Boll distribution patterns in Bt and non-Bt cotton cultivars

Cotton (Gossypium hirsutum L.) cultivars containing the cryIAc gene accounted for 81% of South Africa's cotton production in 2004. Current research on Bt cotton in developed countries has revealed that this transgenic crop provides a key means for enhancing yields and boosting profits. Our study was designed to assess the agronomic efficiency of Bt cotton particularly in South African commercial (Paper 1) and small-scale (Paper 2) farming systems. Over several consecutive years, we compared yield boll distribution patterns (using a plant mapping analysis) of a Bt cultivar (NuOpal) with a conventional nearly isogenic line (DeltaOpal). Compared with the non-Bt cultivar, Bt cotton had better early boll retention rates at the first and second positions on the fruiting branches. Beyond the third position this trend was reversed, indicating that non-Bt cotton varieties offset losses occurring at the first two positions by producing fruits at further positions. Bt cotton thus had a higher average boll weight, shorter vegetative cycle, earlier boll opening, and 13% higher yield potential on average than the conventional control. These results indicate that using the transgenic variety provided better protection against bollworm as compared to chemical treatments—which are likely not effective at the onset of bollworm infestation and do not protect the first bolls. These observations demonstrate the advantage of transgenic Bt cotton when grown under near optimal conditions.