Effect of increasing level of spineless cactus (Opuntia ficus indica var. inermis) on intake and digestion by sheep given straw-based diets

The effects of spineless cactus (Opuntia ficus indica var. inermis) supply on digestion of wheat straw was studied in rumen cannulated sheep. In addition to urea (10 g) and mineral and vitamin mixture (30 g), the animals received wheat straw alone or supplemented with graded levels of spineless cactus (150, 300, 450 or 600 g dry matter (DM) per head per day). The diets were studied in five successive 25-day periods from February to June 1992 in Tunisia. Water consumption, food intake, digestibility, diurnal variations of ruminal fermentation (pH, ammonia nitrogen, volatile fatty acids, protozoal concentrations in rumen fluid) and cellulolytic activity in the rumen were measured. Spineless cactus had high contents of ash (260 g/kg DM) and water (926 g/kg fresh weight) and low neutral-detergent fibre content (185 g NDF per kg DM). Crude protein (CP) content of spineless cactus was 2.5 times greater than that of wheat straw (58 v. 23 g/kg DM, respectively). Drinking water consumption was substantially reduced (P < 0.002) as the level of spineless cactus increased. When spineless cactus supply exceeded 300 g DM per day, sheep consumed practically no drinking water. The voluntary intake of straw increased significantly with spineless cactus level. Total diet apparent digesibilities of DM, organic matter and CP tended to increase with spineless cactus supply. Such increase was significant only with the 450 and 600 g levels of cactus. NDF and acid-detergent-fibre apparent digestibilities were not affected by spineless cactus supply. Addition of spineless cactus up to 300 g DM significantly increased ruminal ammonia nitrogen (P < 0.001) but there was no additional effect with further supplementation. When sheep were supplemented with spineless cactus, total rumen volatile fatty acid increased and acetate : propionate ratio decreased significantly. Moreover, spineless cactus supply increased total protozoa number and reduced significantly rumen cellulolytic activity measured as DM and NDF disappearance of wheat straw from incubated nylon bags.