Nutritional status of cattle grazing natural pasture in the Mid Rift Valley grasslands of Ethiopia measured using plant cuticular hydrocarbons and their isotope enrichment

The seasonal diet composition, digestibility and nutrient intake of cattle grazing on natural pasture in the Mid Rift valley region of Ethiopia were determined using an improved n-alkanes method. Sixteen local Borana and Arsi cattle (8 bulls and 8 heifers, 175±10 kg weight) were randomly selected from herds at two sites; a moderately grazed ranch and a heavily grazed, communal grassland area. Grazing behaviour was observed and herbage species consumed sampled during five periods (early-dry, dry, short-rainy, main-rainy and end-of-rainy seasons) throughout the year at the two grazing sites. During each period, animals were dosed twice daily with 152±4 mg of C32 and 150±3 mg C36 alkanes for 10 consecutive days, with faeces samples collected in the morning during the last five days to determine dry matter intake (DMI). The proportion of consumed herbage species in the diet was determined using n-alkanes and their carbon isotope enrichments as markers, while the energy and nutrient intakes were derived from the DMI, digestibility, and diet composition of the DM consumed. Marked seasonal variations (P<0.05) were observed in the species diversity of diets consumed as well as intake of DM (65–98 g/kg0.75/d), crude protein (222–448 g/d), metabolisable energy (20–37 MJ/d) and minerals. Energy intake was more limiting than crude protein for weight gain during most of the seasons. During the dry period, animals were in negative energy and nutrient balance with a predicted body weight loss of approximately 110 g/d, whereas in the main rainy season the intakes supported 500–800 g daily weight gains. Predicted weight gains agreed well with the body condition recorded for the same period. The n-alkanes method coupled with isotope enrichment in n-alkanes and visual observations as used in the present study provided realistic nutritional data for free-ranging cattle which correlated well with changes in body conditions.