Summer weeds

Introduction

Uncontrolled heavy weed growth during the summer fallow period can reduce the yield of the subsequent crop by:

• robbing subsequent crops of available soil nitrogen
• decreasing the amount of stored soil moisture
• reducing crop emergence due to the physical and/or chemical (allelopathic) interference at seeding time.

Controlling summer weeds early will conserve valuable soil nitrogen and moisture for use by the crop during the following season. A Western Australian grower at Salmon Gums can demonstrate an average farm crop yield increase of 400 kilograms per hectare since the adoption of consistent summer weed control.

A study by the Cooperative Research Centre (CRC) for Australian Weed Management found that summer weeds can lock away large amounts of nitrogen in the weedy biomass, rendering it unavailable for crop growth. Weed burdens of 2.5 tonnes per hectare can cause a net loss of available soil nitrogen and burdens of more than 3t/ha can reduce subsequent wheat yields by as much as 40%.

In another Grains Research and Development Corporation (GRDC) supported study in South Australia, it was found that the major impact of summer weeds was on soil moisture. Complete weed control increased available soil moisture at one site by over 11 millimetres. The CRC study also found that as weed biomass increased, water losses increased. The magnitude of the water loss and its importance to the subsequent grain yields however, varied from site to site.

Summer weeds can also impede crop emergence. Moderate to heavy uncontrolled weed growth can result in reduced crop emergence in minimum tillage systems due to the impenetrable stover layer left on the soil surface. Wireweed for example, has long tough and wiry stems which can get caught in the tynes at seeding.

Some weeds are allelopathic, where toxic substances are released directly from plant’s roots, or during the decomposition of their residue. These toxic substances can inhibit the subsequent germination of the crop. In the CRC study, allelopathic weeds (such as caltrop) reduced subsequent wheat emergence by as much as 25% due to the chemicals that were exuded from the roots.

Controlling summer weeds

Effective weed control can reduce weed numbers in subsequent years and run down the seedbank. Uncontrolled weeds contribute massively to the soil seed bank, creating increased costs of control and future weed burdens. This may limit crop choice and reduce flexibility in systems.

Summer weed control can be expensive but is necessary to prevent problems with excessive growth and/or moisture and nitrogen loss from the soil. When using herbicides:

• Water rates should be kept high (at least 60 litres per hectare).
• Add a surfactant and/or spraying oil to all post-emergent treatments unless otherwise directed on the label.
• Do not spray stressed plants.
• Spray grazing can be effective at high stocking rates.
• Glyphosate, 2,4-D, metsulfuron, atrazine and triclopyr are the most common herbicides used for summer weed control.
• Where summer grasses are present, glyphosate at rates around 2L/ha are generally required.
• Metsulfuron provides cheap control of wireweed, triclopyr is generally preferred for melon control and atrazine for small crumbweed (also known as mintweed or goosefoot).
• 2,4-D controls a wide range of broadleaved weeds and is preferred if stock are available for spray grazing. The ester formulations are usually more effective for summer weed control because they are oil soluble and more able to penetrate the waxy surfaces or stubble.
• Moisture stress in weeds is common in summer and reduces the effectiveness on most herbicides. This can be partially overcome by spraying early in the morning. However at this time of day, inversions may be present which could lead to excessive drift. Avoid spraying during still conditions.

Grazing summer weeds

• Summer weeds can provide stock feed.
• Summer weeds can be toxic.

Summer weeds can provide quality feed for sheep, especially when there is no other green feed around. Windmill grass for example has a moderate forage value and has a digestibility of 35-68%. Perennial grasses such as windmill grass maintain some quality feed into summer especially with summer rainfall and when the flowering stage is delayed. Annual grasses such as soft brome (Bromus hordeaceus) and barley grass have moderate digestibility but quickly lose quality as they become reproductive.

Some summer weeds create grazing problems. Caltrop is toxic to sheep and can cause photosensitization (abnormal sensitivity to sunlight) leading to inflammation of exposed skin and sometimes death. If seed set is not prevented, the spiny burrs from infestations can cause lameness and infection, particularly in young lambs because their hoofs are soft.

Crumbweed (Chenopodium pumilio) can also be toxic to sheep causing cyanide poisoning, profuse scouring and sudden death. It emerges in spring and summer and can also reduce crop establishment in the following season (allelopathic). It is native to Western Australia.

Lesser Loosestrife

In the wetter regions, lesser loosestrife (Lythrum hyssopifolia) can also be a problem. It has been implicated in occasional large mortalities of sheep grazing stubbles in summer. It causes severe hepatic (liver) and renal (kidney) tubular necrosis. It is thought that the late senescence or greenness of this plant in summer stubbles may contribute to its attractiveness to stock during this time. It is a sprawling annual with angular, ribbed stems and pink flowers.

Melons

Paddy melon (Cucumis myriocarpus) and Afghan melon (Citrullus lanatus) are both prostrate annual melons germinating in spring and summer. Their growth is favored by good moisture relations and bare or fallowed paddocks. Melons can stabilise areas prone to wind erosion and provide stock feed when food is scarce (although opinions vary greatly). Horse, sheep and cattle losses have been associated with eating the melon but the smell of the plants generally makes them unpalatable.

Prickly Paddy Melon seedling

Afghan Melon seedling

The simplest distinguishing feature is the size of the fruit and the seeds. Paddy melon fruit are bristly and 2-3cm in diameter with small pale seeds. Afghan melon has large smooth-skinned fruit up to 15cm in diameter and large brown seeds.

Afghan melon seeds (left), Paddy melon seeds (middle) and colocynth seeds (rare in WA) (right) 

Control

• Grazing is an effective control method after applying low rates of a hormone herbicide to make the melons more palatable.
• Another example for melons control in summer might be a mixture of triclopyr, 2,4-D and metsulfuron in the early morning when plants are not stressed. Graze heavily five days after spraying. Increase rates if grazing not possible.
• Prevention of seed set by mechanical removal is feasible on small areas.

Fleabane

In Western Australia, there are three species of fleabane present, flaxleaf fleabane (Conyza bonariensis), tall fleabane (C. sumatrensis) and Canadian fleabane (C. canadensis).

Two species of fleabane (flaxleaf fleabane (L) and tall fleabane (R) at Woogenellup, South Coast Western Australia

Canadian fleabane at Esperance

Flaxleaf fleabane at Esperance

All species are very difficult to kill in summer, when they become large and hairy, have a decent tap-root and are under moisture stress. It is recommended to try and control them directly after harvest and not wait until mid-summer.

• Flaxleaf fleabane is the most common, prevalent along the south coast but is found to be spreading across much of the wheatbelt. It is a weed of both pasture and cropped paddocks but appears to be worse in areas that have recently been cropped.
• Both tall fleabane and Canadian fleabane are more prevalent in the wetter areas. There has been some indication of the species hybridising in the field although this is as yet unconfirmed.

Fleabane seeds have greatest germination at 20°C (although some will germinate at temperatures ranging from 4.2-35°C). Therefore, in WA, germination is reduced in late autumn and early winter, and greatest germination occurs in spring and early summer. Following rainfall (in optimal temperature conditions), there is an initial rapid germination of seed after rain followed by a slow but steady decline in seedbank numbers over time. However, the seed bank can last for over three years.

Research in southern Queensland suggests that most fleabane seeds germinate from the soil surface with very few seeds germinating from below 1cm. This suggests that the recent fleabane problem in WA is a result of the switch from conventional to minimum tillage systems. These low disturbance tillage systems are less likely to bury seeds below 1cm depth and provide moist conditions for better emergence of seeds that germinate on the soil surface.

In the WA wheat belt, fleabane often germinates in spring and early summer prior to harvest. Once the crop is removed, the fleabane has no competition for light or moisture and can grow rapidly, especially with further summer rain. By the time there is a window for control, the fleabane are often mature, with a large root system, a reduced leaf area and a high tolerance to most herbicides. Large infestations of summer weeds have been implicated in reductions in available soil moisture for the following crop resulting in yield losses.

Control

• Fleabane can most effectively be controlled when they are emerging, mainly in early spring while they are still small. Small fleabane plants are relatively easy to kill and a late post-emergent application of some Group 1 (phenoxy) herbicides during spring can control them in cereals.
• Mature fleabane can be difficult and expensive to kill, especially in mid to late summer.
• Trials conducted by Department of Primary Industries and Regional Development researcher Sally Peltzer in 2009 indicated that the best control of large fleabane in stubble used a ‘double-knock’ approach with a range of primary herbicides followed by paraquat 7-10 days later.
• The most effective treatment was an application of glyphosate (540g/L) at 2L/ha or a mixture of glyphosate (540g/L) at 2L/ha and 2,4-D amine (625g/L) at 2L/ha with a follow up spray of paraquat at 2L/ha seven days after the initial treatment

Windmill grass

Windmill grass (Chloris truncata R.Br.) is a native species, and is the tenth most common summer weed species in the WA wheatbelt. Plants generally commence germination in spring, when the winter annual crops are still standing, and most seed production occurs during spring and early summer.

Windmill grass

Windmill grass plants in the wheatbelt germinate in response to spring and summer rainfall. Small plants (less than 1g dry biomass) can produce seed heads. A single seed head holds 294-522 seeds.

Plants commence germination in spring, when the winter annual crops are still standing and most seed production occurs in spring and early summer. Plant density is usually less than 20 plants per m². Maximum seed production is approximately 60 000 seeds per m² and maximum dry biomass production is approximately 1500kg/ha

Windmill grass may reduce crop yield potential by:

• Utilising soil moisture and nutrients over summer/autumn that would otherwise be available to the following winter crop. The degree to which stored soil moisture from summer impacts crop yield depends on rainfall and soil type. However, windmill grass is a common summer weed on the heavier loam or clay-loam soils, which have a greater capacity to store summer soil moisture than light sandy soils.
• Causing a delay to sowing due to the time taken to remove windmill grass in autumn.
• Hosting crop pests or disease over summer, including aphids and the cereal leaf diseases barley yellow dwarf virus and wheat streak mosaic virus.

Control

• Windmill grass can be controlled by non-selective (knockdown) herbcides over summer or pre-seeding.

Windmill grass is a useful pasture species in some areas, providing forage over the summer/autumn feed gap. It is tolerant of drought and saline conditions and can be grazed heavily and repeatedly. Windmill grass growing in the central wheatbelt over summer has 61-63% digestible dry matter, 10.4-14.2% crude protein and 8.8-9.1MJ of metabolisable energy per kg dry matter.

Button grass

Button grass (Dactyloctenium radulans) is a native species found throughout Australia. It is a common summer weed species and like most summer grass weeds, it depletes soil moisture and nutrients, reducing the yield potential of the subsequent crop. Summer weeds also act as a green bridge for crop pests and disease. The rapid emergence and growth of button grass after rainfall makes it important for the Australian plague locust. It can be a valuable pasture species in arid areas, although overgrazing of button grass can result in toxicity for sheep and cattle.

A mature button grass plant

Button grass is difficult to control, as the stressed, dusty plants are poorly responsive to herbicides. Button grass is often the first species to germinate in summer, but it is also the first species to experience heat or moisture stress. Therefore, it is important to spray herbicides before the plants become stressed.

For further information refer to Button grass.

Roly poly

Roly poly, also known as prickly saltwort or tumbleweed (Salsola australis), is a native species found throughout Australia. The dead plants can break off at ground level, forming the ‘tumbleweeds’ seen in movies. Like most summer weed species, roly poly utilises soil moisture and nutrients that would otherwise be available to the following crop. The time taken to clear uncontrolled plants can delay seeding. Livestock will graze the young plants. However, they will not eat the mature plants and are injured by the prickly leaves.

Young roly poly growing along a fence line

Roly poly, along with a range of other species in the Salsola genus, is a prominent weed of agricultural systems internationally. Like most summer weed species, roly poly utilises soil moisture and nutrients that would otherwise be available to the following crop. The time taken to clear uncontrolled plants may delay seeding. Livestock will graze the young plants. However, they will not eat the mature plants, and are injured by the prickly leaves. This species has tentatively been linked to oxalate poisoning, but most tests indicate that oxalate levels are insufficient to poison sheep. Levels of oxalates and nitrates in roly poly may increase in the presence of nitrogen fertiliser or legume species. The dead, mobile tumbleweeds can become a significant fire hazard, particularly when too many plants pile up against fences or buildings. Note that as a native species, this species is not a problem in areas of native vegetation, and plays a valuable role in revegetation of disturbed sites.

For further information refer to Roly poly.

Tar vine

Tar vine (Boerhavia coccinea) is found throughout Australia, with the exception of Tasmania. It is a common summer weed species and like most summer weeds it depletes soil moisture and nutrients, reducing the yield potential of the subsequent crop. Tar vine also acts as a green bridge for crop pests and disease. It can be a beneficial and highly palatable pasture species.

A mature tar vine plant at Mullewa

The origin of this species is unknown, but it may be native to the Kimberley, Pilbara and desert regions of Western Australia. There are twelve species of Boerhavia in Australia, but B. coccinea is the most common in disturbed areas like agricultural fields or roadsides. It is a common summer weed species and like most summer weeds it depletes soil moisture and nutrients, reducing the yield potential of the subsequent crop. This weed also acts as a green bridge for crop pests and disease, including scab disease, melon viruses, Xylella fastidiosa disease of grapes and plague locusts. It can be a beneficial and highly palatable pasture species. Some species of Boerhavia are consumed by people, but B. coccinea should not be harvested for human consumption. Leave it for the stock to eat. While tar vine contains oxalates, the levels are unlikely to be high enough to be toxic to livestock. However, livestock should be removed if toxicity is suspected. Tar vine has sticky seeds, which may contaminate wool. This weed is difficult to control, as the stressed, dusty plants are poorly responsive to herbicides.

For further information refer to Tar vine.