Effects of multinutrient blocks and polyethylene glycol 4000 supplies on intake and digestion by sheep fed Acacia cyanophylla Lindl. foliage-based diets

Abstract

The effect of multinutrient block supply and polyethylene glycol 4000 (PEG) on intake, digestion and rumen fermentation was studied in sheep fed with air-dried Acacia cyanophylla foliage (acacia)-based diet. In Experiment I, six Noire de Thibar breed sheep ( BW=46 kg ) were used in double 3×3 Latin square design. All diets included about 360 g of dry matter (DM) of oat–vetch hay and acacia ad libitum. Diet D0 was without a block supplement. Diet D1 included a urea–molasses–mineral block (B1). While D2 included another type of block (B2) that differed from B1 essentially by adding polyethylene glycol 4000. Each experimental period lasted 33 days (21 days for adaptation and two periods of 5 days for measurement separated by 2 days for rest). Feed intake, apparent digestibility of organic matter (OM), crude protein (CP) and crude fibre (CF) and retained nitrogen (Nr) were measured by total faecal collection. In Experiment II, four Noire de Thibar sheep ( BW=53 kg ) fitted with rumen cannulae were fed sequentially D0, D1 and D2, respectively, to 90% of intake levels as measured in Experiment I on metabolic weight (MW) base. Fermentation parameters in rumen liquid (pH, NH 3-N, volatile fatty acid (VFA)) were measured at 0, 2, 5, 8, 10, 13, 16 and 21 h after the morning meal. Protozoal number and composition in rumen fluid were determined at 2 h sampling time. Solid digesta kinetics through the rumen was measured using chromium (Cr) mordanced acacia. The DM and CP degradation of acacia was determined using the nylon bag technique. Acacia had a relatively high content of condensed tannins (41 g kg −1 of DM) and acid detergent lignin (176 g kg −1 of DM). B1 and B2 were high in CP (381 and 369 g kg −1 of DM, respectively) compared to acacia (127 g kg −1 of DM, 20% bound to fibre) and hay (75 g kg −1 of DM). The two kinds of blocks improved similarly ( P<0.001) acacia DM intake by 195 g. Block 1 increased ( P<0.001) only diet CP digestibility and Nr. Block 2 increased ( P<0.05) by a low extent DM and OM digestibility compared to D1, and remarkably ( P<0.001) CP digestibility and Nr compared to D0 and D1. Block supplies considerably increased water intake. Both B1 and B2 increased ( P<0.001) NH 3-N and VFA concentrations in the rumen liquid with a positive specific effect of B2 (PEG). VFA molar proportion was significantly modified by B1 and B2. B1 and B2 decreased acetate proportion and increased propionate and butyrate proportions as compared to D0 ( P<0.001). Protozoal number in rumen fluid was increased significantly by B1 and B2 ( P<0.001). PEG-containing block (B2) increased protozoal number as compared to B1. Both B1 and B2 increased ( P<0.001) solid outflow rate, with a specific increasing effect of B2 (D2) when compared to B1 (D1). Blocks supply did not modify in situ DM degradability of acacia, but B2 improved ( P<0.05) effective degradability of CP when compared to D0 and D1 which were similar. It is concluded that both B1 and B2 improved the nutritive value of acacia-based diet. A further positive effect was noted in D2 (PEG), especially for N metabolism.