by Professor Simon J Davies FRSB, Editor, International Aquafeed, UK, Derek Balk and Melissa Jolly-Breithaupt, FHR Biofuels & Ingredients Nutrition Specialists, Flint Hill Resources LLC, USA



The quest for alternative proteins in aquaculture is of increasing importance, primarily due to the rapid expansion of the industry and the constraint being imposed on marine ingredients like fishmeal and terrestrial plant ingredients such as soybean meal, due to concern as to their long-term sustainability. The rearing of high value fish species places demands on feed formulations that are high in protein to meet the stringent nutritional requirements of these fish. Salmon are of particular importance, considering the iconic image they portray in fish farming.

Total global production of farmed Atlantic salmon (Salmo salar) is estimated to have increased by some seven percent in 2019, to around 2.6 million tonnes. This marks the third consecutive year of strong supply growth, following an increase of around five percent in 2018. The three major contributors to this expansion were Norway, Chile and Scotland.

It is estimated that the global salmon industry is worth in excess of EU €13 billion (US $14.7 billion) and is highly dependent on a large investment and supportive infrastructure that includes a well-established feed manufacturing platform. In this respect, it is critical we examine the utilisation of novel feed ingredients for a sustainable industry to meet consumer demands and reliable production figures. The range of options is large and one such area of active interest is the potential of distillers' dried grain enhanced protein by-products, resulting from the bio-ethanol industries.

Previously, there has been much interest in the more traditional sources of distillers' dried grains with and without solubles as feed stock for a wide range of production animals, including fish to some extent, depending on species. DDGS from potable alcohol fermentation (i.e. beer and whisky distillation) was utilised mainly in rations and the issue of high fibre content placed limitations on their use for monogastric animals, with most of the by-products being directed towards ruminant feeds in concentrate fractions of the diet. Some applications in aquafeed for tilapia and carp (Omar, 2009) proved quite successful and were efficiently digested by these omnivorous fish species, compared to dietary constraints in carnivorous fish such as trout and salmon.

The exploitation of DDGS and yeasts for aquaculture diets has now become of significance as a strategy to use alternative proteins and high energy sources. Relatively few studies have focused on the feeding of diets containing DDGS and yeasts with freshwater fish and prawns (Stone et al., 2005; Schaeffer et al., 2009; Zhou et al., 2010).

More recently, there have been considerable advances in the processing of DDGS as a consequence of the expansion of the biofuel industry in many parts of the world, based on the controlled fermentation of corn and wheat. This process leads to a significant biomass of spent grains and residual yeast that can be de-fibred and dried into a milled product of high protein content and excellent balanced essential amino acid content. Recent work by Goda et al (2019, 2020) has shown that a high protein DDG derived from corn fermentation for biofuel with the added inclusions of enzymes worked well in diets for European seabass (Dicentrarchus labrax).

One such leading company is FHR Biofuels & Ingredients located in Kansas, USA, with their advanced product NexPro that has proven benefits in feed formulations for farmed fish species. NexPro has many attributes in terms of its nutritional profile being of consistent product quality and availability with a high protein content of 50 percent from the combined yeast (25%) and corn gluten composite in the co-product after ethanol removal. The material offers superior digestibility of dry matter, energy and protein compared to first generation DDGS products and is an excellent source of EAA's and lysine and methionine usually limiting in other free-standing plant derived feed ingredients and is a proven cost-competitive alternative to many other terrestrial proteins as well as fishmeal. Additionally, NexPro has a very good shelf life and stability, due to low fat levels. These attributes have been fully tested under varying conditions for fish and more work is now being undertaken to extend our knowledge base.

We report on preliminary trials conducted recently to evaluate NexPro in diets of post-smolt (grower stage) Atlantic salmon. The study addressed growth and feed utilisation performance under controlled conditions. The effects of graded inclusion levels of yeast product (NexPro) on growth, feed efficiency, nutrient utilisation, flesh pigmentation, blood biochemistry and gut histology were assessed.

Atlantic salmon trial protocol

Five research diets were formulated as shown in Table 1, i.e. One control diet (Diet 0, Table 1) with no NexPro was formulated to meet the known nutrient requirements of Atlantic salmon (NRC, 2011) with four additional diets containing varying levels of NexPro at 5, 10, 15 and 20 percent respectively.

Ingredients were mixed and extruded using a co-rotating, intermeshing twin-screw cooking extruder (ZSK-57, Werner & Pfleiderer, Ramsey, NJ, USA). The oils were added to the extrudates using a vacuum coater UAS Canada Inc. (Abbotsford, BC, Canada). The proximate composition of the test diets are presented in Table 2, analysed according to standard AOAC 2019 protocols and essential amino acids were also determined for each diet.

Atlantic salmon (St John River strain; initial body weight 304.0±10.7g) randomly assigned to 35 fish-per-tank were reared in saltwater (25ppt) in a recirculating aquaculture system (RAS) equipped with fifteen 750-litre circular tanks. The water temperature was maintained at 14.2±0.6°C and dissolved oxygen was maintained at >90 percent saturation. These were optimum conditions for the species, allowing for excellent growth and feed utilisation.

Salmon were hand-fed to satiety, three times daily for the duration of the 12-week study. Blood samples and distal intestine samples were obtained from salmon on termination of the feeding trial. Blood was analysed for a range of haematological indices to assess health and indictors of stress. Gut sections were fixed and examined histologically for any dietary induced lesions such as enteritis and associated morphological changes.

Results and discussion

It was shown that NexPro was of high nutritional value and an acceptable feed ingredient for Atlantic salmon and had no significant adverse impact on growth, feed intake of salmon, up to 15 percent inclusion level in this study (See Table 3).

The Thermal Growth Coefficient (TGC) gap converged between salmon fed the Control and Diet D (NexPro-20) over the study period. Salmon fed Diet C (NexPro-10) had the highest final body weight after 84 days (See Figure 1).

NexPro incremental inclusion in experimental diets of post-smolt Atlantic salmon resulted in Feed Conversion Ratio (FCR) values in compliance with those expected for the species under the experimental conditions encountered. FCR was excellent at below 1.0 and varied between 0.93 and 0.98 for all respective treatments. There was no indication of changes in pellet quality or the palatability of feed when presented to the fish.

Growth and feed intake responses were not significantly driven by dietary lipid and/or protein contents. Salmon performance was, therefore, affected mostly by the inclusion level of NexPro within the diet formulation.

It was interesting to find that the inclusion level of NexPro had no significant effect on proximate composition of whole-body and amino acid content of fish (See Table 4). Whole-body protein, lipid, ash and amino acid retention results were in line with data reported in the literature (e.g. Shearer et al., 1994; NRC, 2011). These findings were in accordance with a previous salmonid evaluation of DDGS with rainbow trout (Oncorhynchus mykiss) conducted by Overland et al. 2013.

In our study with salmon, whole-body composition, nutrient utilisation, fillet pigmentation and gut histology were comparable to salmon fed a practical Control diet. The fillet colour of salmon was also not affected by the inclusion of DDG and fish retained a good expression of pigmentation with consistency across the range of diet formulations. This showed that overall carotenoid flesh distribution and the retention of astaxanthin added to feed as the principle pigment was not masked by NexPro inclusion.

The histological examination of gut morphology with a focus on distal gut health showed interesting results confirming the neutrality of NexPro when morphometric measurements were applied. We know from experience that certain categories of soybean meal can promote distal gut enteritis in salmonid fish, especially in salmon, due to various anti-nutritional factors present in soybean such as saponins and non-starch polysaccharides and, together, these effects can be synergistic to cause localised inflammatory reactions and lesions.

These ANF's are not present in High Protein DDG and have no deleterious effects on the gut health of salmon. These are shown clearly in Figure 2, depicting both a typical healthy distal gut cross section of NexPro-fed fish and an example of enteritis in a comparative image of the distal gut region of Atlantic salmon sampled at the end of the trial. Additionally, Table 5 provides data on the respective length and width of salmon mucosal folds with no adverse effect recorded of increasing NexPro on overall intestinal morphometrics.


incorporation into a standard Atlantic salmon diet formulation was able to sustain growth and feed efficiency contributing to both dietary protein and energy assimilation. There were no indications of any adverse effects of incorporation into diets on blood indices or metabolic parameters relating to the health and stress status of salmon under the experimental conditions. These values were within the normal ranges for farmed Atlantic salmon, showing that metabolic and physiological homeostasis was not compromised during the trial period. Similar results for the successful inclusion of high protein HPDDG was reported for European seabass by Goda et al, 2019 and 2020. Jin et al. 2014 also found no detriment and excellent performance of Juvenile Red Seabream (Pagrus major) when fed similar levels of a rice-based distillers grain, as in this salmon evaluation.

It is likely that the continual expansion of the ethanol biofuel industry will provide a steady and growing DDGS output and improving processing technologies will provide interesting economic opportunities for an optimum level of inclusion in animal feeds. These new proteins offer a next generation of products for animal nutrition with aquaculture being at the top of the agenda. This work confirms the feasibility of including up to 15 percent NexPro within feeds for salmon and meeting with growth and feed utilisation expectations to fill the growing 'protein gap'.

Future work will be directed towards further optimisation for different stages of development and studies that would involve taste panel assessments for retail value and consumer acceptance. The use of NexPro will likely have positive significance for fish welfare, especially in terms of gastrointestinal performance and improved nutrient absorption, compared to soybean and other plant ingredients, due to various yeast-related components. An arsenal of high value ingredients can make a major contribution to least cost formulation strategies to mitigate the use of both fishmeal and soybean protein concentrates.

A further strategy would be to determine if NexPro can strengthen the immune system of salmon by improving the gut mucosal interface and to conduct disease challenges to test its potential functionality in the diet. Various pathogen(s) would be selected as a function of possible modes of action of NexPro at the metabolic and physiological level in Atlantic salmon during crucial stages in production. It would be most interesting to explore if the product could positively modulate the gut microbiome and influence the immune response at the gut mucosal interface.

has, therefore, considerable potential as a feed ingredient of choice for farmed fish and the Atlantic salmon responded well as a typical carnivorous species. This augers well for other marine fish species such as seabass, seabream and flounder. It is also a more sustainable ingredient when one considers the cyclic economy and requirements for environmental stewardship of resources in the expansion of global aquaculture. It will have cost benefit advantages and added value in farmed fish diets.

*This work was undertaken by Flint Hill Resources LLC as part of a strategy to develop NexPro and validate the product for different fish species and shrimp for aquaculture diets.

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