Options for increasing yield and water use efficiency in high retention Bollgard II cotton were identified from research that studied the growth and development of Bollgard II and conventional cotton varieties. Options include increase plant size via managing for a larger plant either at first flower or at maturity, breeding for a larger plant and avoiding water stress late in flowering. The former option would involve changes to early water management and possibly early insect management to increase early leaf area.

A further outcome of this research is changes to the OZCOT model and HydroLOGIC irrigation support tool which will assist growers with management decisions when growing the Bollgard II varieties Sicot 71BR and Sicot 289BR.

This research found that high fruit retention in the absence of early main-stem tipping combined with a lower leaf area index late in flowering were characteristics of Bollgard II. As a result, boll growth was earlier and often faster than conventional cotton. Potential yield could be less due to smaller plants in Bollgard II crop with high retention because harvest index (the ratio of boll weight to total plant weight) was the same as conventional cotton However, yield differences are likely to be confined to regions with a long growing season and full irrigation, where the later fruit set and larger plant size of conventional or lower retention crops will allow them to mature a bigger crop.

The need to monitor fruit load and avoid moisture stress late in flowering of Bollgard II varieties was identified from this research. Due to the rapid increase in boll growth, Bollgard II was more determinate than conventional cotton, hence less capable of recovering from water or other stress late in flowering.

Leaf nitrogen and photosynthesis on leaves lower in the canopy was not affected by high retention, suggesting that rapid boll growth was the major cause of growth differences due to high retention in these experiments.

Future research identified from this project includes, optimal water management of Bollgard II including options to increase early plant size, the contribution of upper leaves to yield in high retention cotton and further enhancements to OZCOT that will improve simulation of crop water use and requirements, and ultimately lead to an improved HydroLOGIC DSS.