In recent years, the reduced protein diets balanced with crystalline amino acids (AA) for broiler chickens have been developed due to economic, environmental and bird welfare advantage. However, this reduced protein diet quite often results in adipose fat accumulation and sometimes compromises chicken performance. In practice, reduced protein diets should be formulated by experienced nutritionists.
Ideal amino acids profile
In reduced protein diets, digestible lysine concentration should be the first limiting AA. Based on the ideal AA profile, other essential AA concentrations including Methionine, Threonine, Valine, Isoleucine and Arginine need to be increased accordingly while the dietary lysine concentration is increased. When the dietary lysine concentration is fixed, increasing any other essential AA will not improve chicken performance but depressed the liter quality due to increased N excretion. On the other hand, when any other essential AA is deficient, Lysine becomes surplus and increases carcass fat. Figure 1 clearly showed that in reduced protein diets, when dietary Arginine and Histidine are deficient, it is very likely resulting in fatty carcass. Therefore, in recent protein diets, digestible Arginine and Histidine to digestible lysine ratios were increased to be 115% and 40%, respectively.
Figure 1. Effect of essential AA on abdominal fat content in reduced protein diets
Methionine sources
In broiler chicken feed ingredients, Methionine is the first limiting AA and practically be overused in chicken production. Figure 2 clearly showed that compared with DL-methionine and DL-HMTBA, L-Methionine significantly reduced fat composition in breast meat.
Figure 2 Effect of Methionine from different sources
The optimal digestible Lysine to apparent metabolizable energy (AME)
In reduced protein diets, if dietary AA is not balanced based on the ideal AA profile, the extra energy is required to excrete surplus energy. In contrast, if dietary AA is balanced based on the ideal AA profile, surplus energy will be accumulated in chicken body. Figure 3 is shown that reducing 100 kcal/kg in reduced protein diet significantly reduced abdominal fat in chicken carcass. It is suggested that the optimal Digestible Lysine to AME in reduced protein diets will be 100, 90, and 80% in the starter, grower, and finisher periods, respectively.
Figure 3. Effect of higher digestible Lysine to AME ratio on abdominal fat contents in chicken carcass
Supplementation of β-Mannanase
Soybean meal, canola meal and faba beans contain β-Mannan about 0.8, 0.4 and 0.4%, respectively. Β-Mannan can provoke the activity of innate immune system which leads to unnecessary energy cost for broiler chickens. Therefore, adding the exogenous β-mannanase to the reduced protein diets could save some energy cost and reduce carcass fat. Figure 4 is shown that in the reduced AME diets, adding β-Mannanase could further reduce the abdominal fat contents.
Figure 4. Effect of β-Mannanase on the abdominal fat contents in high and low AME diets