Improving the accuracy of digestibility estimates for cattle grazing tropical pastures when sampling with oesophageal fistulated cattle

Abstract

The in vitro digestibility analysis of extrusa samples collected from oesophageal fistulated cattle is widely used to estimate the digestibility of diets selected by resident grazing cattle. In the present study, oesophageal fistulated steers were used to sample a range of grass–legume pastures continuously stocked with small herds of resident cattle at two sites in northern Queensland at different times of the year. Pepsin–cellulase in vitro analysis was used to measure in vitro dry matter loss (IVDML) during incubation. Additional laboratory analyses were used to demonstrate that unless appropriate corrections are applied, serious inaccuracies can occur in the estimates of in vivo digestibility as a result of (1) differences between IVDML of extrusa (feed plus saliva) and the IVDML of the feed consumed (feed without saliva), (2) differences in the botanical and chemical composition between the extrusa samples and the diets of the resident cattle, and (3) differences between grasses and legumes in the linear-regression relationships used to estimate in vivo digestibility from IVDML. Methodologies to minimise such errors are described and results illustrating the magnitude of potential errors are presented. In the dataset from the present study, it was clear that differences between extrusa and resident diets in grass–legume proportions consumed were the major cause of error but other causes of error were also important. Appropriate methodologies to improve accuracy are recommended.