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. In addition, amino acids can impact productivity by affecting metabolism and immune function.

Amino acid requirements

Requirements for absorbed essential amino acids can be defined using the classical factorial method and by ideal protein methods. 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 by Rulquin and Verite 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 percent methionine and 7.3 percent lysine. 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 percent methionine or 6.5 percent lysine, these levels are considered minimums. Inspection of graphs presented by Rulquin and Verite suggest responses of milk protein to methionine may be negative if lysine is limiting (i.e., lysine in metabolizable protein less than 6.5).

Production responses to methionine and lysine

Production responses to supplemental methionine and lysine have been variable. We selected four reports where diets were evaluated with a nutrition model to illustrate balancing for methionine and lysine in metabolizable protein can be beneficial.

Sloan et al. used CPM-Dairy to examine responses to methionine and lysine in the data set compiled by Garthwaite et al. Increases in yield of milk (1.7 kilograms per day), yield of milk protein (90 grams per day) and concentration of protein in milk (0.10 percent) only occurred when methionine in metabolizable protein was greater than 2.2 percent, lysine in metabolizable protein was greater than 6.8 percent and lysine-to-methionine ratio exceeded 3.

Chalupa et al. used CPM-Dairy to formulate amino acid-enriched rations for fresh cows. Methionine in metabolizable protein was increased from 1.89 to 2.35 percent. Lysine in metabolizable protein was increased from 6.38 to 7.45 percent. The lysine-to-methionine ratio in the amino acid enriched ration was 3.2.

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.

Both Garthwaite et al. and Chalupa et al. reported production responses were greater when rumen protected amino acids (RPAA) were provided both prior to and after calving.

Noftsger and St-Pierre showed that cows fed rations that contained RUP sources with high (greater than 89 percent) intestinal digestibility produced more milk, more dietary nitrogen was captured in milk (gross nitrogen efficiency) and less nitrogen was excreted per kilogram of nitrogen in milk (environmental efficiency) than cows fed rations that contained RUP sources with low (55 percent) intestinal digestibility.

Supplementing a ration that contained RUP sources with high (greater than 89) intestinal digestibility with methionine allowed crude protein to be decreased from 18 to 17 percent with no drop in milk yield, an increase in the concentration of protein in milk and improvements in gross nitrogen efficiency and environmental efficiency.

Schwab et al. examined the impact of increasing concentrations of methionine and lysine in metabolizable protein in six commercial dairies. 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-to-methionine ratio of 3-to-1. Methionine in metabolizable protein ranged from 2.01 to 2.35 percent. 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. Based upon evaluation of published research, we propose 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.

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.

Efficiency of utilizing metabolizable proteinfor 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.

Calculations by Piepenbrink et al. and Sloan et al. showed increasing the concentrations of methionine and lysine in metabolizable protein increases the efficiency of milk protein synthesis. When formulating rations, one might increase the metabolizable protein efficiency in the constants screen to .69 or formulate for a metabolizable deficiency of about 100 grams.

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.

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. 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.

Reducing nitrogen 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 showed a 35 percent improvement in nitrogen efficiency when a ration was balanced for methionine and lysine in metabolizable protein.

Application of diets for ruminant animals should first be 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 productivity of growing and lactating cattle can be optimized.

Amino acids are the only nitrogenous nutrients 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.

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 percent 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 nonfiber 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. Optimum milk protein is obtained with 2.5 percent methionine and 7.3 percent lysine. 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 percent methionine or 6.5 percent lysine, these levels are considered minimums. To see larger increases in milk protein, 2.20 percent methionine and 6.9 percent lysine may be needed.

Both Rulquin and Verite and NRC (2001) indicated it is important to have methionine and lysine balanced with respect to each other. Keep lysine to methionine at 3.10-to-1. Even when 6.5 percent lysine cannot be achieved, supplemental methionine should be provided to obtain a lysine-to-methionine ratio of 3.10-to-1. PD

References omitted but are available upon request at editor@progressivedairy.com

—From 21st Annual Southwest Nutrition & Management Conference Proceedings