Supplementation of Zebu cattle grazing Sahelian pasture. I. Diet selection and intake

From 1990 to 1992, diet selection and nutrient intake of Zebu cattle grazing Sahelian pasture were studied in Central Mali. Forty-five intact males and 12 oesophageally fistulated animals were separated into three groups of 15 intact and 4 fistulated animals. The control group (C) grazed natural pasture only, while the moderately supplemented group (M) additionally received 0.8-1.5 kg OM/day of crop by-products during the dry season (November-June) and the first month of the rainy season. The third group (H) was highly supplemented with 1.2-2.7 kg OM/day in the dry season and 0.8-1.2 kg OM/day in the rainy season. Oesophageal extrusa was collected during 5 consecutive days at intervals of 4-5 weeks. Samples were analysed for contents of organic matter (OM), crude protein (CP) and neutral detergent fibre (NDF). Digestible organic matter (DOM) and metabolizable energy content (ME) were calculated from in vitro gas release. Intake of organic matter (IOM) of the fistulated animals was determined from faecal organic matter excretion (FOM) and extrusa DOM content. Intake of CP (ICP) and ME (IME) were calculated from IOM and the respective nutrient contents of extrusa samples. Extrusa CP, DOM and ME contents did not differ significantly between the three groups either in the dry or in the rainy season. The CP content was c. 230 and c. 197 g/kg OM during the rainy season, and declined to values less than or equal to 70 and less than or equal to 95 g CP/kg OM at the end of the dry season of 1990 and 1991, respectively. Average DOM and ME contents were c. 668 g DOM and c. 9.9 MJ ME/kg OM during rainy seasons. At the end of the dry seasons, these values decreased to < 550 g DOM and < 8.0 MJ ME/kg OM. Per kg of metabolic body mass, IOM of group C was c. 89 g/day during the early dry season. It was accompanied by a daily ICP of 8-10 g and by an IME of 691-765 kJ/day. Due to a higher nutrient content in the selected diet, energy intake was slightly increased and protein intake was significantly higher during the rainy than during the dry season, although IOM was only 77-81 g/day. At the end of the dry season, unsupplemented animals ingested < 70 g IOM/day and the concomitant CP and ME intake were < 6 g CP/day and < 500 kJ ME/day, respectively. Feed intake from pasture was stimulated by a moderate supplementation, but reduced by a high supplementation. The results indicate that on slightly degraded Sahelian rangeland, the nutrient intake of cattle is in the first place limited by biomass availability and only secondly by the quality of the vegetation.