Meeting the protein requirements of high-producing dairy cattle has been a long-time goal of many nutritionists across North America. Lately this has been particularly important due to the pressure dairy producers are feeling from environmentalists, as ruminants are blamed for releasing high levels of nitrogen emissions such as ammonia and nitrous oxide into the atmosphere. Overfeeding of protein has also been associated with reproductive failure as well as increasing feed costs, making this an important issue for producers today as well as in the near future.
Proteins are made up of amino acids; as the protein in the feed is broken down by the rumen microbes of the cow, ammonia is released into the bloodstream. This ammonia is quickly converted to urea in the liver, and released back into the bloodstream.
Once in the bloodstream, this urea nitrogen has two fates: It can either be recycled back to the rumen and be incorporated into microbial protein, or, when in excessive amounts, it can be excreted into the urine by the kidney. This can be costly to the cow, as the kidney will utilize energy to perform this task, taking energy away from milk production.
Studies examining the efficiency of milk protein production have noted that milk protein synthesis varies between 20 and 35 percent of the cow’s total dietary protein intake; the remaining 65 to 80 percent exits the animal through the urine and feces. From these studies, cows that are more efficient don’t have higher milk protein yields, but excrete less urea nitrogen in feces and urine.
These studies also highlight that while underfeeding protein will reduce milk protein yield, overfeeding of protein only results in a higher amount of urea nitrogen being excreted into the urine. This results in higher nitrogen emissions as well as increasing the cost of feed, as feeding excess protein yields little return to both the cow as well as the bottom line.
Formulating for protein requirements
When formulating a dairy ration for protein, there are three types of protein that must be accounted for. The three types differ on how quickly they are degraded and absorbed by the rumen microbes and the cow, and are as follows: non-protein nitrogen, rumen-degradable protein and rumen-undegradable protein.
The most quickly degraded form of protein is actually called non-protein nitrogen (NPN), and is often added to dairy rations in the form of urea. This provides the rumen microbes with a readily available source of nitrogen, as the microbes have the unique ability to create their own protein if they are provided with a nitrogen source as well as carbohydrates such as starch and sugar.
Rumen-degradable protein (RDP) is more slowly degraded by the rumen microbes to provide them with preformed amino acids, typically met through the addition of soybean meal or canola meal as well as alfalfa silage.
The slowest-degraded type of protein is rumen-undegradable protein (RUP), which is poorly utilized by the rumen microbes, allowing this protein to be absorbed by the cow. It is generally provided in feedstuffs such as corn gluten meal or fish meal.
When formulating dairy rations, it is important to maximize the microbial protein synthesis, where bacteria are washed out of the rumen into the lower digestive tract, providing an excellent source of protein for the dairy cow. The amino acids used to form rumen microbial proteins are actually very similar in quantity and quality to those used to form milk proteins, which is why it is so imperative to meet the animal’s dietary needs.
However, meeting your cows’ protein requirements at peak lactation can be very difficult as feed intake lags behind milk production. This has led many nutritionists to formulate early lactation rations that contain upwards of 18 percent crude protein on a dry matter basis.
Due to the high rate of digestion in these cattle, a large portion of their dietary protein needs to be in the form of RUP, approximately 35 to 40 percent, in order to meet the demand for milk protein synthesis. The remainder of the protein needs can easily be met by supplementing RDP sources.
Late in lactation, the crude protein can be dropped to between 14 and 16 percent, with RUP levels between 30 and 35 percent of crude protein, without decreasing the milk protein yield.
Recently there has been a movement toward low-protein diets for early-lactation dairy cows – approximately 16 percent crude protein without limiting milk protein yield. Formulating low-protein diets focuses closely on optimizing the rate of rumen microbial growth by balancing the rate of digestion between proteins and carbohydrates, decreasing the need for supplementation of excessive RUP or bypass protein.
In other words, balancing the levels of NPN with sugars and RDP with starch to allow the rumen microbes to more effectively transform these nitrogen and plant protein sources into high-quality microbial protein. Researchers examining low-protein diets have noted that crude protein levels of 16 to 16.5 percent are able to maximize milk and milk protein yield, while minimizing the excretion of nitrogen.
Working closely with your nutritionist is essential to achieve the ideal protein-to-carbohydrate balance, as an increase in readily fermentable carbohydrates increases the risk of digestive upsets such as subacute ruminal acidosis, if not properly managed.
Determining your protein requirements
How do you know if you are accurately meeting the protein needs of your herd? The first thing is to look at the milk yield as well as the milk protein yield. As a rule of thumb, the ratio between milk protein and milk fat should be between 0.8 and 1, i.e., for a cow with a 3.6 percent butter fat, her protein should be between 2.9 and 3.6 percent depending on her stage of lactation.
If milk protein yield is much outside the expected range, further investigation is required. Milk urea nitrogen (MUN) is a useful tool for quickly estimating the overall nitrogen status of a particular cow or group of cows, as it is very tightly associated with the blood urea nitrogen levels in the cow. MUN values vary depending on stage of lactation; however, typical values fall between 10 and 18 mg/dl.
Below this range is a good indication that your cows’ protein requirements are not being met, while above this range often tells nutritionists that protein is being overfed, or carbohydrate and protein digestion are not properly balanced. Herds considering using MUN as a tool should test their milk regularly to create a set of baseline values; this allows producers to effectively evaluate ration changes by comparing production and MUN values on different feeding programs.
Working with your nutritionist and veterinarian can help you accurately meet the protein needs of your herd, thereby maximizing milk and milk protein production, reducing nitrogen emissions and reducing the cost of feed.
Finally, as with any feeding program, success depends on maximizing feed intake through the production of high-quality forages, timely delivery of a well-balanced and consistent ration and adequate feedbunk space to all cows. PD
References omitted due to space but are available upon request to firstname.lastname@example.org .
- Ph.D. Student, Ruminant Nutrition and Physiology
- University of Saskatchewan
- Email Matthew Walpole