Pro­tect knock-down her­bi­cides

AgLife - - News -

Most crop­ping sys­tems rely fairly heav­ily on a small group of non-se­lec­tive or ‘knock-down’ her­bi­cides.

Since the wide­spread adop­tion of zero and min­i­mum tillage, these her­bi­cides have pro­vided ef­fec­tive control of many grass and broadleaf weeds – but these use­ful her­bi­cides could be lost to the in­dus­try if steps are not taken to in­crease the di­ver­sity of weed man­age­ment tac­tics used.

In­de­pen­dent Con­sul­tants Aus­tralia Net­work se­nior con­sul­tant Mark Con­greve said the highly ef­fec­tive dou­ble-knock tac­tic, which com­bined an ap­pli­ca­tion of glyphosate fol­lowed by paraquat, was at risk if grow­ers failed to re­main vig­i­lant and en­sure re­moval of any sur­viv­ing plants.

“The dou­ble-knock strat­egy of glyphosate, plus a group I her­bi­cide for weeds such as flaxleaf flea­bane, fol­lowed by paraquat has pro­vided ex­cel­lent control of weeds that are dif­fi­cult to kill with glyphosate alone,” he said.

“Re­cent con­fir­ma­tion of a flea­bane pop­u­la­tion that is re­sis­tant to paraquat, found in a New South Wales vine­yard, is a clear warn­ing to grain pro­duc­ers there is no room for com­pla­cency fol­low­ing a dou­ble-knock op­er­a­tion.”

Paraquat is a widely used her­bi­cide, be­ing an ac­tive in­gre­di­ent in more than 100 her­bi­cide prod­ucts reg­is­tered for use in broad­acre crop­ping.

It is a group L her­bi­cide and as such is con­sid­ered a ‘mod­er­ate risk’ for her­bi­cide re­sis­tance.

A mod­er­ate risk rat­ing means re­sis­tance gen­er­ally takes longer to oc­cur, not that it will not oc­cur.

“Paraquat re­sis­tance typ­i­cally takes more than 15 years of con­sis­tent use be­fore re­sis­tant weeds are no­tice­able in the field,” Mr Con­greve said.

“This crit­i­cal pe­riod has now elapsed on many farms where paraquat is used in ce­re­als and broadleaf crops, and for gen­eral weed control around the farm.”

Paraquat re­sis­tance has been present and wide­spread in bar­ley grass in lucerne pro­duc­tion sys­tems for many years in Vic­to­ria and south­ern NSW. While paraquat re­sis­tance is still rel­a­tively rare out­side of lucerne sys­tems, high-level re­sis­tance to paraquat was con­firmed in three weed species – crows­foot grass, black­berry night­shade and cud­weed – taken from sug­ar­cane and tomato blocks around Bund­aberg in 2015.

In the event of wide­spread re­sis­tance to paraquat, Mr Con­greve is con­cerned there are no new modes of ac­tion likely to be com­mer­cialised within the next 10 years or more, mean­ing farm­ers needed to pro­tect what they had.

“It is es­sen­tial farm­ers do ev­ery­thing in their power to pre­serve the ef­fec­tive­ness of the her­bi­cide groups avail­able,” he said.

“The key is to take a di­verse ap­proach to weed man­age­ment and, im­por­tantly, remove weeds that sur­vive her­bi­cide ap­pli­ca­tions. This is the best way to keep weed num­bers low and when num­bers are low, re­sis­tant weeds can be con­trolled more ef­fec­tively. It’s a num­bers game.”

Check­ing steps

Mr Con­greve sug­gested grow­ers check the results of every spray ap­pli­ca­tion, look­ing for in­di­vid­ual plants ‘sur­viv­ing’ or ‘re-grow­ing’ af­ter a spray ap­pli­ca­tion that had killed ad­ja­cent weeds.

This might be a sign the sur­viv­ing plants carry the ge­netic mu­ta­tion that ‘pro­tects’ them from the her­bi­cide’s mode of ac­tion.

“If this is ob­served, the first step is to remove those in­di­vid­ual plants be­fore they shed seed,” Mr Con­greve said. “It is rec­om­mended to have the plants, or their seed, tested to con­firm re­sis­tance and de­ter­mine what her­bi­cides those in­di­vid­u­als are still sus­cep­ti­ble to.”

There are 10 weed species with con­firmed re­sis­tance to paraquat – Group L – and 13 species re­sis­tant to glyphosate – Group M – in Aus­tralia.

• In­for­ma­tion from Weeds­mart, an in­dus­try-led project that aims to en­hance on-farm prac­tices and pro­mote the long-term, sus­tain­able use of her­bi­cides in Aus­tralian agri­cul­ture. For more in­for­ma­tion about re­duc­ing the risk of her­bi­cide re­sis­tance, visit the Weeds­mart web­site: www.weeds­mart.org.au.

LOOK­OUT: ICAN se­nior con­sul­tant Mark Con­greve says grow­ers need to look for sur­vivor weeds af­ter every her­bi­cide ap­pli­ca­tion.

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