Weed Management in Horticultural Crops

HERBICIDE TOLERANCE AND RESISTANCE

Herbicide Resistance: Naturally occurring inheritable ability of some weed biotypes within a population to survive a herbicide treatment that would, under conditions of use effectively control the weed population (Rubin, 1991)

• Senecio vulgaris resistance to triazine group of herbicide was noticed during 1970.
• Worldwide 183 weeds have developed resistance to herbicides till 1997.
• In India the most common example is Phalaris minor .
• The highest resistance in 61 weed species was recorded for Atrazine.
• USA alone found to have 49 herbicide resistant weeds, the highest in the world.

o Tolerance: The term tolerance refers to the partial resistance and presently the usage of the term is discouraged due to inconsistency in quantifying the degree of tolerance.
o Gross Resistance: When a weed biotype exhibits resistant to two or more herbicides due to the presence of a single herbicide mechanism.
o Multiple resistance: It is a situation where resistant plants posses two or more distinct resistant mechanism to a single herbicide or groups of herbicides.

Basic principles of herbicide resistance

1. Time, dose and method of application of herbicide variation
2. Variation in phenotypes of a population
3. Genetic variation by mutation or activation of pre-existing genes

Conditions favourable for development of Herbicide resistance

• Repeated use of same herbicide or use of herbicide with same mode of action due to the practices of monoculture
• Areas where minimum/zero tillage is followed
• Fields where farmers rely on only herbicides for high degree/level of weed control including nurseries, orchards
• Non-crop situations like road sides, railway tracks etc. where herbicides are repeated used may be at higher doses than cropped situation

Resistance was exhibited in crop is due to:

• Herbicide metabolism by crops making them inactive
• Absence of certain metabolic process in crops compared to weeds and thus tolerating the herbicides
• Crops couples the herbicide molecule