Managing yields of high fruit retention in transgenic cotton (Gossypium hirsutum L.) using sowing date

Recently, genetically engineered (transgenic) cottons expressing genes from Bacillis thuringiensis (Bt) have been made available to cotton growers throughout the world. In Australia, cotton growers have access to Bt cotton that contains genes expressing the insecticidal proteins Cry1Ac and Cry2Ab (Bollgard II®). Bollgard II offers significant potential to reduce pesticide use for the control of major Lepidopteran pests (particularly Helicoverpa spp. in Australia). As a consequence of the improved insect control, retention of squares (flower buds) and young bolls is higher in Bollgard II varieties than in non-Bollgard varieties. A concern raised by Australian cotton growers is that in some regions, yield potential for Bollgard II may be limited because the demands of earlier high fruit retention reduce resources for continued growth and fruiting, thus leading to earlier maturity and reduced yield. Non-Bollgard crops with high early retention are known to mature earlier sometimes reducing yield. Three field experiments over three seasons, which varied sowing date and compared non-Bollgard II and Bollgard II cotton cultivars, were conducted to test the hypothesis that delaying sowing date in Bollgard II will increase canopy size (without delaying crop development) and alleviate the potential concerns for the effect of higher fruit retention reducing canopy size and the time to maturity, limiting the yield of Bollgard II. In non-Bollgard II crops, larger canopies resulting from early loss of fruit or apical meristem damage can support more fruit growth for longer, provided season length allows fruit to mature. Results showed that delayed sowing did not increase the yield of the Bollgard II cultivar through increased leaf area index at flowering compared with normal sowing dates. However, in comparison with the conventional cultivar, which had yields that became lower with later sowings, Bollgard II maintained its yield presumably through the shorter fruiting cycle (because of its consistently higher earlier fruit retention), allowing time to support growth of the same number of bolls as earlier sowings. Improvements in fibre quality were also recorded with later sowings for both cultivars. Varying sowing dates for Bollgard II in different production regions may offer opportunities for Australian growers to help optimise yield, fibre quality, and reduce risks associated with poor crop establishment when crops are sown too early.