Plant proteins in pig diets: With processing comes variability

The current European Union ban on the use of mammal-sourced proteins in livestock diets is unlikely to be changed in the near future. Milk proteins are available but their expense limits them primarily to diets for newly weaned piglets. Accordingly, there is an increased reliance on plant proteins with a higher rate of inclusion and greater amounts used.

There are a number of home-grown plant proteins, including peas, faba beans, rapeseed, sunflower, maize gluten and lupins. There have been concerted attempts to promote the use of home-grown plant proteins but there is still considerable, and increasing, reliance on imported raw materials. By far, the most important are soybeans (either as whole beans or oil-extracted meal) which account for over 50 percent of EU protein equivalents in livestock diets.

Anti-nutritional factors and processing

Raw soybeans contain a number of heat labile anti-nutritional factors (ANFs) and it is therefore essential that they are heat-processed before being added to pig diets. Additional benefits of processing include optimising availability of oil (promoting higher DE values), destruction of lipid oxidation enzymes and denaturing protein (plant proteins are inherently less well-digested than animal proteins) and non-starch polysaccharides (NSPs). These may cause adverse effects on overall digestibility.

It should not be forgotten that over-processing can lead to a reduction in nutritional value.

In the past, processing has been considered simply in terms of whether or not raw materials have been processed. Thus, there is still frequent reference simply to ‘cooked' in descriptions of raw material type. This ignores the fact that there are a large number of processes available and that each one has a range of operating conditions. Even referring to a cooked raw material by the name of the process itself is of little value.

While temperature is the most important aspect of processing, humidity, time and particle size may also be changed. Thus there is the risk of considerable variability in ultimate nutritional value and the problems of ignoring this become greater when processed raw materials are used more frequently at higher rates of inclusion. This is precisely what has happened with use of soybeans.

In vitro assessment

Quality control in the animal feed industry is of major importance. There are a number of techniques which are available for assessing the degree of processing. Colour is perhaps the simplest but is not particularly quantitative. Cresol Red is very quick and easy, and is a good initial test. As the major ANFs are trypsin inhibitors, then measuring them should be a useful guide.

Urease activity is widely used but is of limited value. It is unable to detect overprocessing and is less reliable than trypsin inhibitor activity (TIA) at predicting FCR/LWG. Protein dispersibility index (PDI) is unreliable for extrusion, and cannot differentiate between different shear and mixing levels; it is not possible to disperse large protein aggregates formed and its usefulness is confined to differentiate the quality of solvent extracted soybean meal (SBM).

Nitrogen solubility in potassium hydroxide is the most reliable for assessing protein quality of full fat soybeans (FFSB) and SBM. As with all in vitro methods, there is some variation due to type of material and particle size. Ultimately it is in vivo tests which are the only really reliable means of assessing nutritive value.

Effect of full fat soybeans

Not only do trypsin inhibitors reduce digestibility, but they also have an adverse effect on gut morphology by reducing villus height. Data reports a trial with full fat soybeans showing a close correlation between trypsin inhibitor intake and villus height.

It appears that younger animals are more sensitive to the adverse effects of the inhibitors. The data are a good indication of the importance of considering both concentration and intake of the inhibitors; the latter is influenced by both the concentration and the rate of inclusion of the FFSB and feed intake. Thus, the trial in question added FFSB at a rate of inclusion of 300g/kg and intake of animals averaged 735g/day across treatments. The problems of trypsin inhibitors become more serious the higher the rate of inclusion and feed intake.

Processing is vital to denature anti-nutritional factors. But there is frequent variability in processing conditions, so controlled processing to optimize the value is essential.