Influence of crop development and fruit retention on the timing of crop maturity in Ultra-narrow row cotton

Ultra-narrow row (UNR) cotton, a production system with rows spaced less than 40 cm apart, has been proposed as a system for earlier maturity without substantial yield loss. However, trials in the U.S.A. and Australia have found maturity benefits difficult to achieve consistently. Studies undertaken in high input cotton systems that compared UNR to conventionally (1 m) spaced cotton found yield differences but failed to demonstrate differences in crop maturity. This paper examines crop development and fruiting dynamics of the two systems in more detail to understand why there were no differences in maturity. Results showed that lack of difference in maturity between the row spacings was not influenced by differences in the time to reach crop development stages nor by lower fruit retention of early bolls in the UNR plants. Node production and fruiting site production on a per plant basis were significantly slower in the UNR plants from early in the growing season. Slower node and fruiting site production delayed maturity in the UNR plants as they set fewer fruit within the same period of time compared to plants grown in conventional spaced rows. The number of fruiting sites produced per plant was highly dependent on the amount of dry matter per plant. Fruiting site production was reduced in the UNR crop because each plant produced less total dry matter, and hence plant development was slower. To further help understand the reasons for outcomes generated in these studies, and how UNR could possibly mature earlier without impacting yield, a conceptual modelling framework was developed to integrate seasonal patterns in fruiting site production, retention, and boll growth. A key component of this framework is translating from a per plant basis to an area basis. The analysis showed that for individual UNR plants to mature earlier and maintain yield on an area basis, early node production and fruiting site production must proceed at a similar rate to conventionally spaced crops. In this scenario 90% of final yield would be present 14 days earlier in the UNR crop compared to conventionally spaced crops. To realise any benefits of potential earlier maturity further research is needed to explore genetic or management interventions that might avoid the early competitive stress response that slows node development in UNR.