The understanding of protein nutrition in dairy cows has been something of a challenge for dairy scientists and researchers over the years. This is largely due to the fact that a cow is a ruminant which causes her to have two sets of protein requirements: one for the microbes in the rumen and the second for her general metabolism. Protein metabolism in a ruminant goes through a different process than that of mono-gastric species such as swine and poultry. A better understanding of protein nutrition in our dairy cows has become necessary in recent years to:

•control feed cost, which is the single, largest expense on a dairy farm
•increase milk protein, which is one of the milk components for which the value of your milk is based upon
•limit excessive nitrogen release into our natural environment

It wasn’t that many years ago that our basic understanding of protein nutrition for dairy cows consisted mostly of supplying a certain level of crude protein (CP) in the dairy diet. The 1970s brought research that began to focus on how much protein was needed for the rumen and how much was needed for the cow’s general maintenance and milk production. Even though there was a basic understanding of rumen degradable and undegradable protein beginning to emerge, the industry still focused mostly on total CP levels.

Today we evaluate protein nutrition based on the levels of amino acids and peptides (chains of amino acids) and ammonia (NH3) balances in the rumen, how much is used, or degraded, in the rumen and how much bypasses rumen degradation and continues on to the small intestine to be metabolized for maintenance needs. In the last few decades, we’ve learned the amino acid composition of the protein being absorbed in the small intestine plays a critical role in milk production and milk protein synthesis.

It’s all about the rumen …
Nutrition, health and the subsequent milk production of a dairy cow starts in the rumen – a muscular organ that’s essentially a big fermentation tank for feedstuffs. Cows, which are herbivores, are designed to eat forages that are indigestible to most mono-gastric species. The rumen which is the largest of four individual “stomachs” in a cow, is populated with millions of bacteria and other micro-organisms that accomplish the job of breaking down the cellulytic material contained in grasses, haylages, hay and corn silage. In order for all of that to happen, bacteria require “food” of their own. They convert some of the dietary proteins from the feedstuffs in a cow’s feed ration for their own needs. This portion of the protein, both in the complex form of peptides or a basic form of ammonia, are utilized by the microbes as they go about the business of fermenting the feed introduced into the rumen. This protein (and more correctly, nitrogen) is referred to as rumen degradable protein (RDP).

Dr. Chuck Schwab of the University of New Hampshire is one of the world’s leading researchers in protein metabolism for dairy cows. His research has brought to the dairy industry a much better understanding of supplying adequate RDP as well as balancing diets for amino acids (AA) that are metabolized in the small intestine. At a meeting of nutritionists and feed professionals in New Hampshire in March this year, Schwab emphasized the importance of meeting the microbial protein requirements of the rumen with adequate RDP before focusing on AA balances.

Rumen microbial protein has an AA profile similar to that of milk, bone and tissue. Schwab said that the more microbial protein that can be moved to the small intestine, the less need there is for more expensive bypass proteins to meet the metabolizable AA needs in the small intestine. The primary focus on balancing dairy cow diets at any stage of lactation should be on maximizing microbial protein efficiencies in the rumen. This is accomplished through the proper use of protein supplements such as soybean and canola meals, adequate effective fiber from forages and the proper amounts of carbohydrates from feedstuffs such as corn, barley or wheat midds.

Unfortunately, even at best, the microbial protein that escapes the rumen to move on to the small intestine provides only slightly more than half of the metabolizable protein (MP) needs of a cow. The rest of the MP and AA can come from dietary protein sources such as animal and fish byproducts, corn germ meal or distillers grains. But, with the exception of animal and fish, none of these feeds have a proper AA profile. Once the protein escapes the rumen, the digestive system of a cow is similar to mono-gastric species and that protein, in whatever proportions of AA it happens to be, is then absorbed in the small intestine.

The limiting amino acids…
A driving force behind the research to understand the metabolizable amino acid needs of our dairy cows is to increase the protein in the milk, thereby increasing the value of the milk. A second, and equally worthy reason, is to reduce the amount of excess nitrogen being released into the environment through the overfeeding of excess protein in dairy diets. The more closely the absorbed AA in the small intestine can be to “perfect,” the better job we do in managing and controlling feed costs and excessive nitrogen pollution.

Schwab, along with others, has concluded that it is both an imbalance of specific amino acids and limited amounts of those essential amino acids in the rumen-escape protein that prevents our cows from realizing their full productive potential.

The challenge for nutritionists is to formulate and feed a diet in which the rumen undegradable protein (RUP), flowing to the small intestine, contains an AA profile that more closely resembles that of the microbial protein and lean tissue of the cow. In order to accomplish this we must know the amino acid composition of our feedstuffs along with the percentage of rumen undegradability it has. With this information we can calculate how much of a particular amino acid will be passing through the rumen and be available for digestion in the small intestine.

This task may seem daunting since we know that there are about 20 amino acids that make up most of the proteins. Added to that, degradation rates vary for different feedstuffs and are also affected by rates of passage in the rumen. Schwab’s work has determined, however, that there are only three of those 20 amino acids that a cow is often deficient in and is not able to synthesize in the rumen. Those three amino acids are lysine, methionine and histidine. Lysine and methionine are considered to be the first- and second-limiting amino acids in dairy cow nutrition. In rare instances, histidine can be limiting in high haylage diets.

Microbial protein contains about 7.9 percent, 2.6 percent and 2.0 percent of lysine, methionine and histidine, respectively, as a percentage of crude protein. Most of the grains and byproducts we feed our cows contain varying amounts of these three amino acids and, with the exception of fish, blood, meat and bone, none of them have enough of or the proper proportions of those three essential amino acids. Since microbial protein that escapes the rumen is close to being “balanced,” it then becomes necessary to ensure RUP flowing to the small intestine also has adequate lysine and methionine.

As with all amino acids, lysine and methionine are easily degraded in the rumen which requires the need to provide them in a form that will bypass the rumen. Technology now exists for a rumen bypass methionine (rumen-protected methionine) that is very effective in getting methionine levels where they need to be in the small intestine. Technology for bypass lysine is neither affordable nor effective at this time. Often, the only practical way to get adequate levels of bypass lysine into the small intestine, especially for very high-producing cows, is with fish meal or animal protein products, which are both expensive and politically incorrect these days.

Schwab has established that intestinal lysine levels should be about 180 to 200 grams per day to support milk production near 100 pounds or more and intestinal methionine levels should be about 60 to 70 grams per day. You will notice that the ratio of lysine to methionine is 3 to 1. It’s very important that the 3-to-1 ratio be observed very closely as well as the total amounts of lysine and methionine flowing to the small intestine. Feeding too much methionine relative to lysine will have a negative effect on milk production.

Yes, it pays in several ways…
Even though the concept of balancing rations for AA has been around for two decades, our industry continues to struggle with volatile milk prices that have contributed to the slow implementation of AA balancing. Even when milk prices are high and feed prices are low, many producers question the value and appropriateness of some of the more expensive protein supplements (such as blood and fish meals) as well as the rumen-protected methionine products. More recently there are enzyme products coming on the market showing promising signs of increasing rumen microbial efficiencies, reducing or eliminating the need for animal byproducts.

Balancing feed rations that take into consideration necessary lysine and methionine have been shown to produce over a half gallon of milk (4+ pounds) as well as increases in milk protein of 1 to 2 percentage points (e.g., 3.0 to 3.2). These rations often cost no more than a current ration because by improving the amino acid profile in RUP we can reduce the need for excessive RUP, which ultimately will cost less money. Your income-over-feed-cost can easily increase by 40 cents per cow per day, which if your cows are producing 100 pounds of milk per day is a 40-cent-per-hundredweight (cwt) increase in your milk price. Irregardless of whether milk is worth $10 per cwt or $17 per cwt, every dairy farmer should jump at the chance to increase his bottom line by 40 cents with little impact on the cost of the ration.

Getting dietary protein into balance has also been shown to reduce metabolic disorders at the time of calving and improve conception rates later in lactation. Excessive nitrogen in a cow’s system must be expelled as urea, which has an energy cost associated with it as well as depositing more nitrogen in the waste stream and environment. Pushing CP levels too high in a fresh cow’s ration can affect her energy status and keep her in a negative energy balance for a longer period of time. Schwab also noted that methionine plays a clear role in liver metabolism, helping to minimize fatty liver syndrome and ketosis. Excessive urea in the blood and milk (high BUN or MUN) has also been shown to negatively impact conception which we all know will have a negative effect on cash flow in the future.

When it comes to protein nutrition in your dairy herd, there’s no need to be shy in embracing the science behind AA balancing in your herd’s diets. You’ll see the benefits very quickly with the increased value of your milk, and you’ll be doing both your cows and the environment a big favor. PD