Effects of dietary digestible fibre and soybean oil level on the odd-numbered, branched-chain and hydroxy-fatty acid composition of caecotrophs in rabbits

The study aimed to characterize shifts in caecotroph microbial FA induced by different dietary digestible fibre (DF) and soybean oil (SO) contents, in fattening rabbits. Following a 2 × 2 design, diets with either a low (LDF) or high (HDF) digestible fibre content (180 and 260 g DF/kg DM, respectively) supplemented with or without 20 g of SO/kg, were fed to 12 rabbits (n = 48 in total) for a period of 77 days. At an age of 63 days, a light plastic flat collar was applied on animals for 24 h to prevent caecotrophy. Gas chromatography was used to determine the odd-numbered (ONFA), branched-chain (BCFA), hydroxy (OHFA) and other FA in freeze-dried caecotroph samples. A high dietary DF level increased ONFA (P<0.01) and OHFA (P<0.05) contents, the ONFA:BCFA (P<0.05) ratio and cyclopropane 17:0Δ weight percentage in caecotrophs, suggesting that higher numbers and (or) proportions of bacterial populations were most likely associated with those FA. SO addition increased the total FA content (P<0.001) in caecotrophs, resulting in decreased ONFA (P<0.001), BCFA (P<0.001) and OHFA (P<0.05) weight percentages, which suggests a dilution effect of the dietary derived FA. Other data, such as the increased (P<0.001) conjugated linoleic acid (CLA) for HDF diets and the significant DF by SO interactions (P<0.05) observed for vaccenic (trans11-18:1) and oleic (cis9-18:1) acid, suggest other roles for FA metabolism in the caecum. Microbial FA can potentially be used to describe dietary-induced changes in the caecum, but further studies are necessary to verify their importance in rabbit nutrition research.