The world population is growing, and improving living standards and urbanisation are driving factors in the global increase in meat, egg and milk consumption (Boland et al., 2013). Poultry production is a key factor in providing sufficient animal protein. To achieve efficient poultry production, highly digestible diets need to be fed, often resulting in competition for resources for human consumption (the food-feed competition). This intensifies the search for novel dietary ingredients that neither compete for resources, nor for arable land and freshwater. Seaweed is such a novel dietary ingredient of interest. This thesis describes studies into the nutritional value of various seaweed species (an alternative protein source), with a focus on nutritional composition, digestibility and health in broiler chickens.In Chapter 2 the composition of intact seaweeds obtained from coastal waters in Northwestern Europe were studied as source of macronutrients for livestock. The chemical composition, in vitro digestibility and in vitro gas production were determined. The seaweed species included in this study were brown: Saccharina latissima, Laminaria digitata and Ascophyllum nodossum, red: Palmaria palmata and Chondrus crispus, and green: Ulva lactuca. The nutrient content was highly variable between and within species. Based on the protein content and amino acid pattern, intact P. palmata and U. lactuca could be a valuable protein source for farm animals, with the high non-starch polysaccharides and non-protein nitrogen contents and a low in vitro digestibility potentially limiting their use as a feed ingredient for monogastric species. Furthermore, the fermentability of L. digitata, S. latissima and P. palmata indicate that these intact seaweeds may have a higher nutritional value in ruminants. The high ash content hampers the use of intact seaweed for both ruminants and simple stomached animal species.To be able to include seaweed in broiler diets, the ash content needs to be reduced and the digestibility and shelf life increased. In Chapter 3, the effects of ensiling, washing and extraction processes were evaluated using S. latissima, L. digitata and U. lactuca incubations in an in vitro simulated digestibility model. In an in vivo study, broilers were fed S. latissima silage or silage residue. Washing and ensiling reduced the ash content, but also the in vitro organic matter digestibility. Washing reduced in vitro nitrogen digestibility. Extraction processes decreased in vitro organic matter and nitrogen digestibility. Feeding seaweed diets to broilers resulted in a higher feed conversion ratio (FCR), without increasing body weight (BW). Feeding S. latissima silage residue compared to silage resulted in a slightly better broiler performance and a higher pre-caecal amino acid digestibility. In conclusion, washing, ensiling and extraction processes reduced the nutritional value of the seaweed products, and did not make seaweed suitable for inclusion in broiler diets. To create suitable seaweed products for inclusion in broiler diets, a further reduction in the ash content and increase in digestibility are necessary.To increase the economic viability of the use of seaweed in livestock diets, products resulting from a biorefinery approach can be included in broiler diets. To investigate methods to improve seaweed digestibility, Chapter 4 discusses the effects of a proteolytic enzyme pre-treatment of green seaweed species U. laetevirens and red seaweed species Solieria chordalis co-products, resulting from a biorefinery process, before inclusion in a broiler diet. Broilers were fed diets including U. laetevirens or S. chordalis co-products, with or without proteolytic enzyme pre-treatment of the seaweed. The seaweed diets increased BW gain, and feed intake in the third week of the experiment. Birds fed the U. laetevirens compared to the S. chordalis diets had a higher BW gain, and a lower FCR. Seaweed inclusion reduced diet digestibility. Birds fed U. laetevirens vs. S. chordalis diets had shorter villi. Untreated versus treated seaweed in the diets led to lower plasma Interleukin-13 levels. In conclusion, the proteolytic enzyme treatment of the seaweed co-products did not improve performance or health-related parameters, and reduced digestibility of the diets. U. laetevirens co-product inclusion did improve performance based on growth and FCR. It had a slight negative effect on duodenal villus length, and a positive effect on crypt depth. The inflammation response was strongly reduced in birds fed the untreated U. laetevirens diet, making the U. laetevirens co-product of interest for further research.The experiment described in Chapter 5 investigated a proteolytic enzyme pre-treatment to improve digestibility of U. laetevirens seaweed biorefinery co-products when included in a standard diet or a diet based on European protein sources. The effects on health and performance were further investigated. Broilers were fed a soy or EU-based diet, including the U. laetevirens co-product treated without or with a proteolytic enzyme. In the last two weeks, birds fed the soy vs. EU-based grower diets showed a higher BW, BW gain and feed intake, as well as a lower FCR in wk 3. Heavier gizzards and more gizzard contents were observed in birds fed the EU vs. soy-based diets, as well as longer villi. U. laetevirens supplemented diets had a higher water holding capacity than the basal diets. Enzyme treatment did not affect nutrient digestibility. U. laetevirens inclusion in soy-based diets led to higher, and in EU-based diets to lower apparent pre-caecal digestibility of all nutrients. Dietary U. laetevirens inclusion had different effects when added to a soy or EU-based diet. No obvious health effects were observed, leading to a conclusion of the absence of performance of health promoting bioactive components in the U. laetevirens co-product, or of diminishing of these effects due to the proteolytic enzyme pre-treatment.In the final Chapter 6 the potential of seaweed for broiler nutrition is discussed based on its nutritional composition and the effects on digestibility, performance and health. The water holding capacity and viscosity of seaweed are discussed, as well as the limitations and future of seaweed for broiler nutrition. The potential of seaweed as dietary ingredient for broilers is discussed, in light of a biorefinery approach and seaweed availability. Recommendations for further research are presented, as well as implications of this research for the industry, finalized by the conclusions drawn from this thesis.