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) sup ply on digestion of wheat straw was studied in rumen cannulated sheep. In addition to urea (10 g) and mineral and vitamin mixture (30 g), th e animals received wheat straw alone or supplemented with graded level s 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 fr om February to June 1992 in Tunisia. Water consumption, food intake, d igestibility, diurnal variations of ruminal fermentation (pH, ammonia nitrogen, volatile fatty acids, protozoal concentrations in rumen flui d) and cellulolytic activity in the rumen were measured. Spineless cac tus had high contents of ash (260 g/kg DM) and water (926 g/kg fresh w eight) and low neutral-detergent fibre content (185 g NDF per kg DM). Crude protein (CP) content of spineless cactus was 2.5 times greater t han that of wheat straw (58 v. 23 g/kg DM, respectively). Drinking wat er consumption was substantially reduced (P<0.001) as the level of spi neless cactus increased. When spineless cactus supply exceeded 300 g D M 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 significa nt only with the 450 and 600 g levels of cactus. NDF and acid-detergen t-fibre apparent digestibilities were not affected by spineless cactus supply. Addition of spineless cactus up to 300 g DM significantly inc reased 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 aceta te:propionate ration decreased significantly. Moreover, spineless cact us supply increased total protozoa number and reduced significantly ru men cellulolytic activity measured as DM and NDF disappearance of whea t straw from incubated nylon bags.