Nutritional evaluation of rendered animal by products and blends as suitable partial alternatives for fishmeal in diets for rainbow trout (Oncorhynchus mykiss)

Abstract

The current European legislation states that animal by-products should not be used due to the problems associated with BSE in the 1990’s. Consequently, a ban was imposed in 2000/01 to prohibit the potential for BSE to be transferred to other species. One of the problems associated with older abattoirs and particularly working practices within this industry at this time was the potential for contaminated meat to come into contact with meat fit for human consumption or by products which were bound for rendering for animal feed. Since the first restriction with regard to feeding mammalian animal protein in 1994 within the UK to ruminants, the occurrence of BSE within Britain has reduced significantly. Since the EU curtailment in 2001, the rendering industry has improved drastically with HACCP principles applied to all UK facilities. Animals must be fully traceable and are indeed tested for BSE before being declared fit for human consumption. As a result of these significant changes there does seem to be a certain degree of scope to at least start including some of these products back into certain parts of agriculture. As such, aquaculture has probably the least risk associated due to the species barrier not being crossed with respect to cultured fish. As such the suitability of products derived from this industry is evaluated within this thesis for rainbow trout as an important farmed species of salmonid. The potential of replacing high quality fishmeal with terrestrial animal by-products on a digestible protein and energy basis was established using preliminary digestibility trials, followed by a comprehensive 12-week nutrition trial with sub-adult class rainbow trout. The use of high quality fishmeal was an important standard when comparing terrestrial animal protein sources for their replacement value. The digestibility of these terrestrial animal protein sources was established by substituting 40% of the reference protein and adding an inert marker (chromic oxide) to the feeds. Diets were manufactured using a California pellet mill to produce pellets of suitable size. A series of test diets were fed at 1.5% bodyweight to rainbow trout (in triplicate groups) for three weeks prior to the fish being stripped for faecal material, this ensured the fish were well acclimated to their respective feeds prior to assessment. The fishmeal control diet performed well and values for Crude Protein (CP), Energy, (E), and Essential Amino Acids (EAAs) were all around >90%. The best performing test terrestrial animal protein source in terms of digestibility was Spray Dried Haem (SDH) with values for (CP), (E), and (EAAs) all above 95% whilst, Poultry Meat Meal (PMM), Steam Hydrolysed Feathermeal (SHF), and Enzyme Treated Feathermeal (ETF) all provided good CP and E digestibility values (>66% digestibility). EAA digestibility values ranged from PMM (82-93%), SHF (52%-98%), ETF (71-91%) and blends of these ingredients PMM/SDH ranged from 73-100% and SHF/SDH (87-98%). The values obtained from the digestibility trials were then utilised to improve the diet formulations for the 12 week growth trial. All diets were formulated on a digestible CP and E basis so that each ingredient could be compared on an equal basis. The growth trial included a digestibility evaluation towards the end of the investigation. Haematocrit and haemoglobin levels were also established as a health indicator for each treatment. At the end of twelve week feed trial, there were no significant differences (p>0.05) with respect to growth performance, however it should be noted that the fishmeal control feed, SDH, and the PMM/SDH feeds all numerically out performed the other test diets in relation to growth performance, Specific Growth Rate ranged between (1.71-1.82). Diet composition did have a significant effect with respect to Protein Efficiency Ratio (PER) (p<0.05) with the control (2.00), SDH (1.94), PMM/SDH (1.87) and SHF (1.91) out performing other test ingredients. Blood indices such as haematocrit and haemoglobin indicated significant effects on rainbow trout (p>0.05) when fed the test diets containing SDH as an ingredient, though these values were within the normal range expected for rainbow trout. At the end of the feeding trial a further digestibility trial was undertaken; on this occasion fish were fed to apparent satiation (circa 4% body weight) to assess the efficiency of the test feeds compared to the control fishmeal diet. These digestibility results indicated that the fishmeal diet was less efficiently utilised in terms of (CP) digestibility (86%) when compared to the initial digestibility trials (92%), though this was to be expected due to the excessive feeding regime. The SDH diet indeed followed a similar pattern to the control feed with a digestibility (CP) value of 87% for the growth trial and a value of 95% for CP in the earlier trials. This pattern was not reflected for the other test diets which actually showed a marked improvement with respect to CP digestibility ranging from 75% for PMM to 82% for ETF. The EAA digestibility in the feed trial also displayed marked improvement over the initial digestibility trials for SHF (59% for tryptophan, all other EAAs 74%-92%), whilst all other test feeds had a similar digestibility pattern compared to the control feed (77-89%). In summary, the digestibility trials demonstrated the importance of establishing the value of test feed ingredient inclusion within aquafeeds prior to incorporation into balanced diets. It can be elucidated from the test data that generally an ingredient with a high digestibility for CP, E and EAAs generally will allow for a higher inclusion within a practical diet with no loss of performance or indeed the potential to exceed a control feed formulation. The findings of this thesis verify that terrestrial animal proteins are a sustainable source of protein that can be effectively included in aquafeeds for rainbow trout without loss of growth performance or any apparent adverse effects on feed utilisation and health.