Spray-topping with paraquat improves grass-cereal rotations

Annual ryegrass ready to set seedParaquat can be used to improve the species composition of pastures for livestock grazing and reduce the carryover of grass weed seeds into following cereal crops. The technique used is known as spray-topping. Rotation of grass pastures for sheep and other livestock with wheat is a common cropping system in Australia. Spray-topping with paraquat is used to control problem grass weeds such as barley grass (Hordeum glaucum and H. leporinum) and annual ryegrass (Lolium rigidum).

Why spray-top?

Spray-topping involves spraying paraquat at a low rate to prevent the setting of viable seed1,2. The species composition of a pasture is important for maximum livestock productivity. Weed grasses

Description

The leaves are "narrow" as opposed to the "broad" leaves of broadleaved weeds. Also called 'monocots' having one seed leaf opposed to 'dicots' having two seed leaves.

Authoritative On-line References and Resources

The International Weed Science Society represents individual associations around the world. 
are of inferior nutritional value; compete with desirable species; are hosts for pests and diseases; and can even be harmful to livestock. For example, the awned seeds of barley grass can damage sheep’s eyes and contaminate wool3; annual ryegrass is the main host for the seed-gall nematode (Anguina funesta) that carries a toxin-producing bacterium (Rathayibacter toxicus) into the seed heads4. To minimise problems with annual ryegrass toxicity, spray-topping should be done as early as possible to prevent the formation of the nematode galls.
The seed heads of grass weeds usually emerge over a certain period, so it is important to time paraquat application such that seed in the most advanced ears are still susceptible and as many of the younger tillers as possible have extended enough to be accessible to the spray. Following the short withholding period after spraying paraquat (generally 1 day for livestock), heavy grazing can help to reduce any problem with late heading tillers. Spray-topping may improve the digestibility and protein levels in the sprayed pasture. In the course of normal development, stems lignify reducing digestibility and nitrogen compounds are exported from leaves to developing seeds. These processes are arrested immediately when paraquat is sprayed. Glyphosate can also be used for spray-topping, but paraquat has the advantages of being faster acting and less damaging to subterranean clover (Trifolium subterraneum). Spray-topping with paraquat means that the pasture is of better quality and higher nutritional value the following spring. Spray-topping is also used when a pasture is to be followed by a crop of wheat. Yields may be increased by reducing competition from grass weeds such as annual ryegrass, and also through a lower incidence of the root disease take-all (Gaemannomyces graminis) carried on the ryegrass and higher soil nitrogen availability.

Benefits of spray-topping

Field trials were carried out by Western Australia’s Department of Food and Agriculture to investigate the benefits of spray-topping in a grass-wheat rotation5. The trials were conducted at two locations over several rounds of the rotation. Treatments comprised combinations of various herbicide treatments to selectively control pasture weeds in the autumn followed by spray-topping with paraquat in the spring. After either one or two years of pasture, plots were sown to wheat for one season. Data were collected on soil nitrogen, disease incidence and grain yield and protein levels.

Soil nitrogen

The soil concentrations of ammonium and nitrate nitrogen, ie the forms available to plants, were measured each season immediately before planting a wheat crop. In the vast majority of comparisons, spray-topping with paraquat increased available soil nitrogen, sometimes by up to 50%. This was thought to be due to the ease by which plant tissue desiccated with paraquat degrades in soil or due to higher soil moisture levels promoting bacterial action on organic matter.

Take-all

The incidence of the root disease take-all which is carried over from pasture to wheat by annual ryegrass, was reduced by up to 40% when the pasture had been spray-topped, but a longer period of treatment may be needed for reliable control of take-all.

Yield and Quality

Figure 1. Effect of paraquat spray- topping on yield of following wheat crop (mean of three crops in rotation).Spray-topping increased grain yield in most cases, the margin over untreated depending on site and season. The mean effects over all seasons for both sites are shown in Figure 1. Grain protein levels also improved following spray-topping.

Glyphosate resistance

Annual ryegrass was the first weed recorded as being resistant to glyphosate6. Resistant populations were first recorded in Victoria, Australia in 1996. Preventing seed-set is, therefore, important to constrain the spread of resistant populations. The use of paraquat in spray-topping provides an important opportunity to achieve this. Even if target weeds are still susceptible to glyphosate, using paraquat brings another mode of action to bear on the weed and this has been demonstrated to markedly delay any onset of resistance. The best approach is to use the `Double Knock’  technique.

References

  1. Western Australia Department of Food and Agriculture: pasture spray-topping
  2. Western Australia Department of Food and Agriculture (2009). Spray-topping. e-weed newsletter, 17 September 2009.
  3. Western Australia Department of Food and Agriculture: Barley grass.
  4. Western Australia Department of Food and Agriculture (2008). Farm Note 258. Annual ryegrass toxicity (ARGT) – control through management of annual ryegrass pasture.
  5. Western Australia Department of Food and Agriculture: Pasture manipulation and spray-topping - just what are the likely yield benefits?
  6. International Survey of Herbicide Resistant Weeds

Notes

The brand names of the leading paraquat products for spray-topping are Gramoxone and Spray.Seed.