting and MET supply is increased by 7g per cow per day, a reasonable expectation should only be an increase in 50g of milk protein which is still a very attractive financial proposition (+25 cents).

 

Nevertheless, even though the marginal utilization of the limiting amino acids may be low, the utilization of all the other amino acids is improved. The consequence is that the efficiency of utilization of MP for milk protein synthesis of a poorly balanced ration can be as low as 0.55, but can be as high as 0.70 when a ration is balanced for LYS and MET.

 

The challenge is to take amino acid balancing of dairy rations to the next stage, where we can predict more accurately the benefit to a change in dietary supply of a limiting amino acid in conjunction with minimizing dietary N inputs. To do this a better understanding is needed of the factors (physiological status, stage of lactation, BST use, energy supply/sparing effect, vitamin co-factors…) that can modulate the response to increasing the supply of limiting amino acids.

 

Sloan, Brian K., Manager Methionine Products for Ruminants, Adisseo North America, 3480 Preston Ridge Road, Suite 375, Alpharetta, GA 30005. Tel. 678-339-1501. Fax. 678-339-1601. E-mail brian.sloan@adisseo.com

 

PREDICTING DIGESTIBILITY OF DIETARY RUP

AND ITS CONSTITUENT AMINO ACIDS

Marshall D. Stern, University of Minnesota and Sergio Calsamiglia, Universitat Autonoma de Barcelona

 

In the last twenty years, elaborate models have been developed to formulate diets with greater accuracy in meeting the protein and amino acid requirements of lactating dairy cattle. Models for feeding protein to dairy cattle in the USA evolved from basic crude protein (NRC, 1978) to more complex systems based on rumen undegraded protein (RUP) and intestinal digestion of RUP (NRC, 2001).  The NRC (1989) recognized that intestinal digestion of protein supplements differs; however, empirical data were lacking and as a result, a constant value of 80% was used for all feeds.  The main problem was the lack of reliable techniques for estimating intestinal digestion of proteins.  With improved techniques, the NRC (2001) assigned estimates of intestinal digestion to RUP fractions for each feedstuff.  Data were obtained mainly from literature

 using the mobile bag technique and a three-step in situ/in vitro procedure (TSP) using pepsin-pancreatin to predict digestibility of RUP.

 

Various criteria can be used to select the protein supplement for lactating dairy cows including palatability, RUP, protein quality, intestinal absorption of protein, availability and consistency of product, cost of protein and impact on animal performance. If RUP and intestinal protein/amino acid digestion data were readily available, an alternative method could be to determine cost of intestinally absorbable dietary protein (IADP) or amino acids.

 

Because a considerable amount of variation in intestinal protein digestion has been observed among and within feedstuffs using the TSP procedure, there is a need to expedite the procedure.  A study was conducted to modify the TSP procedure using protease in vitro to eliminate the in situ rumen fermentation step, thereby eliminating the use of ruminal cannulated cows. Results indicated that the modified TSP procedure could accurately predict intestinal protein digestion for barley, oats, wheat and heat-treated soybeans compared with the in situ mobile bag technique. However, when the modified TSP procedure was compared with the original TSP procedure using 36 different feedstuffs, there was a poor relationship (R2 = 0.26) between procedures. A shortcoming of the TSP procedure is the inability to determine individual amino acid absorption because of the use of trichloracetic acid (TCA) to precipitate protein. As a result, the TSP was modified by adapting it to a Daisy II incubator with the objectives of reducing cost and labor involved in determination of intestinal digestion of proteins and to be able to estimate intestinal digestion of individual amino acids.

 

Questions that need to be addressed: 1) How do we communicate to the feed industry that there is a need to improve quality control, providing consistent products to producers? 2) Can cost/IADP or individual amino acid be used as a tool for determining protein supplementation to the dairy cow diet? 3) Can we replace in situ ruminal digestion with an in vitro procedure and what can we do to expedite and improve estimation of intestinal digestion of RUP? 4) Is the modification of the three-step procedure using the Daisy II incubator the current and future method of choice for estimating intestinal protein digestion?

5) Can the modified three-step procedure using the Daisy II incubator be used to provide reliable information on individual amino acid absorption?

 

Stern, Marshall D., Department of Animal Science, 180A Haecker Hall, University of Minnesota, Tel: (612-624-9296), email: stern002@umn.edu.

 

REGULATION OF AMINO ACID UPTAKE AND UTILIZATION

 IN THE PORCINE MAMMARY GLAND

 

Nathalie L. Trottier, Department of Animal Science, Michigan State University

 

Milk yield and composition are two of the most important factors limiting neonatal growth.  In particular, because of high selectivity for piglet growth potential, decreasing neonatal deaths due to improved management practices, and early weaning, sow milk production and milk protein yields during the early phase of lactation is insufficient to maximize growth of nursing piglets.  In recent years, nutritional approaches to maximize milk production, in particular dietary amino acid supplementation and ideal amino acid profile, have either provided inconsistent responses.  This is due in part to our lack of mechanistic understanding behind dietary amino acid utilization. With increasing pressure from environmental organizations and future legislations to reduce nutrient losses from livestock operations, amino acid supplementation will play a critical role in sustainability of swine production.  Thus, our understanding of the role of dietary amino acids beyond empirical work will become essential.  Part of this mission will require a coordinated effort to elucidate the mechanisms regulating the rate limiting steps during milk protein synthesis processes, namely, amino acid delivery to tissues, amino acid transport, intracellular metabolism, transcription and translation. 

 

Despite the central role of amino acids, little is known about the regulation of amino acid transport in tissue, including in important biomedical models of protein synthesis, secretion and cancer cell growth. The intracellular availability of amino acids for protein synthesis, cellular remodeling, and growth is determined in part by the presence and activity of a range of very complex amino acid transporter systems and proteins.  There is a dearth of information on amino acid transport and utilization in the porcine mammary gland.  Yet, of all organs, the mammary gland has the highest demand for amino acids to meet the requirements of both milk protein synthesis and tissue growth for the nursing neonate. In recent years, studies have shed some light on the regulation of amino acid transport across the porcine mammary gland.  Amino acid transport responds differentially to physiological and nutritional factors such as litter size, stage of lactation, and protein/amino acid nutrition as evidenced by arteriovenous difference, mammary tissue explant, and molecular studies.   Of particular physiological and nutritional importance is the response of amino acid transport to amino acid availability and(or) protein. Amino acids themselves act as inhibitors or stimulators of amino acid uptake via competitive and non-competitive inhibition, adaptive regulation and signal initiation.  Interaction between large neutral amino acids, in particular isoleucine and leucine, and transport of cationic and small neutral amino acids by mammary tissue may impact the efficiency of dietary protein and amino acid utilization.  Low milk yield, amino acid arteriovenous difference, and changes in amino acid transporter gene expression responses to overfeeding and underfeeding protein, and to dietary amino acid imbalance in lactating sows must be in part related to the complex interaction between amino acids and their transport sites.  Knowledge of transport regulation will (1) elucidate the biological significance behind balancing amino acids relative to the first limiting amino acid in diets; (2) enhance our understanding of the factors controlling milk protein synthesis in both agriculturally valuable animals and humans; (3) allow the development of potential interventions to stimulate or inhibit expression of transporters and other related proteins that have selective effects on mammary secretory cell synthetic and proliferative activities.

 

Nathalie L. Trottier, Associate Professor, Department of Animal Science, Michigan State University, East Lansing, MI 48824. Phone: (517) 432-5140; Fax: (517) 432-0190; e-mail: trottier@msu.edu.

 

 

 

RUMEN NITROGEN DYNAMICS

R. John Wallace, Rowett Research Institute, Aberdeen, UK

 

The forestomach anatomy of the ruminant evolved as a mechanism by which the passage of plant fibre is slowed, in order to give the symbiotic microorganisms present in the rumen and reticulum time to digest their recalcitrant substrate.  An additional advantage was that the microbial protein formed as the result of the fermentative process passed from the rumen, to be digested and its component amino acids absorbed further down the intestinal tract.  In this respect, ruminants have an advantage over hindgut fermenters, where microbial protein is voided in feces.  The long retention time has unwelcome consequences, however, when dairy farmers wish to increase amino acid flow to the cow by increasing the protein content of the feed, because the protein is subject to degradation in the rumen.  In order to maximise nitrogen retention in ruminants, for environmental as well as economic reasons, we have to understand the properties of proteolysis by ruminal microorganisms and the amino acid requirements of the microorganisms for optimal fermentation rate and growth.

 

There are four broad categories of proteolysis in the rumen: breakdown of feedstuff protein by microbial activities, breakdown of bacterial protein by ciliate protozoa, bacterial invasion and digestion of ruminal epithelial tissue, and proteolysis by endogenous plant proteinases. Some peptides released by proteolysis accumulate, but for the most part the breakdown of peptides and amino acids to ammonia is a rapid process.  Various methods are available for slowing all of these processes.

 

Although ammonia can be used as the sole source of N for the growth of the mixed ruminal population, individual species have an absolute requirement for various amino acids.  Perhaps of greater relevance to dairy cow nutrition is that amino acids increase both the fermentation rate and growth yield of the mixed community of ruminal microorganisms.  The stimulation is highly dependent on the energy source: if an energy source is so slowly degraded that it permits only slow growth rates, amino acids may not enhance growth rate; the opposite may be true if the energy source is degraded more rapidly. 

 

Isotope-incorporation experiments suggest that certain amino acids may be more limiting than others, however no single amino acid or group of amino acids can reproduce the stimulatory effects of a complete amino acid mixture.  Tracer studies using combined 15N and 13C labels illustrate that the N- and C-components of amino acids are metabolised and incorporated in different ways.  Thus, conclusions based on only one type of label need to be interpreted with caution.

 

John Wallace, Rowett Research Institute, Bucksburn, Aberdeen, Scotland, AB2 9SB UK, PH: 1224-716656, FX: 1224-716687, EM: John.Wallace@rri.sari.ac.uk