- Author: Brad Hanson
Today's post is a) long, b) recycled from another use, c) of high interest due to current weather conditions, or d) all of the above? The correct answer, I think, is "D" all of the above.
As an aside, the length of this post reminds me of one of my favorite Mark Twain quotes " I didn't have time to write a short letter, so I wrote a long one instead" - there's a lot of truth in that.
Ok, this is actually an article I prepared for the meeting booklet for the Plant and Soil Conference held each year by the California Chapter of the American Society of Agronomy. In the pest management session a couple weeks ago, I gave a presentation on the effects of drought on weeds and weed control and I thought I'd share the accompanying writeup here as well for posterity and broader distribution.
Effects of drought conditions on weed control performance and herbicide fate
Brad Hanson, Cooperative Extension Weed Specialist,
Plant Sciences Dept. One Shields Ave, Davis, CA 95616
Environmental conditions before, after, and during herbicide applications can have a dramatic impact on the performance of the herbicide and on the environmental fate of the product. The environmental condition that is most on people's minds the past couple seasons, of course, is drought. While specific weed management situations will be affected differently, or not at all, by drought, it's useful to think about how dry conditions can impact weeds and herbicides in and on the plant as well as in the soil.
Weed pressure: First, the good news about drought and weeds. Under drought conditions, we typically have fewer weeds. Weeds often have inherent or enforced dormancy mechanisms that protect seed from germinating into inhospitable environments; so overall weed germination typically is reduced under dry conditions. This is especially true for small-seeded weeds because they usually germinate on or near the soil surface; however, deep-rooted perennial weeds or larger-seeded weeds that can emerge from deeper in the soil are less affected. Second, under dry conditions or periodically dry conditions, fewer of the seeds that do germinate are able to successfully establish which further reduces weed pressure under dry conditions. Now the bad news: those weeds that do establish because of timely germination relative to rain or irrigation or those that are in low spots that hold water longer may actually be more competitive because they have less competition with other weeds for resources (weeds compete with each other too).
Dry conditions can also affect the proportion of the various weed species present in a field – some weeds do better than others under dry conditions. For example, we often see more drought weeds like field bindweed (deeply rooted perennial), prostrate knotweed, and puncturevine under drought conditions because the water-loving plants like common purslane, nutsedges, and spurges don't do as well under moisture stressed conditions. This effect is much more noticeable with the winter-germinating weeds because our irrigation practices in cropped fields overcome this stress for summer-germinating weeds.
Preemergence herbicides: The performance of preemergence (aka PRE or residual) herbicides can be affected in several ways by drought conditions. First, to be effective, these herbicides must be incorporated into the soil, in many permanent crops and non-crop situations, we count on winter rains to incorporate or “activate” the herbicide. Lack of incorporation or delayed incorporation can lead to poor performance with many PRE herbicides because most of them have activity on the germinating seed or the very small seedling. In these instances, weeds that germinate below the herbicide layer at the soil surface may be able to grow roots and push shoots through the herbicide zone before it can be washed into the layer of soil where germination occurs. In the past few years with little winter rain, we've seen erratic performance with some PRE herbicides in orchards and vineyards – growers who happened to time PRE applications ahead of a rain often had very good performance while others who missed the rain did not get adequate incorporation for several weeks and had less than stellar performance. A second way that lack of incorporation can affect PRE herbicide performance is through “photodegradation”. This refers to the chemical degradation of some herbicides after long exposure to sunlight on the soil surface. Fortunately, most of the modern PRE herbicide chemistries and formulations are not very prone to this; however, long periods on the soil surface will never increase PRE herbicide performance either.
On the other hand, in some cases dry conditions can extend herbicide life in the soil. The primary mechanism for the degradation of most herbicides in the soil environment is microbial activity. Conditions like drought and excess heat or cold that are not good for microbial life often lead to slower herbicide degradation while conditions such as warm, moist soils can increase microbial activity and lead to faster degradation. As an illustration of this, think about where PRE herbicide performance first breaks in crop and non-crop situations. Usually herbicides fail first under the drippers in a vineyard, nearly leaky valves in a field or lawn, and in low areas where water stands longer because the herbicide degrades more quickly in these areas relative to the dryer areas – drought presents the same issue on a larger scale. Conversely, dry areas such as those between drip emitters in a vineyard, the tops of berms in furrow-irrigated orchards, or non-irrigated industrial sites may have much longer herbicide persistence.
If possible, timing applications ahead of even a small rain event will help reduce these problems. Second, if solid set sprinkler or microspray irrigations can be used, even a very short irrigation set will greatly improve herbicide efficacy. Finally, in drip-irrigated orchards or vineyards, some growers delay PRE applications until late winter trying to time the next rain event, some apply the PRE materials normally and plan on a followup POST application to pick up any weeds that emerge before the PRE herbicides are incorporated, and some forgo using PRE programs altogether in really dry years.
Slowed degradation of PRE herbicides caused by drought conditions can have both positive and negative results for weed management. In 2013, for example, we saw situations where weed control duration was actually much better than expected from herbicides generally considered to be “short residual” materials. If incorporated by an early rain or irrigation event but then surface soils remained relatively dry for much of the winter, materials like Goal and Matrix had much longer effective weed control in orchards than typically expected in a wetter winter season and long-lasting material lasted even longer. While that scenario presents a bit of a cloud-with-a-silver-lining for perennial crops, excess persistence due to drought can be a big problem in annual crops with complex rotations. Abnormally long carryover of PRE herbicides can sometimes lead to crop injury in a rotational crop because rate recommendations and rotational crop restrictions based on “normal” degradation rates may not provide a high enough margin of safety in really dry conditions. The worst case for this in our systems probably are those fields that were prepared, sprayed, and planted in spring but had the crop terminated due to irrigation cutbacks – full rate herbicides, followed by a non-irrigated, dry summer conditions could lead to carryover problems in some areas and should be considered in the coming season. For these growers, selection of appropriate rotational crops can minimize carryover issues and deeper tillage can be used to dilute the herbicides; if in doubt, consider conducting a soil bioassay with the planned rotational crop seed to verify an acceptable level of crops safety.
Postemergence herbicides: The efficacy of postemergence (POST) herbicides also can be greatly impacted by drought conditions. First, although it seems a little counterintuitive, stressed weeds are harder to kill than unstressed plants. It's always easier to kill weeds with POST herbicides when they are healthy, happy, and growing quickly because all biosynethetic processes (photosynthesis, synthesis of amino acids, proteins, and other cellular components, and meristematic growth) are all proceeding at full speed. When we disrupt one or more of those processes with a POST herbicide, the negative impacts on plant growth are usually faster and more complete. Conversely, a plant that is growing very slowly, or not at all, due to drought, cold, heat, etc will have all those same physiological processes occurring at a very slow rate and symptom development may be very slow and possibly incomplete if there is sufficient time for the herbicide to be metabolized or if it does not move to other parts of the plant.
Drought stress can reduce efficacy of POST herbicides due to physical changes to the plant architecture, leaf surfaces, and spray droplet behavior. When plants are subjected to sufficient moisture stress, leaves can droop which can change the angle at which spray droplets are intercepted. This can lead to a greater proportion droplets bouncing off the leaf or running off after they are deposited on the surface. This can be mitigated to some degree by timing POST herbicide applications within a few days after an irrigation event so that plants are fully hydrated and by selecting appropriate adjuvants to reduce droplet surface tension.
Second, plants grown under hot, dry conditions typically produce a much thicker cuticle with a higher proportion of waxy constituents. The plant cuticle functions to keep water inside the plant from being lost to evaporation; however, it also functions as a barrier to the spray droplets applied to the leaf surfaces. This is usually mitigated with spray adjuvant selection; often times more non-polar (wax-loving) adjuvants such as COC or MSO can be used to solubilize the surface waxes and increase absorption. Additionally, higher application rates are often used to compensate for the reduced absorption efficiency. Morning applications rather than afternoon or evening applications may also help overcome this problem to some degree as the plant is usually as hydrated as it can be at that time and will have a slightly more open cuticle due to cracks and channels that may close as the plant dries later in the day.
Finally, dry conditions can greatly impact the spray droplet performance itself. Droplets are subject to evaporation between the time they leave the spray nozzle and being absorbed by the leaf surface – this is much more dramatic under hot, low humidity conditions. Drying of the spray droplet in the air can increase the chances for drift as larger droplets become smaller. More importantly, the droplets may dry very quickly once they are on the leaf surface leaving insufficient moisture to facilitate transfer of the herbicide (which is usually a crystalline form) from the droplet into the plant. Ways to overcome this problem include spraying in the morning when humidity is higher and temperature is lower; add higher rates or use a high oil surfactant to slow droplet evaporation, and avoiding silicone surfactants because they reduce droplet surface tension to increase spreading which can increase evaporative losses on the leaf surface.
Like most aspects of crop production, weed management operations are affected by moisture conditions. Weed emergence and growth, herbicide performance and longevity, and weed/herbicide interactions all can be affected by California's seasonal moisture availability as well as the dramatic drought conditions currently impacting the state. Understanding how drought can affect weed management can lead to informed decisions on technology and practices to minimize herbicide performance problems./span>