Wageningen UR Livestock Research
Wageningen, August 2016
Estimating requirement values for apparent faecal digestible and standardised ileal digestible tryptophan in broilers by a meta-analysis approach
T. Veldkamp, J.W. van Riel, R.A. Dekker, S. Khalaji, H. Hashemipour, M.M. van Krimpen and M.C. Blok
Wageningen UR (University & Research centre) Livestock Research, Livestock Research Report 969.
Requirement values for apparent faecal digestible and standardised ileal digestible tryptophan of broilers at different ages were estimated by a meta-analysis approach. This study was part of a project to estimate the apparent faecal (AFD) and standardized ileal digestible (SID) amino acid requirement values of the first limiting amino acids in both broilers and laying hens. Peer reviewed papers were selected, describing experimental results of dose response studies in which the effect of graded levels of free amino acids supplemented to a basal diet on body weight gain and feed conversion ratio in broilers was studied. The papers searched for were published during the period 1994 – 2012. Subsequently, a stepwise process was applied for the selection of the research data to be used in the meta-analysis. In total 11 criteria were set. Two of these criteria are: at least three graded levels of supplementation of the amino acid of interest to the same basal diet; maximal supplementation of the amino acid of interest was at least 10% higher compared to the concentration of the amino acid of interest in the basal (non-supplemented) diet. Feed ingredient composition of the experimental diets should be present in each paper; this information was included in a separate database and nutrient composition of the experimental diets was recalculated by using data on the nutritional composition of the individual feed ingredients according to the CVB Feed Table (2007). When the determined level of the amino acid of interest (and – when presented – of other amino acids) on an AFD basis was published, this information was used. In case this information was not presented, the level of the amino acid of interest (and of other amino acids) was calculated by using either the total amino acid levels in the basal diets as analysed by the authors or as calculated by using the CVB Feed Table (2007) in combination with the digestibility on an AFD basis as published by CVB (CVB Table, 2007) In addition, the concentrations of standardized ileal digestible (SID) amino acids in the diets of each study were also calculated using the digestibility on a SID basis as tabulated bij CVB (Dekker and Blok 2015) and included in the database. These (calculated) dietary concentration of AFD and SID amino acids were used in the present study for the regression analyses to derive requirement values for AFD and SID amino acids in broilers. The responses of body weight gain (BWG) and feed conversion ratio (FCR) to supplementation of the free amino acid of interest to a basal diet were analysed for each individual experiment included in the database by regression analysis. Mean data for BWG and FCR per experimental group as provided in the original paper were used as response parameters. Response of BWG and FCR to supplementation of the free amino acid of interest was determined by use of an exponential model. For each individual experiment the estimated requirement (Req) for the amino acid of interest was calculated as the amino acid concentration at which 95% of the response (BWG and FCR) between intercept and asymptotic value was reached. Data of studies that could not be fitted with the exponential model were excluded from further evaluation. Also studies, where the estimated requirement value was over 110% of the maximum concentration of the amino acid of interest in the diet with the highest supplementation level, were excluded from further evaluation. Studies in which a non-test amino acid might have been co-limiting (< 90% of CVB 1996) at higher supplementation levels of the amino acid of interest were also excluded from the dataset. After estimation and evaluation of the amino acid requirement values for the individual studies, an overall regression model was used to fit the requirement values of the amino acid of interest on an AFD and SID basis for BWG and FCR as a function of age of the broilers. For this purpose the mean age of broilers in each experiment was calculated as (age at start of the experimental period + age at the end of the experimental period)/2. In total, 10 studies each containing one or more experiments, were judged. In total, 11 experiments from 6 papers met the criteria for inclusion into the database. One of the criteria of inclusion of experiments in the overall analyses was that the concentrations of these amino acids relative to the amino acid of interest on an AFD basis were at least 0.90 of the ratio of the requirement of the same amino acids on an AFD basis according to CVB (1996). For tryptophan, none of the studies did met the criterion and therefore this criterion was not used. At the used tryptophan requirement of 20% relative to lysine instead of 16% relative to lysine (according to CVB (1996)) all studies were included. Requirement values of tryptophan on AFD and on SID basis at different ages for BWG and FCR were restricted to the age period of 9.5 to 12.5 days of age. Therefore, it was not possible to estimate the requirement values for the total production period.