Browsing by Subject "Broiler chicken"

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Bestimmung präcecaler Verdaulichkeitskoeffizienten für heimische Energie- und Proteinfuttermittel für die Bio-Hühnermast
(2016) Ritteser, Carolin; Grashorn, Michael
To improving the supply of slow growing broiler chicken in organic poultry production with essential amino acids the ileal digestibility (ID) of 15 organically cultivated feedstuffs was determined using a linear regression approach. The ID was determined for three and six week old chickens. The feedstuffs included common cereal species such as wheat, rye, spelt and summer barley, hull-less cereals such as naked barley and naked oats, forgotten cereals or pseudo-cereals such as brown top millet, pearl millet and buckwheat but also corn silage, strip waste of lentils, alfalfa leaves, and clover grass silage (extruded and untreated). The cultivars were added to a basal diet in 3 concentrations in exchange to corn starch. Therefore, the increase for crude protein (CP) and amino acids (AA) resulted from the cultivar only. Titanium dioxide was used as an indigestible marker. Each diet was fed ad libitum between day 15 and 21 d of age to 6 pens of 15 chickens or between day 36 and 42 d of age to 6 pens of 6 chickens, each. On day 21 and/or day 42 birds were killed by carbon dioxide, the chymus was collected and pooled for the 15 or 6 chickens per pen, respectively and analyzed on pen basis. Contents of crude protein, amino acids and titanium dioxide have been determined. The slope of the regression line between the amount of AA intake by feed and the amount of AA digested up to the ileum is considered to be the true ileal digestibility. The crude nutrient contents of the organically grown feedstuffs show different results in comparison to conventionally grown ones. The highest crude protein content was determined for lentils, followed by clover grass silage and dried alfalfa leaves. However, the alfalfa leaves also showed the highest content of crude fiber. Brown top millet, pearl millet, clover grass silage and alfalfa leaves contained the highest amount of methionine. The highest lysine contents were found for lentils, clover grass silage and alfalfa leaves. Values for digestibility of crude protein and amino acids varied strongly among the feedstuffs. Rye, spelt and the clover grass silages were digested only poorly by both, the three and the six week old broilers. This was probably caused by an increased content of antinutritional substances and crude fiber. Extrusion didn´t have any positive effect on the amino acids digestibility of the clover grass silage, it rather impaired digestibility. For buckwheat and clover grass silage the high contents of crude protein and amino acids were compensated by poor digestibility. Brown top millet, pearl millet, strip waste of lentils and especially alfalfa leaves turned out to be appropriate feedstuffs for an adequate protein and amino acids supply in broiler nutrition. Due to the high fiber content alfalfa leaves should be used carefully in the diet. Their high crude fiber content causes an increase in feed volume and this may result in a too low feed intake of broilers to fulfill their nutritional requirements. This again can cause growth disturbances. In general, digestibility values for the organically grown feedstuffs were found to be similar to the values of conventionally grown feedstuffs.
Bi-objective optimization of nutrient intake and performance of broiler chickens using Gaussian process regression and genetic algorithm
(2023) Ahmadi, Hamed; Rodehutscord, Markus; Siegert, Wolfgang
This study investigated whether quantifying the trade-off between the maxima of two response traits increases the accuracy of diet formulation. To achieve this, average daily weight gain (ADG) and gain:feed ratio (G:F) responses of 7–21-day-old broiler chickens to the dietary supply of three nutrients (intake of digestible glycine equivalents, digestible threonine, and total choline) were modeled using a newly developed hybrid machine learning-based method of Gaussian process regression and genetic algorithm. The dataset comprised 90 data lines. Model-fit-criteria indicated a high model adjustment and no prediction bias of the models. The bi-objective optimization scenarios through Pareto front revealed the trade-off between maximized ADG and maximized G:F and provided information on the needed input of the three nutrients that interact with each other to achieve the trade-off scenarios. The trade-off scenarios followed a nonlinear pattern. This indicated that choosing target values intermediate to maximized ADG and G:F after single-objective optimization is less accurate than feed formulation after quantifying the trade-off. In conclusion, knowledge of the trade-off between maximized ADG and maximized G:F and the needed nutrient inputs will help feed formulators to optimize their feed with a more holistic approach.
Evaluation of the availability of different mineral phosphorus sources in broilers
(2012) Shastak, Yauheni; Rodehutscord, Markus
Inorganic feed phosphates are an indispensable supplement for compounding poultry feed. The requirement of available P in broiler chicks cannot be covered only with plant ingredients as P in plant feedstuff is largely presented in form of phytate which is only partially available in avian species. Due to the increase in prices for feed phosphates and environmental concerns associated with excessive excretion of P by livestock, the knowledge about the availability of P from mineral sources has gained in importance during the last decade. However, there is still no standardized method available for assessing the P availability of inorganic feed phosphates. Without knowledge of the exact quantitative values of the P availability for different P sources, it is not possible to formulate adequate diets without the risk of deficiency or excess supplementation. There are various approaches which are used by different laboratories for the determination of P availability. The main problem is, however, that it is not clear how the differences between approaches affect the results. The development of a standardized method of P evaluation, which allows obtaining quantitative values for P availability, is the basis for optimizing the dietary P concentration in broiler diets. The major objective of this thesis was to compare various methodological approaches that are used internationally to determine P availability in terms of their suitability. Therefore, firstly the P availability of two mineral phosphates was determined in 3- and 5-wk-old broilers based on data for P retention and prececal digestibility. The P availability of both mineral sources was calculated for both ages of birds by regression analyses for comparison of both response criteria. Secondly, the tibia bone ash and other bone criteria were determined. A comparison of these bone response criteria was then carried out by relating these data to measurements made on P retention. Thirdly, the suitability of tibia P retention for the estimation of the whole body P retention was investigated at both ages of birds. Variation in P retention of birds in these studies was additionally caused by the level and the source of P in the diet. In a fourth study, the effect of the basal diet composition on the availability of a feed phosphate was investigated based on quantitative P retention. A phytin-containing corn-SBM-based as well as a purified basal diet was used. Moreover, the impact of the inorganic phosphate level on the IP6 hydrolysis of the corn-SBM-based diet was assessed on the basis of excreta collection. In the first study, a corn-SBM-based basal diet was used (0.35% P on dry matter basis). MSPa or DCPa was supplemented to increment the P concentration by 0.08%, 0.16%, and 0.24%. Two balance trials (n=8 birds per diet) and two digestibility trials (n=8 pens with 10 birds per diet) were conducted (8 treatments per diet). In 3-wk-old broilers, P retention for MSPa was 70% and significantly higher (P < 0.001) than for DCPa (29%), as calculated by linear regression analysis. Values determined for P pc digestibility at the same age were very similar (67% for MSPa and 30% for DCPa; P < 0.001). In 5-wk-old broilers, P retention was 63% (MSPa) and 29% (DCPa) (P < 0.001), and pc digestibility was 54% (MSPa) and 25% (DCPa) (P = 0.002). In conclusion, in 3-wk-old broilers results obtained with both approaches were the same. In 5-wk-old broilers, the ranking of the two P sources was the same for both approaches. Values differed not greatly between the two age periods. The second study was linked to the first one, and the experimental design was the same. The study comprised two periods with birds of different ages, but from the same hatch. The response criteria evaluated were tibia, tarsometatarsus, toe ash, and P, as well as the Quantitative Computed Tomography measurements of tibiae, blood Pi concentration, and body weight gain. Responses were evaluated and compared based on linear regression analysis. In general, MSPa had a greater slope than DCPa for all criteria studied. For the different bones, the ratio of slopes was very similar based on the amount of ash in both periods. Foot ash was proved to be as sensitive as tibia ash in both periods. Blood serum Pi and body weight gain were not sufficiently sensitive criteria for P evaluation. We concluded that the ranking of both mineral P sources based on bone criteria differed from the ranking that was based on P retention or pc digestibility. The third study was also linked to the first one. Thus, the experimental design was the same. On days 21 and 35, two chicks per treatment were randomly chosen. Contents of P and Ca were determined in tibiae-free bodies and tibiae. The whole body P to tibia P ratio was 21.3±1.3 at d 21 and 19.8±1.1 at d 35 of age. The slope of linear regressions between the tibia P and the whole body P for both ages was identical (17.7). Results indicated that changes in tibia P may be suitable to predict changes in whole body P retention. In the last experiment, a phytin-containing as well as a purified basal diet, both containing 1.8 g available P per kg feed dry matter, was supplemented with MSPa to increment the P concentration by 0.05%, 0.1%, and 0.15%. A retention trial with excreta collection from d 20-24 was conducted (n=7 birds per diet). The level of P did not significantly affect the total P retention either of the corn-SBM-based or of the purified basal diet (P > 0.05). However, increasing the P level significantly reduced (P = 0.015) the IP6 hydrolysis for the corn-SBM-based diets. Percentage P retention for MSPa was calculated by linear regression analysis. P retention for MSPa was 50% for the corn-SBM-based diet and 51% for the purified diet. We concluded that there was no difference in P retention from MSPa between corn-SBM-based and purified diets. It can be concluded from the results of the present thesis that both retention and pc digestibility can be used for evaluating mineral P sources in broilers based on a regression approach. The ranking of mineral P sources based on bone criteria differed from the ranking that was based on P retention or pc digestibility. There was no difference in P retention from MSPa between corn-SBM-based and purified based diets, but a significant effect of the P-level on the IP6 hydrolysis in corn-SBM-based basal diets was found.
High-throughput sequencing techniques to analyze microbial communities in the gastrointestinal tract of broiler chickens
(2018) Borda Molina, Daniel Enrique; Camarinha-Silva, Amélia
Broiler chicken represents an excellent case-study to elucidate the inter-communication between the host and its microbial communities. The general aim of this thesis was to describe the changes in bacterial community structure that occurred in chickens, in response to different experimental diets. An update of the state of the art of the chicken gastrointestinal microbiota was done in chapter 2. The composition and functionality are described through the most recent technologies that provide taxonomic information at DNA level using 16S rRNA genes. Gene catalogs and their abundance are deciphered through shotgun metagenome sequencing, which is still at its infancy and only eight publications have been published so far. At the protein level, only two studies were found that contribute metaproteomic information. Thanks to these technologies many studies were able to focus on answering how feed supplementations altered the microbes in GIT sections. The second part presented in chapter 3 comprises an extensive investigation of the broiler chicken microbiota composition in digesta and mucosa of individual samples under varying supplementation of calcium, phosphorus, and phytase. The dietary impact on the distribution of the microbial communities was studied in the crop, ileum, and caecum through illumina sequencing of the 16S rRNA gene. One important outcome was the high variability in the microbial composition between individual samples. Significant differences were observed between the digesta and mucosa samples, supporting the hypothesis that being close to the host, mucosa-associated communities show a different composition. A calcium effect on the performance was observed, where values for body weight gain and feed conversion were lower in comparison to the other treatments. Microbial communities in the crop mucosa revealed a dietary effect, while in the digesta samples no significant changes were seen. Regarding the ileum mucosa, there was an effect of P addition on the microbial distribution. As expected, caeca-derived samples showed an increase in the diversity indexes when compared to the ileum and crop and butyrate producers were detected in higher abundance. A lower microbial diversity in the crop was linked to lower growth performance regarding the supplementation of Ca. Hence, each dietary treatment affected the microbial communities; nevertheless, none of the dietary treatments displayed a consistent effect across the studied gut sections. Additionally, the effects of supplementing different proteases and one phytase on the microbial community of the ileum of broiler chickens was assessed. Thus, the specific aim of chapter 4 was to determine how enzyme supplementation affects the microbiota composition in the ileum of broilers and whether these effects were related to differences in pre-caecal AA digestibility. Three different protease sources at a low and high level were included. The microbial taxonomy was assessed through 16S rRNA gene Illumina amplicon sequencing. Performance results revealed a significant increase in growth and feed efficiency in broilers fed with phytase only and the high dosage of protease C, in comparison to the control. Most of the AA showed a significant difference between the control diet and protease C at high dosage and phytase diets. Effects on microbiota composition were observed at the genus level for some protease and phytase supplementations. The genera Streptococcus, Lactobacillus, and uncultured Clostridiaceae were responsible for these differences. This study demonstrates that effects of enzyme supplementation were evident in the terminal small intestine microbiota composition, and, to a lesser extent, in pc AA digestibility. However, the changes in microbiota composition and pc AA digestibility could not be correlated which may indicate the absence of a causal relationship. Finally, an outlook with metagenome sequencing is presented in chapter 5, to further characterize the result of feeding strategies. Metabolism information, essential to microbial activities registered 50% of abundant genes in the supplemented diets while being reduced to 40% in the control samples Phosphatases pathways and butyrate production increased in the supplemented diets while calcium signaling pathway was higher in the control. In conclusion, within this project a method of standardization to study the microbiota along the gastrointestinal tract of broiler chickens was successfully established. The obtained results revealed a significant impact of both, enzyme and mineral supplementation in the individual sections of the GIT. Also, it was proved that even if the GIT works as an interconnected system, its compartmentalization creates different environmental conditions which influence the microbiota. This study provides insights into the responses of the bacteria and their functionality which were stimulated by the feed supplementations.
Interactive effects of feed particle size, calcium, and exogenous phytase on gastrointestinal phytate degradation and related traits in broiler chickens
(2025) Wolfrum, Stephanie; Rodehutscord, Markus
Phosphorus (P) is an essential element in maintaining the physiological mechanisms of poultry, making an adequate dietary P supply crucial. In plant-based feed ingredients commonly used in poultry nutrition, the primary form of P is phytate, any salt of phytic acid (InsP6). Enzymes that hydrolyze InsP6, such as phytases and phosphatases, are needed to release P from InsP6 for digestion and absorption. The extent to which P is released depends on the limited and variable endogenous enzyme capacity of poultry. Consequently, diets are often supplemented with costly mineral P sources derived from finite global rock phosphates to meet P requirements. Any unabsorbed P is excreted and contributes to environmental impacts. Incorporating exogenous phytases into poultry diets enhances the P release from InsP6, thereby enabling reduced use of mineral P supplements. To further improve P utilization and completely replace supplemental mineral P, it is essential to understand the effects on phytase efficacy, both single and interactive. Therefore, three studies were conducted to gain deeper insight into the InsP6 degradation and related characteristics along the digestive tract of broiler chickens, as influenced by dietary ingredients and their physical properties. Using coarsely ground feed particles in broiler diets improves digestive tract development and nutrient utilization. Coarse particles may therefore be beneficial for InsP6 degradation. Moreover, dietary coarse particles might mitigate the inhibitory effects of high dietary calcium (Ca) concentrations by lowering the pH in the digestive tract. The first study aimed to investigate the single and interactive effects of feed particle size and Ca concentration in the absence and presence of exogenous phytase on InsP6 degradation, prececal digestibility of P, Ca, and amino acids (AA), and retention of P, Ca, and nitrogen (N). Ross 308 broiler chickens received experimental diets from day 7 to 22/23 of life. Experimental diets differed in feed particle size (fine and coarse; 222 and 309 μm), Ca concentration (low and high; 4.9 and 7.2 g/kg), and phytase supplementation (0 and 1,000 FTU/kg). Prececal InsP6 disappearance increased with coarse particle size without phytase supplementation. Supplemented phytase removed such differences. Coarse particles were associated with higher gizzard weights and lower gizzard content pH compared to fine particles, whereas high dietary Ca concentration increased the pH of the gizzard content. Thus, the antinutritive effects of the higher dietary Ca concentration were not mitigated by using coarse feed particles, resulting in a reduction in prececal InsP6 disappearance and digestibility of P and most of the AA. Prececal AA digestibility increased with dietary coarse particles compared to fine particles. In the presence of phytase, high dietary Ca enhanced InsP6 disappearance in the crop. The first study found that dietary coarsely ground feed particles without supplemented phytase increased prececal InsP6 disappearance, but phytase supplementation compensated for this particle size effect. Accordingly, the objective of the second study was to determine the single and interactive effects of feed particle size and phytase level. Effects on growth performance, prececal InsP6 disappearance, prececal digestibility of P and Ca, and tibia characteristics were examined. From day 10 to 38 of life, Ross 308 broilers were fed experimental diets containing fine and coarse feed particles (434 and 729 μm) and without and with phytase supplementation (300, 600, and 1,200 FTU/kg). Particle size and phytase did not interact significantly concerning any measured trait. Contrary to expectations, finely ground feed particles increased prececal InsP6 disappearance and P digestibility, suggesting coarse grinding of rapeseed meal may not benefit birds. Growth performance, prececal InsP6 disappearance and related P digestibility, as well as tibia ash and breaking strength, were increased by phytase supplementation. This indicated that diets met the bird’s P requirement at the given total P level and 1,200 FTU phytase/kg, allowing the renouncement of feed phosphate in the grower and finisher phases without adverse effects on performance and bones. Based on a previous study that found a linear relationship between the dietary InsP6 concentration and the prececal InsP6 disappearance caused by supplemented phytase, the third study aimed to verify this indication of a constant prececal InsP6 disappearance per unit of supplemented phytase. This suggested that the efficiency of supplemented phytase was not affected by dietary InsP6 concentration. The study investigated the effects of dietary InsP6 concentration and phytase supplementation on gastrointestinal InsP6 degradation, prececal digestibility of P, AA, and Ca, and N-corrected metabolizable energy (MEN) in broiler chickens. Ross 308 broilers were provided with diets from day 14 to 22/23 of life. Dietary InsP6 was increased by substituting corn starch with a mixture of 50 % soybean meal, 20 % rapeseed meal, 20 % sunflower meal, and 10 % rice bran (oilseed meal-rice bran levels (ORL)). Experimental diets included four dietary InsP6-P concentrations (ORL1, ORL2, ORL3, and ORL4) and three phytase levels (500, 1,500, and 3,000 FTU/kg). Feed ingredient exchange led to additional diet alterations beyond changes in InsP6 concentration, including variations in P, crude protein, Ca, and fiber concentrations. Increasing ORL decreased the InsP6 disappearance in the crop. Prececal InsP6 disappearance and P digestibility decreased linearly with increasing ORL at 500 FTU/kg, whereas ileal myo-inositol concentration was unaffected. This indicated that the supplemented phytase was the limiting factor for the complete dephosphorylation of InsP6 to myo-inositol. At 1,500 and 3,000 FTU/kg, prececal InsP6 disappearance and P digestibility differed hardly among ORL but decreased with increasing ORL. A non-linear relationship was found at 500 FTU/kg when prececal InsP6 disappearance or ileal myo-inositol concentration relative to FTU was regressed against dietary InsP6. Such relationships were linear at 1,500 and 3,000 FTU/kg, suggesting that the efficiency of supplemented phytase to fully hydrolyze InsP6 to myo-inositol at 1,500 FTU/kg or higher was impaired by phytase level but not ORL. The cecal InsP6 concentration increased with ORL and decreased with phytase. Increasing ORL decreased the prececal digestibility of all AA (except cysteine) and MEN. In conclusion, conflicting effects on InsP6 degradation concerning feed particle size and its effects on endogenous and supplemented enzymes were found. This suggests that using coarsely ground particles in the feed may not be equally effective for all ingredients. Rapeseed meal is potentially less suitable than wheat or corn in coarsely ground form. Regardless of phytase level, coarse feed particles increased prececal AA digestibility. Due to the inconsistent results and a lack of studies regarding feed particle size effects on InsP6 degradation and prececal AA digestibility, further research is needed. The antinutritive effects of dietary Ca on InsP6 degradation and prececal AA digestibility could not be compensated for by using coarse feed particles. To maximize nutrient utilization, feed particle size should be adapted to the respective raw material, and dietary Ca should be as low as possible without risking a deficit, combined with phytase. Supplemented phytase at 1,500 FTU/kg or higher does not impair InsP6 degradation and prececal AA digestibility in response to increasing ORL. Thus, adjusting the phytase level to increased dietary InsP6 concentrations was not necessary under the conditions of this study, and further research is necessary.
Investigations on factors influencing the response of broiler chickens to low crude protein diets with specific regard to nonessential amino acids
(2020) Hofmann, Philipp; Rodehutscord, Markus
Nitrogen (N) excretion caused by animal husbandry can have negative effects on the environment. Lowering dietary crude protein (CP) concentrations can reduce these negative impacts by lowering the N excretion of the animals. However, reduction of dietary CP concentrations for broiler chickens may be accompanied by reduced growth. This thesis focused on the effects of dietary CP reduction on growth of broiler chickens and influencing factors that need to be considered in low CP diets. The first study was carried out to investigate to what extent dietary CP concentrations can be reduced when dietary glycine equivalent (Glyequi) and essential amino acid (AA) concentrations are adequately supplied. Further, it was the aim to determine the response of broiler chickens to dietary Glyequi concentrations at varying CP levels. Ten male broiler chickens each were housed in 1 of 84 metabolism units. Diets with three CP levels of 16.3% (CP16.3), 14.7% (CP14.7), and 13.2% (CP13.2) each containing four Glyequi concentrations of 12, 15, 18, and 21 g/kg were used. Quantitative excreta collection was carried out from days 18–21. The reduction of dietary CP concentrations decreased average daily gain (ADG) and gain:feed ratio (G:F) from days 7–21 and increased the nitrogen-utilization efficiency (NUE). Supplementation of Glyequi increased ADG and G:F at CP13.2. The ADG at CP14.7 and G:F at CP14.7 and CP16.3 increased up to 15 g Glyequi/kg. These results indicated that the minimum to which dietary CP concentrations can be reduced in broiler chickens up to three weeks of age is between 16.3 and 14.7% when dietary Glyequi and essential AA are adequately supplied. Further, these findings showed that the growth-response of broiler chickens to dietary Glyequi is influenced by dietary CP concentrations. The second study was conducted to determine whether supplementation of single nonessential AA (neAA) can diminish or overcome the growth-decreasing effect of a diet with reduced dietary CP and neAA concentrations. Further, the effect of non-protein nitrogen supplementation in a diet with insufficient neAA concentrations was investigated. Nine male broiler chickens each were kept in 1 of 81 metabolism units. Two diets with different neAA concentrations, except Glyequi, and adequate essential AA concentrations were mixed resulting in CP levels of 17.8% (CP17.8), and 15.6% (CP15.6). The dietary Glyequi concentration was 15 g/kg in each diet. Other diets were mixed by supplementing either L-Alanine, L-Proline, L-Aspartic acid, a mix of L-Aspartic acid and L-Asparagine·H2O, L-Glutamic acid, or a mix of L-Glutamic acid and L-Glutamine to CP15.6 to achieve the respective neAA concentration of CP17.8. Ammonium chloride (NH4Cl) was added to CP15.6 to achieve the CP concentration of CP17.8. Excreta were collected quantitatively from days 18–21. Highest ADG and G:F from days 7–21 were found at CP17.8 and decreased at CP15.6. Supplementation of aspartic acid and asparagine (Asp+Asn), glutamic acid (Glu), and glutamic acid and glutamine (Glu+Gln) increased ADG and G:F to a similar extent, but not to the level of CP17.8. The NUE was highest at CP15.6, and CP15.6 supplemented with alanine, proline, and Glu. Lower NUE was observed at CP17.8 than at CP15.6 without and with neAA supplementation. Overall lowest ADG, G:F, and NUE were found upon NH4Cl supplementation. These findings showed that individual supplementation of Asp+Asn, Glu, and Glu+Gln could partly overcome the growth-reducing effect of very low CP diets. NH4Cl was found unsuitable to increase growth. The aim of the third study was to investigate interactive effects among dietary Glyequi, cysteine (Cys), and choline (Cho) on the growth of broiler chickens. Ten male broiler chickens each were housed in 105 metabolism units. Excretion of N was determined from days 18–21. Five levels each of dietary Glyequi, Cys, and Cho were tested in 15 dietary treatments. Another diet was provided to 15 birds each in another 5 metabolism units to measure prececal AA digestibility. The G:F from days 7–21 increased with digestible Glyequi intake. Differences between low- and high-digestible Cys intake were low. Hardly any effect of Cho intake on G:F was found compared to digestible intake of Glyequi and Cys. The NUE was very high with low variation among treatments. These results showed that the interactive effects among dietary Glyequi, Cys, and Cho on growth were slightly pronounced. This was likely an effect of high NUE and its low variation that caused the Glyequi requirement to be low. In conclusion, the lowest level to which dietary CP for broiler chickens up to three weeks of age can be reduced is between 16.3 and 14.7%. The growth-decreasing effect of a diet with reduced neAA concentrations can be slightly overcome upon supplementation of Asp+Asn, Glu, and Glu+Gln. Moreover, dietary CP concentrations and the related amounts of excreted N influenced the response of broiler chickens to dietary Glyequi.
Nutrient utilisation and metabolic adaptation of broiler chickens to high levels of free amino acids in the diet
(2024) Ibrahim, Ahmad; Rodehutscord, Markus
Reducing dietary crude protein (CP) content in broiler chickens is a strategy used to reduce nitrogen excretion and its impact on the environment caused by the broiler industry. This requires an increased proportion of free amino acids (AA) in the diet to avoid insufficient AA supply. An upper limit of dietary free AA inclusion has often been suggested but could not be derived from the current literature. The overall objective of this dissertation was to determine the upper limit of free AA content in CP-reduced diets. Additionally, the aim was to investigate the effects of adding free AA on the acid-base status in the blood of broiler chickens. Furthermore, the physiological and metabolic responses to substituting peptide-bound AA with free AA were investigated. In a preliminary experiment of the first study, the precaecal AA digestibility of soya protein isolate (SPI) representing a peptide-bound AA source was determined. One diet with 80 g SPI per kg and another with the basal mix, including maize starch in substitution with SPI, were tested. The diets were offered from day 18–21 of age and tested in eight units with 15 animals each. On day 21, the birds were slaughtered and the content of the last distal ileum was sampled. The amount of precaecal digestible AA derived from SPI was calculated as the difference in AA digestibility between both diets. In the main trial of the first study, the amount of digestible AA derived from SPI was incrementally substituted with a free AA mixture, i.e. 0, 25, 50, 75, and 100 % to determine the maximum amount of peptide-bound AA that can be substituted by free AA without reducing growth performance. At each substitution level, the amounts of digestible aspartate (Asp) and glutamate (Glu), which, per analysis, also contained asparagine (Asn) and glutamine (Gln), were either substituted with free Asp and Glu or with a 50/50 mixture of Asp/Asn and Glu/Gln. Each diet was offered from day 7–21 of age and tested in 7 units of 10 animals each. Based on growth performance, there was a maximum limit of dietary free AA inclusion dependent on the inclusion of Asn and Gln. This indicates that Asn and/or Gln were limited in the diets including only Asp and Glu and high levels of free AA. Asn and Gln supply allowed for increased AA substitution from 10 to 19 % of CP in the diet and increased free AA concentration from about 37 to 54 g/kg without reducing growth. Additionally, as the AA substitution was done in 25 %-unit increments, the maximum amount of dietary free AA was estimated between 54–71 g/kg. The broken line linear regression estimated value of 57 g free AA/kg in the diets with Asn and Gln without impairing growth and 61 g free AA/kg without impairing feed intake. Blood data related to the acid-base balance indicated a compensated acidosis 14 days after the diet change, which was attenuated by including Asn and Gln in the diet. In the second study, three diets were selected based on the results of the first study. The first diet included SPI with 0 % AA substitution. The second diet included the maximum free AA level with 50 % AA substitution without negative effects on growth and nitrogen utilisation. The third diet with 100 % AA substitution had evidence of an acid-base shift. The second and third diets considered all 20 proteinogenic AA including Asn and Gln. The change to one of these three diets occurred on day 7 post-hatch and feeding continued until day 22. Excreta were collected on days 7–8, 8–9, 9–10, 11–12, 14–15, and 21–22. Repeated blood samples were withdrawn on days 7, 8, 9, 11, 14, and 21. There was no evidence of affected nitrogen utilisation by substituting peptide-bound with free AA, especially 3 days after the diet change. It was confirmed that up to 50 % AA substitution from SPI did not affect growth. The reduction in growth at 100 % AA substitution was mainly due to the reduced feed intake immediately after the diet change. Adaption of nitrogen utilisation and AA in blood plasma to high levels of dietary free AA occurred within 3 to 7 days after changing to 50 % and 100 % AA substitution diets, respectively. This was suggested by the data on feed intake, nitrogen accretion, nitrogen utilisation efficiency in the first 3 days and plasma free AA concentrations in the first 7 days after the diet change, which remained unaffected by the treatments thereafter. A significant increase in ammonia excretion was also observed with increasing levels of free AA at all points in time. The blood samples were also investigated for responses in the acid-base balance and plasma metabolites directly after the diet change. An acid-base shift was determined in the birds that received 100 % and 50 % AA substitution diets. An acute reaction of the acid-base balance was on days 4 to 7 and day 4 after a change to the 100 % and 50 % AA substitution diets, respectively. Thereafter, a compensated acid-base acidosis was determined on day 7 for 50 % AA substitution and on day 14 for 100 % AA substitution. This likely explained the increased ammonia excretion found for these diets as an adaptation to excrete acids after an acid-base shift. The untargeted metabolomics analysis effectively determined the reduction in the use of plant feed ingredients with increasing free AA inclusion. However, no changes in metabolic pathways by increasing free AA inclusion were detected. It can be concluded that there was a maximum limit to including free AA in diets for broiler chickens, which was dependent on the supply of Asn and Gln. A reduced feed intake was the primary response of the birds to this upper limit. One of the likely reasons for the reduced feed intake was the acid-base shift caused by the diets with AA substitution higher than 50 %. The dietary inclusion of Asn and Gln attenuated this acid-base shift. The knowledge gained from this dissertation can contribute to the targeted use of more free AA, representing a potential alternative to protein sources for the future and reducing the CP content of the feed. The free AA level used to date in practical feeding is below the limit identified in this dissertation. Nonetheless, such levels of free AA are often used in research to answer specific questions or to cover certain AA interactions for a more accurate estimation of the individual AA requirements for contemporary poultry breeds.