The relationship between bulk density and median chop length of chopped wheat straw samples

Abstract

The paper describes research conducted to establish the relationship between the bulk density of chopped straw and the median value of its length. The bulk density was determined by a defined procedure and the median length by a chop length analyser. Samples were prepared by cutting wheat straw to uniform lengths from 15 to 250 mm and the bulk density determined. The relationship which was found between bulk density and length was of a form which could be described by a two-dimensional geometrical model, based on a simple rhomboidal element. The work was extended by establishing the relationship between bulk density and median length of samples of chopped straw, using data from a national survey of the performance of combine-mounted straw choppers. The geometrical model was modified to a three dimensional form and a relatioship developed which gave a good fit to the observed data from the survey, over a range of median chop lengths from 30 to 80 mm. It was also found that a modification of the model gave a good description of the relationship between bulk density and median length for unchopped material for median values up to 250 mm. A combine chopper was used for controlled experiments in selected varieties of straw over two harvest reasons. The results showed that the relationships between bulk density and median chop length were similar to those which were established from the survey data and the three-dimensional model. Significant differences were found, however, between wheat varities and moisture content was shown to have a marked effect on bulk density. It is important to establish the relationship between bulk density and median chop length for specific crops and varieties. This should be done at a controlled moisture content. Sufficient data should be obtained to determine the standard error of the estimate of the median chop length estimated from the bulk density.