Understanding the types and mechanisms of resistance, along with how it develops at the cellular and field scale, can help you to develop an effective black-grass control strategy.
What exactly is resistance?
Herbicide resistance is defined as ‘the inherited ability of a weed to survive a rate of herbicide that would normally kill it. (Moss SR, 2006).
3 key criteria must be fulfilled for a plant to be classified as ‘resistant’:
- Resistance must be heritable (passed on to offspring)
- The plant must occur naturally and not be the result of deliberate/artificial selection
- Resistance must be confirmed using acceptable scientific protocols
How does resistance spread?
Once a mutation has occurred, the trait (e.g. herbicide resistance) is embedded in the genetic make-up of the plant and can be passed to offspring. Only 1 plant has to survive and produce viable seed for the traits conferring resistance to be passed on to the next generation.
Beware the false alarm
It is worth remembering that several other factors could cause a plant to survive herbicide treatment; it may not always be due to resistance.
Other factors which must be eliminated first include:
- Application problems
- Rain after application
- Too cold or too dry for herbicides to work
- Germination from outside the zone of activity (pre-ems)
Resistance development at the genetic level
The repeated use of herbicides with the same mode of action has led to the evolution of resistant weed populations, but herbicides themselves are not to blame for causing resistance.
Natural gene mutations occur spontaneously in all plants and have the potential to change how proteins function. It is largely down to chance where any mutation strikes and how it alters that gene.
The evolution and spread of herbicide resistance through subsequent generations is best explained by the Mendel theory of inheritance.
During the breeding process, the 2 alleles segregate during gamete production, with 1 allele going to the male part (pollen grain) and the other to the female ovum. If different alleles are present then 50% of gametes receive the dominant resistant allele while 50% receive the recessive susceptible allele.
Field-scale development and spread
During herbicide treatment, the active ingredient will only be effective against those individuals which are susceptible. Those which are resistant will survive treatment and be free to set seed and reproduce in following seasons.
Because black-grass is a cross-pollinating species, resistant genes can therefore be spread aerially in pollen, as well as via seed return in the soil.
Once resistant plants have established within a field, it can be relatively easy for seeds from these plants to be picked up and moved within the field by cultivation, harvesting or baling equipment.
Long-distance development and spread
It is possible for seeds from resistant plants to be picked up and transported longer distances by machinery, straw and manure, slurries or digestate, moving between fields or farms which can result in a wider geographic spread of resistant individuals.