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Herbicide Carryover Following Dry Conditions the Previous Year

By: Clark Brenzil, PAg, Weed Control Specialist

Crop selection for any given field may be more challenging for the 2018 season as a result of unexpected herbicide residues. Moisture after application is critical to the degradation of herbicides. With 2017's exceptionally dry season, producers need to be very conservative when selecting crops to follow potentially residual herbicides used in 2017.

The majority of herbicides are degraded through the process of microbes using them as a source of food. Microbes break the herbicides molecules down of the into smaller and smaller pieces. The result is fractional inactive molecule portions initially, but eventually nothing is left but carbon dioxide and some elemental salts. Some herbicides (certain Group 2s and Group 5) also degrade through chemical hydrolysis. In this process, water cleaves herbicide molecules into less active or inactive pieces.

Microbes that break down herbicides are like any other living organism in that they need moisture to survive and reproduce. They also need sufficiently warm temperatures to grow, so no breakdown occurs while soil is cold or frozen, generally from early to mid-October until the first seeding begins the following year.

Larger version of this
map can be downloaded
from image link above
As illustrated in the Herbicide Carryover Risk Map, very little rainfall fell after herbicide application timing in many areas of Saskatchewan in 2017. The exception to this is the far north-west region of the grain belt and localized areas of the northeast and east-central regions. During this rainless period, surface soils dried out to the point where microbial activity would be slow or stopped. Most herbicides have no other major degradation route. With little to no microbial activity or hydrolysis, there will be little loss of herbicide from the soil.

Herbicides typically considered non-residual will not likely have a problem, since the little moisture that will come from snow melt and rains in the spring will be enough to break them down before seeding takes place. However, any field in 2017 that saw the application of product that has a recropping restriction in the Guide to Crop Protection 2017 will have experienced little herbicide breakdown and will contain higher levels of that product than would normally be expected.

Residual herbicides can be found in Groups 2, 3, 4, 5, 7, 8, 11, 13, 14, 15, 26 and 27, although not all herbicides in these groups are residual. An overview of residual herbicides can be found in the Guide to Crop Protection 2017 on pages 77 through 79. This chart will also show which crops are the most sensitive to soil residue for the products listed. The Guide to Crop Protection can be picked up from your local retail outlets or Regional Office, or viewed online.

Several things can influence the amount of herbicide that will affect plants the following year. Both organic matter and clay particles act as buffers in the soil by binding herbicide molecules and making them less available to plants. Crops grown in soils with low levels of clay and organic matter are more susceptible to injury from herbicide residue. Eroded knolls are known to show herbicide damage at lower residue levels than the remainder of the field because they are drier and have lower organic matter.

Residual herbicides in Group 2 and Group 5 also have pH as a critical factor in the degradation process. This is due to the chemical hydrolysis element of breakdown within these groups. While microbial degradation continues at a more or less constant pace, depending on the herbicide and the climate, chemical hydrolysis is affected greatly by soil pH. The combined effect of both microbial degradation and chemical hydrolysis determines the degradation profile of these products.

  • Within the Group 2 herbicides, imidazolinones (IMI) persist longer under acid (low pH) soil conditions, whereas sulfonylureas (SU) persist longer under high pH soils.
  • Of the newer Group 2 herbicides, flucarbazone-sodium (Everest) has increased persistence under high pH conditions whereas florasulam (Frontline/Spectrum) does not appear to be influenced by the normal range of soil pH. Florasulam is not expected to have problems with carryover being similar to Refine Extra in speed of decay.
  • In Group 5, the triazines atrazine and cyanazine are affected by pH in such a way that degradation is slower and therefore persistence longer under high soil pH.

Having soil analysed at a chemistry lab is an expensive proposition and few people can give you a definitive answer about what the herbicide concentration values mean or whether they are going to be toxic to a crop grown in that soil. A more reliable method of determining the risk of herbicide injury from soil residue is a plant bioassay. A plant bioassay is simply growing a sensitive plant in the soil in question and comparing it to a plant grown in known untreated soil of the same type. While producers could test their own fields to see if residues are present by using a plant bioassay technique using garden pots, these tests can be a challenge due to low light and inconsistent watering. Therefore, sending samples to a lab that conducts plant bioassays, such as A&L Laboratory in London, Ont., will produce a more reliable result. Even then the predictability of these tests is not perfect and may give both false positives, restricting opportunities to plant a crop that would have been fine, and false negatives, giving the impression everything is safe when the risk of injury is present.

Sampling can be a critical source of error in a plant bioassay. Ideally an untreated soil sample should be included that closely matches for soil being tested and preferably for each slope position within the field being tested. Untreated soils could come from nearby in a neighbouring field known not to have a residual herbicide used or the corners of fields treated where large spray equipment often miss (avoid corners near entry points to fields). Most herbicide residues will remain in the top one to two inches of soil, so soil samples should be taken from this layer. If fall tillage was conducted, collect soil from the top half of the tillage layer. Collect separate samples from treated areas on knolls, mid-slopes and low areas as well as the untreated area, and keep it in separate clean containers. Do not use old chemical containers to collect soils.

If herbicide residues are detected, producers should take measures to lessen their potential impact on the 2018 crop by adjusting their rotation to seed a known tolerant crop in that field. When growing similar crops, disease pressure is always a concern to keep in mind. Under conditions as dry as was experienced in many areas of the southwestern part of the province (red areas), some of the crops that would normally show tolerance may also have problems in 2018. If you used a residual herbicide in 2017 in these areas, check with the manufacturer of that herbicide to find out the tolerance levels of each of the allowable crops and choose one of the most tolerant ones.

The final concern with dry soil conditions and herbicide breakdown is accumulation of residues of a herbicide type under repeated use of that group. The same soil microbes are utilized to break down herbicides in the same herbicide group. If the herbicide applied in one season is not completely broken down by the next, another application of a product from the same group in subsequent years can result in a build-up of that chemistry in the soil over time. Sometimes the rate of degradation of a herbicide or herbicide group is faster when lower rates are applied vs. higher rates. Under dry conditions, when there is little degradation of herbicide from the soil, application of a herbicide to soils with residues could result in the herbicides in that group degrading even more slowly when they do begin to degrade. Remember that repeated applications of the same herbicide group year after year could result in the development of weeds that are resistant to that group of herbicides.

Producers need to be extra careful about what choices they make with respect to their rotations in 2018. Unfortunately if persistent herbicides were applied in 2017, the choice may already be made for them.

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