Herbicide residues can persist in soil and injure crops planted in subsequent seasons. The rate of herbicide breakdown depends on factors like the chemical properties of the herbicide, soil microbes, moisture, temperature, and tillage practices. Farmers can minimize carryover risks by selecting herbicides with short half-lives, applying the minimum effective rate, timing applications early in the season, and using crop rotations and soil additives. Determining residual herbicide levels involves field bioassays, chemical analysis of soil samples, or commercial plant bioassays.

1. HERBICIDE RESIDUES &
THEIR MANAGEMENT

2. 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.

3. 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

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

5. Sulfonylurea damage on chickpeas

6. Herbicide Recropping Restrictions
2,4-D No restrictions
Absolute wheat ,barley , canola
Accord Wheat, barley, canola, field peas, and
sunflowers
Assert Black and grey soil zones – wheat, barley
canola, peas, flax, sunflowers
Brown 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

7. Factors Affecting Herbicide Carryover
• Herbicide Factors
o 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

9. • Microbial Decomposition
o Soil bacteria, algae and/or fungi metabolize
some herbicides.
o Their action is enhanced by conditions that favor
the growth and multiplication of microorganism.

10. • 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.
• Photodecomposition
o Some herbicide will degrade when exposed to ultra-violet
light of sun
o Eg: Herbicides such as trifluralin, ethalfluralin and the
cyclohexanones (Poast, Achieve, Select) can
degrade in sunlight. Specific management strategies

11. • Plant Uptake
o Some plants absorb specific herbicides
and metabolize it.
o This effectively removes residues from the
soil.
• Soil Factors
o Soil pH-affects the stability of the herbicide
o 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 texture
o Soil moisture-The higher the soil moisture levels
the higher the rates of leaching, volatilization
and microbial/ chemical degradation

12. • Management Factors
o Application Rate
o Time of Application
o Uniformity of application/incorporation
o Tillage System

13. 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

14. • 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.

15. 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.

16. • 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

17. • 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.