- Author: Lynn M. Sosnoskie
- Author: Rachel Long
- Author: Mariano Galla
- Author: Thomas Getts
The old saying, “Everything's fine until it's not,” comes to mind when dealing with some tough to control perennial weeds in alfalfa production during the summertime. Such was the case for an alfalfa field in the Sacramento Valley, where weed control seemed good up until mid-summer, and then it wasn't. Perennial weeds that started off small and overlooked, grew through the season, persisting through multiple cuttings, including curly dock, plantain, and nutsedge. For tough to control weeds in alfalfa fields, one needs to determine: 1) What types of perennial weeds are present, and 2) How many of them are there, to make a decision on how to manage them. These sorts of weed issues can creep up quickly in older alfalfa stands.
When growing hay, poisonous weeds should be high on any grower's radar. While plantain and nutsedge are benign, curly dock has potential to cause problems. Exposure to soluble oxalates that curly dock may contain are a low risk for renal disease for livestock because typically it is not that abundant in alfalfa hay fields. However, curly dock is also known to be able to accumulate toxic nitrate concentrations if grown under certain conditions, such as high levels of nitrogen fertilization (not typically applied in alfalfa), or frost. Regardless, high enough concentrations of any weeds in hay, poisonous or not, will lower the value and quality of the product produced, and should be addressed.
Grass weed control is relatively easy, with Poast (sethoxydim) or Select Max (clethodim) available for both annual and perennial grasses (e.g. barnyardgrass, foxtail, and Johnsongrass). Post and Select Max are both systemic herbicides which move through the plant and are more effective when the weeds are actively growing. Middle of the season applications after cutting may be more effective following an irrigation, to ensure the weedy grasses are not drought stressed, so herbicide effectiveness is maximized.
Often mistaken as a grass, nutsedge can be difficult to control any time of the year. Applications of EPTC (Eptam) in irrigation water or use of a granular formulation can give suppression of nutsedge. The liquid formulation of EPTC requires uniform metering of the herbicide into water during irrigation, which may not be effective on heavy soils. Sandea (halosulfuron) can be used for post-emergence control of nutsedge in established alfalfa. However, Sandea can cause temporary stunting and yellowing of the crop when applied during the growing season in the Central Valley. The use of herbicides in rotational crops prior to alfalfa stand establishment is important in helping to reduce populations of nutsedge.
Perennial broadleaf weed control is challenging during the summer, particularly with well-established weed populations. Chateau (flumioxazin), or Shark (carfentrazone), might be of some help by providing a burndown of the aboveground foliage for perennial weed species. While this may sound desirable, most perennial weed species will regrow from the roots, just like the alfalfa, so control would be short lived and may only last for a single cutting. Both products have labels for weed control in established alfalfa between cuttings with language directing applications to be made as soon as possible after hay is removed from the field, and before there is 6 inches of regrowth on the alfalfa stand. It should be noted that both products will also burn back alfalfa in addition to the weeds.
Raptor (imazamox) can also provide control of some broadleaf and annual grass species. The label directs applications between cuttings to be made before there are 3-inches of alfalfa regrowth. Unlike Chateau and Shark, Raptor is a systemic product which will not burn back the weeds or alfalfa on contact, but can still cause a reduction in crop growth, while controlling small weeds. Unfortunately, out of the three problematic weeds in this field, only curly dock is listed on the Raptor label and only suppression, not control, can be expected from a successful application.
Butyrac 200 (2,4-DB) could be an option for control of either curly dock, or common plantain. According to the UC IPM herbicide sensitivity chart for alfalfa, Butyrac 200 is able to partially control established broadleaf plantain. Where the label indicates, applications to small curly dock plants less than 3-inches can also be expected to provide suppression. However, potential crop injury may also be a deterrent for use in established stands. Crop safety of alfalfa to Butyrac 200 is better for seedling alfalfa as compared to established plants, which are likely to display more leaf deformation and yellowing. If applications of Butyrac 200 are made, healthy, actively growing alfalfa will be more tolerant to the applications than stressed plants. As Butyrac 200 is a restricted material, it is important to follow the regulations and best management practices for use, in order to reduce the potential for off target movement.
Should a treatment be made?
At this time of year, alfalfa foliage burn back from products like Shark or Chateau is temporary, lasting about a week; visual effects from dormant season applications with these herbicides may last a month or more. There may be a 7 to 10 day loss of growth when using these herbicides mid-season (right now). If using these contact herbicides between cuttings, either let the alfalfa grow longer before harvesting (our recommendation) or expect approximately a 10% yield loss for a scheduled 28-day interval cutting. Other post-emergence selective herbicides are either not registered for use during the growing season, or they have too long of a pre-harvest interval (PHI) to be utilized in most California systems.
Certainly, if there are poisonous weeds out there like sow thistle, the decision to treat is made easy, but if they are standard run of the mill weeds, a decision needs to be made on what is tolerable to both the consumer, and the producer. Recently germinated summer annual weeds in large enough numbers can be prime examples of when mid-season applications between cuttings may be necessary and cost effective. Treating well-established populations of most perennial weeds between cuttings may be a waste of time, as perennial weeds will be suppressed, not controlled, making crop yield loss from herbicide injury much less acceptable.
Mid-season alfalfa herbicides treatments can be further confounded by the cutting cycles. Weeds are most susceptible to chemical treatments right after they have germinated when they are small and actively growing, which is why most alfalfa weed control measures take place in the late fall or early winter. Established annual and perennial weeds, which have been cut in an alfalfa field, can be less susceptible to herbicide control, as they become ‘hardened-off' from being cut and drought stressed during the harvesting process.
The dormant season is when most alfalfa weed control applications occur targeting winter annual weeds and the first flushes of summer annual weeds. The dormant season applications can also be effective at suppressing or controlling perennial weeds in the field. Applications of Gramoxone (paraquat) and Velpar (hexazinone) are standard dormant season treatments with a wide spectrum of weed control. Seedlings of most perennial species, including curly dock, will be controlled by Velpar applications. More established curly dock plants and other perennial species can be suppressed by the dormant season treatments but are typically not eliminated. Other perennial species such as yellow nutsedge which are not active at time of application can be missed by dormant season applications.
The best time to control perennial weed populations in alfalfa is before the stand is established. Crop rotation allows the ability to target difficult to control species in crops that utilize more effective weed control practices, for example, broadleaf weed control in wheat. In fields with heavy perennial weed populations, RR alfalfa may be a good choice for producers not able to address the perennial weed problem before establishment. With conventional varieties, mid-season perennial weed control may require a trade-off between weed control and crop injury concerns. Growers need to determine 1) what types of perennial weeds are present and 2) how many of them are there? These factors will determine if a bit of injury to the alfalfa would be worth trying to clean up the field with an herbicide application.
Glyphosate was commercialized in 1974. Since then, it has become one of the most widely used and studied herbicides. According to Duke (2018b), almost 20,000 scientific publications and patents have included glyphosate as a focus; only 2,4-D surpasses it with respect to citations. The articles in the 5th issue of the 74th volume of Pest Management Science are no different; they all focus on this important (and controversial) molecule.This special issue arose from a day long symposium organized by Stephen O. Duke (University of Mississippi) and Keith Solomon (University of Guelph) that was held at the 252nd annual meeting of the American Chemical Society (Duke 2018a).
Figure 1. The cover of the 5th issue of the 74th volume of Pest Management Science, which is dedicated, entirely, to articles about glyphosate. The cover photos are based on the research article: Glyphosate resistance in Ambrosia trifida: Part 1. Novel rapid cell death response to glyphosate.
Stephen Duke (2018) updates a previous publication describing glyphosate's history and use (Duke and Powles 2008) in the first article. Of interest to the California weed science community is how consistent glyphosate use appears to be in the Central Valley between 1994 and 2014 as compared to the rest of the United States (Figure 2). Glyphosate has been a relative staple for weed control in our tree and vine systems over this time whereas the development and release of glyphosate-resistant crops (GRCs) has expanded the molecule's utility in row crop agriculture.
Figure 2. An image showing changes in glyphosate use in the US between 1994 and 2014 from the review article: The history and current status of glyphosate.
Jerry Green (2018) describes the history and discusses the future of GRCs, which currently account for more than 50% of the glyphosate applied in the world. While recognizing the influence of GRCs on weed control, both Green (2018) and Heap and Duke (2018) acknowledge that the widespread use of this active ingredient over space and time has lead to the development of glyphosate-resistant weeds. Worldwide, there are 41 weed species (24 dicots, 17 monocots) with resistance to glyphosate. In the United States, 17 weed species have documented resistance to glyphosate; in California, glyphosate resistance has been confirmed in: Amaranthus palmeri, Conyza bonariensis, Conyza canadensis, Echinochloa colona, Lolium perenne, Lolium rigidum, and Poa annua. Heap and Duke (2018) note that the development of glyphosate resistance in species with resistances to other herbicides may precipitate a management crisis for growers that rely heavily on chemical tools for weed control. Considering the rise in the numbers of species with multiple resistances and the lack of a.i. introductions, the authors advocate for the adoption of integrated management efforts (e.g. physical and cultural practices), the judicious use of effective herbicides, an evaluation of herbicide a.i.s for their suitability in additional crops, and a continued focus on advancing computing and engineering to facilitate precision weed removal.
The remainder of the articles in the issue are split between reviews and research papers. The reviews focus on: the impacts of glyphosate resistant sugar beets (Morishita 2018); the effects of glyphosate on plant disease development (Hammerschmidt 2018); and the hermetic effects of glyphosate on plants (Brito et al. 2018). The original research articles describe: a novel mechanism of resistance to glyphosate in Ambrosia trifida (Van Horn et al. 2018; Moretti et al. 2018); the effects of temperature on glyphosate absorption in Kochia scoparia (Ou et al. 2018); the spread of the glyphosate resistance trait in Palmer amaranth (Molin et al. 2018), among many others.
Considering the importance of glyphosate to crop production in California, the United States, and the world, this issue of Pest Management Science is likely to be an importance reference. It can be accessed at the journal's webpage: https://onlinelibrary.wiley.com/toc/15264998/74/5
Brito, et al. 2018. Hormetic effects of glyphosate on plants. Pest Manage Sci 74:1064-1070.
Duke, SO and SB Powles. 2008. Glyphosate: A once-in-a-century herbicide. Pest Manage Sci 64:319-325.
Duke, SO. 2018a. Glyphosate: The world's most successful herbicide under intense scrutiny. Pest Manage Sci 74:10-26-1026.
Duke, SO. 2018b. The history and current status of glyphosate. Pest Manage Sci 74:1027-1034.
Green, JM. 2018. The rise and future of glyphosate and glyphosate-resistant crops. Pest Manage Sci 74:1035-1039.
Hammerschmidt, R. 2018. How glyphosate affects plant disease development: it is more than enhanced susceptibility. Pest Manage Sci 74:1054-1063.
Heap, I and SO Duke. 2018. Overview of glyphosate-resistant weeds worldwide. Pest Manage Sci 74:1040-1049.
Molin, WT et al. 2018. Survey of the genomic landscape surrounding the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene in glyphosate-resistant Amaranthus palmeri from geographically distinct populations in the USA. Pest Manage Sci 74:1109-1117.
Moretti, ML et al. 2018. Glyphosate resistance in Ambrosia trifida: Part 2. Rapid response physiology and non-target-site resistance. Pest Manage Sci 74:1079-1088.
Morishita, DW. 2018. Impact of glyphosate-resistant sugar beet. Pest Manage Sci 74:1050-1053.
Ou, J et al. 2018. Reduced absorption of glyphosate and decreased translocation of dicamba contribute to poor control of kochia (Kochia scoparia) at high temperature. Pest Manage Sci 74:1134-1142.
Van Horn, CR et al. 2018. Glyphosate resistance in Ambrosia trifida: Part 1. Novel rapid cell death response to glyphosate. Pest Manage Sci 74:1050-1053.
The United States Geological Survey (https://www.usgs.gov/) defines drought as a 'period of drier than normal conditions that results in water-related problems.' Short-term effects of drought include: declines in surface water flows, which can impact water supplies for agricultural, urban, and environmental uses. Long term effects of drought can include aquifer overdrafts and land subsidence. California has experienced drought events in the past (most recently: 1976-1977, 1986-1992, 2007-2009, and 2011-2017) and is expected to experience many more in the future. While a changing climate and variable weather patterns are contributing to the California's water issues, a lack of new water projects, a complex water distribution network, and a complicated system of water rights also factor into the equation, magnifying the concerns of many parties.
Below are some useful web page links for both monitoring and responding to drought conditions.
The California Department of Water Resources (DWR) was established in 1956 and 'is responsible for managing and protecting California's water resources. DWR works with other agencies to benefit the State's people and to protect, restore, and enhance the natural and human environments.' The DWR manages a significant portion of the state's water supply including the State Water Project. The department's website provides information about the California Water Plan, flood management, dam safety, and groundwater protection, as well as other topics. The California Irrigation Management Information System (CIMIS) is a program unit of the Water Use and Efficiency Branch that manages over 145 weather stations throughout the state that collect publicly available data about soil and air temperature, relative humidity, precipitation, and evapotranspiration. The California Data Exchange Center, which is also housed at the DWR site, maintains an extensive collection of hydrologic data including: current river conditions, snow reports, and reservoir storage.
Current California reservoir conditions as of August 1, 2018, according to data collected and maintained by the California Data Exchange Center.
Drought.gov houses the United States Drought Monitor, a map that shows the current location and intensity of drought across the country. The site also provides drought forecasts for several months in advance. The data is updated each Tuesday and released on Thursday. Additional links provide information about the seasonal wildfire outlook and agricultural impacts and reports.
Drought conditions in the United States as of August 1, 2018. The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Lincoln-Nebraska, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map Courtesy of NDMC-UNL
The National Weather Service Climate Prediction Center provides operational predictions of climate variability over time scales ranging from a week in advance to an entire season (as is technically feasible).
Precipitation forecasts from NOAA. Predictions range from a few weeks to a year in the future.
The State of California as well as the California Department of Food and Agriculture have web pages that provide California-specific drought resources (such as links to the state water project) as well as portals to both state and federal assistance programs.
There are several meetings featuring the research of UC ANR Staff coming up this month (August 2018). Directions to sites can be obtained through the UCCE Merced Agronomy and Weed Science Upcoming Events web page: http://ucanr.edu/sites/agronomyweedscience/Upcoming_Events/
1. West Side Pest Management Update
This meeting will cover a variety of topics including vegetable insect and disease management, the biology and ecology of field bindweed, and the management of field bindweed in agronomic and specialty crops.
Date: August 15, 2018
Time: 8:00 AM - 12:00 PM
Location: UC Westside Research and Extension Center
8:30 Welcome and introduction Tom Turini, UCCE Advisor Fresno
8:35 Recent research on vegetable insect and disease management Tom Turini
8:55 Field bindweed biology and ecology Lynn Sosnoskie, UCCE Advisor Merced/Madera
9:15 Bindweed management in annual systems Scott Stoddard, UCCE Advisor Merced/Madera
9:35 Bindweed management in perennial crops Kurt Hembree, UCCE Advisor Fresno
9:55 Bindweed control discussion
10:40 Recent research on Fusarium diseases of tomato Cassandra Swett, UC Davis Department of Plant Pathology
11:10 Resistance breaking strains of TSWV and Beet curly top virus research Robert Gilbertson, UC Davis Department of Plant Pathology
2. Warm Season Cover Crops Field Day
Warm season cover crops are an underused tool in California agriculture. Their use can “add value” during a fallow “window” within a production cropping system.Field Day participants will get a close look at a replicated warm season cover crop trial at the PMC with 11 species and four cowpea cultivars after 60 days of growth.
Date: August 22, 2018
Time: 11:00 AM - 12:00 PM
Location: Lockeford Plant Materials Center
10:00 - Margaret Smither-Kopperl, PMC Manager - Welcome
10:05 - Scott Park - A Producers Experience with Warm Season Cover Crops
10:25 - Valerie Bullard, PMC Agronomist - Warm Season Cover Crops
10:50 - Gill Costa, Sentek Technologies - Moisture Sensors for Irrigation Water Monitoring
11:10 - Michelle Leinefelder-Miles and Brenna Aegerter, UC Extension - Cover crops, Soil Health and Greenhouse Gas Study
11:30 - Z. Kabir, NRCS Regional Soil Health Specialist - Timing of Warm Season Cover Crops
11:45 - Wendy Rash, District Conservationist - NRCS Programs and Cover Crops
11:55 - Margaret Smither-Kopperl - Conclusions and Questions
3. Annual California Rice Field Day
The purpose of the Rice Field Day is to give rice growers and opportunity to observe and discuss research in progress at the RES. Program highlights will include information about variety improvement, disease resistance, and insect and weed control.
Date: August 29, 2018
Time: 7:30 AM - 1:30 PM
Location: California Rice Experiment Station
7:30-8:30 a.m. Registration, posters and demonstrations
8:30-9:15 a.m. General session: CCRRF membership report, rice research trust report, California rice industry award
9:30-12:00 p.m. Field tours of rice research: variety improvement, disease resistance, insect control, weed control
12:00 p.m. Lunch
Cotton lint stickiness is a significant problem, worldwide. It is also an issue here in California, and a major focus of attention for cotton growers and ginners in the state. Sticky cotton loads can physically slow down the processing speeds of gins, even to the point of shut down. Stickiness may necessitate the special handling of contaminated bales, which can result in increased costs. The milling process can also be affected by stickiness, as work stoppages may be required to clean combs and rollers. Millers may choose to blend sticky fibers with clean ones as a way to mitigate the situation or they may elect to engage with different cotton merchants altogether in order to avoid persistent problems.
While free plant sugars (from immature fibers) can contribute to stickiness, most lint contamination results from honeydew, a sugar-rich liquid that is excreted by sap-sucking insects such as sweet potato whiteflies (Bemisia tabaci) and cotton aphids (Aphis gossypii). For more information about identifying whiteflies in Central Valley cotton, please visit this web page: http://cottoninfo.ucdavis.edu/files/133231.pdf. Droplets of honeydew can be deposited onto open bolls from these pests as they feed on the cotton plant; it is these sticky secretions that cause lint to adhere to metal surfaces throughout the harvesting, ginning, and milling process. Don't underestimate the amounts of honeydew that can be produced...results from a study conducted in 1999 (Henneberry et al, Southwestern Entomologist 24:207-231) found that each adult sweet potato whitefly feeding on cotton could excrete between 25 and 64 droplets of honeydew, every day.
The California Department of Food and Agriculture's (CDFA) Plant Health and Pest Prevention Service began their whitefly monitoring program on July 2, 2018. CDFA personnel from the Pink Bollworm Program will be regularly evaluating cotton leaf samples from pre-selected screening sites, located in 10% of all cotton fields in each county in the San Joaquin Valley, to describe changes in whitefly numbers over space and time. Reports will be posted beginning July 16th at the CDFA Pink Bollworm web page: https://www.cdfa.ca.gov/plant/ipc/pinkbollworm/pbw_hp.htm
Whitefly monitoring reports will be compiled and released by the Pink Bollworm Program.
For more information about whitefly biology and management in cotton, please visit the following University of websites: http://cottoninfo.ucdavis.edu/files/133231.pdf and http://ipm.ucanr.edu/PMG/r114300311.html, or contact the UC Cooperative Extension office. Cotton aphid control guidelines can be accessed at: http://ipm.ucanr.edu/PMG/r114300111.html. Stickiness doesn't have to be a part of California cotton production. The monitoring and management of the primary pests responsible for sticky cotton (as well as their alternate, weedy hosts, like field bindweed (Convolvulus arvensis) and black nightshade (Solanum nigrum)) can help prevent reductions in both lint yield and quality.