Effects of the dietary proportions of lipids and carbohydrates on milk fat, blood metabolites, and the fecal microbial community in lactating jennies

The donkey has been receiving increasing attention due to its milk production, and the manipulation of milk fat content has become one of the key current topics. We tested the hypothesis that the dietary proportions of lipids, non-fiber carbohydrates (NFC) and neutral detergent fibers (NDF) affect milk composition in jennies. Twenty-four lactating jennies of similar age, weight (180 ± 4.1 kg) and foaling date were assigned randomly into three dietary groups: high lipids and NDF with low NFC (HLF); medium lipids and NDF with intermediate NFC (MLF); and low lipids and NDF with high NFC (LLF). The three diets were isoenergetic and isonitrogenous, and the study consisted of 14 days of adaptation and 56 days of measurements. Milk, blood and fecal samples were collected to determine the effects of dietary intake on milk composition, blood metabolites, and fecal short-chain fatty acids and microbiota. The percentage (P = 0.007) and yield (P = 0.013) of milk fat in the HLF group were greater than in the MLF and LLF groups, while the percentages of milk protein (P < 0.001) and lactose (P < 0.001) were greater in the MLF group than in the HCS and LLF groups. The concentrations of milk medium-chain fatty acids (FA) (P = 0.047), polyunsaturated FA (P = 0.021) and unsaturated FA (UFA) (P = 0.036) were greater in the HLF than LLF group, and the feed conversion ratio (FCR) of ∑≤ C14 FA was greater (P = 0.003) in the HLF and MLF groups than the LLF group, while the FCR of ∑≥ C18 FA in the LLF group was greater (P = 0.046) than in the HLF group. Most differential blood metabolites between HLF and MLF or between HLF and MLF groups were related to lipid metabolism. Fermentation parameters in the rectal feces did not differ among groups. The abundances of the phylum Spirochaetes (P < 0.001) and the genus Treponema (P < 0.001) in the feces of the MLF and LLF groups were lesser, but the abundances of Prevotella (P < 0.001) and Ruminococcus (P = 0.009) in the MLF and LLF groups were greater than in the HLF group. A high proportion of dietary lipids and NDF increased milk fat content, UFA concentrations, feed conversion ratio of ∑≤ C14 FA, and abundance of fecal cellulolytic bacteria; whereas, a high proportion of dietary NFC increased blood FA metabolites and the abundance of pro-inflammatory bacteria. Dietary composition can influence milk composition in jennies, and high proportions of lipids and NDF can increase milk fat content.