Digestive enzyme physiology and fibre nutrition in layer strain chickens

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Agricultural Sciences, School of Life Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Growth of layer pullets before point of lay is a key factor in their later performance and health and this is influenced by the feeding and managing of the pullets before the onset of egg production. Improving the early development of digestive function in growing pullets will enable them to utilise nutrients and grow efficiently. Inclusion of fibre to the diet of chickens (both broilers and layer strain poultry) has been shown to induce positive changes on transit time of ingested food and can also stimulate organ development such as the gizzard. However, most of the research on the effects of fibre on growth and development of the digestive tract has been conducted in rapidly growing young broiler strain poultry before they reach maturity. The experiments described in this thesis were designed to gain information on the effects of fibre supplements on gastrointestinal tract (GIT) development and function in layer pullets at different stages of growth. Because there is little information on rates of development of organs of the GIT and the functional activity of enzymes involved in digestion of protein, in the first experiment measurements of GIT organs and activities of proventricular pepsin, pancreatic general proteolytic (GP) enzymes, chymotrypsin and trypsin, and small intestinal dipeptidase and aminopeptidase were carried out from 1 to 18 weeks of age in Hy-Line Brown strain of poultry. Results showed that the weights of supply organs of the GIT tended to reach their maximum weight at 8 to 12 weeks of age whereas live weight had not reached a maximum by the end of the experiment when pullets were 18 weeks of age. Also, activities of digestive enzymes tended to peak before live weight gain. In a second experiment, 0.8 percent insoluble fibre in the form of the commercial product, Arbocel RC, was added to the diet of point of lay hens (19 weeks of age) kept under commercial conditions. After 12 weeks, the gizzards were significantly heavier than in controls and pepsin activity, expressed per organ, was significantly greater. Pancreatic GP and trypsin activities were significantly increased when expressed /g tissue or /organ. Based on the positive response of mature hens and knowledge of GIT growth and function from the first experiment it appeared that there could be a link between the growth of internal organs and the addition of fibre which might result in improved weight gain of young pullets. The third experiment determined the effects of feeding 1 percent insoluble fibre (Arbocel) at different ages and for different lengths of time to pullets between 8 weeks of age (when growth rates of GIT organs and increases in rates of digestive enzyme activities were decreasing) and 18 weeks of age when pullets are at the point of lay. Results showed significant increases in live weights and weights of supply organs, liver, proventriculus and gizzard and in activities of pepsin, GP, trypsin and chymotrypsin. Gene expression for pepsinogens A and C in proventricular tissue of pullets given the 1 percent IF diet for 10 weeks were also significantly increased compared to controls. A shorter period of feeding (5 weeks) and removing IF from the diet after 5 weeks did not result in equivalent beneficial effects as in pullets supplemented for 10 weeks with IF. When two different forms of fibre, mixed (soluble/insoluble) and insoluble, were compared in young (4 to 8 weeks) or growing (8 to 16 weeks) pullets the results showed that in the younger pullets IF (Arbocel) had a greater effect on increasing the enzyme activities than the mixed fibre (MF) supplement (OpticellC5) however, both treatments resulted in increased mRNA expression for pepsinogens A and C though pepsin activity in the MF pullets was not increased. When IF was added to rations of the 8 to 16 week old pullets, it resulted in increased body weight and that of liver and gizzard: pepsin and GP activities were also increased. On the other hand, MF supplementation did not cause increases in organ weights or increases in activities of pepsin or pancreatic enzymes, however dipeptidase and aminopeptidase activities were significantly increased. Thus different responses to supplementation were observed with the different fibre types and at different ages. Efficient digestion of protein in the upper digestive tract may be more important for protein utilization and growth than that in the small intestine. The beneficial effects in IF pullets could have contributed to the weight gain of the pullets. The findings reported in this thesis show that the age at which fibre supplementation is started, the length of time it is added to diets and the type of fibre supplement used, can all affect responses. It is therefore necessary to carefully match feed supplementation with growth of pullets in order to improve weight gains before maturity in order to improve health and productivity of commercial layer hens.