Weed Control

Weed Control in The South Australian Wheat Industry

With axe and fire the first South Australian colonists set about gardening and sowing small areas of wheat on the Adelaide Plains in a mind frame which attempted to recreate old England’s rolling hills uncluttered by scrub.
Their activities opened the land to alien plants accidentally introduced as seed hidden in imported food, fodder, packing materials and ballast or in seed for sowing. The process was hastened by the relative high fertility of the virgin soil and the frenzied introduction of seed for European food plants and ornamentals
Within five years of colonization wheat growers were complaining that, three introduced plants, hedge mustard,(Sisymbrium officinalis) perennial ryegrass,(Lolium perene) and wild oats (Avena fatua) were serious weeds in wheat crops. It is interesting to note that, 185 years later, wheat growers in South Australia still list wild oats and ryegrass as serious problems.
In the early days of the Colony the obvious blue flowers of the thistles drew the most serious attention and. the graziers, who had most say in the legislature, enacted The Noxious Thistles Act 1852 aimed at eradicating them. By the 1880’s blue star thistle (Centaurea calcitrapa) was added to the legislation to help the wheat growers and then saffron thistle (Carthamus lanatus) was added by the turn of the century but that legislation gave little help, if any. For more than a 100 years growers had no alternative for controlling weeds in their crops but to hand pull or hoe, burn the stubble, cultivate and harrow or turn the sheep onto the fallow.

In 1948 weed control in wheat crops began to embrace two miracle selective herbicides, 2-4D and MCPA These herbicides could remove the broad-leaved weeds such as wild turnip, (Brassica tournefortii) radish, (Raphanus raphanistrum) mustards (Sinapsis and Sisymbrium spp) and saffron thistle (C calcitrapa) as they were growing in the wheat. They enabled growers to stop competition from those weeds early in the life of the crop.
Growers suddenly had to learn new skills. These herbicides had to be accurately mixed and applied through new equipment, the boom spray, which had to be calibrated and the spray applied at the correct rate per hectare and at the safe wheat growth stage, not earlier than first tillering or later than early jointing. They also had to contend with different percentages of the active ingredient in different trade named products. For example, Amoxone 50® was marketed by ICI with 50% active ingredient while Geigy Australia marketed its product, Weedar 77®, with 43.4% active ingredient.

To help growers make the best use of this new technology a weed science unit was formed in the Agronomy Branch in the Department of Agriculture with the appointment of agricultural science graduates and other qualified staff in 1958. A new Weeds Act was proclaimed which provided vastly improved resources and funding and involved local government.
The unit had three tasks. Research for the control of specific weeds, including the use of herbicides in wheat crops with account needed to be taken of the weed patterns, cropping systems and soil types. Secondly, the unit provided a weed identification service, conducted weed control schools and other extension programs. Thirdly, fulfilled regulatory requirements. For example, all herbicides had to be assessed for efficacy, persistence and safety under the terms of the Agricultural Chemicals Act before field use was approved.
By the middle of the 1970’s the unit comprised 17 staff members with its core research operation at the Department’s Northfield centre and field advisers at the major country offices.

While the 2-4D and MCPA herbicides selectively controlled the easy-to-kill, broad-leaved weeds the grassy weeds, wild oats and rye grass remained serious problems. To deal with these the agricultural chemical industry introduced a pre-emergent herbicide, diallate, marketed as Avadex® and an early post-emergent herbicide  barban marketed as Carbyne.®. The introduction of these herbicides required yet another set of skills involving soil incorporation (Avadex® had to be incorporated within two days of sowing) and the complication of matching application rates to particular soil types and wheat varieties.
By the beginning of he 1960’s broad scale use of  2-4D and MCPA had changed the weed patterns on some properties leaving the harder to kill weeds such as sheep weed, (Buglossoides arvensis) dead nettle (Lamium aplexicaule) and the fumitories (Fumaris spp). Their control was made possible by the introduction of the triazines such as atrazine, marketed as Linuron® and simazine as Gesatop®. To be effective these needed to be applied between the three leaf stage and tillering.

At the beginning of the 1970’s over 80 commercial herbicides had been registered for sale in South Australia for weed control in wheat and were available from 49 companies selling agricultural chemicals.
How did wheat growers adjust to such far reaching technical changes which occurred over those 20 years? Firstly, they had their own sources of information from the press, the radio and the Agricultural Bureau but, significantly, the weed science officers, the district agronomists and the agricultural chemical industries’ staff worked closely together to prepare annual weed control charts and more detailed herbicide recommendations in booklet form. This was a great step forward, not easily taken by the agricultural chemical industry whose companies were marketing in competition and the Department of Agriculture which needed to protect its independent role. Never-the-less it was achieved and these charts and booklets found their way into almost every farm office. Since the demise of the weed science unit in the 1990’s herbicide recommendations in the Wheat Industry have been largely left to the marketing companies with the risk of unbalanced advice.

Early in this period of such far reaching technical changes the South Australian Wheat Industry was seriously threatened by progressive infestations of skeleton weed (Chondrilla juncea). Skeleton weed was first identified on a property owned by Mr. C. Gilbertson near Parilla in March 1947. The weed had already caused serious losses to wheat growers in southern New South Wales and in the Victorian Wimmera. A deep rooted perennial, skeleton weed vigorously competed for soil moisture and nitrogen at a time when the wheat was getting established and by harvest time it had created a wiry mass of stems which virtually prevented any harvest. The seed of skeleton weed, spread by wind and stock, enabled it to disperse rapidly, particularly on the sandy soils across South Australia. Initially the only treatment was to spray with Atlacide or cover patches of the weed with salt.

By 1960 skeleton weed had spread across the Murray Mallee to the Murray Plaines and over the next five years more than 120 outbreaks were found on the prime wheat growing lands of the Mid North. In 1963 the first outbreaks were appearing on Eyre Peninsula.
Wheat farmers rose to the occasion. Outbreaks, when found, were treated using more sophisticated herbicides as they became available on the market. Importantly they contributed funds through their production levies to support an intensive research program conducted across southern Australia.
Disaster was avoided in the latter half of the 1970s when a successful biological control agent, a rust, (Puccinia chondrillina) became widely established. CS I RO estimated that between 1974 and 2000 this biological control made a net saving to the South Australian wheat farmers of more than $ 1 billion.
Throughout the period when skeleton weed was a threat to the South Australian Industry more and more useful herbicides became available but not without a price. As mentioned, with each additional herbicide came the challenge to learn how to efficiently apply it. The results were worth the effort. No more so than with the use of Glean, a herbicide which has enabled wheat farmers to virtually remove the fence to fence infestations of sour sob (Oxalis pes-caprea) on the better wheat growing soils in the mid and upper northern areas of the state, The obvious iridescent yellow displays of flowering sour sobs in May and June which threatened the productivity of crops and sheep grazing, have disappeared.

The South Australian Wheat Industry, having survived critical soil nutrient deficiencies, water and wind erosion, rust, weeds and changing market requirements had yet to face two more hurdles before the 21st century.
Unexpectedly, with the persistent use of herbicides, some weeds developed resistance. In 1982, an infestation of annual rye grass near Bordertown was found to be resistant to diclofop-methyl.  Later, other herbicides, not necessarily chemically related, faced resistance. Wild oats has developed similar resistance at sites around the state. In some cases farmers have been able to avoid the consequences of resistant weeds by using alternative herbicides, a challenge requiring more and more knowledge of the chemicals they are using. But even that technique is failing It is now evident that annual rye grass is resistant to dissimilar chemicals which means the problem cannot be controlled by simply changing herbicides. Wheat farmers have therefore had to start concentrating on integrated weed management, a further call on their knowledge and skills. Integrated weed management is not new to wheat farmers but now much more emphasis is required achieving control by a number of methods including cultivation, pasture management, spray topping, crop rotations and avoiding continual use of the same herbicide.

Finally wheat growers in South Australia have widely adopted minimum tillage to sow their crops. This method of sowing wheat has in effect replaced mechanical energy traditionally needed to plough and work the soil to control weeds and prepare the seed bed with chemicals which destroy the weeds but do not persist in the soil. With a one-pass machinery operation the wheat is sown with fertilizers following the herbicide application. This obviously saves time and costs but again has called upon the growers to respond with new knowledge and skills, a process of adaption which has occurred throughout the life of the industry in South Australia.

References.
1.  Department of Agriculture. South Australia. Cereal Weed Spraying Chart. First Edition 1968.
2. Department of Agriculture. South Australia. Herbicide Recommendations 1968.
3. Department of Agriculture. Declared Weeds of South Australia. Bulletin 453. 1959.
4. Elders Cropping Guide. Herbicide Resistance in Australian Weeds.
5. Parsons W, Cuthbertson E. Noxious Weeds of Australia.1992
6. Tideman. A. F.  Ten Acts For Weed Control in South Australia
7. Tideman A. F. History of Skeleton Weed in South Australia.
8. Tideman A. F. The History of Weed Control Legislation in South Australia 1850 – 1990

* 2-4D is a phenoxy acetic acid growth hormone formulated as the sodium salt or in the amine or ester form.
MCPA is 2-methyl-4-dichlorophenoxy acetic acid formulated as the sodium or potassium salt or in the amine form.