Proteins are chains of amino acids which can be essential or non-essential. Essential amino-acids are those that are not produced by the body and so must be obtained by the animal through its diet. Animals can get non-essential amino acids from their diet or they can be synthesised by their body. Proteins and energy are some of the most important components of an animal's diet and the effects of a protein deficiency are quite rapid. Thus, it is important that animals receive enough protein in their diet.
How much is enough? Well, the amount of dietary protein required by an animal depends on its species, physiological state (eg reproduction, lactation, growth etc), level of exercise, and disease status. The amount of protein needed increases with growth, breeding, reproduction, lactation and work.
Fates of Dietary Protein in Monogastric Animals
The protein that is provided in its diet is known as ration protein. Some of the ration protein is lost as faecal nitrogen. Faecal Nitrogen consists of: unabsorbed ration protein, spent enzymes, sloughed cells and intestinal microbes. The remainder of the ration protein is digested and absorbed and is now known as metabolisable protein because the body is able to use it.
Some of the metabolisable protein is used to make digestive proteins which contribute to the ration protein because the enter the digestive tract where they can be digested and absorbed by the body. Some of the metabolisable protein is used to generate energy. The by-products of this process are carbon dioxide, water and urinary nitrogen. The remainder of the metabolisable protein is used to make other proteins to make more tissues and to produce milk, eggs, wool, etc. Some protein is lost through tissue catabolism and takes the form of endogenous urinary nitrogen.
What About Ruminants?
Ruminants use their dietary protein differently to monogastrics in two main ways:
- Rumen microbes ferment the dietary nitrogen.
- Ruminants have a high efficiency of nitrogen recycling through saliva and conversion to absorbable amino acids.
The bacterial and protozoal protein as well as the undigested dietary crude protein are broken down into amino acids in the abomasum and small intestine. The amino acids are used to make tissue protein.
Thus, ruminants are able to utilise non-protein nitrogen (eg nitrates, nitrites and other nitrogen from plants) and protein for the synthesis of microbes which they then digest. This microbial protein also provides the full range of essential and non-essential amino acids. How cool is that!
Rumen Degradability
The degradability of a foodstuff is dependent on several factors:
- Rumen Solubility: more soluble proteins are more degradable because microbes are able to use them more rapidly.
- The Amino Acid Sequence in the Polypeptide: this is because the rumen microbes only have enzymes which can cleave particular types of bonds.
- Rumen Residence Time: The longer the protein is in the rumen, the longer there is for it to be broken down.
- Intake of Feed: the higher the intake, the less time the proteins can stay in the rumen.
- Particle Size: smaller food particles increases the surface area of feed available to the microbes and so can be expected to increase rumen digestibility. However, increased particle size also decreases the time spent in the rumen which decreases digestibility. Thus, these two factors need to be balanced.
The amount of protein produced by the microbes in the rumen depends on a few factors:
- Energy supply to the microbes
- Nitrogen supply to the microbes
- Level of feed intake by the animal
- Outflow rate: this is affected by the level of feeding and also lactation.
Protein Requirements of Ruminants at Different Stages of Production
The protein supplied by the microbes is generally enough for the protein requirements for maintenance and slow growth. However this supply is not enough for early growth, late pregnancy, parturition and lactation.
Once the dietary crude protein reaches 14% the ruminal microbes are at their crude protein cap - they are using as much of this dietary protein as they possibly can. When the dietary crude protein exceeds 16% the protein is converted to ammonia. Increasing levels of ammonia can be dangerous because it puts the animal at an increasing risk of ammonia toxicity.
So How Do We Supply Protein Beyond Microbial Capacity?
The solution is to increase the level of dietary protein that can't be degraded by the microbes (Undigestible Protein). This can be done by decreasing the size of the food particles, increasing the amount of food available or treating the food with chemicals or heat.
Protein Quality
There are three factors that affect the quality of the protein given to an animal:
- Amount of protein
- The mix of essential amino acids relative to the target protein: for example, if you want to maximise egg production in a chicken you should feed it a diet that contains a essential amino acids that mimic the essential amino acid mix in eggs.
- The digestibility of the protein.
- This is the percentage of protein digested in the GI tract.
- Apparent CP digestibility (%) = ( (N Intake - Faecal N) / N Intake) x 100
- True CP Digestibility (%) = [(N Intake - (Faecal N - Endogenous Faecal N) ) / N Intake] x 100
The rate-limiting amino acid is the essential amino acid that is in greatest deficit in regards to the target protein. Lysine is usually the rate limiting amino acid in most production systems.
That's all for now, if you have any questions let me know in the comments section :)
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