Gastrointestinal passage of Sahelian roughages in cattle, sheep and goats, and implications for livestock-mediated nutrient transfers

Green and dry pasture vegetation and millet crop residues are the main feed for domesticated ruminants across the West African Sahelian zone. Studying their gastrointestinal passage is important for modelling livestock-mediated nutrient flows in these agro-pastoral systems. Intake, passage, digestion and excretion were studied in 16 cattle, 16 sheep and 16 goats offered these feeds at levels corresponding to 0.8 and 1.3 times voluntary feed intake. Parameters of particle passage were derived from faecal excretion of a pulse-dose of short (1–2 mm) and long (>2 mm) particles labelled with Ytterbium, using age-dependent Gamma-2 single-compartment models.Across the three species, mean retention time of particles in the mixing compartment (CMRT) varied between 39–50 and 54–66 h in green and dry pasture vegetation and between 66 and 88 h in millet leaves, and was always longer in cattle than in small ruminants (P<0.05). Particle passage through the mixing compartment (λ), particle half life (T50) and total tract mean retention time (TMRT) were correlated to the dry matter digestibility and to the contents of nitrogen and neutral detergent fibre of the ingested diet, while particle size had no and feed intake only a weak influence on these parameters.The results suggest that seasonal and species-specific differences in gastrointestinal passage of feed particles should be taken into account when addressing the temporal aspects of livestock-mediated matter and nutrient flows across the agro-pastoral landscapes of the Sahel.