Extrusion of ingredients and complete diets is a thermal processing method that involves progressively increasing pressure, friction, and attrition as material passes through the extruder barrel.
Extrusion offers numerous benefits in diet preparation for young pigs, among which the following are included: starch gelatinisation, protein denaturation, destruction of certain antinutritional factors, microbial decontamination, and dehydration.
Reduction of nutritive value
However, overprocessing may actually reduce nutritive value of extruded products because of protein destruction (i.e., formation of Maillard products), starch retrogradation (i.e., starch resistant to digestion), and vitamin destruction.
For example, poorly extruded full-fat soybeans had urease activity three times higher than that of typical soybean meal and severely depressed growth performance in weaned pigs compared to soybean meal during the first week post-weaning.
Additionally, mildly extruded wheat (68% starch gelatinisation) supported better growth performance than excessively extruded wheat (77% starch gelatinisation) in 21-day old pigs during the first three weeks post-weaning, although both were superior to untreated wheat. In another study, growth performance of weaned pigs actually decreased and then increased as starch gelatinisation in extruded maize increased from 39 to 89%, perhaps because of excessive starch retrogradation in the intermediate levels of gelatinization (from added water during extrusion).
Earlier studies also indicated that overprocessed maize (retrograded) may actually increase the incidence of diarrhoea in young pigs.
Proper extrusion of cereals improves starch digestibility and marginally improves protein utilisation. Thus, cereals with more naturally resistant starch and higher protein concentration, such as barley and wheat, benefit most from extrusion processing than cereals with more readily digested starch and lower levels of protein, such as maize and rice.
Additionally, extrusion appears to increase solubility, and thus digestibility, of non-starch polysaccharides, which prevail in cereals such as rye, triticale, barley and wheat, and promote bacterial proliferation in the gastrointestinal tract.
In terms of growth performance, cereal extrusion may enhance growth rate, depending on cereal source and processing conditions, but the most consistent effect is improved feed efficiency, due to enhanced energy and nutrient digestibility. For example, extrusion improved feed efficiency by 3% in weaned pigs fed diets based on maize or sorghum, but had no effect on growth rate or protein utilisation.
Similar results were observed with extruded maize and wheat. Extruded barley also improved weight gain and feed efficiency by about 14% in weaned pigs compared to raw barley, but extruded maize actually reduced performance (perhaps due to overprocessing, 75% starch gelatinisation). In protein-rich ingredients, such as soybeans, peas, lupins, and rapeseed, the main effects of extrusion processing are mainly the destruction of antinutritional factors and secondary any improvements on carbohydrate utilisation.
In conclusion, proper extrusion improves starch digestibility and enhances feed efficiency, but excessive thermal processing may actually reverse any beneficial effects of extrusion.