Effect of forage to concentrate ratio in complete diets offered to sheep on voluntary food intake and some digestive parameters

Twelve mature ewes were used to study the effect of forage:concentrate ratio in complete diets on voluntary intake and some digestive characteristics. Diets consisted of four combinations of chopped lucerne hay and a concentrate (390 g cracked barley grains, 440 g cracked maize grains and 170 g soya-bean meal per kg of concentrate) in the following proportions (fresh matter basis): 0.8:0.2 (C20), 0.6:0.4 (C40), 0.4:0.6 (C60) and 0.2:0.8 (C80). Diets were offered over two 42-day periods and, in each of them, three sheep received one of the four diets, with the restriction that no animal received the same diet in both periods. Chromium III-mordanted fibre was used as a marker to estimate passage rate of digesta and microbial nitrogen supply (MNS) was estimated from the urinary excretion of purine derivatives. The increase in the proportion of concentrate affected linearly (P < 0.05) the voluntary intake of food, the mean values being 36.8, 37.9, 36.3 and 30.0 g dry matter (DM) per kg live weight per day for C20, C40, C60 and C80 diets, respectively. Apparent digestibility of DM, organic matter (OM) and crude protein increased linearly (P < 0.01) with the proportion of concentrate in the diet, whereas that of cellulose evolved quadratically (P < 0.05), reaching a minimum value in the C80 diet. Digestible OM intake was unaffected (P > 0.05) by the proportion of concentrate in the diet. Both particulate passage rate from the rumen and through the caecum and proximal colon decreased linearly (P < 0.05) as concentrate proportion in the diet increased. MNS (g/day) was not affected (P > 0.05) by the diet, whereas its efficiency (g/kg digestible OM intake) tended (P < 0.10) to increase with the proportion of concentrate in the diet.