The changing weed spectrum under no-till
By Dr Chris Preston, CRC for Australian Weed Management, and Dr Gurjeet Gill, University of Adelaide
Adopting no-till does not mean "no weeds" but different weeds, and the need for alternative systems of weed management. If good weed management is to be achieved, an understanding of the dormancy characteristics and preferred germination sites for these weeds is required.
No-till impacts on weeds in several ways. It results in no pre-seeding disturbance and more rapid drying of the soil surface that can reduce germination. Conversely, it results in shallow or no burying of weed seeds. This encourages the germination of seeds stimulated by light.
No-till can also result in poor weed seed soil contact, resulting in delayed germination and consequently more in-crop germination.
Weed species that prefer to germinate on or close to the soil surface include prickly lettuce, Indian hedge mustard, sow thistle and fleabane. In broader terms, these species tend to have small seeds that are often dispersed by the wind.
Wild oats, radish, silver grass and ryegrass all require a degree of soil incorporation for germination to occur. With these weeds no-till is either providing ideal conditions for germination or, where weed seed soil contact is poor, it is promoting delayed germination.
Indeed, research has shown that emergence of annual ryegrass is delayed by seven days in no-till systems compared to conventional systems that cause ryegrass seed to be buried prior to seeding.
Some species have seeds that are inhibited from germinating in the light. Work by researchers at the University of Adelaide has found that germination of Ripgut brome or Rigid brome (Bromus rigidus) is strongly inhibited by light (see Figure 1). This means that seed left on the soil surface may not germinate until burial by the seeding operation, promoting a larger in-crop flush of brome grass.
Figure 1: Emergence patterns of Bromus rigidus under light/dark and continuous dark regimes. This clearly shows germination of Bromus rigidus is strongly inhibited by light.
Weed seeds formed in pods may require the seed pod to decay, and sometimes abrasion of the seedcoat before germination. These are less likely to germinate under no-till as tillage can meet both of these requirements by the physical action of mixing the seed in the soil and increasing seedpod/soil contact. However, no-till does favour perennial weed species and those which germinate soon after the opening rain.
Work by Dr Sally Peltzer at the WA Department of Agriculture in Albany demonstrated that barley grass shows little dormancy, with more than 99 percent germinating within the first two months of the break of the season, and most germinating in the first few weeks
.With few in-crop selective herbicides available, a potentially bad barley grass paddock could be left until last in the seeding program, or at least left for about 10 days after the break of the season to allow barley grass control at seeding. This last example clearly shows how a more detailed understanding of weed germination and dormancy can help in no-till weed management.
Some of this information is available and could be used to predict weed spectrum changes. However, for some weeds further research may be required.
[Photo: No-till is causing more in-crop germination of some weeds. Germination of annual ryegrass is delayed by seven days in no-till systems compared to conventional systems.]
GRDC Research Code UA60.
For more information: Dr Chris Preston, 08 8303 7237, Christopher.Preston@adelaide.edu.au
Region North, South, West