Mochammad Fahmi Habibi

Global demand for animal-based protein is rising rapidly, with poultry production playing a key role due to its high efficiency and relatively low production costs. Genetic selection has substantially increased growth rates and egg production in modern poultry breeds, enhancing productivity but also increasing nutritional requirements and reliance on highly digestible feed ingredients. At the same time, sustainability challenges—particularly competition for high-quality feed ingredients and the environmental impacts of feed production—have intensified interest in using agricultural by-products as alternative feed ingredients. These by-products are typically rich in dietary fiber (DF), whose diverse physicochemical properties can markedly influence gastrointestinal function, nutrient digestibility, and overall performance in poultry. This highlights the need for a deeper understanding of the role of DF in poultry nutrition. The main aim of this thesis was therefore to investigate how DF with differing physicochemical properties (solubility, particle size, viscosity, and hydration capacity) influences digestion-related processes in chickens, including digesta passage behavior, endogenous protein losses (EPL), and nutrient digestibility.

Chapter 2 investigated the effects of coarse or fine soybean hulls (SBH), as insoluble DF sources, with or without purified arabinoxylans (AX), as viscous DF source, on digesta mean retention time (MRT) and nutrient digestibility in broilers. The addition of AX markedly influenced gastrointestinal physiology and digestion by increasing digesta retention time in several segments of the gastrointestinal tract and reducing the apparent digestibility of dry matter, starch, and protein. In contrast, the particle size of SBH had relatively minor effects on digestion processes. However, an interaction was observed whereby AX reduced ileal protein digestibility only when combined with finely ground SBH. This finding suggests that coarse fiber particles may partly counteract the negative effects of soluble viscous fibers, potentially through changes in digesta viscosity.

Chapter 3 focused on quantifying total endogenous protein losses (basal + diet-specific) in broilers fed the same diets as in Chapter 2, as well as diets in which fine soybean hulls were replaced by fine sugar beet pulp (SBP), which DF with a high hydration capacity or enzymatically hydrolyzed casein (EHC). A minimally invasive 15N-isotope dilution (ID) method was studied using urinary 15N-enrichment as a proxy for the enrichment of the precursor pool involved in endogenous protein synthesis. Additionally, because no gold standard method exists for measuring EPL, the ID method was compared with the commonly used EHC and EHC-ultrafiltration methods. Although the ID approach resulted in lower estimates of EPL than the other methods, the results showed that the ID method can reliably quantify EPL under near-practical feeding conditions across diets differing in ingredients, including those varying in fiber composition.

Chapter 4 further investigated total EPL and protein digestibility in broilers fed the same diets as in Chapter 3, excluding the EHC diet. It was found that purified AX increased EPL in feces and reduced true ileal protein digestibility, regardless of the particle size of SBH. However, apparent ileal protein digestibility decreased only when AX was combined with finely ground SBH, again highlighting interactions between viscous soluble DF and particle size of insoluble DF. In addition, replacing fine SBH with fine SBP, increased endogenous protein losses without affecting true protein digestibility. These findings demonstrate that different DF properties influence endogenous losses and protein digestibility in distinct ways.

Chapter 5 examined the effects of coarse or fine oat hulls (OH), as insoluble DF sources, or purified lignocellulose, a fine insoluble with notable hydration capacity on digesta MRT, EPL, and nutrient digestibility. Diets containing purified lignocellulose reduced apparent and true protein digestibility as well as digestibility of starch and fat compared with OH diets. Purified lignocellulose also prolonged digesta retention time in the small intestine, without influencing EPL. In contrast, the particle size of oat hulls had minimal influence on digestion processes. These findings indicate that hydration properties of insoluble fiber ingredients may play a more important role than particle size in determining digestion responses in broilers.

Chapter 6 investigated whether laying hen breeds differing in genetic background respond differently to insoluble DF particle size. Two breeds, Dekalb White and Bovans Black, were fed diets containing either fine or coarse OH. Clear differences between breeds were observed in gastrointestinal tract development, digesta retention time, and nutrient digestibility. Bovans Black hens showed larger gastrointestinal organs, longer digesta retention time, and greater ileal protein digestibility compared with Dekalb White hens. However, the particle size of OH had limited effects on most digestion parameters. These results suggest that genetic background can influence digestive physiology and nutrient utilization, potentially affecting the response to DF characteristics.

Overall, this thesis demonstrates that the physicochemical properties of DF, including viscosity and hydration capacity, strongly influence digesta passage, EPL, and nutrient digestibility in poultry. These findings highlight the importance of considering dietary fiber properties in poultry feed formulation. Therefore, incorporating these properties into ingredient databases and formulation may help to optimize fiber inclusion without compromising nutrient digestion. The implications for nutritional strategies, the broader impact of this research to the poultry industry, and the key take-home messages have been extensively discussed in Chapter 7.