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Factors affecting the utilisation of dietary energy Kese, Adu Gyamfi

Abstract

Seven different but integrated experiments were conducted to study the factors affecting the utilisation of dietary energy. The first two experiments involved feeding broiler chickens diets containing either corn oil or corn starch as the supplementary source of energy at two protein levels. Herring meal was used in all diets because of the high biological value of its protein. All diets were calculated to be isocaloric and to contain the same balance of amino acids; minerals and vitamins were added to meet the requirements for these nutrients. Live weight gain and the efficiency of food utilisation within calorie:protein regime were not improved when fat was substituted for starch in isocaloric diets. The superiority of the low-fat high-protein diet in promoting the highest metabolisability of energy, questions the validity of the claim that added dietary fat has an "extra-caloric" effect. Birds fed the high-fat low-protein diet which had a lower calorie:protein ratio, deposited more abdominal adipose tissue, indicating that in evaluating growth performance, the balance between energy and protein is of greater significance than the source of supplementary energy. Formulation of isocaloric diets has necessitated the inclusion of the so-called nutritionally-inert ingredients such as cellulose. Since the diets used to test the main hypothesis of "extra-caloric effects" attributable to dietary fat incorporated cellulose, the effect of the latter on the physiological parameters under study, was tested. At high levels of inclusion cellulose depressed body weight gain. Adverse effects of added dietary cellulose on food conversion efficiency and energy metabolisability were also evident. Another effect of .added dietary cellulose which is particularly interesting is that it decreased abdominal adipose tissue. Results of the above studies have shown significant differences among the different treatment groups in body weight gain, efficiency of food utilisation and metabolisability of energy within the first three weeks posthatching. The possibility that the residual yolk may influence the metabolic parameters in question was considered and tested. Absence of the yolk sac, excised surgically, did not influence the performance of birds on the basis of growth and energy utilisation as measured by body weight gain and metabolisable energy values, respectively. The residual yolk did not influence food conversion efficiency in the first and third weeks of the experimental period. However, depression of food conversion efficiency resulting from the removal of residual yolk was found to occur in the second week. It was noted that birds without residual yolk retained a greater amount of nitrogen compared to birds with residual yolk in the first week posthatching. The residual yolk does not contribute significantly towards the nourishment of the chick in the first week posthatching as evidenced by the fact that abstinence from food resulted in the death of both the groups retaining yolk sacs and those without yolk sacs at approximately the same time. The presence of a large bacterial population in the avian caeca and extrapolation of the features associated with bacteria-host symbiosis in ruminants and other animals to the domestic chicken have led to speculation that the avian caeca perform some cellulolytic and proteolytic functions. The relevance of the concept of caeca-mediated nutrient utilisation to the topic under study prompted an experiment using intact and caecectomized chickens to investigate the effect of the excision of the caeca on the utilisation of dietary energy and protein. Caecectomized and intact control New Hampshire cockerels were fed diets used in the previous studies. Metabolisable energy values and uric acid excretion were used as the criteria for measuring dietary energy and protein utilisation. Caecectomy did not affect the metabolisability of the diets. Metabolisability of the diet incorporating corn starch was more variable and slightly lower with the caecectomized birds. Uric acid excretion was similar for the caecectomized and the intact birds. Macroscopic and histological examination of sections of the caeca revealed that regeneration of the caeca had occurred in four caecectomized birds that survived until autopsy 85 weeks later. Although the onset of the regeneration of the caeca was not observed, it would appear that the degree (or the absence) of regeneration of the caeca in caecectomized birds may be responsible for the discrepancies in the findings reported with such birds.

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