Basic principles for experimental diets

In growing pigs, protein deposition is closely associated with deposition of water and bone development, resulting in a weight increase of 4.4 kg for each kg deposited protein (Boisen and Verstegen, 2000). Growth and feed utilization are therefore often used as response parameters in studies on amino acid requirements. However, many different factors influence these responses, in particular the growth of the pig and, consequently, its actual amino acid requirements.

The most fundamental factor is the dietary energy supply which usually limits protein deposition up to at least 40 kg live weight (Black, 2000). Consequently, amino acid requirements should basically be related to the optimal composition of available amino acids relative to available energy in the diet. On the other hand, feed evaluations throughout the world are based on many different energy evaluation systems (Boisen and Verstegen, 1998) and a number of factors may influence the correct determination of available energy in the actual experimental diets. Furthermore, it is necessary to consider that the amino acid requirements relative to the energy supply rapidly decrease during growth (Fig. 9.2).

Therefore, in order to simplify the presumptions on which the experiments are based, experimental determinations of individual amino acid requirements relative to a reference amino acid, i.e. lysine, using the concept of an ideal dietary amino acid profile, may generally improve validity of the obtained results on amino acid requirements.

For a proper experimental determination of the optimal ratio between a specific amino acid, e.g. threonine, and lysine, five diets, only varying in the composition of threonine, are needed. Figure 9.3 illustrates the principle in the prediction of the optimal Thr:Lys ratio in a theoretically perfect experiment with five levels of threonine and each series is performed at two different levels of lysine. In both series lysine is limiting in relation to energy supply. However, threonine is only limiting at the two lowest levels, which describe the slope of the response curves, whereas the two sets with the highest levels of threonine describe the plateau when lysine is limiting, and threonine is in a relative surplus. The intercept between the two linear curves corresponds exactly to the ratio at which threonine and lysine are equally limiting and, consequently, the optimal ratio between the two amino acids. This ratio is not considered to be influenced by the concentration of the amino acids when they are only slightly below their requirements relative to the energy requirement.

For establishing the optimal ratio between a specific amino acid, e.g. threonine and lysine, the latter needs to be undersup-plied throughout the experimental period. Furthermore, all other essential amino acids need to be slightly oversupplied. Otherwise, the determined ratio between the investigated amino acid and lysine will be underestimated. According to Fig. 9.2 this is a particular problem in younger pigs where the amino acid requirements relative to the energy requirement are rapidly decreasing. The practical solution for this can be a 10% deficit of lysine and a 10% surplus of each of the other essential amino acids. This may be performed by using a multiphase feeding system in which

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Thr: Lys ratio

Flg. 9.3. Theoretically perfect experimental data for determining the optimal Th r: Lys ratio in diets for growing pigs at two different limiting levels of lysine (for further details see text).

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Thr: Lys ratio

Flg. 9.3. Theoretically perfect experimental data for determining the optimal Th r: Lys ratio in diets for growing pigs at two different limiting levels of lysine (for further details see text).

the protein concentration is gradually decreased (Boisen, 1993).

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Dieting Dilemma and Skinny Solutions

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