Tillage changes the weed environment

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A better understanding of weed biology and the factors that trigger germination is helping in the development of improved early weed control

As farming systems have evolved to use less cultivation and more herbicides for weed control, the type of weeds dominating the system and the timing of their germination has gradually changed. No-till systems favour weed species with smaller seeds that germinate close to the soil surface. These changes, combined with increasing resistance to herbicides, have resulted in the need to change our focus of weed control. To do this effectively we have had to improve our understanding of weed-seed biology.

Our focus needs to move from concentrating on killing weed seedlings to managing weed populations. Historically, weeds that emerged before crop seeding were killed with cultivation or herbicide, and those that emerged within the crop were controlled with a selective herbicide. With the development of single-pass seeding systems, dry-seeding and herbicide resistance, cultivation and selective herbicides are used less often. As a consequence, the emphasis for weed control now must focus on minimising weed seed-set.

[Photo (left) by Emma Leonard: A better understanding of weed biology and the factors that trigger germination is helping in the development of improved early weed control]

Weed-seed survival in the soil is influenced by a range of factors - seed size and whether the seed is contained in a pod; the depth of burial; the need for periods of darkness or light to break dormancy; and seasonal conditions.

Experiments conducted at the University of Adelaide showed that small-seeded weeds such as Indian hedge mustard, sowthistle and silver grass emerged in much greater numbers under a no-till with a knife-point, rather than a minimum-tillage, system using two passes with a wide shear (Table 1). Conversely, larger-seeded weeds such as bedstraw, wild radish and annual ryegrass were favoured by minimum-tillage systems.

Wild oats did not respond significantly to a particular seeding system, because the awns on wild oats allow it to bury itself in the soil with wind movement, or soil movement caused by temperature changes.

Most weed species have a narrow depth preference for successful emergence. If placed outside this preferred depth range, germination will be reduced (Table 1). However, annual ryegrass will germinate from significantly smaller depths in no-till compared with minimum-tillage systems. This means that annual ryegrass will remain a problem weed in no-till systems. The good news is that annual ryegrass is not likely to be a bigger problem in no-till systems, provided effective control can be maintained.

For most of the species reported in Table 1, tillage type did not influence the preferred emergence depth. For example, under both tillage systems, all the Indian hedge mustard emerged from three millimetres. However, annual ryegrass and turnip weed both showed the ability to emerge from a range of depths.

Germination of species that prefer to emerge from deeper in the soil is generally inhibited by light, while those that emerge from near the soil surface require light to stimulate germination. For all weed seeds, deep burial induces dormancy and inhibits germination by reducing the oxygen concentration, but deep burial also increases seed survival.

Sally Peltzer, of the Department of Agriculture and Food, WA, assessed the use of one pass with a mouldboard plough to achieve deep seed burial for annual ryegrass. The results for weed control were positive, providing seeds were left at depth. However, the implications for soil structure, soil erosion and so on make this practice useful for only a small number of soil types. Unfortunately, with continuous cropping systems weeds can be buried one year and be brought to the surface the following year, and may germinate.

Weed seeds left on the soil surface typically suffer more seed death and predation than buried seeds. Several species of ants have been found to be valuable in the predation of young weed seeds. As seeds age, decay or failed germination is more important for seed bank reduction. For example, for common sowthistle seed left on the surface, 82 per cent decayed over winter; burying the weed seed to five centimetres reduced decay to 55 per cent. 8.Tillage table

Seed pods and impermeable seed coats complicate the situation, as they can substantially inhibit seed germination. Generally, germination cannot proceed until water has penetrated the seed. The pod needs to weather and the seed coat be cracked before water can penetrate. This phenomenon results in extended dormancy in wild radish, with most seed germinating two years after it was set.

The previous spring conditions can also influence the level of dormancy. This is true, for example, of annual ryegrass and wild radish. Typically, warm dry spring conditions result in less natural dormancy. If summer conditions are wet and hot, dormancy release is faster in annual ryegrass. The result of both of these situations is that a greater proportion of the seed bank will emerge at the break of the season.

Ultimately, weed control is a numbers game: the fewer weed seeds set, the fewer that can germinate. Preventing weed seed-set is of particular importance for weeds such as annual ryegrass, wild oats and wild radish that have a long survival period in the seed bank. Driving weed seed banks down may require more than one season of seed-set control, depending on the persistence of the seed in the soil seed bank and the size of the seed bank.

For species like common sowthistle that have a short-lived seed bank, one year of seed-set control may be sufficient. For species with longer-lived seed banks such as wild oats, seed-set control will need to occur for three or more consecutive years.

When managing weed seed-set, the best results are obtained if control occurs before the formation of a mature seed. For grass weeds, control needs to be at anthesis. Cutting grass weeds later than this will allow some seed to mature.

For broadleaf weeds, timing needs to be before seed formation. Recent work by Aik Cheam of the Department of Agriculture and Food, WA has found that seed-set control of wild radish needs to occur before the embryo forms. At this stage, wild radish pods will be green and watery and no embryo will be found inside the ovule.

More details of the conditions that favour survival and germination of 20 common broadacre weeds can be found in the Integrated Weed Management in Australian cropping systems manual (see back page).

GRDC Research Code CRW5

More information: Dr Christopher Preston, 08 8303 7237, christopher.preston@adelaide.edu.au

Region North, South, West