Importance of feed structure (particle size) and feed form (mash vs. pellets) in pig nutrition – A review
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Pigs are monogastric animals with simple, single-chambered stomach and require easily digestible, high quality feed. One of the most important factors that determine feed utilization by pigs is the particle size distribution. The reduction of particle size of feed improves pigs’ performance due to increased specific surface of feed particles allowing better contact with digestive enzymes. In this respect, fine grinding could be recommended in production of pig feed. Additionally, in modern pig production dry feed is predominantly used in pelleted form, which is mainly due to improved (i.e. decreased) feed conversion ratio (FCR) of pigs fed pelleted feed, but also due to other advantages of pelleted over mash feed. Size of feed particles is strongly reduced during pelleting process. Consequently, digestibility of nutrients in pig feed could be improved. On the other hand, presence of high quantities of fine particles in pig feed (both mash and pelleted) negatively affects the health of gastro-intestinal tract (GIT) leading to higher incidence of stomach ulceration and other negative alterations of gastric mucosa (keratization, erosions). Gastric ulcers are one of the most important causes of sudden death on farm that can result in large economic losses in pig production. Concerning that the animal therapy is expensive, labor-intensive, and mostly non-effective due to late recognition of ulceration, prophylactic recommendations are required. Thus, according to literature data, decreasing the quantity of fine particles in pig feed is strongly recommended. Particle size distribution of the pigs’ feed has a strong influence on presence of pathogen bacteria in GIT of pigs. Feeding pigs with coarse mash feed decreases pH value of stomach content compared to pigs fed finely ground mash feed and compared to pigs fed pelleted feed. This can be explained by slower passage rate, increased dry matter, and more dense consistency of stomach content in pigs fed coarse mash diets. Consequently, feed acidification in stomach is better, number of lactic acid bacteria and concentration of organic acids is higher, and pH of stomach content is lower. These conditions create additional “barrier” against pathogen bacteria. According to available data, optimal particle size of diets for pigs is in the range between 500 and 1600 μm, while particles smaller than 400 μm are considered as undesirable with high ulcerogenic capacity. Optimal particle size could be designed in the grinding process, and it was shown that the most convenient grinding method is to combine roller mill and hammer mill. Concerning that nowadays pigs are mainly fed pelleted feed, and that pelleting causes strong additional grinding of feed particles, particle size distribution (PSD) obtained within the grinding process would be dramatically changed during pelleting. The possibilities to decrease the intensity of grinding of particles during pelleting, by variation of parameters of pelleting process, are very limited. Modified extrusion process (i.e. processing using expander) followed by shaping element, is suggested in the literature as an alternative for pelleting in order to obtain agglomerated pig feed with preserved PSD, but this process is not extensively studied so far.