**Conflict of interest**

*Advances in Poultry Nutrition Research*

are subjected to amino acid imbalance [28].

CSMFD was the most efficient.

the digestible form [24].

That a reduction in protein content and the supplementation of the diet with industrial amino acids can substantially decrease N excretion. Excretion of N can be significantly reduced by an amino acid balance that meets the requirements of birds but with minimum excess, and especially through the supply of these nutrients in

The composition of excreta is directly influenced by the nutritional composition of the diet consumed by birds. Thus, when formulating diets with high CP levels based on TAA, with the utilization of a margin of safety or by the unfamiliarity of the nutritional composition of the feeds, a high protein content in the excreta is expected [25]. According to Lemme [26], the greater the inclusion of raw materials of low amino acid digestibility based on formulations of TAA, the less reliable the estimation of bird performance. This makes the poultry industry use an onerous margin of safety at the offer of nutrients. The largest portion of N excreted by birds is related to the material that was not digested and the excessive amino acids left after requirements for maintenance and egg production have been met [4].

CST diets differed significantly from the others, presenting lower plasma uric acid concentrations. This may be explained by the diets containing only CST having

The diets based on corn and soybean meal showed a slight elevation in cost about those that contained meat and bone and feather meals, regardless of the base formulations (**Table 11**). This accrual may be attributed to the utilization of soybean meal being higher and the supplementation of industrial amino acids being lower in these diets to meet the amino acid requirement during this phase. The diets which contained the inclusion of the meat meals, however, resulted in a lower cost when compared with those using vegetable protein sources. The cost of the diet varied by 7.5% (US\$0.03) and the cost of feeding had a variation of 11.94% (US\$0.10). The impact on the reduction of soybean meal in the diet was 7.45%. In diet CSMFT, in which meat meal is included, the variation was 2%, whilst for the feather meal, it was 2.28%. In diet CSMFD, this inclusion was 3.35% and 1.81% for the meat and feather meals, respectively. The cost of feeding per bird varied by 10.85%, showing the lowest discrepancies in the diets that received the inclusion of the meals. Diet

The number of eggs produced during the laying phase (84 days) varied by 5.68% among the treatments, with a gross income at around US\$2.58 per bird during the experimental period. The data on the relative gross income of the treatments were obtained through the percentage of the gross income of diet CST, formulated with corn and soybean meal and based on TAA. Diet CSMFT showed to be more efficient, reaching 1.94% over the relative gross income of diet CST. The relative gross

income of diet CSMFD, however, was just 0.28% below that of diet CST.

Improved efficiency at FC was observed at the starter phase for the diets formulated based on DAA and when protein ingredients of animal origin were used. From 22 to 42 days of age, diets formulated based on DAA did not influence the

a higher amino acid digestibility than those containing the animal-origin byproducts (CSM, CSMF), thus increasing the usage efficiency of these amino acids. Schmidt et al. [27] state that approximately 60%–80% of the uric acid excreted in birds is N. Uric acid is related to kidney and liver metabolism and is the main source of excretion of nitrogenous bases. Uric acid excretion is utilized as an indicator of protein quality ingested by birds; less protein is deposited in the muscle and more uric acid is excreted when birds receive low-quality protein in the diet, or when they

**56**

**4. Conclusions**

The authors declare no conflict of interest.
