Importance of the quality evaluation of Soybean meal as a feed ingredient for poultry

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Introduction

Soybean meal is the inevitable vegetable protein source for poultry, and it is the most common, widely used feed ingredient. It has also become world-wide standard against which other protein sources for comparison. Protein content varies from 44 to 48% which depends on seed variety, oil extraction processing and harvesting time. Its amino acid profile is excellent and highly digestible, which helps in cost efficient feed formulation for animal or birds. In addition, it is also rich source of potassium and isoflavones. Isoflavones have been implicated in enhancing immunity and improving growth performance and carcass traits.

Soybean processing 

Soybean meal is a byproduct of soya oil industry; after cracking the seed, oil is extracted at 70°C using hexane. The hexane is then removed completely by evaporative cooling; otherwise, the residual hexane may cause liver problems in birds.

This highly valued feed ingredient contains many anti nutritional factors (ANF) which is one of the most important restrictions in the use of soybeans and their products in monogastric animal diets. The presence of these factors is also the main reason why different technological treatments are applied to soybeans or their products.

Anti-nutritional factors

Following are different ANFs which are easily destroyed with proper heat treatment (except mycotoxins). First five are intrinsic, while mycotoxins are formed because of high moisture content and improper storage of the soya seed or soya doc.

1. Trypsin inhibitors: The residual trypsin inhibitor in soyabean products combines with the trypsin in the small intestine and forms an inactive complex thus reducing the availability of proteins by reducing the digestibility of specific amino acids. It induces pancreatic hypertrophy causing increased secretion of trypsin (loss of endogenous nitrogen). Independent of this, trypsin inhibitors have been correlated with the occurrence of “rapid feed passage” syndrome in broilers. The combined effect on the bird is a reduction in nitrogen retention, growth and feed conversion.
2. Antigenic compounds: Glycinin and B conglycinin are the allergens present in soya beans, which may induce inflammatory response in intestines.
3. Agglutinin:Its non-fibre carbohydrate related glycoprotein. The structure is similar to lectins found in other leguminous plants but is more stable. It causes negative effects on animal’s intestinal health by influencing the intestinal structure, barrier function, mucosal immune system, and the balance of the intestinal flora.
4. Anti-vitamin factors against vitamin A, B2 and D3.
5. NSP and OS: Soyabean meal contains approximately 30% carbohydrates (DM basis), of which only a small part is in the form of starch. A significant proportion (7-10%) comprises indigestible OS (e.g. raffinose, stachyose, verbascose). Up to 20% of dry matter is in the form of NSP such as arabinogalactans and galactomannans.

OS increase osmotic pressure and therefore fluid flux into the gut. This increases digesta passage rates and reduces the efficiency of digestion and absorption of nutrients. On reaching the hind gut, OS are subject to microbial fermentation, which causes bloat and concomitant reductions in feed intake and production efficiency. As a result, these are commonly referred to as “flatulence factors”.

Insoluble NSP directly prevent access to nutrients due to the well-documented “cage effect”. In addition, soluble NSP cause increased viscosity of the gut digesta, leading to rapid through-put and reduced availability of nutrients for digestion and absorption.

Importantly, both OS and soluble-NSP are implicated in wet litter and its associated effects on health and carcass quality.

6. Mycotoxins: Soya doc generally do not contain significant levels of mycotoxins, however, ochratoxin (produced by the molds Aspergillus ochraceous or Penicillium verrucosum under poor storage conditions) and zearalenone (produced by the fungus Fusarium graminearum) are commonly found.

Under-toasting retains the intrinsic ANFs of soybean and over toasting may denature proteins. Over toasting also makes protein unavailable by a chemical reaction between protein and carbohydrate in the soybean, called as Maillard reaction, in which lysine is involved. Reactive lysine content (NIRS calibrations are available) should be more than 90% of total lysine (Reactive + heat damaged lysine).

Quality tests to assess soybean meal processing efficacy

There are few tests available to check whether soybean is properly processed or not. They are 1) Urease activity test 2) Soluble protein and 3) Protein Dispersibility Index (PDI).

1) Urease Activity: It is an indirect method to detect whether trypsin inhibitors are destroyed to the maximum extent by measuring the presence of urease enzyme, which normally is present in soybean but destroyed by heat treatment along with trypsin inhibitor. Urease is measured in terms of change in pH during assay. The change in pH that is normally accepted in the industry is between 0.05 and 0.2. As pH test is time consuming, commonly phenol red indicator method is used, here number of red spots are correlated with the change in pH units.Higher values mean there is still residual urease (and trypsin inhibitor) indicating undercooked meal. One drawback of this test is that absence of any pH change just indicates the absence of urease activity in the soybean meal, which could be indicating either optimum cooking (up to the exact level of destruction of urease and thereby the trypsin inhibitor) or over cooking or over-processing of soybean meal.

2) KOH protein solubility: Since there is no indication of over-cooking by urease activity method, Araba and Dale from Canada developed an alternative method. In this method, solubility of protein in soybean meal is measured using a weak alkali solution of potassium hydroxide (0.2%). After confirming good correlation between the amount of protein from soybean meal soluble in weak KOH solution and chick growth, the method is followed in many countries as a tool for assessing the quality of soybean meal. Heating tends to make the protein less soluble in weak alkali solution. Correctly heat-processed soybean meals should have a protein solubility around 78-85%. Raw soyabean meal is 100% soluble.

3) Protein Dispersibility Index (PDI): Among the available tests for determining protein quality in soybean products, the PDI is the simplest, most consistent, and most sensible method. This test measures the solubility of soybean proteins in water and is probably the best adapted to all soy products. The PDI method measures the amount of soy protein dispersed in water after blending a sample with water in a high-speed blender. PDI value between 15-30% is considered as good.

Adulterants in soybean meal

Adulteration is addition of inferior/cheaper quality materials to superior quality materials. High quality feed ingredient such as soybean meal is usually prone to adulteration intentionally or unintentionally. Adulteration not only changes the chemical composition but also reduces the original nutritive value.

The following are the common adulterants with soybean meal:

1. NPN compounds
2. Soil/Sand silica
3. Hulls (fibre)
4. Other cheaper meals
5. DDGS
6. Over/under cooked soya

NPN compounds: Non protein nitrogenous compounds like urea, melamine may be adulterated in soya DOC to increase the crude protein (CP) content. It boosts the N content in soya DOC which is reflected in CP. It can lead to formulation mistakes leading to short supply of the amino acids to poultry. It also creates burden on liver and kidney for excretion of unutilized N. Ultimately poultry performance will be poor.

Urea contains 46% N (287% CP equivalent). Urea toxicity can cause respiratory dyspnea, staggering gait and watery diarrhea in birds.

For urea spot test phenol red indicator is used whereas for NPN (other than urea) spot test Nessler’s reagent is used.

Melamine is an organic nitrogenous compound used in the production of plastics, dyes, fertilizers and fabrics. It contains 67% N (418% CP equivalent). Feed level of 5000-10000 ppm is toxic to birds, also it has human health significance because its residue are observed in liver and kidney of birds. In humans it causes nephrotoxicity.

Soil/ sand silica/LSP: It is added to soya doc to increase weight. It reduces nutrient value of the soya doc and may contain mineral/heavy metal contaminants. Addition of limestone powder (LSP) can increase digesta pH, increases feed calcium levels and reduce the nutrient digestibility.

QC test: Analysis of the Acid Insoluble Ash (AIA)%.

Hulls: Soya hulls may be added which dilutes the nutrient levels (energy, amino acids) and increases the fibre content. Non starch polysaccharide content of hulls may cause increased digesta viscosity ultimately reducing the digestibility of different nutrients.

QC tests: Analysis of the crude fibre, neutral detergent fibre and acid detergent fibre %.

Other cheaper raw materials: Raw materials like guar meal, mustard seed doc, DORB and DDGS can be used as adulterants in soya DOC. Guar meal has similar protein content as that of soya DOC, it is high in NSP (mannans, gums). In case of mustard doc, ANF glucosinolates can reduce the iodine utilization in animals, also, because of poor palatability, feed intake may reduce. It may contain high mycotoxin levels. DORB may be used as a diluent which may contain high aflatoxin levels and less digestible protein. DDGS may contain very high levels of mycotoxins. Maize DDGS contains high sulphur levels.

QC test: Physical examination, microscopy

For DDGS: the following test can be performed.

1. Take the whole sample (sample straight from truck or lot, without grinding)
2. Sieve the sample through laboratory standard sieve mesh size 20 (850 microns)
3. Observe the material which has passed through 20 mesh. If Corn DDGS is present, it will have characteristic strong smell to the powder.
4. Take 10 gm of this sieved powder and add distilled water to make the volume 100 ml. Take pH of this 10% suspension.
5. pH of soybean meal will be around 6.6 to 6.7
6. pH of 10% suspension of Corn DDGS will be around 5.0 to 5.1
7. If 1% Corn DDGS is mixed in soybean meal, the powder of sample passing through 20 mesh will show pH of 6.5 or lower.

Under toasted or overtoasted soya doc: Both over and under toasted soya doc because of the mistakes in processing, may be mixed with the normal soya doc by manufacturer. Tests are already discussed above.

Conclusions

Soybean meal quality is affected by its processing methods and its adulterant content. Feed millers/Animal Nutritionist should check soybean meal quality before included in feed formulation. By performing the above tests one can choose quality soybean meal, there by ensure the optimum bird’s productivity and performance. Apart from stringent quality control measures, use of enzymes may help the nutritionist to alleviate adverse impacts of indigestible carbohydrates (NSP, OS) and TI.

References are available upon request.

Dr Ravindra Jadhav and Dr Shweta Dhawan, Premium Chick Feeds
Dr M. Manobhavan, Madras Veterinary College, TANUVAS