Electron transport in plant cells inevitably results in the creation of reactive oxygen species (ROS) that can hinder metabolic processes and cause cellular damage. In spite of the gains additional antioxidants may impart to the cotton (Gossypium hirsutum L.) photosynthetic apparatus, single time point measurements on live plants have not refl ected distinguishable phenotypes for key physiological measures. Transgene effi cacy may also be quantifi ed by measuring cumulative agronomic performance (lint yield and quality) under conditions that promote photorespiration and ROS production. To test this hypothesis, cotton lines constitutively expressing the ascorbate peroxidase (APX) or glutathione reductase (GR) protein were examined in field trials (2005, 2006, and 2007) under three different irrigation treatments. Yield, gin turnout, and 23 measures of fi ber quality were assessed. Transgene (APX and GR) effi cacy and in vitro culture effects were estimated by comparing a line's performance with the null and wild-type checks based on an analysis of variance using a linear mixed model to estimate general least squares and variance components. Signifi cant effects were observed in 18 traits. Despite no signifi cant changes in fi ber maturity, all GR and APX transgenic lines produced signifi cantly fi ner fi bers than the wild-type. Fiber maturity is a problem in western Texas that affects most fi ber quality measures especially the fi neness measurement micronaire.