Effect of sward attributes on legume selection by oesophageal-fistulated and non-fistulated steers grazing a tropical grass-legume pasture

A grazing trial was carried out in the Eastern Plains of Colombia to compare legume selection by transiently grazed oesophageal-fistulated steers and non-fistulated steers permanently grazing pastures. The 6 pasture types investigated were Arachis pintoi-Brachiaria humidicola mixtures which differed in legume and grass mass per hectare and grass:legume ratios. The sward attributes explaining the greatest variation in legume selection by fistulated and non-fistulated steers were legume percentage in the forage on offer (r2 = 0.67) and available legume mass (r2 = 0.56), respectively. Legume selection by both groups of steers was correlated with legume bulk density (kg DM/ha/cm). However, the relationship was linear (r2 = 0.53) for fistulated steers with maximum dietary legume proportion reaching 92% and exponential (r2 = 0.52) for non-fistulated steers with a plateau at 25% dietary legume with individual values never exceeding 35%. This study confirms that legume selection under grazing differs between oesophageal-fistulated steers transiently grazing the pastures and resident intact steers, and indicates that this difference is associated with their contrasting response to changes in sward attributes.