Tuesday, February 17, 2015

17/02/2015: Soluble phosphorus in salmon feed: Suitable as a measure of available phosphorus?

by Sissel Albrektsen, senior scientist Nofima

First published in International Aquafeed, January - February 2015.

Phosphorus (P) is an essential mineral that has to be added to salmon feed to achieve normal growth and skeletal development. P from marine ingredients, plant protein and crystalline P salts provide respectively 46, 30 and 24 percent of dietary P in Norwegian salmon feeds. Hence, fishmeal is an important P-source, even though the level of fishmeal in the feed has dropped significantly from 64 percent in 1990 to 20 percent in 2012 (Ytrestoyl et al., 2014). About 40 percent of P in the fishmeal originates from the bones, and is present as calcium (Ca)-phosphate salts in hydroxyapatite. P in hydroxyapatite has low solubility and is poorly available to salmon.

The digestibility of P in different fishmeal reflects this and further shows great variation (20 – 60 percent), depending on the fish raw material and seasonal variations. No reliable direct measure of available P exists, and this makes it difficult to know how much available P is present in any given fish feed. In this article we will focus on the possibility of using a new method on soluble P as a measure of available P in ingredients and feeds, and what opportunity this gives to ensure better control of available P in the feeds.

Industrial fish that traditionally has been used as raw material for fishmeal production, such as herring in the Nordic countries, is today used for consumption. As a consequence, an increasing amount of fishery byproducts is used as raw material for fishmeal production. The global by-product material used for fishmeal and fish-oil production has been rising by 1–2 percent year−1, and represented 25 percent of world production in 2010 (Shepherd and Jackson, 2013).

In Norway, fishery byproducts accounted for 30 percent of the raw material in fishmeal in 2013 (Ytrestoyl et al., 2014). Fishery byproducts contribute with high content of total P from the fish bones, while at the same time, the percentage of P that is available for the salmon is actually reduced compared with a traditional fish meal produced without trimmings.

Fig. 1 shows total P and soluble P in herring meal produced with different levels of trimmings from herring, and other fish meals produced from blue whiting and capelin and also from Antarctic krill. High inclusion levels of trimmings reduce the proportion of soluble P from 61 to 35 percent of total P, which means that the level of soluble P is reduced despite an increase in total P. The analysis of total P tells little about the proportion of P that is available for the salmon, i.e. how much of P that is present as free phosphates, and thereby available for digestion.

The amount of trimmings used in global fishmeal production varies greatly, from 0 to 100 percent, which increases the unpredictability with respect to how much of dietary P that is available. In some commercial smolt feeds, total P ranged from 0.7 to 1.77 percent, soluble P from 0.36 to 0.7 percent and the proportion of soluble P from 31 to 70 percent of total P.

The plant ingredients used in fish feed today contain 60-80 percent phytic acid. Phytic acid contains P (phytate-P) but many fish species, including salmon, has no or little phytase activity and cannot utilise this P source. Phytic acid also acts as an anti-nutrient by inhibiting the absorption of available P and other minerals by forming poorly soluble mineral complex in the intestine. Nofima has analysed total P and soluble P in many plant proteins (Fig. 2), and the main finding is that most of the total P in plant proteins will be analysed as soluble P. The analytical method for soluble P apparently cannot distinguish between phytate-P and other soluble forms of P in plant proteins.

Total P and the proportion of phytate P and other P-components are known and quite stable for most plant ingredients. By analysing soluble P in the plant ingredient, it is easy to correct for the proportion of P that is present as phytate-P, an approach that is also applied in the aquafeed industry today. Overall, the method will give much more reliable measures of available P in salmon feeds and feed ingredients compared to the current total P analysis.

The P requirement is affected by a variety of biological and environmental factors, and it can vary with life stage and growth rate of fish, diet composition and temperature. When available P in the diet is low, the fish will regulate this by increasing the P uptake in the intestine, reduce the excretion of P in the kidney, and mobilise P from the skeleton to cover vital functions in other parts of the body.

Sustained demineralisation of the bones over a long period will weaken the skeleton and eventually cause deformity. In fast-growing Atlantic salmon fed 0.3, 0.5 and 0.7 percent soluble P in the diet following seawater transfer, the fish developed 30, 15 and 0 percent deformity in the lower jaw bone concomitant with increased mineral content of the spinal bones within a feeding period of only 12 weeks.

In another smolt trial with similar size fish, reduced mineralisation was found in fish fed 0.5 percent soluble P in the diet compared to fish fed 0.7 percent soluble P, while no sign of external deformity was observed. This illustrates the risk of feeding slightly sub-optimal P (0.5 percent soluble P in the diet), it is cost effective and environmentally friendly, but may occasionally affect fish welfare due to variation in the fish material, growth rate, feed or the environmental conditions which is not easy to control.
The digestibility of P depends on dietary P level, irrespective of measuring total P or soluble P in the feeds. In salmon fed diets with different P contents, the digestibility of total P varied between 30 and 50 percent (Fig. 3a), while the digestibility of soluble P in the same feeds were higher and varied between 60 and 80 percent (Fig. 3b). Other trials have shown that the digestibility of soluble P in the feed can be as high as 90 percent in salmon, indicating a very efficient digestion of soluble P.

The amount of dietary P that is retained (deposited) in the body is about 30 percent, which means that about 70 percent of dietary P will be released to the environment. As this is a major global concern, dietary P is usually added close to or slightly below the requirement in order to obtain maximum P utilisation and minimum P load to the environment, as well as to keep the cost as low as possible. This is a reasonable strategy, but requires better feed control with available P than is realistic to achieve today by analysing dietary total P.

The new method distinguishes between insoluble and soluble P, which is to be seen as indigestible P and digestible P, respectively. It does not only distinguish between the P in hydroxyapatite and other P-forms, but can also be used to distinguish between P from inorganic salts with different solubility, which is the main criterion for P absorption. Mono-Ca-P salt was found to contain about 65 percent soluble P, while mono-Na-P salt contained more, about 94 percent soluble P, demonstrating that the mono Na-P salt contain higher levels of available P than mono Ca-P despite similar levels of total P. Inorganic mono-salts of P will also be more soluble than di-salts of P and this will contribute to different P digestibility and thus different availability of P from the feed.

Nofima has conducted experiments that indicate that feed that contain 0.7 percent soluble P provide adequate P in salmon at the smolt stage, while a higher dietary P content of 0.8 percent soluble P is required in Atlantic salmon fry during early start feeding. More research is needed to understand the potential for using dietary soluble P when analysing commercial high plant protein diets with variable phytate levels, although it is possible to correct for this. The soluble P method has been developed and validated by Nofima and found to have high accuracy, resembling the analytical method for total P (Hovde, 2013).

Read the magazine HERE.

The Aquaculturists
This blog is maintained by The Aquaculturists staff and is supported by the
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