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herbicides are literally poison. and these poisons remain even for longer period after application, hence they are actual

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  2. 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. 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. 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. 5. Sulfonylurea damage on chickpeas
  6. 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. 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
  8. 8. • 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.
  9. 9. • 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
  10. 10. • 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
  11. 11. • Management Factors o Application Rate o Time of Application o Uniformity of application/incorporation o Tillage System
  12. 12. 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
  13. 13. • 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.
  14. 14. 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.
  15. 15. • 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
  16. 16. • 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.