The Development of Sustainable Cotton Farming Systems for Coastal North Qld

Abstract

The reintroduction of cotton to the Australian semi arid tropics (SAT) was prevented by insect pests that are dominant during the wet (summer) season and a perception that the crop could only be grown in the wet season. Growing cotton during the dry (winter) season could avoid these pests provided an integrated pest management system was adopted. However the photothermal pattern of the dry season is the reverse of the wet season and that of spring sown cotton in temperate latitudes. Cold night temperatures are possible mid season and high temperatures are likely early and late in the season. Solar radiation is 20% less than at temperate latitudes mid season and could also limit crop growth. It was not known what yield or fibre quality was possible or whether the crop could be reliably sown and picked within the confines of the dry season. Over three seasons two Gossypium hirsutum (upland) Bt transgenic cultivars and one Gossypium barbadense cultivar were sown from March to June in field experiments at the Ord River (15.5oS) in Western Australia. A pot experiment was conducted at Katherine, Northern Territory (14.5oS) where biotic stresses were removed and over two seasons: ambient and ambient plus 5 to 6 oC night thermal conditions were imposed from 1 wk prior to first flower to 2 wk after last effective flower. Day temperatures were the same. Average ambient minimum temperature for the treatment period was 2 to 4 oC less than the Ord River at the same growth stage. The OZCOT cotton simulation model was validated then applied to simulate gross margin, yield and quality, with enhancements to predict fibre length and colour grade developed in this thesis, at the Ord River using 53 years of historic climatic records for sowing dates from March 1st to May 25th. Experimental results at the Ord found for the upland cultivars, the highest lint yields of 1900 to 2300 kg/ha were for March and April sowings and were at the high end of Australian and international benchmarks. The lint yield of the Gossypium barbadense cultivar was highest at a March sowing, at least 87% of the upland cultivars, which is comparable with temperate climates. For the March and April sowings both the lower temperatures and radiation during early boll growth reduced the crop growth rate during this phase compared with cotton grown at temperate latitudes. However, assimilate supply was adequate because boll demand was also lower due to early flowers having slower boll development, lower retention and smaller bolls. Increasing late season temperature and radiation permitted yield compensation via an extended flowering period and a greater contribution to yield from later pollinated flowers on the top and outside of the plant. The number of temperatures >35oC and or <11oC affected time to squaring, requiring modification of development models derived in temperate climates. Radiation use efficiency (RUE) was similar to Gossypium hirsutum grown at temperate latitudes. The RUE measured for the Gossypium barbadense cultivar was the first reported for this species. The linear decline of RUE with average temperature up to first flower has not been reported previously in cotton and explains some of variation in RUE measured here and elsewhere. Due to cool temperatures during fibre development fibre length and strength at March and April sowings were low to marginal compared with market preference values. The cultivar differences observed here suggest wider screening may identify upland cultivars with suitable fibre length and strength in these conditions. The commercial prospects for Gossypium barbadense are doubtful unless longer and stronger fibre types are identified. The pot experiment confirmed that flowers were damaged by low ambient minimum temperatures near anthesis which led to shedding or reduced boll size due to lower seed number. The latter could be due to poor pollination and competition for assimilates. Importantly this experiment demonstrated that full yield recovery from minimums <11oC during flowering and boll growth is possible provided they are episodic. The OZCOT cotton simulation model was validated for lint yield and average time-to-maturity in response to sowing date and N fertiliser rate. Further research was required to reduce the variability of maturity predictions. There was only a 14 day sowing period from March 19th where the simulated gross margin (GM) was maximised at $2378/ha. Poor trafficability combined with the Bt resistance sowing window, reduced the number of sowing days in the optimum period. Hence to reliably sow a commercial area it was likely sowing would extend beyond April 3 and reduce median GM by 9-15% due to lower fibre quality. Future research should apply this type of analysis throughout the Australian SAT. Cotton management in the dry season should aim to increase the flowering period, to ensure yield compensation from later flowers and to adopt practices that can improve trafficability within the optimum window e.g. minimum tillage.