Irrigation is crucial for the production of melons in California. It facilitates seed germination, it is essential for crop growth and fruit production, and, for growers that apply pre-emergence herbicides, it is necessary for product activation.
Pre-plant irrigation (pre-irrigation) is used to develop an optimal planting bed for the crop, however, it can also stimulate weed seed germination. Knowing this, growers must be prepared to use pre-emergence (i.e. soil-applied, residual herbicides) or post-emergence (i.e. flaming, or foliar-applied herbicides) to reduce crop-weed competition. Early weed control is important; to maximize crop yields, young melons should remain weed-free for up to eight weeks.
The length of the interval between a pre-irrigation event and a pre-emergence herbicide application can significantly affect weed control. In 2013, our field site received irrigation 2-3 days before melon seeding. Residual herbicides were applied and then incorporated (with at least 0.5 inches of water) directly after crop planting. The timely activation of the residual products resulted in excellent weed (i.e. purslane and pigweed) control for up to six weeks (Figure 1).
Figure 1. Weed cover in melons in response to pre-emergence herbicides in 2013. Command (clomazone) = 0.5 pt/A, Curbit (ethalfluralin) and Strategy (clomazone + ethalfluralin) = 4 pt/A, Sandra (halosulfuron) = 1 oz/A, Dual Magnum (S-metolachlor) = 1.3 pt/A, Zeus (sulfentrazone) = 3.2 oz/A. NOTE: COMMAND, DUAL MAGNUM, STRATEGY and ZEUS are NOT LABELED for use in melons in California.
In 2014, melons plots were pre-irrigated 5-7 days before seeding. Similar to 2013, residual herbicides were applied and then incorporated directly after melon planting. The level of weed control observed in 2014 was significantly lower than what was observed in the preceding year (Figure 2). Reduced herbicide efficacy may have been due, in part, to the increase in elapsed time between pre-irrigation and herbicide application/activation.
Figure 2. Weed cover in melons in response to pre-emergence herbicides in 2014. Command (clomazone) = 0.5 pt/A, Curbit (ethalfluralin) and Strategy (clomazone + ethalfluralin) = 4 pt/A, Sandra (halosulfuron) = 1 oz/A, Dual Magnum (S-metolachlor) = 1.3 pt/A, Zeus (sulfentrazone) = 3.2 oz/A. NOTE: COMMAND, DUAL MAGNUM, STRATEGY and ZEUS are NOT LABELED for use in melons in California.
Pre-emergence herbicides are only active against newly germinated seedlings that that are emerging from/through the chemical barrier (top 1-3 inches of soil). In 2013, this barrier was established no later than 72 hours after the field soil was pre-irrigated; weed seedlings that were stimulated to germinate were susceptible to the residual products. In 2014, the herbicides were applied and activated up to 168 hours after the pre-irrigation event; by this time, many weeds were probably close to breaking through the soil surface and, therefore, less likely to be controlled by soil-applied products. In hindsight, the use of a post-plant, but pre-melon-emergence, burn-down treatment would have helped with season-long weed control in 2014.
Moral of the story? Apply and activate residual herbicides as soon as is reasonable after a pre-irrigation event. The longer you wait, the more likely it is that emerging weeds will be able to escape residual control measures. Weeds that do emerge should be managed so as to avoid crop injury and to maximize the potential for control. Once a residual herbicide barrier is established, avoid disturbing it!
"If you don't water in the San Joaquin Valley, you're not getting a yield," Larry Williams, a professor in the Department of Viticulture and Enology at UC Davis and based at Kearney Agricultural Research and Extension Center, told Pierson.
Last month Sacramento Bee columnist Mike Dunne used Williams' study of water use of chardonnay grapes in the Carneros Region to refute the amount of water a Dutch researcher claimed was required to produce a single glass of wine. “In California vineyards and cellars, is 29 gallons of water to produce a single glass of wine a realistic estimate?” Dunne asked Williams, who explained that California grape yields per gallon of water are much higher than in Europe.
“The mean yield of wine grapes in Europe ... is around 1.8 tons per acre using data I've gleaned from research papers,” Williams says. “The mean chardonnay yields across California are 7.4 tons per acre.”
Based on Williams' research, Dunne wrote, “Vines of the dry-farmed portion yielded 4.9 tons per acre, while vines on the irrigated portion produced 6.3 tons per acre. The upshot was that 14.2 gallons of water was needed in the dry-farmed block to produce a typical 4-ounce pour of wine, while 15.3 gallons of water was needed in the irrigated parcel to produce a 4-ounce pour of wine, totals far lower than the figure calculated by the Water Footprint Network.”
It's that time of the year when we're planning preemergence (aka "residual") herbicide programs for orchard and vineyard crops in California. Typically, these are the herbicides that are applied in the fall, winter, or early spring BEFORE weeds emerge (preemergence) and they usually affect weeds as they germinate or are just beginning to emerge from the soil. [often, people mistakenly think these herbicides kill seeds or sterilize the soil which is not actually the case].
As you're planning the specific program for the weed problems in your orchards and vineyards (or any site, really), I thought it would be a good time to review some of the concepts of residual weed control with preemergence herbicides. Hopefully the concepts and ideas presented in the following line drawings will help us think about what PRE herbicides can and can't do, and how to best use them in the orchard and vineyard production system.
Herbicide dissipation - Dissipation (or disappearance) is a word that describes both degradation processes and transfer processes. Degradation is those processes that actually change the herbicide molecule into something else. Transfer processes are those that change the availability and could include binding to soil, leaching, volatilization etc. I won't dwell on those today but here's a link to a little longer writeup on that if you're interested. For today's discussion, I'll just make the point that all herbicides dissipate in the soil environment and that this usually follows what is called "1st order" or "2nd order" degradation kinetics - basically a curved line (see figure 1 below). This means that the processes happen faster at first and then slow down over time (red dashed line). However, to more easily illustrate this (and because it's easier to draw!), I'll use straight lines instead (red solid line below).
The whole point of residual herbicides is that they persist in the soil for a period of time and affect weeds that germinate after the application (days or months later, perhaps). There is a threshold of activity for the herbicide, basically this is a concentration in soil, above which the herbicide is effective on the weed and below which it is not (green dashed line in the figure below). This threshold will vary considerably among specific weeds and herbicides, though. [I've discussed this concept previously when talking about soil bioassays]. Another important concept on figure 2 is that residual herbicides can have very different dissipation rates which can result in different persistence in the soil, and different duration of residual weed control. For example, a short residual herbicide (red line) will fall below the activity threshold (green line) well before a very persistent herbicide (blue line).
To a certain degree, the length of weed control duration with a preemergence herbicide is a function of rate. A higher rate will remain above the activity threshold for longer than a lower rate of the same herbicide (see figure 3 below). In this slide, I also added in the concept of winter and spring germinating weeds. To me, this is one of the biggest challenges of using PRE herbicides in the orchard system - we typically apply our PRE herbicides in the winter when we'll get rainfall to incorporate them but the herbicides may dissipate too fast to control the late winter weeds or the summer-emerging weeds. Higher application rates in the winter is one approach to addressing this issue.
Often times, we'll use a tankmix of two or more PRE herbicides in the winter to broaden the weed control spectrum and reduce selection pressure for herbicide resistant weeds. While this is a good approach to managing diverse weeds, it does not really do much to "stretch" the weed control duration later into the summer because the dissipation processes of the individual herbicides are really independent of one another (see figure 4 below). In this figure, you can imagine that the weed control spectrum is much broader during the winter season than if only one herbicide was used; however, by the spring, there is probably not enough of the 2nd herbicide to provide much control.
Sequential applications of PRE herbicides is another way that weed control duration might be extended later into the season. This could be a sequence of two different herbicides as shown in figure 5 below where the first PRE herbicide was applied in the winter and a second PRE herbicide was applied in late spring. This might also be done with a repeat application of the first herbicide which could make sense in some situations (for example one application to target winter grasses (eg ryegrass) and another to target summer grasses (eg junglerice).
I think integrated weed management programs that include preemergence herbicides make a lot of sense for orchard and vineyard cropping systems. They can broaden the spectrum of weed control, they can reduce selection pressure for resistance to our POST herbicides, they can minimize spray trips through the field compared to multiple applications of POST herbicides, and can save management time. Just like any management tool, though, it's important to recognize what the limitations of PRE herbicides and plan accordingly to avoid over- or under-treating in any specific orchard/weed complex.
Effective this fall (2014) there will be a fairly significant change to the Alion herbicide label for California orchard and vineyard crops. Growers and PCAs will want to be aware of this as you're planning your dormant-season herbicide programs now that many areas of the state are getting some rain.
The use patterns for Alion (active ingredient: indaziflam) has been modified for tree nuts, grapes, stone fruit, pome fruit, and olive (citrus uses were not changed).
Most important changes include:
- Maximum use rates now have a restriction based on soil organic matter (OM) content
Grapes: if soil less than 1% OM, max rate is 3.5 oz/A (0.045 lb ai) and if over 1% max rate is 5 oz/A (0.065 lb ai)
Nuts, Pome, Stone, Olive: if less than 1% OM, max rate is 3.5 oz/A; If 1-3% OM, max rate is 5 oz/A; if OM above 3%, max rate 6.5 oz/A
Previously there was a 6.5 oz rate max for all crops except grape which had a 5 oz max rate. Although it presents another thing to think about when writing recommendations, a soil OM restriction is not unusual. Because of the charge characteristics of organic matter (as clay particles), soil OM content can greatly affect the proportion of herbicide in "soil solution" - that is, herbicide that is not bound to soil. Rate refinements based on OM can avoid the situation where lighter soil is over treated and also should increase the margin of crop safety because light or coarse soils do not hold herbicides in the surface zone (where the weeds are) as well.
- The label now restricts Alion use in flood-irrigated orchards. It also prohibits irrigation within 48 hrs after the applications.
This is designed to ensure crop safety by giving the herbicide sufficient time to bind to surface soils before a large amount of water is intentionally applied. It should also help maximize weed control because any residual herbicide that is moved too deeply into the soil is likely to lose some efficacy on some weeds - this is especially true for herbicide like Alion that primarily affect weeds as they first germinate but have less of an impact on established weeds.
- The manufacturer has also offered some best use guidelines for this herbicide that are very positive (in my opinion):
- Use the highest rate for local conditions for best performance
- Consider tank mixes with other PRE herbicides such as Matrix, Chateau, Goal, GoalTender (this is good for both broadening the weed spectrum and managing selection of herbicide resistant weeds)
- Tanks mix with burndown herbicides if emerged weeds are present (this was always the case as Alion as almost no activity on germinated weeds)
- Apply from Nov-Jan, avoid spring applications for best weed control (good idea also to increase crop safety and get the greatest performance out of this chemistry)
- Soil should be free of large trash and clods at application (this is true for best performance of ANY of our PRE herbicides)
- For best weed control, rainfall or sprinkler irrigation within 3-4 weeks is ideal. If irrigation is used to activate, 0.5 inch of water is ideal (the idea is to incorporate the herbicide into the surface inch or so, where the weeds germinate, but not go too deeply. This is also pretty true for all PRE herbicides).
In my opinion, the prohibition on use in flood irrigation orchards is probably the most important change as those sites simply cannot use the herbicide - these growers will have to use other products. The soil OM restriction is much less of a problem and may actually be beneficial from both a product stewardship and resistance management standpoint if growers use tankmixes and good integrated strategies. I had several trials with a range of Alion rates in 2014 and we observed very good weed control with the reduced Alion rates in most instances but control at 2.5 or 3.5 oz/A was definitely more dependable and long-lasting when a tankmix partner (selected based on field scouting) was used in the management program.
Check out the updated Alion label on your favorite herbicide label source.
Application instructions and a full position description are available on the UC Agriculture and Natural Resources employment website (https://jobs.ucop.edu/applicants/Central?quickFind=57905).
This position is with the University of California Statewide Integrated Pest Management Program (UC IPM). UC IPM develops and promotes integrated and ecologically sound pest management programs in California (www.ipm.ucanr.edu).
The Pesticide Safety Educator works under the direction of the Pesticide Safety Education Program (PSEP) Coordinator. This position coordinates with UC ANR advisors and specialists, government agencies, professional organizations, and others to plan, develop and deliver local pesticide safety educational programs for fieldworkers, pesticide handlers, pesticide applicators, as well as other trainers of these clientele. This position supports development and delivery of programs that provide objective information about pesticide use and safety in order to reduce pesticide risks to human health and the environment. The Educator participates in program planning and supports the efforts of the PSEP Coordinator to identify priorities, engage cooperators and disseminate resources to achieve program goals. In addition, the Educator develops or assists in the development and delivery of outreach materials and training programs and also conducts systematic review of program materials to assure that they are up-to-date and meeting clientele needs.
If interested in finding out more about this position and/or to apply, visit https://jobs.ucop.edu/applicants/Central?quickFind=57905