Browsing by Subject "Peptide-bound amino acids"

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    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.