- Author: Brad Hanson
Happy New Year!
A quick post today to share an article I wrote last fall for Western Fruit Grower (Nov/Dec 2012) It was in a section called "California Nut Report" which is about California nuts, not by them.. I think). The online, and professionally edited version of the article can be found here.
Nut harvest is over and, barring another dry winter, the fall rains should be starting up in the western tree nut production regions. Now is the time when nut producers and their pest control advisors should be developing or fine tuning weed management programs for the upcoming growing season.
Over the past decade, many tree nut growers shifted to weed control programs based on postemergence (POST) herbicides such as glyphosate. Although this strategy has been fairly cost effective in the short term, the need for multiple POST treatments, shifting weed populations, new herbicide registrations, and the evolution of glyphosate-resistant weeds have increased interest in residual herbicide programs in recent years.
Why consider a residual herbicide program?
Residual herbicides (also known as preemergence (PRE), or soil-applied herbicides) are usually applied in the orchard before weeds are present; the material persists in the soil at high enough concentrations for some period of time to control weeds that germinate after the treatment. Conversely, herbicides with primarily POST activity control only weeds that are present at the time of application and do not affect subsequent flushes of germinating weeds. A few important orchard herbicides have both PRE and POST activity.
Residual herbicides, typically applied in the late fall or early winter and incorporated into the soil by winter rains, can provide an important start for a season-long orchard weed management program. With appropriate product selection, application rates, and tankmix partners, residual herbicides often can extend control of winter and summer annual weeds well into the growing season.
There are several benefits to a chemical weed management program that includes both PRE and POST herbicides either in a tank mix or sequential application during the year. Most herbicides with PRE activity are taken up by the roots or shoots of very small weed seedlings shortly after germination and affected weeds usually fail to emerge or die shortly after emergence. Because any weeds controlled at the seedling stage by the residual herbicide do not have to be controlled later by POST applications, residual herbicides can sometimes reduce the number of POST applications needed. Additionally, the efficacy of later POST applications is often improved following PRE applications because of better spray coverage in less dense weed canopy. This is especially important for POST herbicides that are not translocated (ie. “contact” herbicides). PRE herbicides applied during the late fall or early winter when orchard access is more certain, also can help keep tree rows clear of weeds during periods where timely POST applications are delayed due to weather or soil conditions later in the winter.
Reducing the number of POST applications applied to the tree rows may increase crop safety due to fewer opportunities for non-selective POST herbicides to come in contact with green bark or foliage due to direct application or drift. With the exception of the grass-specific herbicides, POST products registered in orchards have little inherent crop safety; instead, safety depends on placement to minimize tree exposure. Because crop safety is dependent on placement, low hanging branches, rootstock suckers, trunks with green or thin bark, and herbicide drift all can lead to tree injury from POST herbicides.
The recent increase in herbicide resistant weeds is one of the most important reasons to consider a residual herbicide program. The primary recommendation for managing herbicide-resistant weeds is to include multiple herbicide modes of action (MOA) in rotation or in sequence. Because many of the currently available residual herbicides have different MOA than commonly used POST products, residual herbicides can help manage existing herbicide-resistant populations and reduce selection pressure for addition resistant species. Very importantly, there are no PRE products with the same MOA as glyphosate; thus including an appropriate residual herbicide can help reduce the impact of glyphosate-resistant weeds in tree nut orchards. A listing of common orchard herbicides and mode of action information can be found at: http://wric.ucdavis.edu/PDFs/herbicide_registration_on_horticultural_tree_and_vine_%20crops_2012_01.pdf
Things to think about when selecting residual herbicides:
When considering residual herbicides for nut orchard weed management, it is important to keep a few things in mind in order to optimize efficiency.
Herbicide selection: Just like POST herbicides, residual herbicides have different spectrums of activity and should be selected based on the weeds known or suspected to be in the orchard. Because PRE applications are made before the weeds are actually emerged, a weed monitoring program and record keeping is even more important. Knowing what weeds have been problematic in the past will help with herbicide selection and will help identify new weed patches while they are small so that management plans can be adjusted accordingly.
Application rate: The application rate of a residual herbicide can have significant impact on the length of time weeds are controlled. Higher rates generally provide longer duration of weed control efficacy; however, this can be greatly affected by seasonal differences in rainfall or other factors that affect microbial or chemical degradation of the herbicide. Many residual herbicides have a range of rates on the label and, in some cases these vary among soil types, organic matter content, or rainfall regimes. Because residual herbicides bind more strongly to clay and organic matter, application rates may need to be adjusted lower in light, sandy soils or higher in clays or soils amended with compost, ash, or other organic amendments. Read the label for these recommendations.
Incorporation: PRE herbicides work on newly germinated seedlings in the shallow layers of soil. To be effective, these materials need to be uniformly applied to the soil and then incorporated into the top inch or two of soil within a reasonable amount of time. However, the amount of time an herbicide is stable on the soil surface can vary considerably among herbicides with different propensity to volatilize or degrade in sunlight. Some herbicides must be incorporated almost immediately to avoid significant losses in efficacy while others are stable for several weeks. However, even if the compound is stable on the surface, it may not control weeds that emerge from below the treated zone. Consult the product labels for information on incorporation requirements.
Crop residue: Crop residues on the soil surface or disturbance of the soil after application can impact residual weed control. Many growers apply residual herbicides before leaves are dropped or before pruning operations to ensure that the herbicide reaches the soil surface and is not tied up or blocked by crop residues or other debris. This is a greater problem in crops with heavy residue or persistent leaves, but can affect almost any deciduous tree crop at times. Some growers use a blower right ahead of the sprayer to ensure that residual herbicides reach the soil.
Tank mixes: Residual herbicides usually control only very small seedlings at or before emergence. If weeds are present at the time of application, most PRE herbicides will need a POST herbicide as a tankmix partner to control the existing weeds. Tank mixes can also be used to broaden the spectrum of weeds controlled as well as to provide greater opportunities to control known or suspected herbicide-resistant species.
Leaching and runoff: Because residual herbicides are applied to the soil and persist for several months, there can be serious concerns about movement into ground or surface waters. Some residual herbicides are relatively weakly bound to soil while others are more strongly bound and less likely to leach or move off site.
Young trees: Care should be taken in young orchards and when planting replacement trees in established orchard treated with residual herbicides. If herbicide treated soil is used to backfill planting holes or if large cracks or soil settling allows residual herbicides to contact the young trees, significant injury can sometimes occur. Make sure that untreated soil is used to fill planting holes and that the soil has settled before making applications of residual herbicides.
Perennial weeds: While seedlings of annual and perennial weeds often can be controlled with various PRE herbicides, very few residual herbicides will provide satisfactory control of established perennial weeds. In particular, established field bindweed, nutsedges, johnsongrass, and bermudagrass will not be controlled with these materials and a tankmix partner will likely be required.
Cost: When compared to low-cost POST herbicide programs, some residual programs can be considerably more expensive. However, when considering the relative costs of residual programs, remember to include cumulative costs of each POST application (herbicide, adjuvants, time) required to provide season-long weed control. Additionally, also consider the longer-term benefits of reducing herbicide-resistant weed populations or shifts to tolerant weed species. In the long run, the cost differences may not be as great as it may appear at first glance.
Current residual herbicide research:
As a part of our ongoing research in orchard and vineyard crops, we conduct field trials each year to evaluate residual herbicides alone and in various tankmix combinations. Many of our recent residual herbicide comparisons have included: flumioxazin, indaziflam, isoxaben, oryzalin, oxyfluorfen, pendimethalin, penoxsulam/oxyfluorfen, and rimsulfuron. Several of these materials have provided good to exceptional weed control for two to six months after treatment in some cases depending on the weed spectrum present. Other recent trials have focused on late applications of tank mixes of residual and POST herbicides to control existing weeds while extending residual control further into the season. This work has highlighted the importance of selecting the right tank mix partners for the weed spectrum present in a given orchard.
While a complete rundown of our research results is not possible here, this information is routinely presented to California tree nut growers and pest control advisors at UC Cooperative Extension meetings and field days around the state, at industry-sponsored events, at the California Weed Science Society annual meeting, and at the annual UC Weed Day. Many of these results are also available online at:
- the UC Weed Science blog (http://ucanr.org/blogs/UCDWeedScience/)
- the UC Weed Research and Information Center (http://wric.ucdavis.edu/)
- or at my UC Davis lab web page (http://www.plantsciences.ucdavis.edu/plantsciences_faculty/hanson/main/Recent%20Outreach.html)
Developing a tree nut orchard weed management program that includes residual herbicides can provide excellent broad spectrum weed control. Including residual herbicides with different MOA than your POST herbicides can help manage glyphosate-resistant weeds in tree nut orchards as well as increase management flexibility. If you haven’t already, consider including residual herbicides as a part of an integrated weed management program.
Photo caption: A herbicide screening trial in a non-bearing almond orchard treated in early January 2011. The middle plot is a glyphosate-only program whereas the foreground and background plots had residual herbicides. The photo was taken four months after the January treatments./span>