SiroMat is an automated version of the polarized light microscopy technique, which analyses the area of particular interference colors transmitted by mature and immature fibres when they are placed between crossed polars and a first order retardation plate. The percent areas of these colors relate directly to fibre maturity. Moreover, because fibres are analyzed on an individual basis a maturity distribution for a sample can also be measured. In this study SiroMat measurements are reported from blends comprising different proportions of cotton fibre picked from plants subjected to differential defoliation timing treatments. The proportion of immature fibres in each sample is correlated with measurements on the same samples of neps per gram made by the AFIS PRO. The motivation behind this study is based on the desire to manage the amount of immature fruit included in a crop prepared for harvest before nep problems are generated in the cotton at the gin and the mill. The study showed that samples with more positively skewed maturity distributions and low average maturities produced more neps.