HERBICIDE RESIDUES & THEIR MANAGEMENT

RESIDUAL HERBICIDE

• Herbicide breakdown requires sufficient time under adequate moisture and soil temperature to support the growth of microbes that degrade herbicide molecules.

• Some herbicides are broken down quickly • Other herbicides take longer to decay,

hence persist into seasons following the year they were applied.

• These can injure sensitive crops that are seeded in following seasons.

Risk of residual herbicides• Gives lower yield on the next season crop.o Eg: spraying imidazolinone-family

herbicides on sunflowers gave lower oil content in next season

• It also shows phytotoxic effect on the crops of next season

• Residue of last season herbicides incompatible to present season crop may cause heavy damage

The first is a check showing canola seedlings in clean soil. The second two photos are canola seeded the same day into soil with residues from common Group-2 products

Sulfonylurea damage on chickpeas

Herbicide Recropping Restrictions2,4-D No restrictionsAbsolute wheat ,barley , canolaAccord Wheat, barley, canola, field

peas, and sunflowers Assert Black and grey soil zones –

wheat, barley canola, peas, flax, sunflowersBrown soil zones – wheat, Clearfield canola, barley, sunflowers

Attain Wheat, barley, oats, rye, forage grasses, flax, canola, mustard, lentils, and peas

Curtail, Prestige

wheat, barley, oats, rye, corn, flax, canola, mustard, forage grasses, sugar beets

Eclipse Wheat, oats, barley, rye, forage grasses, flax, canola, mustard can be grown

Factors Affecting Herbicide Carryover

• Herbicide Factorso Herbicides differ in their physical and chemical

properties such as water solubility, volatility and susceptibility to breakdown by sunlight, and microbes.

o Their method of breakdown also ranges from simple chemical reaction to a more complex microbial degradation.

o Residual herbicide activity is also described in terms of the half life; the half life varies by herbicide and ranges from a few days to a few years

• Microbial Decompositiono Soil bacteria, algae and/or fungi

metabolize some herbicides.o Their action is enhanced by conditions that

favor the growth and multiplication of microorganism.

• Chemical Degradation:o Some herbicides may react with water or other chemicals in the soil,

changing the nature of the molecules responsible for the herbicidal activity.

o Eg: the sulfonylurea (SU) chemically react with water in a process called hydrolysis. Once the SU is in contact with water, the chemical breakdown is initiated, and the herbicide is no longer biologically active.

• Photodecompositiono Some herbicide will degrade when exposed to ultra-violet light of

suno Eg: Herbicides such as trifluralin, ethalfluralin and the

cyclohexanones (Poast, Achieve, Select) can degrade in sunlight. Specific management strategies such as soil incorporation and the use of effective surfactants minimize the impact of photo-degradation.

• Plant Uptake

o Some plants absorb specific herbicides and metabolize it.

o This effectively removes residues from the soil.• Soil Factorso Soil pH-affects the stability of the herbicideo Organic matter-Organic matter (OM) can absorb large

amounts of herbicides, so the less the OM, the more biologically active the herbicide residue

o Soil textureo Soil moisture-The higher the soil moisture levels the

higher the rates of leaching, volatilization and microbial/ chemical degradation

o Soil temperature

• Management Factorso Application Rateo Time of Applicationo Uniformity of application/incorporationo Tillage System

Avoiding or Minimizing Herbicide Carry-over Effects• Integrated weed management• Herbicide rotation with crop rotation• Selection of herbicides with minimum

carry-over potential• Applying minimum & Accurate application

rates of herbicides• Time of application –Early season

application also assists in reducing the carry-over potential to succeeding crops

• Grow a tolerant crop• Soil additives - Adsorption of herbicide

residue can be increased by the addition of adsorbent material such as activated charcoal. Though the use of activated charcoal on a large scale is not economic.

Determining Herbicide Residues

• Field bioassay- means growing to maturity a test strip of the crop(s) intended for production the following year

• Chemical analysis- requires the submission of representative soil samples to a laboratory for the detection of residual herbicide in soil

• Commercial plant bioassay- involves collecting and sending suspect soil samples to a commercial laboratory where a simple, accurate method is used to determine the residues.

• Random Soil Sampling- Random sampling is the most common method of soil sampling. Cores need to be collected from the entire area to obtain reliable estimates

• Problem areas like, saline spots, poorly drained areas, and eroded knolls should not be sampled unless they represent a significant portion of the field

• Topographic Soil Sampling- it involves dividing fields into several areas split by landscape position based on topography.

• Separate samples should be taken from; eroded knolls, midslopes of field and low areas with better moisture conditions.

• It is better sampling system for fields where the majority of the field is rolling landscapes.