用氨基酸平衡奶牛日粮（英文）

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Balancing Rations on the Basis of Amino Acids: The CPM-Dairy Approach



William Chalupa and Charles Sniffen  
Global Dairy Consultancy Co.; Ltd
P.O.  Box 153 Holderness NH 03245
http://www.globaldairy.net/
Corresponding Author:chalupa@cahp.vet.upenn.edu  




Summary 

Amino acid supply to the mammary gland directly affects milk protein synthesis and milk volume and indirectly impacts productivity by affecting metabolism and immune function. The complexity of balancing rations on the basis of amino acids requires computer models.CPM-Dairy calculates amino acid requirements by a factorial method and by an ideal protein method. Balancing rations on the basis of amino begins with optimizing production of microbial protein in the rumen. Next, rumen outputs of amino acids are complemented with dietary sources that escape ruminal fermentation. Feeds with high concentrations of methionine and lysine in metabolizable protein are helpful but single sources of rumen-protected amino acids are usually needed.


Introduction
Like other mammalian species, the dairy cow's requirement for protein is a requirement for specific amounts and balances of amino acids. Amino acid supply to the mammary gland can affect milk protein content and milk volume (Chalupaand Sniffen, 1991; Rulquin and Verite, 1993; Rulquin et al. 1995, Socha et al.2005; Xu et al. 1998). In addition, amino acids can impact productivity by affecting metabolism and immune function (Bauman et al. 1995; Okine et al.1996).  


AminoAcid Requirements
Requirements for absorbed essential amino acids can be defined using the classical factorial method (Chalupa and Sniffen, 1996;O'Connor et al. 1993) and by ideal protein methods (Chalupa and Sniffen, 1996;Rulquin and Verite, 1993; NRC, 2001). 


The factorial method requires knowledge of the amino acid content of products and the efficiency of amino acid use. Amino acid content of milk and tissues can be estimated reliably but an estimate of the efficiency of amino acid use is difficult and variable. 


The ideal protein method proposed Rulquin and Verite (1993) is based on responses of milk protein to methionine and lysine expressed as percentages of PDI (equivalent to metabolizable protein). Optimum milk protein was obtained with 2.5% methionine and 7.3% lysine Figures 1 and 2). However, it is hard to reach these concentrations without single sources of methionine and lysine. Because milk protein appears to be dramatically reduced when rations provide less than 2.10% methionine or 6.5% lysine, these levels are considered minimums.Inspection of graphs presented by Rulquin and Verite (1993) suggest that responses of milk protein to methionine may be negative if lysine is limiting(i.e. lysine in metabolizable protein<6.5). &#160;


Comparisonof the factorial and ideal protein methods Figure 3 shows potential responses of milk protein yield and milk yield to increased metabolizable lysine and methionine. Responses, based on the factorial and ideal protein methods, were estimated using CPM-Dairy (Boston et al. 2000). &#160;


There are considerable differences in responses estimated by the two systems. The constant transfer coefficients for methionine and lysine dictate that production responses are linear regardless of amounts of metabolizable methionine and lysine. The factorial method may describe production responses correctly when nutrients are limiting but will over-estimate production responses when there are excesses of nutrients. &#160;


The ideal protein method gives the curvilinear response to methionine and lysine that is expected in biology. However, the database used to calibrate the ideal protein method (Rulquin and Verite, 1993) was obtained with cows beyond peak production. Milk production was modest and many experiments were short-term switchback designs. Thus, responses to increasing proportions of lysine and methionine might be expected to be higher in early and peak lactation cows. &#160;


FurtherResearch on the Ideal Protein Method
Since the report by Rulquin and Verite (1993), there has been verification of the methionine and lysine ratios (NRC, 2001; Sniffen et al.(2001) and expansion of the ideal protein concept to other amino acids (Sniffenet al. 2001, Rulquin et al.2001). NRC (2001) used an approach like Rulquin and Verite (1993) where responses of milk protein to abomasal or intestinal infusions of methionine and lysine or to feeding rumen-protected forms of methionine and lysine were recorded. Sniffen et al. (2001) applied multiple regression analyses using the JMP discovery software developed by the SAS Institute to transition cow experiments (three weeks before calving to 4-8 weeks postpartum) designed to study the efficacy of rumen-protected methionine and lysine products. Rulquin et al. (2001) used an amino acid profiling prediction system of intestinal contents to enable the inclusion of experiments where amino acids were not infused into the abomasum or small intestine or fed in protected form. &#160;


Methionineand lysine.&#160; NRC (2001) reported that low concentrations of methionine in metabolizable protein limited responses of lysine in metabolizable protein and that low concentrations of lysine in metabolizable limited responses of methionine in metabolizable protein. Whenmethionine was greater than 1.95% of metabolizable protein, the required concentration of lysine in metabolizable protein to maximize milk protein concentration was 7.24%. When lysine was 6.50% or more in metabolizable protein, the methionine requirement was 2.35%. Optimum ratios calculated by the Sniffen et al. (2001) multiple regression approach were 2.02% methionine in metabolizable protein and 7.04% lysine in metabolizable protein. &#160;


Histidine. Increasing histidine in PDIE (metabolizable protein) increased total protein output. However, this was due mainly to an increased milk yield so that protein content in milk (g/kg)plateaued at 3.2% histidine in PDIE (Figure 4). The value suggested by Sniffen et al. (2001) was 2.7%.&#160;&#160; &#160;


Histidine is more likely limiting with rations containing grass silage as a base (Rulquinet al. 2001). &#160;


Leucine. The curves for leucine in PDIE (metabolizable protein) versus total protein output and protein content in milk (g/kg) are similar (Figure 5). Thus, leucine does not appear to affect milk volume but has an impact on concentration of protein in milk. Leucine may limit milk protein concentration when less than 8.8% of PDIE. Sniffen et al.(2001) found that 8.4% leucine was needed. &#160;


Leucine may be below suggested values with grass and barley based rations (Rulquin etal. 2001). &#160;


Valine does not seem to be limiting so long as the concentration in PDIE (metabolizable protein) is greater than 5.3% (Rulquin et al. 2001). Sniffen et al. (2001) reported that concentration of protein in milk was optimized with 5.75% valine in metabolizable protein. &#160;


Isoleucine does not seem to be limiting so long as the concentration in PDIE (metabolizable protein is greater than 5.0% (Rulquin et al. 2001). This is similar to the 4.7% suggested by Sniffen et al. (2001). &#160;


Phenylalanineand tyrosine. The curves for in phenylalanine in PDIE (metabolizable protein) versus total protein output and protein content in milk (g/kg) are similar (Figure 6). Thus, phenylalanine does not have much of an affect on milk volume but has an impact on concentration of protein in milk. Phenylalanine in PDIE of about 5% seems to be sufficient, (Rulquin et al. 2001). The optimum phenylalanine reported by Sniffen et al (2001) was 5.1%. &#160;


Rulquinet al. (2001) suggested that tyrosine might lower the requirement for phenylalanine. Because tyrosine is insoluble in water, it may not require protection to reach the small intestine. &#160;


Threonine. As with histidine, increasing threonine in PDIE (metabolizable protein) increased total protein output.However, this was due mainly to an increased milk yield (Figure 7). Threonine in PDIE of about 4% seems to be sufficient, (Rulquin et al. 2001). This is similar to the 4.5% suggested by Sniffen et al. (2001).&#160; &#160;


Arginine. Post ruminal administration of arginine has not increased milk protein yield or the concentration of protein in milk. Arginine in PDIE (metabolizable protein) of about 4.3% seems to be sufficient (Rulquin et al. (2001). However, Sniffen et al. (2001) reported that about 6% arginine was optimum. &#160;


Tryptophane. According to Rulquin et al. (2001), tryptophane does not appear to be a limiting amino acid (Rulquin et al. 2001) with hay and corn-based rations. Sniffen et al. (2001) reported that 1.37% tryptophane in metabolizable protein was needed. &#160;


Comparisonsof amino acids in PDIE (metabolizable protein) to maximize the concentration of protein in milk are presented in Table 1. Methionine and lysine concentrations reported by Rulquin et al. (2001) and NRC (2001) are similar. Those reported by Sniffen et al. (2001) are a little lower. &#160;


For the other amino acids, ideal values reported by Sniffen et al. (2001) and Rulquin et al. (2001) are similar. &#160;


Production Responses to Methionine and Lysine
Production responses to supplemental methionine and/or lysine have been variable. We selected four reports where diets were evaluated with a nutrition model (CPM-Dairy, 1998; NRC, 2001) to illustrate that balancing for methionine and lysine in metabolizable protein can be beneficial. &#160;


Sloanet al. (1999) used CPM-Dairy (1998) to examine responses to methionine and lysine in the data set compiled by Garthwaite et al. (1998). Increases in yield of milk (1.7 kg/d), yield of milk protein (90 g/d) and concentration of protein in milk (0.10%) only occurred when methionine in metabolizable protein was greater than 2.2%, lysine in metabolizable protein was greater than 6.8% and lysine:methionine ratio exceeded 3 (Table 2).&#160;&#160;


Chalupaet al. (1999) used CPM-Dairy (1998) to formulate amino acid (Ajinomoto Corp.Inc., Tokyo) enriched fresh-cow rations. Methionine in metabolizable protein was increased from 1.89 to 2.35%. Lysine in metabolizable protein was increased from 6.38 to 7.45%.The lysine:methionine ratio in the amino acid enriched ration was 3.2 (Table 3). Feeding the amino acid enriched ration increased mammary synthesis of protein in both multiparous and primiparous cows. Because milk yield increased in multiparous cows, the increased mammary synthesis of protein was “diluted”and concentration of protein in milk was not changed. In primiparous cows, milk yield was only marginally increased so the increased mammary synthesis of protein was seen as an increase in the concentration of protein in milk.Feeding the amino acid enriched ration did not affect mammary synthesis of fat in either multiparous or primiparous cows.&#160;&#160;


Both Garthwaite et al. (1998) and Chalupa et al. (1999) reported that production responses were greater when RPAA were provided both prior to and after calving.&#160;


Noftsgerand St-Pierre (2003) showed that cows fed rations that contained RUP sources with high (>89%) intestinal digestibility produced more milk, more dietary nitrogen was captured in milk (gross nitrogen efficiency) and less nitrogen was excreted per kg of nitrogen in milk (environmental efficiency) than cows fed rations that contained RUP sources with low (55%) intestinal digestibility(Table 4). Supplementing a ration that contained RUP sources with high(>89%) intestinal digestibility with methionine (Rhodimet AT-88 and Smartamine) allowed crude protein to be decreased from 18 to 17% with no drop in milk yield, an increase in the concentration of protein in milk and improvements in gross nitrogen efficiency and environmental efficiency (Table 4). &#160;


Schwabet al. (2003) examined the impact of increasing concentrations of methionine and lysine in metabolizable protein in six commercial dairies (Table 5). Lysine was increased by adding blood meal and reducing or eliminating distillers grains or a protected soy product. Methionine concentrations in metabolizable protein were increased with Smartamine. These ration changes resulted in a lysine-methionine of 3:1. Methionine in metabolizable protein ranged from 2.01 to 2.35%. Lysine in metabolizable protein ranged from 6.18 to 6.76. There was no attempt to measure changes in milk yield but in most cases, producers thought they observed higher milk yields. All herds responded with increases in concentrations of protein and fat in milk. &#160;&#160;


Based upon evaluation of published research,we propose that balancing rations on the basis of amino acids will increase mammary synthesis of protein but the type of production response will vary depending upon parity and stage of lactation.&#160;Because growth is a higher metabolic priority than milk secretion,response in primiparous animals may depend upon body size at calving. Amino acids seem to increase milk volume if started at or prior to calving. If delayed until after peak production, milk volume increases are small so the main response to amino acids is increased concentration of protein in milk. &#160;


Efficiencyof Utilizing Metabolizable Protein for Milk Protein Synthesis.
Rats and chickens grow more efficiently when fed diets that contain high quality proteins like casein or egg protein versus diets with poor quality protein like zein. Dairy cattle should respond in a similar manner to metabolizable protein that has good balances of amino acids. &#160;


Efficiencies of milk protein synthesis from metabolizable protein are in Table 6. The CPM-Dairy efficiency is .65 (Boston et al. 2000).The NRC (2001) efficiency is .67. Calculations by Piepenbrink et al. (1999) and Sloan et al. (2002) showed that increasing the concentrations of methionine and lysine in metabolizable protein increases the efficiency of milk protein synthesis. &#160;


When formulating rations with CPM-Dairy, one might increase the metabolizable protein efficiency in the constants screen to .69 or formulate for a metabolizable deficiency of about 100 g. &#160;


Some caution to the above may be prudent for transition cows.In early lactation, amino acids have an important role apart from their use in protein synthesis. The increased whole body demand for glucose after calving requires metabolic adaptations that may be enhanced by protein nutrition(Overton, 1998). Propionate is the main substrate for gluconeogenesis but after calving conversion of alanine (used as an indicator of gluconeogenesis from amino acids) to glucose increases more than the conversion of propionate to glucose (Overton et al. 1998).&#160;


Since glucose uptake by the mammary gland is a major determinant of milk volume,limiting the supply of nonessential amino acids by reducing metabolizable protein may compromise rapid acceleration of milk yield. &#160;


ReducingNitrogen Excretion.
Increasing pressure to reduce nitrogen excretion of dairy herds requires feeding rations that maximize conversion of feed nitrogen to milk nitrogen. There are several routes that can be followed. Matching dietary protein to animal requirements is obvious but this means feeding different rations to groups of cattle at different levels of production. Mainly, because of simplicity, many dairymen are not willing to follow this strategy and choose to feed one ration to all production groups. Another alternative is to maximize the ruminal production of microbial protein. Finally, the experiment by Noftsger and St. Pierre (2003), which was discussed previously, showed a 35%improvement in nitrogen efficiency when a ration was balanced for methionine and lysine in metabolizable protein. &#160;


Applicationof CPM-Dairy
Diets for ruminant animals should first formulated to optimize the supply of nutrients provided by ruminal microbes. The rumen system, however, cannot provide sufficient nutrients for high levels of growth or milk production. Thus, rumen inert (bypass) nutrients (fat, protein, amino acids and perhaps some vitamins) are needed to supplement nutrients from the rumen so that productivity of growing and lactating cattle can be optimized.&#160; &#160;


Amino acids are the only nitrogenous nutrients that are used for synthesis of tissue proteins and milk protein. Amino acids are provided by ruminal microbes and by dietary protein that escapes fermentative digestion in the rumen and, depending upon protein nutrition during the dry period, by body reserves of labile proteins.&#160;&#160;&#160;


Commercial sources of rumen protected methionine include Megalac Plus (Church and Dwight Co., Inc.), Mepron (Degussa Corp.),Smartamine (Adisseo), MetaSmart (Adisseo) and Met Plus (Nisso America). HMB, (2-Hydroxy-4-(Methylthio)Butanoic Acid), is available as Alimet (Novus International) and as RhodimetAT-88 (Adisseo). Koenig et al. (2002) reported that the ruminal escape of Alimet was 40%. Noftsger and St. Pierre (2003) cited a personal communication from Schwab (University of New Hampshire)that the ruminal escape of Rhodimet AT-88 was only 5%. In a subsequent study, Noftsger et al. (2005) concluded that HMB is primarily a rumen degradable source of methionine, and its positive effects are mainly due to stimulation of microbial growth, predominantly protozoa in the rumen. &#160;&#160;


There are no commercial sources of rumen-protected lysine. As shown in Table 7, blood meal, rumen bacteria and fish meal have the highest concentration of lysine. It is easier to achieve high lysine in metabolizable protein with rations that provide at least 50% of the metabolizable protein from bacteria. Fermentability of carbohydrates in the rumen is the main determinant of bacterial growth. While fermentability of total ration carbohydrates can be increased by providing more non-fiber carbohydrate, this can lead to low ruminal pH so that the efficiency of bacterial growth is reduced. High digestibility of forage NDF is a key to obtaining good growth of bacteria in the rumen. &#160;


Optimum milk protein is obtained with 2.5% methionine and 7.3% lysine (Rulquin andVerite, 1993). It is hard to reach these concentrations without single sources of methionine and lysine. Because milk protein appears to be dramatically reduced when rations provide less than 2.10% methionine or 6.5% lysine, these levels are considered minimums. To see larger increases in milk protein, 2.20%methionine and 6.9% lysine may be needed. Both Rulquin and Verite (1993) and NRC (2001) indicated that it is important to have methionine and lysine balanced with respect to each other. In CPM-Dairy, keep lysine:mehionine at 3.10:1. Even when 6.5% lysine cannot be achieved, supplemental methionine should be provided to obtain lysine:methionine of 3.10:1 &#160;


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