Effects of alfalfa, wheat bran or beet pulp, with or without sunflower oil, on caecal fermentation and on digestibility in the rabbit

Abstract

The study was designed to assess the influence of diets, differing in cell wall composition, on caecal fermentative activity and total tract apparent digestibility. The effect of adding fat to these diets was also tested. Six diets with 330 g NDF kg −1, were used. They were based either in dehydrated alfalfa (AL), wheat bran (WB), or beet pulp (BP); for each case 60 g of sunflower oil was or was not substituted for starch. Diets were fed to 6 × 12 rabbits, from 28 to 70 days of age, in a 3 × 2 factorial arrangement. Growing performances, coefficient of total tract apparent digestibilities (CTTAD) (during the 5th and 9th weeks of age), caecotrophy and caecal fibrolytic activity (at 35 and 56 days of age), volatile fatty acids (VFA) concentration and full and empty digestive organs – stomach and caecum were all measured at slaughter at the end of experience. The digestibility of all analytical fractions was affected ( P < 0.0001) by the source of fibre. The digestibility of energy was 20% higher in BP diets (0.763) than in AL diets (0.635), intermediate in WB diets (0.719). The same pattern was seen in the digestibilities of cell wall fractions: at 9 weeks, CTTAD for NDF was 0.584, 0.393 and 0.267, and for ADF was 0.391, 0.230 and 0.174, in BP, WB and AL diets, respectively. Sunflower oil addition increased ( P < 0.001) the digestibility of most analytical fractions by 3–5%. It had a greater effect on fat digestibility (from 0.7 to 0.9), and a more variable, albeit also positive, effect on the digestibilities of the cell wall fractions. Fibre source affected fibrolytic activities (mg sugar h −1 g −1 DM) in both the caecotrophs and the caecal contents. Pectinolytic and cellulolytic activities were higher in BP diets, xilanolytic activity higher in WB diets. Fat addition decreased ( P < 0.05) cellulolytic activity in caecotrophs in the second collection, and the pectinolytic and cellulolytic activities in caecal contents. Fibre source also affected ( P < 0.001) the VFA total level, individual levels, and relative proportions. Both the level and the molar proportion of C2 were higher in BP diets. Fat decreased ( P < 0.05) total VFA level, from 43.7 to 39.9 mmol l −1. Source of fibre affected the composition of caecal contents, both in absolute ( P < 0.01) and in relative ( P < 0.05) terms. In absolute terms, WB diets gave less NDF, ADF and cellulose, and more hemicellulose in the contents. Fat reduced the amounts of NDF from 10 to 8.6 g ( P = 0.014), ADF from 6.3 to 5.1 g ( P = 0.002) and cellulose from 4.5 to 3.7 g ( P = 0.006) in the contents. BP diets gave heavier ( P < 0.001) stomachs and caecums. Fat gave heavier ( P < 0.001) empty stomachs, lighter ( P = 0.019) caecums. It may be concluded that caecal fermentative activity was affected by the composition of the fibrous feed, less so by the inclusion of fat in the diet.