Difference between resistance and tolerance?


the evolved capacity of a previously herbicide-susceptible weed population to survive a herbicide applied at normal use rates and complete its life cycle



the inherent ability of a plant to survive a herbicide treatment at a normal use rate. This implies there was no selection or genetic manipulation to make the plant tolerant, it is naturally tolerant

why resistant weeds can’t be ignored?

Superfast Speed of Growth

Superfast Speed of Spread

Amaranthus spread in argentina

Conyza spread in argentina

Reason for Speed of Spread


A single Palmer amaranth plant can produce as many as 5,00,000 seeds in one season, Marestail, about 2,00,000. Waterhemp generally produces 2,50,000 seeds per plant, but in non-competitive environments can produce as many as 1 million. Marestail seed is small enough to be carried for miles by the wind.


for resistance management

Developing herbicide resistant crops like one new herbicide-resistant variety, glufosinate-resistant soybean.

Genetically modified crops resistant to glufosinate were created by genetically engineering the bar or pat genes from streptomyces into the relevant crop seeds.

New weed management practices are needed to manage the consequences of long-term weed control.

Adoption of an Integrated Weed Management Approach

  • Rotate crops (particularly those with different life cycles)

  • Incorporate non-chemical weed control measures:

    • Use cultivation and mechanical hoeing where possible. Also consider post harvest grazing, mowing, stubble burning, etc to prevent seed set or survival.
    • Delay planting to allow removal of initial weed flushes.
    • Use weed-free or certified crop seed and manage crop to maximize competition.
    • Use tillage as a component of the weed management program.

  • Scout fields regularly:
    Scan the fields carefully to identify potential issues and eliminate weed escapes. Keep good records of weed populations.

  • Cleaning tillage and harvest equipment:
    Prevent spread of resistant weeds by thoroughly cleaning tillage and harvest equipment as it moves from field to field. Also do not allow resistant weeds from total vegetation control areas (ie, fence rows, railroads, public utilities, etc) to spread to cropland.