The demand for seafood is increasing and it is seen by many as one of the most promising options to meet the world's growing population and associated need for protein. The aquaculture industry is still relatively small compared to the more established poultry and swine sectors, but its products are increasingly accepted as a healthy source of protein with a more beneficial omega-fat content. Feed cost and quality are two of the key challenges facing the aquaculture industry as it expands. For optimal performance we need to meet energy and nutrient requirements as cost-effectively as possible and with the least impact on the environment.
 

Standard texts (such as NRC) and published papers can be used along with in-house data to optimise the dietary nutrient requirements and energy content of the targeted species. In addition, Evonik has put a great deal of research effort into developing nutrient recommendation tools to optimise weight gain, feed conversion ratio (FCR) and environmental impact of aquaculture species such as tilapia, shrimp, carp and salmonids. Least cost formulation tools are typically used to meet the target and level of nutrients by mixing various bulk ingredients as well as different micro-ingredients such as vitamins, minerals and supplemental amino acids.
 

Knowledge on the nutrient digestibility values of macro ingredients and right bioavailability value of the micro ingredients are highly important to achieve consistency on the nutritional quality of the feeds. All this data will help us optimise the performance of the aquaculture businesses. Methionine provides a good example of what can be achieved.
 

A key amino acid
Methionine is a key amino acid that cannot be synthesised and must be provided in the diet of aquaculture species. It is a precursor of various metabolites and is involved in several biochemical and physiological systems, including protein synthesis, antioxidant defence, immune response, bile salt synthesis, etc. Methionine deficiency has been shown to affect long-term performance of fish and may even influence the performance of their offspring. Today's commercial feeds are predominantly based on plant protein and so relatively poor in methionine. They can be enhanced by adding fish meal, which is methionine rich, but this increases the overall protein content and is not only costly but also increases the amount of ammonia excreted into the water. A more economical, healthy, and environmentally friendly option is to supplement the feed with the right amount of methionine.

 

Which methionine?
Methionine supplements are available in two chemical forms: DL-methionine and DL-MHA (methionine hydroxyl analogue). Strictly speaking DL-MHA is not an amino acid but is converted to methionine by the body. It is available in liquid and dry forms which contain 88 percent and 84 percent MHA respectively. DL-methionine contains at least 99 percent methionine. The different chemical forms do not have the same bioavailability, so the amount of methionine that reaches the body in a usable form is not the same. Bioavailability is also sometimes called 'biological efficiency', 'bioefficiency' or 'biopotency'; but they all basically mean the same thing, such as how well the nutrient is absorbed in a usable form. Relative bioavailability is a comparative measure of how well two different feeds deliver a given nutrient to an animal and allows nutritionists to decide how much of a given supplement needs to be added to a particular feed. It is determined by feeding with the two products and comparing responses such as weight gain and FCR.

 

The relative bioavailability of the two products is measured by feeding them at graded levels and measuring the animal performance. Depending on the response of the animal, a simultaneous linear or exponential model is developed, and the slopes of two products are compared. For the proper estimate of bioavailability, it is important to make sure that methionine in the basal diet is clearly deficient to at least 25 percent less than the requirement. The term 'relative bioavailability' on a weight-to-weight or a product basis is commonly used but sometimes the same term is also used on an equimolar basis. The first compares the effect of say 1kg of one product against 1kg of another, which makes it easier to compare costs at the same time.
 

The equimolar comparison is based on the amount of active substance in a kg of each product, considering other components in the product, such as water or calcium in case of MHA products. One of the earliest studies comparing the bioavailability of MHA and DL-methionine in fish (Channel catfish) was published in 1978. Since then, more studies have looked at bioavailability in other fish species, including rainbow trout, sunshine bass, carp and Nile tilapia. Data of some of the published studies were later re-analysed with appropriate linear slope models to determine the relative bioavailability (Lemme 2010). A summary of these studies, including 10 that looked at weight gain and seven that included FCR assessment, shows that the relative bioavailability of DL- MHA was 51 percent for weight gain and 59 percent for FCR compared to DL-methionine. The NRC published in 2011 also states that the biological efficacy of MHA for fish is about 75-80 percent of that for DL- methionine on an equimolar basis. If you compared that to a product comparison, such as on a 'weight-for-weight' basis, then liquid MHA is 66-70 percent as bioavailable as DL-methionine and MHA-calcium is 63-67 percent as bioavailable.
 

MHA has a lower bioavailability
Further studies, including research carried out by Evonik in conjunction with the University of Saskatchewan, Canada, have investigated the physiological reason for the difference in bioavailability between the different forms of methionine. The study compared the transport kinetics of the two methionine sources along the intestinal tract of rainbow trout and revealed that the absorption rate of DL-MHA was significantly lower (42-66 percent) across segments of gut compared to DL-methionine.

 

A separate study, conducted together with University of Algarve in Portugal, fed radio-labelled methionine sources to tilapia to find out how the methionine sources were distributed and utilised for protein synthesis in different parts of the body. The study revealed that methionine from DL-methionine feed appeared sooner in all parts of the body compared to that from DL-MHA. In addition, analysing muscle protein, the study found that methionine originated from DL-methionine was more present compared that with MHA, clearly demonstrating DL-methionine was better absorbed, bioconverted into L-methionine and also better utilised for protein synthesis. In contrast, when the incubation water in fish metabolic chamber from each group was tested, there was more radioactivity in the MHA group, indicating that more MHA was catabolised or simply excreted into the water. Both studies confirm the superior bioavailability of DL-methionine compared to MHA.
 

Leaching of nitrogen
The leaching of nitrogen sources into the environment is a considerable concern for aqua farmers. Not only is leaching a potential source of water fouling, but it also represents the loss of valuable nutrient and hence an unwanted cost. Water solubility of MHA is 80g per litre compared to 33g per litre for DL-methionine. We compared the effect of this difference by putting equivalent amounts of MHA and DL-methionine mass and pelleted feed supplements in nylon bags which were then suspended in 200ml of fresh water at 25°C. Samples were taken at regular intervals and analysed for DL-methionine or MHA. Over the same time, 6 percent of DL-methionine leached into the water compared to 31 percent of MHA. After just 10 minutes three times more MHA had leached out of the sample bag, and after 30 minutes it was a fivefold difference. This may be one reason why we experience even lower bioavailability for MHA in fish than that in chicken or pigs.

 

Dry vs liquid
There are good reasons why dry ingredients are preferred at the feed mill rather than liquids. Liquids can be quite viscous and sensitive to temperature and humidity, subject to lumping, and dosing accuracy can be compromised in rough environments. Correct mixing can also be affected by how effectively liquids are sprayed and absorbed by dry mass ingredients and mixing generally takes longer. And longer mixing means lower annual production and that affects profitability. Lower bioavailability can also affect profitability of course because feed will have to include more MHA to achieve the same performance for the fish producers. Depending on the relative cost of different methionine supplements, this could be a considerable amount.

 

Essential for optimal growth & health
Methionine is an essential amino acid for optimal growth and health of farmed fish. The difference in bioavailability between different methionine sources is significant and should be considered if the most effective and profitable aquaculture feed is to be used. The bioavailability of DL-MHA in poultry and pigs is acknowledged to be no more than 65 percent of DL-methionine, and research suggests that it is not more than 65 percent in fish as well and more likely it is even lower. The issue of leaching is an added potential source of loss peculiar to aquaculture. In the case of shrimp which tend to masticate rather than swallow feed pellets whole, this may exacerbate the amount of MHA products dissolved/leached out in the water.

 

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Article contributed by Dr Karthik Masagounder, Head of Aquaculture Research at Evonik, Germany.

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