Regional differences guide southern approach to controls

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With increasing variation observed in winter rainfall, conservation of soil moisture from summer rainfall has become even more important in many cropping districts

Managing summer weeds is vital to improve the growth of subsequent winter crops by preserving stored soil moisture and nitrogen, improving crop establishment and reducing the levels of weed-vectored insects and diseases.

Identifying the most important summer weeds in South Australia will help growers target problem weeds and will inform future research needs. With GRDC investment, the University of Adelaide conducted a random survey of summer weeds across 298 SA paddocks during February to March. All regions were surveyed in 2015 except the Upper and Lower Eyre Peninsula, which were surveyed in 2016. Sites were selected at approximately 10 kilometre intervals, with weed species identified along an 80 to 100-metre-long transect in the paddock.

Distribution

The ranking of different summer weeds in terms of their presence in paddocks varied significantly with cropping regions. Heliotrope was the most prevalent summer weed species when averaged across SA (Table 1) and found in eight of the nine individual cropping districts.

Roly poly, Afghan melon and clammy goosefoot were common summer weeds across most of the cropping regions. Other weeds appear to be more localised in their distribution such as: tares (Lower North); cutleaf mignonette (Yorke Peninsula); tar vine (Upper North); skeleton weed, small burr grass and innocent weed (Mallee); Afghan thistle (Upper Eyre Peninsula) and wild radish (Upper South East). Lincoln weed was only found to occur on the Yorke Peninsula, Upper and Lower Eyre Peninsula.

TABLE 1  Top 10 summer weeds identified in the paddock survey of South Australia.
Rank Common name Scientific name Occurrence across SA
(% of fields)
1 Heliotrope Heliotropium europaeum 57
2 Afghan melon Citrullus lanatus 25
3 Roly poly Salsola australis 18
4 Lincoln weed Diplotaxis tenuifolia 14
4 Common sowthistle Sonchus oleraceus 14
6 Clammy goosefoot Chenopodium pumilio 13
6 Panic grass Panicum spp 13
8 Stinking lovegrass Eragrostis cilianensis 11
8 Fleabane Conyza bonariensis 11
8 Mallow Malva parviflora 11
SOURCE: Ben Fleet, Dr Christopher Preston and Dr Gurjeet Gill

Feathertop Rhodes grass (FTR) was found in four per cent of the sampled paddocks only in the Lower North. It is an extremely vigorous weed in the northern region where it can produce up to 40,000 seeds per plant and cause large yield losses in crops. Some populations of FTR in SA have already been shown to be resistant to glyphosate. The combination of these attributes is likely to result in a greater spread of this weed in the future. Windmill grass was not detected in any of the sampled paddocks in this survey, but is present on roadsides in SA.

Caltrop has been traditionally regarded as one of the most serious summer weeds, but in this survey its presence varied considerably between the regions. The highest presence of caltrop was found in the Mallee (27 per cent of paddocks), the Upper South East (21 per cent) and the Upper North (18 per cent). It was not detected in the survey in the Lower South East and Lower Eyre Peninsula.

Year-round threat

Common sowthistle was ranked number one weed in terms of presence in the Lower South East (21 per cent). This weed species also occurred regularly in the Lower North (44 per cent) and Mid North (26 per cent). Its ability to produce seed in summer as well as in winter, has allowed it to take advantage of higher spring and summer rainfall in these regions. Surprisingly, given its prevalence in lentil crops, common sowthistle was found in less than 10 per cent of survey paddocks on the Yorke Peninsula.

FAST FACT

Heliotrope was the most commonly found summer weed in a survey of South Australian paddocks.

Mallow, another winter weed, was found regularly in many cropping regions over summer. It occurred much more commonly in the Lower North (28 per cent), Mid North (26 per cent), Upper North (14 per cent) and Lower Eyre Peninsula (22 per cent). Mallow plants were not found in areas with low winter rainfall (Upper Eyre Peninsula and the Mallee).

Panic grass (Panicum sp.) was regularly found in the Lower North (32 per cent), Mid North (35 per cent) and Upper North (23 per cent), the Upper South East (29 per cent) and Lower South East (16 per cent). However, it was much less common in paddocks in the Mallee (2 per cent), Upper and Lower Eyre Peninsula (3 to 5 per cent) and Yorke Peninsula (6 per cent)

More information

Dr Gurjeet Gill, University of Adelaide
08 8313 7744,
gurjeet.gill@adelaide.edu.au