Juarez Donzele
Dear Dr. Donzele,
Thank you for the comment. I can explain the logic for our trial design, setting Lys as suboptimal level in all diets (also for the diet at 15% Trp:Lys ratio), as follows.
The requirement of an amino acid (AA), e.g. Trp and Lys, can be estimated on the basis of grams per day. This factorial approach is used in growth model (e.g. NRC, 2021). Another approach, obviously, is empirical dose-response trials which is more frequently used to estimate AA requirement or ratio in pigs.
It is well known that for estimating the absolute requirement of an amino acid (e.g. Lys; % of diet), we have to make sure that all other amino acids (AA) should meet or preferably slightly above requirement in the basal diet to avoid potential their co-limitation and to make sure that the response is only due to graded levels of Lys.
For estimating the optimal ratio of an AA relative to Lys, theoretically it is also possible to provide dietary Lys level right at the requirement (as you are referring to) but the Lys level must not exceed requirement level to avoid the risk of underestimation of AA ratio relative to Lys. In practice, it is not so easy to know exactly Lys requirement for a group of pigs used in the trials.
In our experiment, estimation of optimal Trp:Lys ratio, to avoid underestimation of the determined Trp:Lys ratio, the level of Lys was balanced to be slightly below requirement throughout the experimental period. This means that Lys is second limiting AA while other AA (except Trp and Lys) were supplied at or above requirement.
This application of this methodological approach has been published, and commonly applied in today’s AA:Lys ratio trials. For example, Boisen (2000): Ideal Dietary Amino Acid Profiles for Pigs (Book Chapter 9) explained in detail about the need of setting Lys to be below requirement and gave a practical suggestion to target dietary Lys to be a 10% deficit and a 10% surplus of each of the other essential AA.
In poultry, Knowles and Southern (1998): The Lysine Requirement and Ratio of Total Sulfur Amino Acids to Lysine for Chicks Fed Adequate or Inadequate Lysine (Poultry Science 77:564–569) also validated that the optimal SAA:Lys ratio in broiler chicks was identical when chicks are fed at requirement (1.0% Lys) or slightly below their Lys requirement (0.82%).
I hope my response answers your question.
Best regards,
John Htoo
Thank you Dr. Mroz. You likely know a lot more about Trp and stress than I do. But in some of our experiments with corn protein we demonstrated a close relationship between elevated concentration of Leu in the diet and reduced serotonin in the hypothalamus. This reduction can be partly overcome by adding more Trp to the diet. Because all corn proteins including DDGS have high concentrations of Leu we hypothesized that Trp requirements may be greater in pigs fed DDGS or other corn proteins. So that was the reason for conducting this research.
Dear Dr. Donzele,
Thank you for the feedback and additional explanation. Indeed, I fully agree on the merit of adding an additional diet (we can call it as reference or positive control diet) containing Lys at adequate level. For example, we added such positive control diet which contained both Lys and Trp at adequate level in an experiment in Germany to check/make sure Lys was suboptimal in the Trp:Lys titration diets (published: M. Naatjes et al. / Livestock Science 163 (2014) 102–109). Sometimes, having not sufficient number of pens limited to have one more treatment but ideally, this positive control diet should be included in the AA:Lys ratio trials. Thanks for highlighting this.