Limitations in assessment of forage intake and diet composition of steers grazing tropical pastures with short-term intake rate and oesophageal fistulated steers

Abstract

SummaryTwo grazing trials were carried out to compare the grazing selectivity of oesophageal fistulated and intact steers, and to measure short term grazing behaviour and daily intake of steers in pastures (six pasture types) based on Brachiaria humidicola and Arachis pintoi. During the rainy season, the composition of the pasture affected the proportion of legume in the diet selected by the two groups of steers. The property of the pasture best related to legume selected by the intact steers was the legume mass in the forage available. The proportion selected by the fistulated steers was best related to the proportion of legumes in the pasture. The legume proportion selected by the two groups of steers was related to the bulk density of legumes in the forage, but the type of relationship varied between the groups. The relationship for the fistulated steers was linear, and that for the intact steers was exponential with an asymptote. During the dry season the relationships for the fistulated animals were similar to those observed during the rainy season, but the proportion of legume in the diet of the intact animals was not affected by attributes of the pasture. The pattern of intake in the short term was not affected by the height or amount of forage available during the rainy season, but during the dry season the weight per bite, and short-term intake was related to the height and amount of forage available. Daily herbage intake was not related to short-term intake.The results of the first experiment confirmed that the selection of legumes by grazing steers can vary considerably between intact and fistulated steers. Furthermore it was suggested that the difference resulted partially from the different reactions of the two groups of experimental animals to changes in the attributes of the pasture. The second experiment showed that the daily intake of forage is not necessarily related to intake behaviour in the short term.