Towards a physiological feeding strategy for protein in broilers

Abstract

The present thesis describes the development of a physiological feeding strategy for protein in broilers. This project was initiated to improve the efficiency of deposition of dietary nitrogen into animal product in order to reduce nitrogen excretion to the environment. The physiological feeding strategy was based on the consideration that feeding the birds according to their requirements based on physiological status would result in the highest efficiency. At the start of the project, requirement data for amino acids were considered not adequate. For this reason the amino acid pattern required was estimated based on a factorial approach. Generally in a factorial approach requirements are divided in a requirement for production and a requirement for non- productive purposes. The required amino acid pattern for production was based on the amino acid pattern of body accretion. Due to the high growth rate of broiler chicks and high N-efficiency the largest part of dietary amino acids is used for nett protein deposition. This implies that the required dietary amino acid pattern is mainly determined by the amino acid pattern of body accretion.In the first chapter whole body amino acid composition of broiler chicks was studied with the aim to provide this basis amino acid pattern for diet formulation. Three genotypes were studied; a commercial broiler hybrid (Corn) and two experimental lines: one selected for high growth rate (GL) and one selected for efficient feed conversion (FC). Male and female Corn chicks were compared. Change of whole body amino acid composition with age was studied in male Corn chicks by weekly analysis.Whole body amino acid composition at the consecutive sampling moments showed several small, but statistically significant (PThe three genotypes differed in relative amino acid composition. Analysis of growth pattern revealed no differences between Corn and GL genotype, the FC genotype however had a clearly different growth pattern. Similar growth patterns did not result in similar whole body amino acid composition.Compared to male Corn birds, whole body of female Corn birds contained relative less essential amino acids (cys, met, val, tyr, phe and his) and more non-essential (glu, asp) amino acids. It is suggested that the amino acid pattern of whole body accretion in male Corn chicks can be used as a basis for diet formulation for broiler chicks of different genotypes and sexes.When formulating diets based on the required amino acid pattern it is essential to know the nett availability of nutrients from the diet. For this reason in chapter 2 a digestibility study is included in this thesis. The influences of genotype, age and sex on droppings digestibility coefficients of a compound food were studied using male and female broiler chickens of three different genotypes at 2, 4 and 6 weeks of age. Because the traditional method of determination of droppings digestibility coefficients of nitrogen may lead to systematic errors in estimating the feeding value of foodstuffs, a method is proposed to determine the ileal digestibility coefficients. The ileal method is compared with the droppings method for a mixed food and for two foodstuffs: wheat and solvent-extracted soyabean meal.Birds selected for efficient food conversion showed distinctly higher digestibility coefficients for all nutrients than birds selected for high growth potential or birds from a commercial strain. The influence of age on digestibility coefficients was not consistent. Female birds showed digestibility coefficients which were, in general, 3% higher than those of male chickens. Interactions of genotype and sex and between genotype and age for energy metabolisability were the only interactions observed for digestibility measurements.The method of determination influenced the amino acid digestibility coefficients of the mixed food and the relative feeding values of wheat and soyabean meal.It is important to use well defined animals (genotype, sex, age) in evaluating foodstuffs. The preferred method for determination of digestibility coefficients of nitrogen and amino acids is based on ileal sampling, although the differences in amino acid digestibility coefficients were small between methods.After amino acids are made available from the diet, the metabolic utilisation of the amino acids determines whether they are used for either protein synthesis or for non- productive purposes. The utilisation of amino acids for non-productive purposes was studied with 14CO 2 breath test measurements. In these measurements a tracer quantitiy of a [ 14C]amino acid is injected in a bird with a defined physiological status and exhaled 14CO 2 is collected during 4½ hours after injection. 14CO 2 recovery is a measure for oxidation of the amino acid.In chapter 3 the effect of nutritional status (dietary lysine content, time of injection relative to the meal) on 14CO 2 recovery from intraperitoneal hp) injected [U- 14C] lysine was studied in broiler chicks.It was found that ip injection of [ 14C] labelled amino acids ([U- 14C] lysine) sometimes resulted in unrealistic high recovery values in 14CO 2 breath test measurements. The most probable explanation is that with ip-injection in some cases label is injected in an abdominal air sac. In this case part of the substrate is not converted to 14CO 2 but is straight excreted as an aerosol. For this reason ip-injection of tracer amino acids is considered to be risky in chickens and therefore subcutaneous injection is used in further experiments.When only the realistic, lower, range of 14CO 2 recoveries from ([U- 14C] lysine were considered, lysine content of the diet caused a response in 14CO 2 recovery while time of injection did not give a detectable effect. Low lysine diet resulted in lower 14CO 2 recovery, indicating a restriction of lysine oxidation.In birds conditioned on a lighting regime of 1L:2D, 14CO 2 recovery from subcutaneous injected [1- 14C]leucine or [1- 14C]valine was studied within one cycle of the lighting regime with injections at 30, 60, 120 or 180 minutes after onset of a half-hour meal.For both [1- 14C] leucine and [1- 14C]valine 14CO 2 recovery was higher when injection at 30 or 60 min after onset of a half hour meal is compared to injection at 120 or 180 min. This can be interpreted as that a 14CO 2 breath test measurement should be started at 30 or 60 minutes after onset of a meal when amino acid utilisation in fed animals is to be investigated.In chapter 4 the effect of dietary amino acid content and of length of adaptation period on 14CO 2 , recovery from [ 14C]amino acids was studied in broiler chick breath test measurements.A lower (70% of normal) dietary lysine level increased 14CO 2 recovery from [1- 14C]leucine as well as from [1- 14C]valine during the first growth phase (5-22 d of age). In older chicks this effect was less pronounced. A reduced vs increased dietary methionine level (67 vs 133 % of normal) increased 14CO 2 recovery from [1- 14C]leucine only at about 4 weeks of age. At a low dietary lysine level 14CO 2 recovery from [U- 14C]lysine was lower than at a normal lysine level. Length of adaptation period to a low lysine diet (1, 2, 3 or 5 d) had no statistically significant effect on 14CO 2 2 recovery from [1- 14C]leucineThe utilization of leucine and valine for protein synthesis, as indicated by the complement of the 14CO 2 recovery from [ 14C]amino acids, is affected by dietary limiting amino acid level. The age-dependent effect of dietary methionine level suggests that the method is especially suited for assessment of specific requirements during specific stages of development. The absence of influence of length of adaptation period to a low lysine diet on 14CO 2 recovery from [ 14C] leucine suggests that adaptation is either rapid (14CO 2 recovery from [ 14C ]amino acids indicates amino acid adequacy of the diet fed immediately preceeding (20h period) the breath test measurement.In chapter 5, nitrogen efficiency was studied in birds fed diets based on the amino acid pattern of body accretion at three levels of proteinlenergy (P/E) ratio, supplemented with Met, Arg, Glu and Gly, individually, as a group or not. This study was performed to see whether the amino acid pattern of body accretion could be used as a basis for diet formulation or that specific alterations in the pattern were required. The level of protein/energy (P/E) ratio may affect N-efficiency Furthermore, there may be an interaction between dietary composition and genotype. Growth performance and nitrogen efficiency were measured in male and female chicks of two different genotypes, fed diets differing in amino acid composition and in P/E ratio. There were twelve diets. Three diets, of which the amino acid pattern was similar to the amino acid pattern of body accretion, differed only in P/E ratio (High: 17.8 and 19.3 g CP/MJ AME during 0-21 d of age and 22 d-slaughter age, respectively; Medium and Low: 94 % and 87 % of High). Six other diets were based on the medium P/E ratio but were supplemented with either Met, Arg, Glu, Gly, Glu + Gly or all four amino acids together. Two more diets were based on the low or high P/E ratio and supplemented with all four amino acids. Diet 12 was a control diet with a semi-practical composition. Male chicks of a commercial broiler hybrid (Corn) were fed all diets. Male chicks from a population selected for low feed conversion ratio (FC) were fed the three diets differing in P/E ratio plus the medium P/E diet supplemented with all four amino acids. This latter diet and the unsupplemented medium P/E diet were fed to female chicks of both genotypes.It was shown that no amino acid supplementation improved N-efficiency; Metsupplementation improved growth rate at similar N-efficiency while other supplementations negatively affected N-efficiency. Both genotypes reacted similar to differences in dietary composition. This supports the hypothesis that the amino acid pattern of body accretion is a good reference for dietary amino acid pattern. Female chicks also reacted similar to differences in dietary composition as male chicks but attained a much lower N-efficiency. Growth performance was lower at low P/E ratio while N-efficiency was lower at high P/E ratio. This shows that maximal growth performance and maximal N-efficiency can not be attained simultaneously. It is calculated that, compared to male Com chicks fed the control diet, feeding the diet with highest N- efficiency (low P/E ratio) will result in a reduction of N-excretion of 11 %. When this diet is fed to male FC chicks, N-excretion will be only 67 % of that of male Com chicks fed the control diet. It is concluded that dietary as well as genetic improvement will result in lower N-excretion.Based on the previous studies it was hypothesized that nitrogen efficiency could be further increased by a decrease in dietary supply of specific non-essential amino acids (Glu + Asp + Ala, NEAA) since these amino acids are synthesized as a result of the degradation of essential amino acids. Normally these amino acids function to transport amino acid nitrogen from the sites of amino acid degradation to the site of where they are combined with glycine to synthesize uric acid which is subsequently excreted. If the excretion of uric acid is increased by extra dietary non-essential amino acid nitrogen this will result in a extra demand for glycine since glycine, contrary to the other amino acids contributing nitrogen to uric acid, is completely incorporated. Since glycine can be synthesized from threonine an increase in dietary NEAA level might result in an increased non-productive threonine requirement.In chapters 6 and 7 three experiments are described in which responses of birds fed several different diets are compared. Chapter 6 describes technical performance such as growth rate, feed conversion ratio (FCR) and nitrogen efficiency whereas chapter 7 describes weight and yield percentages of edible and other portions after slaughter. In the first experiment described in chapters 6 and 7 a comparison is made between a practical diet and experimental diets based on either data from the literature or the amino acid pattern of body accretion plus some adaptations of both these patterns. It was shown that the amino acid pattern of body accretion can be used as basis for calculation of amino acid requirements. There were no differences in response between a diet based on literature or a diet based on the amino acid pattern of body accretion. The practical diet resulted in a lower N-efficiency, higher growth rate and lower FCR. In the second growth phase (15-35 d of age) there was a positive effect of extra methionine (relative to the pattern of body accretion) on growth and FCR without negative effect on N-efficiency. This implies that in this growth phase dietary methionine level should be higher than that based on the level of body accretion.It was shown that absolute meat production was only slightly affected by dietary amino acid pattern while for relative yields several significant but minor differences were found. Breast meat percentage was highest in birds fed a diet with extra Met and lowest in birds fed a diet with extra Glu. It is concluded that within three proposed dietary amino acid patterns variation in dietary amino acid pattern had only minor influence on slaughter efficiency.In the second experiment described in chapters 6 and 7 it was tested whether it was possible to promote utilisation of non-essential amino acids formed in the obligatory breakdown of essential amino acids for nett protein synthesis by lowering the dietary non-essential amino acid level. Nine diets with varying levels of glutamic acid + aspartic acid + alanine (NEAA) were fed to male broiler chicks. It was shown that with lower NEAA levels growth rate was lowered, FCR was higher, but N-efficiency was higher. Maximal growth rate and maximal N-efficiency can thus not be reached simultaneously.It was shown that variation in dietary NEAA level had no effect on yields of body parts as percentage of live weight. Total production was related to live weight and was decreased with decreasing dietary NEAA level.The third experiment described in chapters 6 and 7 was based on the theory that high NEAA excess will lead to extra uric acid production which will lead to an increase in glycine requirement. Since glycine can be formed from threonine, threonine requirement might be affected by NEAA level. In an experiment with three threonine and three NEAA levels it was shown that at high NEAA level the effect of low threonine level was more severe than at low NEAA level. A distinct interaction between dietary Thr and NEAA level on yields was observed. This supports the hypothesis that the requirement of threonine is determined not only by the requirement for body accretion but also by a requirement for glycine production to excrete excess nitrogen.In general it can be concluded that the developed physiological feeding strategy can improve nitrogen efficiency. However, the limits to practical application may depend on an economic evaluation. Moreover, an improved knowledge concerning utilisation of non-essential amino acids for uric acid and protein synthesis may result in further improvement in the physiological feeding strategy