A key nutrient for growth and feed intake

In European feed formulas, tryptophan is usually the fourth limiting amino acid after lysine, threonine and the sulphur amino acids (methionine + cysteine) for piglets and very often the third one after lysine and threonine for fattening pigs. As lysine and threonine, tryptophan is an indispensable amino acid for body protein deposition and growth. Thus, deficiency in tryptophan affects the utilization of dietary lysine and threonine and consequently animal growth.

Besides its utilization for protein synthesis, tryptophan is involved in other biological functions such as appetite regulation, immune response and health maintenance (Figure 1).

Figure 1. Global pattern of the biological roles of tryptophan and their implications on animal growth.

"L-Tryptophan 98% Feed Grade" is used to supplement feeds in order to meet the digestible tryptophan requirement of animals. It is particularly suitable for post-weaning piglets, because it helps to improve feed intake and limits the negative impact of unfavourable health environment, leading to improved growth rate and feed efficiency.

Maximising feed intake is a practical challenge in post-weaning piglets and in lactating sows. At both physiological stages, supplementing a deficient diet with tryptophan has been proven to enhance feed intake. This effect should be related to the fact that this amino acid is involved in appetite regulation in animals. A published meta-analysis of 37 experiments estimates the SID Trp:Lys ratio requirement at 22% to maximize feed intake and daily weight gain in piglets (Figure 2).

Figure 2. Meta-analytic process: estimation of the standardized ileal digestible tryptophan to lysine (SID Trp:Lys) requirement for piglets between 7 and 25 kg live weight; average daily gain (ADG) and average daily feed intake (ADFI) are maximised.

During the post-weaning phases, diets have a role to play in the promotion of growth, feed intake, maintenance of a good sanitary status and avoidance of stress. The various tryptophan functions are related to each of these characteristics, therefore it is important to use an adequate tryptophan level in piglet diets.

Concerning grower pigs (25-120 kg), a meta-analysis of 15 experiments allows estimating the SID Trp:Lys requirement at 20% to maximise daily gain and feed efficiency (Figure 3).

Figure 3. Meta-analytic process: estimation of the standardized ileal digestible tryptophan to lysine (SID Trp:Lys) requirement for pigs between 25 and 120 kg live weight; average daily gain (ADG) and gain to feed ratio are maximised.

L-Tryptophan for higher performance under poor health conditions

Tryptophan is a nutrient with numerous biological functions. Different studies show that in pigs the degradation of sanitary status induces a specific decrease in plasma tryptophan concentration and reflects the specific use of this amino acid to satisfy the immune functions of the animal (for details, see "Amino acids nutrition and health maintenance").

These results emphasize the benefits of upgrading SID Trp:Lys levels up to 22% in practical farming conditions for piglets.

The tables below give a summary of Ajinomoto Animal Nutrition Europe proposals concerning tryptophan requirements per species and per growth stage when justified. These requirements, in accordance with the concept of the ideal protein, are expressed relative to lysine on a SID basis for swine and on a TD basis for poultry.

Growing pigs
Piglets Grower Finisher
SID Tryptophan:Lysine (%) 22 20 19
Lactating sows
SID Tryptophan:Lysine (%) 24
Broiler chicken Laying hens
TD Tryptophan:Lysine (%) 17 24

As for L-Lysine and L-Threonine, L-Tryptophan supplementation in monogastric feeds represents a simple, effective and accurate way of adjusting tryptophan content to animal requirements to optimize the technico-economic results of the production, particularly in situations where feed intake is the main limiting factor for performance. L-Tryptophan supplementation also contributes to a better utilization of the other amino acids and of the feed in general. In association with L-Lysine and L-Threonine, the use of L-Tryptophan makes it possible to further reduce the dietary crude protein content and, as a consequence, to reduce nitrogen excretion from animal husbandry in the environment. In piglets, these feeds with increasingly lower protein content are frequently used to limit the occurrence of digestive problems. As L-Lysine and L-Threonine, L-Tryptophan is 100% digestible, so its utilization by the animal is far greater than that of protein-bound tryptophan from vegetable proteins. 

The leaflet here below summarizes the role of tryptophan and the benefits of supplementing feeds with L-Tryptophan. 

The article below presents data on tryptophan requirements in piglets and its factors of variation.


For further information, please read our technical bulletins: numerous experimental results are reported.