## 4 Fermented Products in Chicken Nutrition  
In the last ten years, in part due to the development of green energy production in bioelectric power plants, the price of basic raw materials (wheat, maize) used for chicken feed production has increased. Therefore, cheaper sources are being sought for to fully replace them [43]. One possibility is the utilization of alternative crops [44] or by-products such as bran, spent grains, fruit peels [30,45,46], and oilseed pomace [19] that are produced during food production. These alternatives are more economically viable compared to the main cereals but they contain insufficient protein, high amounts of fibre, and often anti-nutrients, limiting their use in broiler chicken fattening [47]. Several works confirm that the use of fermentation processes either by lower filamentous fungi or bacteria will increase the total protein content and reduce the content of indigestible fibre, antinutrients, and toxic substances in raw materials used for fattening [37,45,47,48]. Thus, the prepared pre-fermented product can be efficiently used for broiler chicken nutrition. The most common problem with non-traditional raw materials in poultry nutrition is the high fiber content, which can be significantly reduced by fermentation; specifically, the indigestible part of fiber can be fermented into digestible fiber [47,49]. Via solid-state fermentation (SSF) without access to free water in the substrate and with the help of the filamentous fungus _Trichoderma pseudokoningii_ (a producer of the enzymes xylanase and cellulase), a fermented product was prepared from wheat bran and fed to broiler chickens [45]. Fermented wheat bran contained significantly more enzymes (cellulose and xylanase) and also reduced sugars after fermentation. The addition of fermented bran, replacing corn at 10% in the chickens' diet, did not cause a reduction in the weight gains of the chickens and, on the contrary, significantly reduced feed consumption, which was reflected in a significantly better feed conversion rate of the chickens in the experimental group. _Trichoderma_ spp., as the most consistent and safe fungi, are used for the synthesis of cellulase and hemicellulaes [50,51]. Fungi have a highly effective enzymatic system that allows them to break down lignocellulosic materials [52]. SSF, using fungi of the genus _Trichoderma_ spp., can increase the utilization of a non-traditional raw material (such as bran) in broiler chicken diets; however, they are only able to break down fiber, increase the amount of reducible sugars, and partially increase the total protein content. This genus is not able to produce fatty acids or influence the fatty acid composition of the fermented raw material. The preparation of feeds based on increasing the proportion of PUFA is based on SSF using lower filamentous fungi [37]. By-products of agricultural origin (spent grain, wheat bran, spelt bran) are used as a substrate and lower filamentous fungi of the genera _Thammidium_, _Cunninghamella_, _Mortierella_, and _Umbelopsis_, which are able to utilize carbon sources and produce unsaturated fatty acids such as GLA, EPA, and AA, which are used as fermentation agents [14]. The aim of the experiments was to use SSF to prepare complete feed component for broiler chickens that can fully replace a part of the chicken feed mixture and, in addition, contain a significant amount of fatty acids, which will subsequently increase their proportion in the fat of chicken meat (Tables 2 and 3). In one of the first experiments, the effect of a fermented product prepared by SSF using the lower filamentous fungus _Thammidium elegans_, was investigated [53]. Wheat bran was used as an effective substrate for SSF [54]. Since _Thammidium elegans_ is mainly a producer of GLA, the FP was mainly enriched with this specific fatty acid. This fermented feed was added at a dose of 3% to the feed of Cobb 500 hybrid broiler chickens from the 20 th day of fattening.