Amino acids

When various feedstuffs are involved, working out pig diets on the basis of amino acids is an effective approach.

Essential amino acids

Most proteins contain 19 or more amino acids. During digestion, the protein in feed is broken down into its constituent amino acids, which the pig absorbs and ultimately uses to build body protein. The pig is unable to synthesise nine amino acids: lysine, methionine, tryptophan, isoleucine, histidine, phenyalanine, threonine, leucine and valine. Therefore, these must be supplied in the feed.

Limiting amino acids

The amino acid that is present in a feed in the least amount relative to pig requirements is said to be the first-limiting amino acid. The extent to which this limiting amino acid is adequate determines pig performance. The following tables list the amino acids that are generally first-limiting in various pig feedstuffs. In most Australian cereal-based diets, lysine is first-limiting.

Total and available amino acids

The amino acid composition of a feedstuff or diet can be analysed. This is usually done by acid hydrolysis followed by ion exchange chromatography with colorimetric or fluorimetric detection of the amino acids. However, this procedure does not determine the amounts of amino acids available to a pig from a feed ingredient.

To be 'available', an amino acid must be absorbed and presented to the tissues in a form that can be used for metabolic functions. The primary factors affecting bio-availability are the efficiencies of both protein digestion and amino acid absorption, and the efficiency of using amino acids at the tissue level after absorption.

Common causes of reduced protein digestion in feedstuffs are excessive heat treatment during processing, indigestible cell walls and the presence of anti-nutritional factors.

Because the use of amino acids in feedstuffs varies, working out diets on the basis of bio-available amino acids should be more accurate than calculating total amino acids. Knowing the proportion of essential amino acids in feedstuffs that is bio-available to the pig is essential for working out accurate diets. This proportion has been estimated through digestion trials, which estimate amino acid absorption, and the slope-ratio method, which combines the effects of digestion, absorption and metabolism.

Digestibilities are reported as either apparent or true - the latter correcting for endogenous amino acid output. Because of the difficulties of accurately measuring endogenous amino acid output, apparent digestibilities are more useful in practical swine feeding. Digestibilities determined near the end of a small intestine (ileal digestibilities) are superior to those determined over the total tract (faecal digestibilities) because they are determined prior to the microbial action in the large intestine. Ileal digestibility has an advantage over the slope-ratio method as it can measure all nine essential amino acids at once.

However, one problem with this method is that it assumes that if an amino acid is digested in the small intestine, it is absorbed in a form that can be used. This assumption may not be correct. Availabilities determined from valid slope-ratio assays more accurately measure differences among feedstuffs in their ability to supply amino acids used for protein synthesis than those from digestibilities.

Research shows that for amino acids in over-processed protein concentrates, ileal digestibility may overestimate availability. For example, for cottonseed meal, ileal digestibility of lysine is normally 60-75 per cent while lysine availability is only 30-40 per cent.

Due to the time and cost involved, slope-ratio assays are not practical for routinely working out feed amino acid availability. In practice, lysine availability values from slope-ratio are preferred for use with protein concentrates. However, for cereals, slope-ratio assays are more difficult and ileal digestibility values are more practical (see Table 3). The labour and cost involved in available assays limits their usefulness as part of a diet calculation system. Clearly, a rapid, cheap, accurate and simple test is needed to evaluate the amino acid-nutritive value of various feedstuffs.

* Based on reactive lysine as an indicator of availability

References

Baker, DH 1993, Amino acid nutrition of pigs and poultry, in DJA Cole, W Haresign and PG Garnworthy (eds), 'Recent developments in pig nutrition 2', Nottingham University Press, Nottingham, United Kingdom.

Knabe, DA 1991, Bioavailability of amino acids in feedstuffs for swine, in ER Miller, DE Ullsey and AJ Lewis (eds), 'Swine nutrition', Butterworth-Heinemann, London.

Lewis, AJ 1991, Amino acids in swine nutrition, in ER Miller, DE Ullsey and AJ Lewis (eds), 'Swine nutrition', Butterworth-Heinemann, London.

Standing Committee on Agriculture 1987, 'Feeding standards for Australian livestock: pigs', CSIRO, Melbourne.

Van Barneveld R 2002, Australian canola meal - a valuable component of pig feed, 'APL technical notes', no. 2002/1.