An experiment with 210 male (Ross 308) 1-d-old broilers was conducted to test the hypothesis that a coarse diet improves performance of broilers fed a poorly digestible protein source. A highly digestible diet based on soybean meal was gradually replaced by a low digestible diet based on rapeseed meal (RSM) in 5 steps (RSM-0%, RSM-25%, RSM-50%, RSM-75%, and RSM-100%). Two diet structures (fine and coarse) were used as an additional factor. These 2 factors and their interactions were tested at different ages in a factorial arrangement with 10 dietary treatments. An increase in indigestible dietary protein negatively affected feed intake (P = 0.003), BW gain (P = 0.008), and feed conversion ratio (P = 0.034). This increase in dietary indigestible protein contents resulted in a decrease (P = 0.001) in total cecal volatile fatty acid concentration from 209.1 to 125.9 mmol/kg of DM digesta in broilers with increasing RSM in diets. Increase in the indigestible protein level, from RSM-0% to RSM-100%, resulted in a decrease (P = 0.042) in villus heights (1,782 vs. 1,574 µm), whereas crypt depths increased (P = 0.021; 237 vs. 274 µm). A coarse diet improved feed intake (P = 0.006), BW gain (P = 0.014), and feed conversion ratio (P = 0.009). Broilers fed coarse diets had approximately 11, 24, and 10% lower relative empty weights of the crop, proventriculus, and jejunum, respectively, whereas a 15% heavier gizzard was found compared with those fed the fine diets. Dietary coarseness resulted in approximately 16% lower gizzard pH, 21% greater villus heights, 27% lower crypt depths, 24% reduced branched-chain fatty acids, and 12% lower biogenic amines in the cecal digesta compared with broilers fed fine diets. In conclusion, feeding coarse particles improved broiler performance irrespective of digestibility of the diet. Hindgut protein fermentation can be reduced by coarse grinding of the diet.

In the field of animal nutrition, butyrate is used as a zootechnical ingredient and can be used as an unprotected salt or in the form of protected derivatives such as butyrate glycerides or butyrate-loaded matrices. Dietary butyrate supplementation has been shown to improve growth performance and resilience of broiler chickens through distinct mechanisms, operating on both eukaryotic and prokaryotic cells. Firstly, butyrate influences endogenous avian cells in multiple ways: it is an agonist of free-fatty acid receptors, an inhibitor of pro-inflammatory pathways, an epigenetic modulating agent and acts as an energy source. Secondly, butyrate influences the microbiota residing in the avian gastrointestinal tract (GIT) as a result of its bacteriostatic properties. The responses, e.g. changes in growth performance, gut morphology, carcass traits or nutrient digestibility of chickens, to dietary butyrate supplementation are inconsistent with factors such as additive inclusion level, diet composition, age and health status of the bird modulating the effects of butyrate and its derivatives. For many derivatives, the precise GIT segment wherein butyrate is released is unclear. Release location may affect the observed responses to butyrate given the diversity of cell types and pH conditions encountered throughout the gastrointestinal tract of poultry, and the differences in microbiota composition in the different gut segments. As a consequence, our understanding of the mode of action of butyrate is hampered. Characterisation of existing derivatives and development of targeted-release formulations are, therefore, important to gain insight in the different physiological effects butyrate can elicit in broiler chickens

The hypothesis was tested that butyrate presence in the digesta of distinct gastrointestinal tract (GIT) segments of broilers leads to differential effects on digesta retention time, gut morphology, and proteolytic enzymatic activities, ultimately resulting in differences in protein digestibility. A total of 320 male day-old Ross 308 broilers were randomly assigned to 5 dietary treatments: 1) control (no butyrate), 2) unprotected butyrate (main activity in the crop and gastric regions), 3) tributyrin (main activity in the small intestine), 4) fat-coated butyrate (activity in the whole GIT) and 5) unprotected butyrate combined with tributyrin, each replicated 8 times. Rapeseed meal was used in combination with a fine dietary particle size in order to challenge the digestive capacity of young broilers. Birds were dissected at 22, 23, and 24 d of age and samples of digesta at various GIT locations as well as tissues were collected. Butyrate concentration varied significantly across GIT segments depending on treatment, indicating that the dietary contrasts were successful. The apparent ileal digestibility of methionine tended to increase when butyrate and/or propionate was present in colonic and cecal contents, possibly due to modifications of GIT development and digesta transit time. Butyrate presence in the digesta of the crop, proventriculus and gizzard, on the contrary, decreased the apparent ileal digestibility of several amino acids (AA). In addition, butyrate presence beyond the gizzard elicited anorexic effect that might be attributable to changes in intestinal enteroendocrine L-cells secretory activities. The present study demonstrates that, in broilers, effects of butyrate on digestive processes are conditioned by the GIT segment wherein the molecule is present and indicates its influence on digestive function and bioavailability of AA.

Dietary butyrate supplementation has however an inconsistent effect on the growth performance of broilers, with factors such as inclusion level, health status, age of the bird or diet composition being influential for the observed effects on growth performance (Cerisuelo et al., 2014; Polycarpo et al., 2017). Several mechanisms underlying the observed effects of butyrate on growth of livestock have been hypothesised (For review, see Guilloteau et al., 2010). At present, a limited number of studies have investigated the existence of such mechanisms in poultry (Zhang et al., 2011; Liu et al., 2017). In addition, most of the poultry studies resorted to only one butyrate additive and did not report butyrate concentrations along the GIT (e.g. Czerwinski et al., 2012; Cerisuelo et al., 2014). Hence, it remains uncertain whether the effects elicited by butyrate are conditioned by the GIT segment wherein the molecule is present. This dearth of knowledge could limit the full use of butyrate supplements as a nutritional strategy to meet the challenges of the poultry industry. Bacteriostatic and bactericidal effects Butyrate presence in the gastrointestinal tract Intracellular effects Sensing by enteroendocrine cells Increased gut integrity and prevention of inflammation Changes in passage rate, gut morphology and enzymatic secretions Changes in nutrient digestibility Shifts in microbiota composition Changes in immune responses Change in growth performance Figure 1. Putative modes of action of butyrate resulting in changes in the growth performance of broiler chickens Literature review