Dietary carbohydrate sources differently prime the microbial ecosystem but not the epithelial gene expression profile along the complete gut of young calves.

BACKGROUND: Recent data indicated similar growth performance of young calves fed solely high-quality hay instead of a starter diet based on starchy ingredients. Yet, providing exclusively such distinct carbohydrate sources during early life might specifically prime the microbiota and gene expression along the gut of young calves, which remains to be explored. We investigated the effects of starter diets differing in carbohydrate composition, that is medium- or high-quality hay and without or with 70% concentrate supplementation (on fresh matter basis), across the gastrointestinal tract (GIT) of weaned Holstein calves (100 ± 4 days of age) using 16 S rRNA gene sequencing and analyses of short-chain fatty acids and host epithelial gene expressions. RESULTS: The concentrate supplementation drastically decreased microbial diversity throughout the gut, which was also true to a much lesser extent for high-quality hay when compared to medium-quality hay in the foregut. Similarly, the factor concentrate strongly shaped the diet-associated common core microbiota, which was substantially more uniform along the gut with concentrate supplementation. The fermentation profile shifted towards less acetate but more propionate with concentrate supplementation in almost all gut sections, corresponding with higher abundances of starch-utilizing bacteria, while major fibrolytic clusters declined. Noteworthy, the n-butyrate proportion decreased in the rumen and increased in the colon with concentrate, showing an opposite, gut site-dependent effect. Both dietary factors modestly influenced the host epithelial gene expression. CONCLUSIONS: Concentrate supplementation clearly primed the microbial ecosystem on a starch-targeted fermentation with characteristic genera occupying this niche along the entire GIT of calves, whereas the microbial differentiation due to hay quality was less distinct. Overall, changes in the microbial ecosystem were only marginally reflected in the targeted transcriptional profile of the host epithelium.

Feeding concentrate with hay of different qualities modulates rumen histology, development of digestive tract as well as slaughter performance and meat quality of young dairy calves.

Concentrate-rich starter diets are commonly fed to dairy calves to stimulate growth performance. However, feeding high amounts of starter feed with low inclusion of forage fibre may jeopardise the development of the gastrointestinal tract (GIT). Moreover, studies investigating the effects of feeding on carcass and meat quality of young calves at rearing are rare. The objective of this research was to investigate the effect of hay quality and concentrate inclusion on the traits of GIT development, slaughter performance and veal quality of young dairy calves. The feeding trial covered the first 14 weeks of life. Seventeen male and three female Holstein calves (n = 20) were randomly allocated to four experimental groups, which received besides acidified whole milk different solid feeds: (1) 100% medium-quality hay (MQH), (2) 100% high-quality hay (HQH), (3) 30% medium-quality hay and 70% concentrate (MQH+C) and (4) 30% high-quality hay and 70% concentrate (HQH+C). The acidified whole milk was fed in the first 12 weeks of life, and calves had ad libitum access to solid feed and water from birth till slaughter. Calves were kept in individual boxes equipped with straw and slaughtered at the end of week 14. After slaughter, gut development traits, rumen histology, slaughter performance and meat quality were assessed. Overall, both concentrate inclusion and hay quality showed major effects on rumen histology and development of the GIT in dairy calves with minimal effects on most carcass cuts and meat quality traits. Concentrate-fed calves had significantly higher average daily gains, final body weights, blood amounts and proportions of organs from the circulatory and respiratory systems. Proportions of liver and kidneys were lowest in MQH-fed calves. The proportion of GIT was significantly lower in groups fed concentrates, but the weight of the reticulorumen was unaffected by solid feed. Concentrate feeding led to thicker keratin layer and epithelium as well as wider papillae in the rumen. Hay quality particularly affected the width of the papilla and epithelium thickness, while feeding hay without concentrate enhanced the thickness of submucosa and muscularis, as well as the size of parotid glands. In conclusion, the type of solid feed affects the development of the GIT with concentrate feeding holding the risk to induce keratinisation of rumen epithelium while enhancing performance and carcass traits.

Shift of dietary carbohydrate source from milk to various solid feeds reshapes the rumen and fecal microbiome in calves.

The transition from milk to solid diets drastically impacts the gut microbiome of calves. We explored the microbial communities of ruminal fluid and feces of Holstein calves when fed milk on d 7 of life, and when fed solid feeds based on either medium- or high-quality hay with or without concentrate inclusion (70% in fresh matter) on d 91. Ruminal fluid and feces had distinct microbial compositions already on d 7, showing that niche specialization in early-life gut is rather diet-independent. Changes between d 7 and d 91 were accompanied by a general increase in microbial diversity. Solid diets differed largely in their carbohydrate composition, being reflected in major changes on d 91, whereby concentrate inclusion was the main driver for differences among groups and strongly decreased microbial diversity in both matrices. Fecal enterotyping revealed two clusters: concentrate-supplemented animals had an enterotype prevalent in Prevotella, Succinivibrio and Anaerovibrio, whereas the enterotype of animals without concentrate was dominated by fibrolytic Ruminococcaceae. Hay quality also affected microbial composition and, compared to medium-quality, high-quality hay reduced alpha-diversity metrics. Concluding, our study revealed that concentrate inclusion, more than hay quality, dictates the establishment of niche-specific, microbial communities in the rumen and feces of calves.