Of 78 biotypes of wild radish (Raphanus raphanistrum) collected from Western Australia (WA), 42% were resistant and 14% intermediate to acetolactate synthase (ALS)-inhibiting herbicides. Based on the LD50 and GR50 ratios, the resistant biotype K96071 was 81-fold more resistant to chlorsulfuron and 114- to 116-fold more resistant to metosulam than the susceptible biotype K96041. More resistant biotypes were found in northern zones than in southern zones of WA. Resistant biotypes evolved after five applications of chlorsulfuron in a predominantly cereal–lupin rotation. Where resistant biotypes were found, ALS-inhibiting herbicides were not rotated with herbicides with different modes of action as frequently as in fields with susceptible biotypes. Cross-resistance to chlorsulfuron and metosulam was found in the resistant biotypes even though only 15% of the 78 biotypes were exposed to two applications of metosulam over a 10-yr period. All 78 biotypes were effectively controlled by simazine and 2,4-D amine.

This study documents the first case of triazine resistance in wild radish and the resistance mechanism involved. The high survival (57 to 97%) of the resistant (R) biotype progeny plants treated at a rate four times higher than the commonly recommended rate of simazine or atrazine clearly established that the R biotype plants were resistant to triazines. All the plants of the susceptible (S) biotype plants were killed when treated at half the commonly recommended rate of atrazine (0.5 kg/ha) or simazine (0.25 kg/ha). The dry weight of the S biotype was reduced by 89 to 96% at the commonly recommended rate of atrazine or simazine, while the dry weight of the R biotype plants was reduced by only 36 to 54% even when treated at a rate four times higher than the commonly recommended rate of atrazine or simazine. The growth-reduction–ratio values indicated that the R biotype progeny plants were 105 and 159 times more resistant to atrazine and simazine, respectively, than the S biotype plants. Leaf chlorophyll fluorescence yield was reduced by 97% in the S biotype 24 h after application of triazine compared with only 9% reduction in the R biotype, indicating that the resistance mechanism involved is target-site based. The R biotype was effectively controlled by herbicides of different modes of action.

Raphanus raphanistrum L has evolved widespread resistance to sulfonylureas in the Western Australia (WA) wheat belt. With the introduction of imidazolinone-tolerant (IT) wheat (Tritcum aestivum L) and IT canola (Brassica napus L) in the WA wheat belt, it is important to understand the status of cross-resistance in this weed to sulfonylurea and imidazolinone (Imi) herbicides. A study was conducted to examine cross-resistance between chlorsulfuron and Imi herbicides (a mixture of imazapic and imazapyr) in 46 R raphanistrum populations collected from across the WA wheat belt. Plants were treated with herbicides and assessed for phytotoxicity under glasshouse conditions. Of the 46 R raphanistrum populations, 32 were resistant to chlorsulfuron and four were resistant to imazapic + imazapyr. Of the 70% chlorsulfuron-resistant populations, 13% showed cross-resistance to imazapic + imazapyr. However, the cross-resistant populations treated with imazapic + imazapyr showed a lower resistance level than the chlorsulfuron-treated populations. These results suggest that weed populations with such cross-resistance will not be controlled effectively by Imi herbicides. Although the resistance levels of the cross-resistant populations to Imi herbicides were low, the cross-resistance levels of R raphanistrum should be determined before growing IT crops, particularly IT canola.

No abstract

Tedera is a drought-tolerant perennial forage legume introduced in Australia in 2006. In October 2018, T15-1218 Lanza®, the world’s first tedera variety, was released by the Department of Primary Industries and Regional Development and Meat & Livestock Australia for commercial use. A key agronomic practise for the successful establishment and adoption of tedera is to have a robust herbicide package to control a range of grass and broadleaf weeds well tolerated by tedera. A total of 9 pre-emergent and 44 post-emergent herbicide treatments were evaluated in eight experiments from 2017 to 2021. To control grasses such as annual ryegrass (Lolium rigidum Gaud.), propyzamide and carbetamide can be recommended for pre- or post-emergent applications and butroxydim, clethodim, and haloxyfop for post-emergent applications. The broadleaf pre-emergent herbicides recommended are clopyralid to control emerged capeweed (Arctotheca calendula (L.) Levyns), fomesafen to control pre-emergent wild radish (Raphanus raphanistrum L.), and the double mix of fomesafen + diuron, flumetsulam + diuron, and the triple mix of fomesafen + diuron + flumetsulam to control pre-emergent capeweed, pre- and post-emergent wild radish, and other broadleaf weeds. The most consistently well tolerated post-emergent herbicides by tedera seedlings and adult plants were diflufenican, diuron, flumetsulam, fomesafen, and their two- or three-way mixes that will provide good control of capeweed and wild radish. Desiccants such as paraquat or diquat were also well tolerated by 1-year-old tedera plants that recovered after being desiccated.