Assessing Protein Quality of Feed

While the estimation of crude protein content (see above) will give a reasonable estimate of the AMOUNT of protein in a feed, of perhaps greater importance is the QUALITY of that protein. The very different digestive systems of pigs and poultry compared with sheep and goats mean that the assessment of protein quality differs for these two classes of animal.

Degradable Protein Digestible Undegraded Protein Digestible Crude Protein Limiting Amino Acids

Degradable Protein

When a sheep or goat eats any protein, it first goes into the rumen, where it is attacked by the rumen micro-organisms. (See under Nutrition for more information). How much of the protein gets degraded by the micro-organisms depends on how fast the protein is broken down by them, and on how fast feed leaves the rumen. If the feed is broken down quickly, and feed flows out of the rumen only slowly, then most if not all of the protein will be broken down (degraded) in the rumen. If the feed is broken down slowly, and feed flows out of the rumen more quickly, then little if any of the protein will be degraded.

Degraded protein will only be of any use to the animal if the rumen micro-organisms are able to build it up into microbial protein. This microbial protein will then eventually leave the rumen and be digested, absorbed and used by the animal. The rumen micro-organisms will only be able to use degraded protein to make microbial protein if they have enough energy. They will get this energy from fibre (if it can be degraded), but also from sugars and starch in the diet. They will not get any energy from fat or oil in the diet, as micro-organisms are unable to ferment fat to any great extent.

Estimation of Degradable Protein

The common way to estimate how much and how quickly a protein will degrade in the rumen is to take samples of the feed and put them in porous bags. These are placed in the rumen of an animal (usually sheep, cow or buffalo) that has been operated on so that it has a hole (with a cap on it) in its rumen. The bags are taken out at different times, and the amount of protein that is left in the feed that is still left in the bag is measured. From the amount of protein that is left in the bag at different times, the rate (and extent) of protein degradation can be calculated.

Protein degradability is usually expressed using three terms, a, b and c.

a is the proportion of protein that is almost immediately degraded, and so is likely to be degraded in the rumen no matter what the outflow rate is.

b is the proportion of protein that is potentially degraded, depending on the relative rates of degradation and outflow.

c is the rate of degradation of b.

To calculate the proportion of protein that will be degraded, you also need to know the outflow rate from the rumen (k). For most tropical livestock, being kept at or near maintenance, this is likely to be 0.02/h. In other words, 2% of the material in the rumen will flow out into the animal’s lower gut every hour.

The proportion of a feed’s protein that is degradable can be calculated from the equation:

Degradability = a + bc / c + k  

and since usually k = 0.02

Degradability = a + bc / c + 0.02

The amount of degradable protein in a feed can be estimated by multiplying its degradability by its crude protein content.

Digestible Undegraded Protein

The protein that is not degraded in the rumen may be digested and absorbed by the animal. This protein is called digestible, undegraded protein (DUP). There are times (particularly in early lactation) when animals may need some DUP to meet all their needs for protein, if the rumen micro-organisms are not able to make enough protein for the animal.

To estimate how much undegraded protein is supplied by a feed, subtract the degradable protein content away from the feed’s crude protein content:

Undegraded protein = Crude protein – Degradable protein

Not all of this undegraded protein will be digested by the animal. With most feeds, however, 95-98% of the undegraded protein will be digested by the animal and if there is no other information available then this is probably a reasonable estimate to use.

To estimate the amount of indigestible protein there is in a feed, a feed might be heated with acid detergent, and the nitrogen content of the residue determined. This gives an estimate of the feed’s ACID DETERGENT INSOLUBLE NITROGEN (ADIN) content, and is taken to be an estimate of the N fraction that will not be digested in the gut.

To estimate the indigestible protein content, therefore, the ADIN content is multiplied by 6.25 (see estimation of crude protein for an explanation).

Indigestible protein (IP) = ADIN x 6.25.

The digestible, undegraded protein content of the feed may then be calculated from:

DUP content = CP content – Degradable protein content – IP

(Where CP is crude protein).

 

Protein digestibility for pigs can be estimated by feeding them known amounts of the feed under investigation and then collecting all the faeces they produce. The feed and faeces are then analysed for protein, and by knowing how much protein has been fed, and how much protein has been excreted, the digestibility of the protein is calculated from:

Since this method is expensive and time consuming, laboratory tests have been developed to predict protein digestibility. The feed can be mixed with digestive enzymes (pepsin and pancreatin, which are available commercially). These will digest the digestible part of the protein, and the feed/enzyme mixture can then be filtered. The digested protein, being by now soluble, will be filtered off with the enzymes. The undigested protein will remain and can be measured and from this, the protein digestibility can be estimated.

This method is also expensive and uses chemicals that can be hard to obtain for some laboratories. A good prediction of protein digestibility can also be achieved (for tropical forages) by estimating the proportion of protein (N) that is water soluble. This is done by putting the sample of feed through three 30 min cold water wash cycles in a washing machine, and then measuring the amount of N that is left in the residual material.

 

All animals are made up of protein, and so they need protein to maintain themselves and also to grow, or produce milk, eggs or young. The building blocks of proteins are amino acids, and it is really the amino acids (which make up the protein both in the feed and in the animal) that the animal needs.

There are about 20 different naturally occurring amino acids. Provided an animal has a sufficient supply of protein (amino acids) in the diet, then it is able to make many of the amino acids that it requires by transforming the amino acids that it has absorbed from the diet. However, there are some amino acids that cannot be made by the animal. These are called ESSENTIAL amino acids, as it is essential that the animal receives a supply of these amino acids in the diet. However, again, many of these essential amino acids are so widely available in most proteins that provided the animal is fed enough protein, it will receive sufficient essential amino acids. There are some essential amino acids, however, that are not in plentiful supply in most proteins that the animal is likely to eat. These amino acids are called LIMITING amino acids, because they are usually deficient in the animal’s diet, and their absence limits the animal’s ability to maintain itself or to grow. The real value of a protein to a monogastric animal like a pig or a chicken, then, is how well it can supply these limiting amino acids.

The amino acid that is nearly always the first one to be limiting for pigs and poultry is an amino acid called LYSINE. Other amino acids that are frequently limiting include CYSTEINE and METHIONINE. The importance of providing these amino acids mean that protein quality for pigs and poultry is often assessed by estimating the available lysine value (ALV) of the feed. A feed with a high ALV will have a much higher quality protein for pigs and poultry than a feed with a low ALV.

Estimation of available lysine value

The feed is mixed with 1 -fluoro-2, 4-dinitrobenzene (FDNB), which reacts with available lysine to produce a coloured compound. The more available lysine is present, the more intense is the colour, and the colour intensity is estimated by measuring the absorbance of the solution using a spectrophotometer (which measures the intensity of colours). This method does not measure all of the lysine that is present in the feed, but only that bit which is likely to be digested and absorbed by the animal.