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Where proteins come from in cow diet?

Before talking about protein in ruminant diet, let me introduce few terms. We should talk about Metabolizable Protein (MP) as the most important source of protein and amino acid for the cow itself. Remember that the cow receives protein from two parts: microbial protein from the rumen microflora, the Microbial Crude Protein (MCP) (primary source) and directly from the diet, the rumen by-pass protein (secondary source). The rumen microflora is able to transforms vegetable proteins into microbial proteins.

We finally can define four nutrients: 

  • RDP: Ruminal Degradable Protein. It is used by ruminal microbes to synthetize ruminal microbial crude protein. It is expressed in % of Crude Protein.

  • RUP: Ruminal Undegradable Protein. It is the remaining protein after degradation of a protein source by ruminal microorganism. It is considered as a by-pass protein. It is expressed in % of Crude Protein.

  • MP: Metabolizable Protein. It is defined as the net quantity of true protein or amino acid (feed and microbial true protein) absorbed in the small intestine. It is express in grams.

The NPN (non protein nitrogen) is another kind of nitrogen source (like ammonia) used by rumen microflora.

Crude protein percentage and feed intake of a ruminant diet will describe the quantity of protein given per day while %RDP and %RUP will describe the degradability of the protein. The ratio RDP/RUP should be like 66/33. Balanced a cow diet To design a cow diet, we always start from the animal needs. Let’s consider a very simple case with a cow in mid lactation fed with concentrate and forage. She needs an energy density in the diet at 2700 kcal Metabolizable Energy (ME) / kg of dry matter (DM) and around 16% CP / kg DM. This ratio should be kept for a cow in mid lactation whatever the milk production. The total quantity of protein and energy given per day will change according to milk production.

This cow is fed with Corn Silage (2600 kcal ME, 8% CP). If you fed the cow only with corn silage, there is a lack of protein. If you complete the diet with a concentrate like it is usually produced by a feed mill (2700 kcal ME, 16% CP = a balanced concentrate), you cannot correct the diet and you are still facing a lack of protein in the diet. What you should do is using a concentrate feed with a high level of protein (>25% CP = a protein concentrate) to firstly balance the diet (= keep good ratio Energy/Protein) and then you add a balanced concentrate according to milk production or targeted daily gain.

Why not a protein concentrate?

It is still very rare to find feed mill who produce protein concentrate to balance ruminant diet on farm. On small scale farm, you usually see a lack of protein in the diet. It can be easily seen by looking at feces consistency. Feces look dry like horse manure.

We can also measure milk urea nitrogen (MUN) which represents quantity of urea in milk. It is measured with infra-red spectrophotometric device or with specialized device.

Forage quality changes according to weather, season, plant, variety…. When farmer uses only one balanced concentrate, it is more difficult to adapt quantity of concentrate to forage quality. By using one balanced concentrate and one protein concentrate and with a look at simple parameters like feces consistency and MUN, farmer can adapt his feeding program to maximise milk production. Protect the protein Beside protein quantity, using a protein concentrate helps to manage the RDP/RUP ratio of the diet. In most of the forage, there is usually more RDP than RUP. It means that RUP or rumen by pass protein are more difficult to supply. This lack of RUP is particularly critical in early lactation when the cow’s feed intake is lower than its needs. Providing RUP is an insurance of a direct protein supply to the cow’s gut. RUP is usually more expensive to supply than RDP. To increase RUP in feed, feed mills can use different raw materials or additives:

  • Heat treated soya bean meal is a common source of protected proteins but the cost can be high.

  • Essential oils are killing some protozoan or bacteria which consume protein. Proteins are spared for the cow but we reduce rumen microflora.

  • Ellagitannins from chestnut are considered as a good way to protect protein. Ellagitannins have capacity to bind protein at pH from 5 to 7, like in a rumen. This binding is reversible at low pH (2-3) like in the abomasum (true stomach of the cow). The protein is available for enzymatic digestion and absorption in small intestine. This way is cheaper and less harmful for rumen microflora.

A farmer should have different kind of concentrate. It is helpful for a daily adaptation or individual management according to practical observations (feces, milk quantity, milk quality). Feed mill can also provide a concentrate with higher level of RUP to counter balance the low RUP forage.  It will lead to higher level of metabolizable protein and high milk production or weight gain.


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