Faba bean and residual herbicides

Posted in Agronomy alert on Dec 04, 2015

In 2015 a significant number of faba bean crops have shown symptoms characteristic of herbicide damage.

Faba bean and residual herbicides

Pulse growers need to be aware:

  • Herbicides used to maintain fallow or to control weeds in previous crops need to be carefully considered if following with a faba bean or other broadleaf crop.
  • That dry conditions may have reduced the rate of herbicide breakdown from previous in crop and fallow applications.
  • In 2015 a significant number of faba bean crops in the northern region showed symptoms characteristic of herbicide damage.
  • Post sowing rain in 2015 resulted in herbicide damage to some crops from post-sow pre-emergent herbicides.
  • Growers and agronomists need to consider the implications of potentially longer plant back periods for herbicides used in the previous year’s crop and during fallow if dry conditions occur.
  • Alkaline vertosol soils in northern NSW present a higher risk for herbicide residues, particularly Group B herbicides.

Residual herbicide implications

Herbicides applied to paddocks in 2014 and early 2015 did not break down adequately for re-cropping because of insufficient rainfall. The plant back periods on herbicide labels may stipulate a minimum rainfall requirement and where they do not the plant back periods may still be prolonged by dry conditions. Herbicides applied up to two years ago impacted crop health in some cases during 2015.

A crop of PBA Warda faba bean north of Wee Waa showed phenoxy herbicide damage in May 2015. Plants showed twisted stems, rolled leaf margins and very slow growth. Symptoms occurred in patches across the paddock and a fallow spot spray application of 2,4-D herbicide in March was found to be the cause, (see Image gallery 1). Fortunately, the total area that received the spot spray was small and the grower achieved good establishment, limiting the potential yield loss. Following in-crop rainfall the affected plants recovered.

Image gallery 1. Herbicide damage in faba beans observed in 2015, caused by fallow spot spray application of 2,4-D herbicide.

A crop of PBA Warda suffered significant damage from tribenuron-methyl (Express®) sprayed ten days prior to sowing after 12 mm of rain was received between herbicide applications and sowing. Symptoms were most severe on headlands where severe stunting occurred and the plant population was significantly reduced, (see Image gallery 2).

Image gallery 2. Group B damage to faba bean near Inverell.

Image gallery 3. Faba beans plants damaged by terbuthylazine (Group C) sprayed post-sowing pre-emergence to faba beans near Narrabri, causing leaf margin necrosis and death of some seedlings.

Implications for the 2016

Whilst winter rainfall in northern NSW has been good the present El-Nino conditions raise the possibility of a dryer than normal spring and summer which may slow the breakdown of herbicides applied in crop and during fallow maintenance. If dry conditions eventuate growers and agronomists will need to consider the implications of potentially longer plant back periods on future cropping plans.

Faba bean will tolerate relatively deep sowing and this can be used to provide some protection from post-sow pre-emergent herbicides.

Key resources

Australian Pulse Bulletin ‘Residual herbicides and weed control’

Pulse Australia

Key contacts

Paul McIntosh, Industry Development Manager Qld/NSW, Pulse Australia, 0429 566 198, paul@pulseaus.com.au

Mary Raynes, Industry Development Manager Vic/SA, Pulse Australia, 0408 591193, mary@pulseaus.com.au

Kedar Adhikari, Plant Breeder, University of Sydney, 02 6799 2231, kedar.adhikari@sydney.edu.au

Bill Manning, Senior Land Service Office – Cropping, NSW Local Land Services, 0428 607 731, William.manning@lls.nsw.gov.au

Joop van leur, Plant Pathologist, NSW DPI , 0427 928 018, joop.vanleur@dpi.nsw.gov.au

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