Deep drainage below the root zone is still the least understood component of the water balance, especially in cracking clay soils. It represents a waste of a valuable resource and can leach nitrogen out of the root zone. It has the potential to cause watertables to rise, with the accompanying risk of salinity. Drainage can move contaminants, such as salt and agrochemicals, into the groundwater.

The lysimeter facility at the Australian Cotton Research Institute, near Narrabri NSW, was used to study drainage, its contaminants and its interaction with groundwater in a heavy clay soil under a furrow-irrigated cotton – wheat rotation from 2006 to 2011.

Drainage during the cotton seasons varied from 0 – 74 mm, under wheat it was negligible and under fallow it was 23 mm. Drainage occurred in two forms: matrix drainage and by-pass drainage. The former occurs when the water storage capacity of the soil is filled due to prolonged rainy periods with any extra water becoming drainage. Drainage rates are not high (<0.5 mm/day) but can continue for periods of a month.

By-pass drainage occurs after furrow irrigation when water flows rapidly down macropores and by-passes the matrix of the subsoil. Peak drainage rates are reached 25 hours after irrigation and can reach more than 3 mm/day. The rate then declines exponentially over a week to about 0.5 mm/day. The amount of by-pass drainage appears to be controlled by the soil water deficit in the upper metre of soil. Drainage increases as the 0 – 0.5 m layer becomes drier, possibly due to greater cracking. However, larger deficits in the 0.5 – 1.0 m layer decrease drainage and appear important in mitigating by-pass drainage.

The risk of by-pass drainage is greatest when irrigation is necessary early in the cotton season, when the crop is too small to create a subsoil deficit between irrigations, especially if the subsoil was already wet before sowing.

The risk of matrix drainage can be minimized by managing both the rotation and irrigation scheduling to ensure there is sufficient deficit to accommodate likely inputs of water and irrigation at any time of year. Nevertheless there will always be times of above average rainfall when the profile is filled to capacity and drainage occurs.

However, some drainage is necessary to leach salts from the irrigation water that accumulate in the root zone. The electrical conductivity (EC) of matrix drainage is greater than by-pass drainage, suggesting matrix drainage is more efficient at leaching salt.

In addition to salt, drainage leaches nitrogen from the topsoil. During the 2008/09 cotton season approximately 9.5 kg N/ha – equivalent to 6% of that applied as fertilizer – was lost in drainage.

Seasonal drainage from the root zone appears to recharge the watertable at 16 m depth within weeks, although this result is still tentative. There is continuous downwards leakage of salty water from the upper, watertable aquifer into the lower confined aquifer, from which water is extracted for a variety of uses. This leakage is exacerbated by pumping from the lower aquifer.

The lysimeter was also used to test less expensive methods of estimating drainage. A barrel lysimeter installed near the lysimeter facility overestimated drainage, whereas the chloride mass balance method underestimated drainage.

Final Report - Rhizosphere biological functions as influenced by GM cotton

The aims of the trial were: 1. To determine whether mating of H. armigera could be disrupted in cotton by mass release of pheromones; 2. If mating disruption was achieved, to determine the effects of this on the densities of eggs and larvae; 3. To observe the nocturnal behaviour of moths in pheromone treated and untreated fields in an attempt to understand the mechanics of mating disruption

Functional soil systems

Final Report - Study of alternaria leaf spot on cotton in Northern Australia

In Australia the two-spotted spider mite, Tetranychus urticae Koch, is regarded as a major pest of cotton. However, neither the pest status nor the ecology of spider mattes on cotton has been studied. Consequently management strategies are based on information derived elsewhere, which may not be relevant in Australia. Therefore the pest status of mites, their patterns of distribution and abundance on cotton and associated host plants, and the factors that influence these patterns were investigated. This project produced a large amount of information from a wide range of experiments. Rather than detail experimental methods, presentation is restricted to the key findings and their implications for mite ecology and management.

Nutrition group activities

This study determined the genetic basis of pyrethroid resistance in Heliothis armigera and characterised those factors that lead to cyclical fluctuations in the frequency of resistance in field populations. The major findings were:

(1) A single major semi-dominant gene, associated with mimed function oxidases, is responsible for most pyrethroid resistance in field populations.

(2) The nerve insensitivity mechanism, which appeared to be important in field failures with pyrethroids in the early 1980's, no longer contributes significantly to the expression of resistance in field populations.

(3) Field application rates kill resistant larvae < 4-days old.

(4) Very small resistant larvae can survive field exposure to pyrethroids as the pesticide decays or is diluted by plant growth.

(5) Field application rates kill susceptible but not resistant adults.

(6) Pupae overwintering under cotton crops have high levels of resistance and high survival.

These findings have been integrated into the resistance management strategy for control of spring/summer pests in broad acre crops in eastern Australia. In particular, these results led to, or validated, recommendations about the commercial use of pyrethroids:

Use pyrethroids only on larvae < 5 mm,

Do not use pyrethroids at low rates against non-Heliothis pests,

Do cultivate cotton crop stubble to destroy overwintering populations of Heliothis,

Use a synergist, piperonyl butoxide, with pyrethroids to reduce selection pressure for resistance.

The cotton industry is one of the most technologically advanced agricultural industries in Australia. However, advisers to cotton growers often lack confidence in advising their clients on decisions relating to soil management. It was thought that training these advisers would increase their knowledge and skills in soil management, thereby helping to improve and sustain cotton production by reducing soil degradation and lessening the number of expensive ineffective machinery operations in growing cotton. Thus the project was aimed at improving the level of soil management advice given to New South Wales cotton growers by Departmental advisers and private consultants. The project consisted of a series of three soil management workshops, based on the use of backhoe pits and SOILpak, the soil management decision support system for cotton production on cracking clay soils. The workshops were held at yearly intervals in the main cotton growing areas of New South Wales, and involved a total of approximately 50 participants, as follows:

In January 1983, pyrethroids failed to give satisfactory field control of Heliothis (Helicoverpa armigera (Hubner) at Emerald in central Queensland. Prior to that, as in the USA (Riley 1989), they had been "heralded as miracle insecticides" as they replaced the resistance prone and environmentally liable organochlorines, cyclodienes and organophosphates (Morton & Collins 1989). When they were introduced commercially in the late 1970's, they had many benefits over what was then available. They were very cost effective at extraordinarily low rates on a broad range of agricultural and public health pests, had no residue problems, were safe to mammals, had low environmental impact and were immobile in the soil (Elliott 1989). Indeed, they were regarded as the almost perfect insecticide (Leahey 1985). In fact, by 1986, their popularity was such that they accounted for around 25%, of all insecticides used in agriculture and public health (Jackson 1989, Hirano 1989). They were particularly favoured in cotton because of their contact mode of action and good efficacy against previously resistant pests and by the mid 1980's accounted for 49%", of the world cotton insecticides market (Walkinson 1989, Riley 1989). So when the breakdown at Emerald was clearly shown to be due to the development of resistance (Gunning et al 1984), there was no disguising the concern of the Australian cotton industry in particular, but also the other field crop industries in which H. armigera was a key pest. Within 6 months of these reported field failures, a strategy aimed at containing the resistance problem, had been formulated and ratified for use in the following season, by all parties concerned (Forrester 1990)