A comparison of the effect of diet extrusion or screw-press pelleting on the digestibility of grain protein products when fed to rainbow trout (Oncorhynchus mykiss)

This study examined the effect of the extrusion process on the digestibilities of whole diets and also the component test ingredients when fed to rainbow trout. Six diets were prepared using either a screw-press or an extruder based on the same batches of raw materials in each case.Correlations between diets were highly significant for all four parameters examined of dry matter, nitrogen, energy and the sum of amino acids. The correlations showed that extrusion significantly improved the energy digestibility of the diets but effects on the other parameters were negligible. Correlations between ingredients for energy and dry matter digestibilities were highly significant, but correlations between the digestibility of nitrogen and the sum of amino acids were poor. The ingredient correlations also showed that extrusion improved the digestible energy value of the test ingredients (e.g. ADE of 0.700 when screw-pressed, but ADE of 0.800 when extruded), but any improvement in the dry matter digestibility was nominal and no advantages were gained for protein digestibility. The results of this study show that diet digestibility responses obtained from screw-press manufactured diets provide a proportional, but not necessarily direct indication of the responses achieved from extruded diets. The ingredient digestibilities showed that while dry matter and energy digestibilities are also proportional the nitrogen and the sum of amino acid digestibilities are not proportional between the two diet manufacturing methods. Observations of pellet stability in vivo showed distinct differences between the reference, L. angustifolius cv. Myallie kernel meal and soybean meal test diets. Changes in pellet integrity were noted after 2, 4 and 8 h. Pellet integrity after 8 h was best in the reference diet and worst in the soybean meal diet. The soybean meal diet lost its structural integrity quicker than that of the reference and L. angustifolius cv. Myallie kernel meal diets. The specific nutritional implications of these observations need to be more fully explored.