SEEDS from soybeans, peas, lupins and other legumes are protein-rich feedstuffs that are fed to livestock during periods of rapid growth or high egg and milk production.
According to an announcement from the Norwegian School of Veterinary Science, salmon require overall less feed, but the protein content of the feed must be maintained at a high level throughout their entire life cycle.
Legumes are, therefore, appropriate ingredients for fish feed, too, and they are easily available on the world market and reasonably priced. However, they contain a number of so-called anti-nutritional substances that are alien to salmon and can have a negative effect on the growth and health of salmon.
Veterinary researcher Elvis Chikwati conducted doctoral research on how ingredients in feed influence salmon's intestinal function and health, which will make it easier to increase production of salmon while at the same time maintaining good intestinal health.
When Chikwati began work on his doctorate degree at the Norwegian School of Veterinary Science, it was well known that the most common and cheapest soybean meal products generally resulted in enteritis in salmon. Chikwati set out to find out more about the mechanisms causing the effects raw materials from legumes have on intestinal function and about what role some of the anti-nutritional agents play in causing enteritis.
His overall objective was to make it easier to use such ingredients in fish feed as he set out to find the answer to three questions:
1. What is the first thing that happens in the intestines of salmon when they are given feed containing soybean meal?
The answer is that after changing to a feed with soybean meal, the way the intestines function changes very quickly, Chikwati said. The first change that occurred during the first two days was that the fish ate less, probably because they did not "like" the new feed. However, after five days, enteritis could clearly be seen under the microscope.
At the same time, pancreatic enzyme activity in the rear intestine increased, perhaps because the distal intestine, where the inflammation arises, could not manage to digest and reabsorb these enzymes, he said. This assumption goes along with a much lower activity of enzymes on the surface of cells in the distal intestine — enzymes that facilitate the last stage in the digestion of proteins and carbohydrates.
2. How quickly is intestinal mucosa renewed in salmon, and is this process affected by temperature and by soybean meal in feed?
The answer is that the maturation of intestinal cells, which occurs while they wander from their "place of origin" down in the folds of the intestines and up to the top, is much slower in cold-blooded salmon than in warm-blooded animals, Chikwati said. The renewal of intestinal cells only takes a few days in mammals but takes several weeks in salmon. Furthermore, the process was slower when the water around the salmon was 8 degrees C rather than 12 degrees C.
The maturation of intestinal cells was delayed in fish that were given soybean meal in their feed. The cells never reached full maturity. They also found their way more rapidly to the top and appeared to divide not just down in the folds of the intestines but also along the whole length of the intestinal fold.
3. How do certain anti-nutritional substances from soybean meal affect development of inflammation and intestinal function?
The answer is that saponin from soybean meal, when given together with peas, resulted in the same inflammatory reaction as when soybean meal was given alone. This means that the saponin is the cause of the inflammation, either on its own or in combination with other components found in legumes, Chikwati said.
When saponins were given to the fish along with broad beans, sunflower meal, rapeseed and corn gluten meal, they did not cause inflammation but tended to reduce the fish's utilization of nutrients in the feed.
Chikwati's studies show that other anti-nutritional agents from soybean meal, e.g., lectin and trypsin inhibitor, also affect intestinal function and that the effects of these substances are different when given alone versus when mixed together.