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Simple Summary: Organic acids (OAs) are proposed as suitable alternatives in animal production. The released protons of OAs can reduce the pH in the gastrointestinal tract, then, nutrient digestion is stimulated by boosting the secretion of endogenous enzymes. In addition, lower intestinal pH suppresses pathogenic bacterial growth and encourages beneficial bacterial growth. In the starter phase, broilers are young, and their digestive tract is not well developed; thus, the first purpose of this study is to evaluate the efficacy of liquid OAs on the growth performance, serum metabolism lipid profiles, chyme pH, and duodenal enzymes activity in broilers during the starter phase. In addition, since a high stocking density (HSD) usually occurs in grower broilers and can lead to some disadvantages, whether supplemental OAs could alleviate the HSD stress condition in grower broilers is the second study aim of this study. Hence, the objective of the current study was to investigate whether the supplementation of liquid OAs in an entire period could alleviate the grower broilers' stress status under an HSD. We found that OAs decreased the chyme pH in the upper digestive tracts and increased the activity of digestive enzymes in broilers during starter and grower phases. In addition, an HSD decreased the growth performance, while OAs showed no efficacy in the growth performance but improved the nutrient utilization in grower broilers. The alterations of gut microbiota were observed in grower broilers supplemented with OAs. In addition, the mechanisms of OAs in regulating the lipid metabolism and gut bacteria need further investigations. This study aimed to evaluate the efficacy of organic acids (OAs) in starter broilers and to investigate whether supplemental OAs could alleviate the high stocking density (HSD) stress condition in grower broilers. A total of 408 1-day-old Arbor Acres broilers were assigned into two groups without or with liquid OAs in the starter phase. In the grower phase, each group in the starter phase was divided into a normal stocking density and an HSD. The OA dose was 0.16% at the starter and grower phases. The results showed that at the starter phase, OAs decreased the chyme pH in gizzard and duodenum and increased the activities of chymotrypsin and α-amylase in the duodenal chyme (p < 0.05). In the grower phase, an HSD decreased the growth performance and the ether extract digestibility (p < 0.01). The supplementation of OAs decreased the chyme pH in the gizzard, proventriculus, and duodenum and increased the lipase and α-amylase activities (p < 0.05). The supplemental OAs increased the dry matter and total phosphorous digestibility and the contents of acetic acids, butyric acids, isovaleric acids, and valeric acids (p < 0.05). For cecal microbial compositions at the genus level, an HSD decreased the relative abundance of Blautia, Norank_f__norank_o__RF39, and Alistipes, while supplemental OAs increased the relative abundance of Norank_f__norank_o__RF39 (p < 0.05). In conclusion, although there were no interaction effects between OAs and stocking densities in the present study, it was clear that the supplementation of OAs has beneficial effects on the chyme pH, enzymes activities, and nutrient digestibility in broilers, while an HSD existed adverse effects on the growth performance, nutrient digestibility, and gut microbiota balance in grower broilers. [ABSTRACT FROM AUTHOR]

Symptoms indicating digestive malfunction have been observed in cultivated salmon over the last 10–15 years, at least partially as a consequence of use of new feed ingredients. The present knowledge on many aspects of digestive functions in fish is limited, in particular for Atlantic salmon. Strengthening is needed to understand the mechanisms underlying challenges to gastrointestinal health and to find means to prevent their development. The present paper supplies information regarding digestive enzyme optima and stability, chyme pH and enzyme activities along the digestive tract of Atlantic salmon. The data presented includes new data from four studies on Atlantic salmon as well as a review of comparative literature from various other fish studies. The four experiments comprised salmon of different sizes, kept in salt or fresh water, and fed high fish meal diets with different protein to energy ratios (P/E). These studies showed an average pH in the stomach chyme, i.e. across diets and experiments, of 4.8 (SD = 0.7), whereas the observed pH optimum for pepsin was 3.0. In the intestinal chyme, pH increased throughout the intestine from a mean of 8.1 (SD = 0.23) in the proximal intestine to 8.4 in the mid intestine (SD = 0.27) and distal intestine (SD = 0.29). Total proteolytic activity of chyme in the proximal intestine, measured as the sum of the work of all proteolytic enzymes on casein as substrate, showed its maximum at pH 8.8, i.e. clearly above prevailing chyme pH. For the individual proteolytic enzymes, pH optimum varied from 7.10 for elastase 1 to 8.98 for trypsin. Chyme pH did not appear to be influenced by fish size, but was significantly higher in the fish raised in fresh water than in those raised in salt water. In the intestinal chyme pH showed a significant negative regression on diet P/E. For the investigated pancreatic enzymes in the intestinal chyme, the activity in the chyme decreased from proximal to distal regions and faecal activities were very low compared to the activities in the proximal intestine. The activity profile for leucine aminopeptidase (LAP), however, trended to increase from mid to distal intestine. Chyme enzyme activities along the intestinal tract did not differ clearly between the experiments and no clear relationship between P/E on enzyme activities along the intestinal tract was observed. Remarkable in vitro stability was observed for the proteolytic enzyme activities. [ABSTRACT FROM AUTHOR]

Acid–base disturbances caused by environmental factors and physiological events including feeding have been well documented in several fish species, but little is known about the impact of dietary electrolyte balance (dEB). In the present study, we investigated the effect of feeding diets differing in dEB ( − 100, 200, 500 or 800 mEq/kg diet) on the growth, nutrient digestibility and energy balance of Nile tilapia. After 5 weeks on the test diet, the growth of the fish was linearly affected by the dEB levels (P< 0·001), with the lowest growth being observed in the fish fed the 800 dEB diet. The apparent digestibility coefficient (ADC) of fat was unaffected by dEB, whereas the ADC of DM and protein were curvilinearly related to the dEB levels, being lowest and highest in the 200 and 800 dEB diets, respectively. Stomach chyme pH at 3 h after feeding was linearly related to the dEB levels (P< 0·05). At the same time, blood pH of the heart (P< 0·05) and caudal vein (P< 0·01) was curvilinearly related to the dEB levels, suggesting the influence of dEB on postprandial metabolic alkalosis. Consequently, maintenance energy expenditure (MEm) was curvilinearly related to the dEB levels (P< 0·001), being 54 % higher in the 800 dEB group (88 kJ/kg0·8 per d) than in the 200 dEB group (57 kJ/kg0·8 per d). These results suggest that varying dEB levels in a diet have both positive and negative effects on fish. On the one hand, they improve nutrient digestibility; on the other hand, they challenge the acid–base homeostasis (pH) of fish, causing an increase in MEm, and thereby reduce the energy required for growth. [ABSTRACT FROM PUBLISHER]