Effects of dietary L-arginine on metabolism and immune response in layer-type chickens of different genetic backgrounds under physiological and pathophysiological conditions

Since the early 20th century poultry industry has changed significantly in the fields of breeding, husbandry, nutrition, and health management. In this context the directional selection for a genetically determined high performance potential has increased the production efficiency in layer-type chickens markedly. However, this process is often associated with undesirable side-effects causing adaptive difficulties of chickens to their surrounding abiotic and biotic environment. For that reason, a suboptimal nutritional supply may cause metabolic stress merging into health disorders more pronounced in chickens selected for high production efficiency than in non-selected chickens. Among essential nutrients required by chickens, the amino acid L-arginine (Arg) plays a pivotal role in growth, performance, neurotransmission, vasoregulation and immune response. Thereby, a dietary Arg supply beyond the requirement for optimal growth elevates the availability of plasma Arg inducing immunomodulation in pathogen challenged chickens and improving nitrogen balance and protein metabolism of endotoxemic pigs, rodents and humans. As the adaptability to varying dietary Arg has not been compared between genetically diverse layer-type chickens yet, the present thesis aimed to investigate the effects of a graded dietary Arg supply on growth, performance and immune response under physiological and pathophysiological conditions in chicken strains differing in phylogeny and laying performance. The experiments of the present thesis based on a chicken model consisting of two white and two brown purebred layer lines with high and low laying performance each. In order to consider the age-dependent variations of nutritional requirements in layer-type chickens properly, chicks (hatch to week 7), pullets (week 8 to 16) and laying hens (week 16 to 41) were fed ad libitum with diets containing Arg equivalent to 70, 100 and 200 % of age-specific recommended supply. To evaluate the adaptability of the layer lines to dietary Arg in their growth and performance response under physiological conditions, 150 one-day-old female chicks of each line were distributed to the three diets for chicks equally. At the end of the rearing period 36 pullets of each experimental group were further used for the evaluation of performance during the laying period. In parallel with the rearing period, additional 12 one-day-old female chicks of each experimental group were reared under the same conditions separately to investigate genotype-specific organ growth as well as organ-specific sensitivities to dietary Arg in 6-weeks-intervals until the 18th week of age. During the rearing period insufficient dietary Arg caused a reduction of feed intake in high performing white and low performing brown chickens as well as a general growth depression and retardation in all four strains. However, the study gave no indication for further growth promotion by surplus dietary Arg in chickens. Insufficient dietary Arg increased the relative weight of the bursa, gizzard and liver and retarded the maturation of spleen and bursa. Yet, surplus dietary Arg decreased relative spleen weight. While the weights of heart, gizzard and liver decreased relatively to the body weight from hatch onwards, proportions of pancreas and lymphoid organs increased until week 12. The overall allometric evaluation of organ growth indicated a precocious development of heart, liver, gizzard, pancreas and bursa independent of chicken’s genetic and nutritional background. In contrast to low performing genotypes, the high performing strains were unintentionally undersupplied with dietary crude protein regardless the grade of dietary Arg during the laying period. As a consequence, high performing hens showed a strong reduction of feed intake and laying performance followed by a severe mobilisation of body mass. On the contrary, low performing genotypes achieved their genetically performance potential and responded to deficient dietary Arg with a decline in laying performance supporting the conclusion that the protein requirement was met. To evaluate the effects of a graded dietary Arg supply on the immune response of the four chicken genotypes under pathophysiological conditions, four 12-week-old cockerels and three 18-week-old pullets of each experimental group were challenged with an intramuscular injection of 2 mg E.coli lipopolysaccharide (LPS) per kg body weight in two separate experiments. In the study on pullets a saline treated, equally sized control group was examined additionally. In cockerels the effects of graded dietary Arg on LPS-induced alterations of feed intake, weight gain, body temperature and differential blood counts were investigated over 48 hours post injection. In the following trial on pullets, the effects of graded dietary Arg on LPS-induced variations in weight gain, nitrogen balance, body temperature and behaviour were examined over 48 hours after LPS injection. In pullets the concentrations of plasma amino acids and the relative organ weights were one-time analysed at 48 hours post injection, too. The differential blood counts in cockerels showed a severe leukopenia characterised by lymphopenia and heterophilia from 4 to 8 hours post LPS injection. Up to 48 hours post injection the leukopenia changed to a marked leukocytosis with longer-lasting monocytosis. Deficient dietary Arg elevated the total leukocyte counts compared with adequate and surplus dietary Arg. Furthermore, the stress indicator heterophil/lymphocyte ratio revealed a high-degreed stress response to LPS which was more pronounced in brown genotypes than in white ones. Additionally, deficient and surplus dietary Arg tended to intensify the LPS-induced stress response by increasing heterophil/lymphocyte ratio. Furthermore, LPS caused higher relative weights of the liver and spleen and decreased those of pancreas, bursa, thymus and cecal tonsils in turn. However, insufficient dietary Arg increased the relative weights of the examined liver and pancreas in pullets further. Moreover, the haematological changes induced by LPS were accompanied by a genotype-specific fever response in cockerels and pullets. Whereas brown genotypes showed an initial hypothermia followed by a longer-lasting moderate hyperthermia, white genotypes exhibited a severe biphasic hyperthermia without initial hypothermia. Beside the fever response, within two hours LPS induced severe, unspecific sickness behaviour characterised by lethargy, anorexia, intensified respiration and ruffled feathers recovering until 12 hours post injection. Whereas high performing pullets exhibited sickness behaviour more pronounced than low performing pullets, dietary Arg did not modulate body temperature and behaviour. Finally, within the first 24 hours post LPS injection both cockerels and pullets reduced their feed intake and lost body weight strongly. Thereby, low performing cockerels lost body weight to a lesser extent than their high performing counterparts and deficient dietary Arg intensified the loss of body weight in cockerels further. However, pullets showed neither a genotype-specific nor a dietary intensification of LPS-induced the loss of body weight. Additionally, the examination of pullets’ nitrogen metabolism and plasma amino acids identified the loss of body weight as being a severe loss of body nitrogen. In the second 24 hours post LPS injection feed intake, weight gain and nitrogen retention recovered, but the analysis of plasma amino acids revealed a persistence of metabolic disorders at 48 hours post LPS injection. In LPS treated pullets the sum of plasma amino acids and the concentrations of Arg, L-citrulline, L-glutamate, L-methionine, L-ornithine, L-phenylalanine, L-proline, L-tryptophan and L-tyrosine were lower than in control birds. On the contrary, the plasma concentrations of L-aspartate, L-glutamine, L-lysine, 1-methyl-histidine and 3-methyl-histidine were increased by LPS. Besides, increasing concentrations of dietary Arg elevated plasma concentrations of Arg, L-citrulline, L-ornithine and 3-methyl-histidine subsequently. In conclusion, the research conducted for this thesis confirms the indispensable character of dietary Arg for growth and performance in chickens, and emphasises the direct dependency of available plasma Arg on provided dietary Arg in general. Among the internal organs, the growth of the liver, pancreas and lymphoid organs thymus, spleen and bursa is most sensitive to dietary Arg. Furthermore, LPS triggers an acute-phase reaction with severe alterations in chicken’s behaviour, body temperature, differential blood counts and protein metabolism. The present research identifies deficient dietary Arg as a potential nutritional stress factor. Referring to the assumed diverse adaptability of high and low performing genotypes to varying dietary Arg under physiological and pathophysiological conditions, the present thesis indicates that the vast majority of examined parameters respond to Arg and LPS largely independent of chicken’s phylogeny and performance level. For that reason, further research is needed on the underlying mechanisms causing different Arg sensitivities of cells, organs and chicken strains under physiological and pathophysiological conditions.