ISSUE 9 June 28, 2001
WEED COMPETITION IN SOYBEAN
Wet weather may prevent or postpone POST soybean herbicide application at the correct weed stage. The question is asked, "How long before weeds begin to reduce soybean yield?". Another way to word the question maybe, "How long can weeds and soybean exist together before soybean yield loss occurs?". In a literature review published in WSSA Reviews of Weed Science Vol 3, 1987, pages 155- 181 weed scientist summarized most interference experiments in soybean published to that date. They and other studies since have found that on a per plant basis weed interference in soybean from greatest to least was:
Obviously, there is never just one plant of particular species in a field. Weed populations vary from light to heavy and exert comparable competition to level of infestation. Foxtail is a good example. Competition from a single foxtail plant is low but foxtail populations as high as 200 to 400 plants sq. ft can occur at emergence and exert very high competition if not controlled.
Rarely will fields have a monoculture of just one weed species. Rather a variety of weeds will compete with the crop. In such cases, the impact of weeds are additive. Weather can influence competition with the crop. Kochia becomes much more competitive in dry rather than wet conditions. Late or early emergence is another factor. Early emerging weeds like wild oat, cocklebur, kochia, and smartweed can have a different affect than late germinating weeds like ragweed and redroot pigweed.
In most situations, soybean yields are reduced only slightly when weeds are present for up to 3 weeks after soybean emergence (WAE). Many weeds, including biennial wormwood, do not reduce yields for 4 to 6 WAE. However, rate of yield loss increases rapidly when weeds are present longer. Weeds present for 12 WAE reduce yields similar to entire season competition. Data suggests that soybean can recover almost completely from a few weeks of interference but cannot recover from more than 12 weeks of weed presence, particularly at high densities.
Studies conducted where weeds are planted at various times after soybean emergence also show interesting results. Soybean yield loss became almost minimal when weed emergence was delayed for 3 weeks after soybean emergence. If soybeans get a head start over most weeds, weed interference was minimal.
It is clear that weed species, weed density, and duration of weed interference are interrelated in a rather complex way and are major factors associated with competition with soybeans. Cultural practices such as crop cultivar selection, row spacing, planting date, crop rotation, tillage, and herbicide usage, directly affect the extent to which weeds interfere with a crop. In addition, climatic factors and geographic location may act directly on interference and subsequently influence the cultural practices employed for a given production area.
NDSU Extension Weed Specialist
ADJUVANTS INFLUENCE HERBICIDE EFFICACY AT LOW SPRAY VOLUMES
Low spray volumes provide savings in time to fill sprayer tanks and travel to and from fields without any additional chemical costs. Herbicides can be applied in spray volumes as low as 2.5 gpa with conventional ground spray equipment fitted with low output nozzles. However, research at NDSU has shown that for several POST herbicides, the adjuvant rate is critical for maintaining excellent weed control at low spray volumes. Herbicide labels often require adjuvants to be added to the spray mixture at a percent-of-spray-volume rate. Herbicide efficacy may be adequate at high spray volumes, but the amount of adjuvant in the spray deposit at low spray volumes likely will be insufficient for optimum herbicide absorption.
Raptor, Accent, and Achieve with methylated seed oil at 1.5 pt/A were equally or more effective when applied in 2.5 gpa compared to 10 gpa spray volume. However, efficacy of Achieve and Accent with methylated seed oil at 1% v/v increased with spray volume. The amount of oil in the spray deposit in low spray volume was probably insufficient for proper Achieve and Accent absorption. Absorption of concentrated Achieve and Accent with methylated seed oil at 1% v/v may have been limited at 2.5 or 5 gpa spray volume due to the increased herbicide-to-oil ratio as spray volume decreased. High oil concentration in the spray deposit also may have increased the deposit drying time, which was perhaps necessary to facilitate absorption of the concentrated herbicide.
Raptor generally was more effective in 2.5 than 10 gpa regardless of adjuvant or adjuvant concentration, but always gave greater grass control when with methylated seed oil at 1.5 pt/A than 1% v/v. The high Raptor concentration at low volumes and high methylated seed oil amount were positive to Raptor efficacy. The high Raptor concentration at 2.5 or 5 gpa spray volume was likely important to maximize absorption of Raptor, a relatively water-soluble herbicide. Glyphosate, a highly water-soluble herbicide, is also known to perform better as spray volume decreases. Several researchers have reported that the high glyphosate concentration in the spray deposit enhances glyphosate absorption.
Accent applied with 0.25% v/v nonionic surfactant was more effective at 10 than 2.5 gpa, but with a basic blend adjuvant at 1 or 2% v/v was equally effective at both spray volumes. Components of the basic blend adjuvant include nonionic surfactant, pH buffer, and ammonium salt. These components adequately facilitated Accent retention and absorption when applied at 1% v/v in 2.5 or 10 gpa spray volume.
Achieve efficacy increased as spray volume increased when applied with 0.5% v/v Supercharge containing methylated vegetable oil plus phosphate buffers. The enhanced herbicide efficacy at high volume with adjuvant rate based on percentage-of-spray-volume was possibly from greater amount of adjuvant applied in high volume. Yet, Achieve efficacy was greater when applied in 2.5 or 5 than 10 gpa when the amount of methylated seed oil was constant at 1.5 pt/A. The concentration of methylated seed oil at 1.5 pt/A would double with each decrease in spray volume from 10 to 5 to 2.5 gpa (1.9% to 3.8% to 7.6% v/v). The high oil concentration for 2.5 and 5 gpa may have increased efficacy by increasing spray retention and absorption.
Assure II, Select, Poast, Pursuit, Assert, Everest, and Discover are other herbicides that performed well at 2.5 gpa, based on NDSU research, provided the proper adjuvant and adjuvant amount was used. Herbicide efficacy at low spray volumes can usually be maintained or improved if adjuvants are applied on an area basis rather than a percentage-of-spray-volume. Additionally, low output nozzles are required for application at 2.5 or 5 gpa spray volume so spray drift may be a concern. However, low-drift sprayer nozzles are readily available and can be utilized to apply herbicides at low spray volumes. NDSU research has shown that several herbicides can be effectively applied at 2.5 or 5 gpa using low-drift nozzles without sacrificing weed control.
The recommended spray volumes stated on the herbicide label should be followed at all times. Unlike applying reduced herbicide rates in which the grower assumes the risk but is a legal application, using spray volumes lower than what the label states would be considered an off-label application. Carefully read and follow the label at all times.
Dr. Brad Ramsdale
NDSU Weed Science, Application Technology
CLARIFICATION OF PLATEAU SECTION 18
Preliminary research has shown good leafy spurge control for more than two years from a spring applied application of Tordon at 1 pt/A + 2,4-D amine at 1 qt/A + Plateau at 4 fl oz/A + MSO adjuvant at 1 qt/A + 28% N at 1 qt/A. However, research has shown that Plateau applied without other herbicides gives greater leafy spurge control when applied in late summer or early fall and the Section 18 directs the use as such. The question has arose is Plateau can be applied in the spring. Below is a clarification of the federal and Section 18 label for use of Plateau for leafy spurge control.
Allows use of Plateau in sping or fall at rates up to 12 fl oz/A on noncropland. Plateau can be applied to CRP (no grazing or haying) at a maximum rate of 4 fl oz/A. Plateau cannot be used on any area where grazing or haying will occur.
Label recommends for best results to apply in late summer or early fall (August through mid-October). However, spring applications are not prohibited.
ND Section Label:
Plateau may be applied to areas to be grazed or hayed such as pasture, rangeland and CRP at rates of 8 to 12 fl oz/A. NOTE: The Section label is effective from August 1 through December 31, 2001. Plateau cannot be applied before August 1 to any area to be hayed or grazed.
The spring application of Tordon + 2,4-D + Plateau + MSO adjuvant + 28% N can only be applied in noncropland! Treated areas cannot be hayed or grazed. Technically, spring applications made in noncrop areas can not be grazing indefinitely. BASF is seeking label amendment to allow grazing and/or haying but the registration has NOT been approved.Plateau does not control Canada thistle.
CANADA THISTLE CONTROL IN CRP
The NRCS has issued notification to land owners of their responsibility to prevent spread of noxious weeds, including Canada thistle, in CRP. Some may choose multiple mowing/ clipping as a means to prevent seed formation because it is the least expensive option for the CRP to still be enrolled in the program. Mowing/clipping will not injure legumes where chemical application may injure or kill legumes. The NRCS has issued policy concerning control of noxious weeds in CRP and allow noxious weed control in grass/legume plantings. Refer to Paragraph T16 on page 109 of the 2001 ND Weed Control Guide for the policy. Mowing/clipping does absolutely nothing to kill roots or prevent underground root expansion. Despite mowing the root system may continue to increase in size within thistle patches.
A few strategies can be used to not only control top growth but also kill roots. These strategies will cost more but results will be much greater.
#1. Apply Redeem prior to thistle bud stage. Redeem contains two products one of which is clopyralid (Stinger) and cost approximately $65.00/gal. Rates range from 2.5 to 4 pts/A. NOTE: You get more clopyralid in Redeem for your dollar than any other product on the market, including Curtail or Curtail M. Cost is $20 to $32.50/A.
#2. Use the Hunter Method or Rosette Method for Canada thistle control. Control thistle until the latter part of July when day length is less than 15 hours. See Paragraph T2 on page 107 of the 2001 ND Weed Guide for more information. Apply a relatively inexpensive but effective top growth control product like 2,4-D amine at 1 to 2 qt/A prior to thistle bud stage. 2,4-D controls top growth slowly. New shoots will emerge after day length is shorter than 15 hours but will not bolt but remain in the rosette stage. Apply Redeem to rosettes in late September or early October. Excellent root kill will result and thistle patches will controlled if not be greatly reduced in size. Followup application will be required in successive years. You cannot kill perennial weeds with one application of any herbicide - except maybe 2 gallons of Tordon/A.
#3. Some have asked if the Rosette method can be used with mowing/clipping. The strategy would include multiple clippings until late July then spray Redeem in late September or October. The key to making the Rosette Technique work is controlling ALL top growth until day length is shorter than 15 hours. When new growth emerges it senses short day length and will respond by not bolting. With mowing, bolted plants are clipped but basal leaves continue to sense day length. New growth emerging after late July are connected to plant roots that have been preconditioned by the long day length and established thistle patches may not respond to Redeem or other herbicide the same as using #2 above. Therefore, success may not be as great. NDSU does not have data to support these assumptions from clipping followed by herbicide application in CRP.
In short, clipping followed by herbicide application will give better control that just clipping alone. Any control strategy other than clipping and included with clipping is better than clipping alone. Wet years and mild winters continue to allow Canada thistle to increase in establishment and expansion of patch size. It appears Canada thistle has become one of ND worst weeds. Areas of little disturbance, like CRP, are fertile grounds for noxious weed establishment. However, land owners are also stewards of their land and should manage their land to prevent weed establishment that can infest surrounding areas. Your neighbors will certainly appreciate it.
Redeem does not control leafy spurge.
ABSINTH WORMWOOD IN CRP
Absinth wormwood is a noxious weed of ND and requires control in CRP. Page 52 of the 2001 ND Weed Control Guide contain information for most effective control. The most cost effective treatment is 2,4-D at 2 qt/A. NOTE: Application should be made when plants are AT LEAST 12 inches tall and actively growing . Herbicides applied in late-June to mid-August have given greater residual control than spring or fall application. Plants can be mowed in early to mid-summer to promote active regrowth prior to fall treatment.
NDSU Extension Weed Specialist