Considerable effort has been put into assessing the effects of various agronomic practices on the stem yield of commercial fibre crops. However, many of these experiments do not report detailed stem characteristics, fibre percentages and retting losses; which are of considerable importance to commercial production. In this study, data has been aggregated and analysed from two separate experiments that took place throughout 2020/21 and 2022/23 in Canterbury, New Zealand. Random plants were selected from within experimental plots and measured for secondary fibre presence, diameter, height, retting losses and fibre proportion. In the first field experiment, secondary fibre tended to increase consistently with total dry matter yield across all sowing dates, where each additional ton of dry matter caused secondary fibre proportion to increase by 3.37%. Plants in earlier sowing dates tended to take longer to develop secondary fibre; potentially due to the longer vegetative period commonly found with earlier establishment. However, secondary fibre was detected at a stubble height of 20 cm when plants reached a mean height of 154 cm; regardless of cultivar or sowing date. In the second field experiment, stem diameter ranged from 5.2 to 11.6 mm with a mean 8.4 mm, whilst mean fibre proportion was 30.2% across five different monoecious cultivars. Stem diameter did not appear to significantly affect the proportion of fibre found in retted, hand-decorticated, samples. In vegetative plants, stem diameter taken at 80 cm above ground level, tended to give the closest correlation (R2 = 0.85) with plant weight when compared to other, more commonly reported, metrics such as height (R2 = 0.76). This suggests that in-field assessments for fibre quality may benefit from objective stem diameter measurements. Retting losses were assessed with traditional dew retting and also under controlled conditions using a small-scale retting bath. Dew-retted stem samples retained 31% of their fresh weight compared with 38% for non-retted samples. Retting losses tended to be higher for water-retted samples, where the retained dry matter portion was 27% after retting compared with 40% for un-retted material. This has important implications for the calculations of commercial businesses, where retting losses of 20 - 35% are possible under field conditions.