Sunday, November 6, 2016

07/11/2016: Safe use of cottonseed protein in aquaculture

by Tom Wedegaertner

Cotton Incorporated Every year, 11 million metric tons (mmt) of protein are produced as a byproduct of worldwide cotton production. This is an equivalent amount of protein to 16 mmt of fish meal.

Unfortunately, this massive protein resource is underutilised due to the presence of a toxin which almost totally restricts feeding of cottonseed protein to ruminant species. Cotton, like many other plants, has evolved a chemical defense mechanism that greatly reduces predation by a wide variety of animals and insects.

Cotton contains the anti-nutritional factor “gossypol,” which is a cumulative toxin that is toxic to most animals, when consumed over an extended period of time. There is a renewed interest in cottonseed protein due to a recent biotechnology breakthrough that offers promise that gossypol can be genetically eliminated from the seed.

This biotechnology derived “proof of concept” will eventually eliminate gossypol as a concern when using cottonseed protein in animal feeds. It will also allow widespread use of ultra-low gossypol cottonseed (ULGCS) in aquaculture feeds, where it is a highly digestible protein, widely available and cost effective. As a bonus, cottonseed protein appears to contain a feeding stimulant for several aquaculture species.

Highly variable processing conditions

In the meantime, cottonseed products that currently are available to the feed trade can be used in aquaculture feeds as long as strict usage guidelines are followed.

The biggest issues when studying the literature on this subject are the highly variable processing conditions and product composition of cottonseed protein products used by the researchers, as well as their failure to not only accurately characterise the cottonseed product used in their research, but also to accurately analyse and report the compositional values for gossypol, iron and lysine.

The single biggest factor limiting the use of cottonseed protein in animal feeds is the presence of gossypol; however, the level of iron and lysine in the diet also has a direct effect on gossypol toxicity and animal performance.

These two compounds are two of the favorites of gossypol for irreversible chemical bonding. The bonding of gossypol to lysine reduces lysine availability. Gossypol binding to iron reduces toxicity, but also offers astute nutritionists the opportunity to double the usage of cottonseed protein in the diet without experiencing reduced performance.

For example, typical CSM might contain .07 percent gossypol (10 pounds per ton). By adding iron at a 1:1 weight ratio to gossypol, the tolerance for many animal species can be doubled. An animal that can be safely fed a diet containing 100 ppm gossypol can consume a diet with 200 ppm gossypol when iron is added to the diet.

Ferrous sulfate is the preferred form of iron for this purpose, but keep in mind that its iron content is variable and it may contain only 20 percent iron. In this case, it will take 50 pounds of this compound to deliver 10 pounds per ton of CSM at a 1:1 ratio of iron to gossypol.

Extreme variability in the toxicity of gossypol
Gossypol is the first concern of aquaculture nutritionists when considering cottonseed products as a protein source. Extreme variability in toxicity to gossypol exists among aquaculture species. Toxicity trials with aquatic species utilizing CSM of known gossypol levels or purified gossypol acetic acid (GAA) have been reported.

Based on multiple research trials, one concludes that the major farmed-fish species (catfish, tilapia, trout), especially adults, are moderately tolerant to gossypol. Shrimp also are very tolerant due to the absence of iron as an oxygen-carrying molecule in shrimp blood.

As mentioned previously, the binding of gossypol with hemoglobin iron is the primary mechanism of gossypol toxicity. As gossypol accumulates in the blood of an animal over time the oxygen carrying capacity of the blood is diminished and fluid accumulates in the thoracic cavity, making it that much more difficult for the animal to thrive.

Additional stress and age exacerbate this toxic process. Fowler (1980) found that salmon could tolerate between 22 percent and 34 percent of their diets as CSM without adverse effects (gossypol levels not reported).

In trout, particularly juveniles, research by Lee et al. (2001) demonstrated that up to 15 percent CSM could be included in the diet without detrimental effects. CSM is digested relatively well by both fish and crustaceans.

However, CSM is reported to be less digestible by several aquatic species than is soybean meal (SBM) with a few exceptions. Red Drum and Palmetto “Wiper” Hybrid Bass appear to digest CSM better than SBM.

Processing conditions at cottonseed oil mills are highly variable, having a direct impact on product quality. If extreme heat is applied in an effort to bind gossypol, protein quality also can be damaged.

It is very important to have a gossypol analysis conducted by a competent laboratory before using CSM in an aquaculture feed. Total amino acids apparent availability coefficients typically are lower for CSM than SBM, averaging 74.4 percent versus 82.2 percent for channel catfish (Wilson et al. 1981) and 78.4 percent versus 95.7 percent for Australian Silver Perch (Allan et al. 2000), respectively.

Lysine availability is severely deficient in CSM (60.4 percent-84.9 percent) as compared to SBM (85 percent-96.7 percent) based on perch, catfish, rockfish and trout research. Lysine binding to gossypol renders it unavailable to the animal.

Therefore, synthetic lysine or combinations of animal and vegetable protein sources typically are used to compensate for CSM’s lower lysine availability in aquatic diets.

Although methionine availability (72.5 percent-89.9 percent) is higher than lysine availability (60.4 percent-84.9 percent) in CSM, some fish research reports infer that sulfur amino acid deficiency may have been responsible for lower growth performance of CSM diets.

Read the full article HERE.

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