ISSUE 10 July 6, 2000
ADJUVANT RATE INFLUENCES HERBICIDE EFFICACY AT LOW
Herbicides can be applied at 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.
For example, efficacy of Assure II plus 1.5 pt/A petroleum oil was similar when applied at 2.5, 5, or 10 gpa. However, efficacy of Assure II with petroleum oil at 1% v/v increased as spray volume increased. Petroleum oil (COC) adjuvants at 1% v/v was insufficient to maintain Assure II efficacy at low spray volumes. Accent plus 1.5 pt/A Scoil applied at 2.5 gpa provided equal control of proso millet compared to application at 10 gpa, while Accent plus Scoil at 1% v/v was more effective at 10 gpa than 2.5 gpa. Pursuit, Raptor, Poast, and Achieve are
also effective at 2.5 gpa spray volume when applied with 1.5 pt/A methylated seed oil adjuvant.
Increasing the amount of adjuvant in the spray mixture can also be beneficial for herbicide efficacy at low spray volumes. Accent at 2.5 gpa spray volume applied with 2% v/v Quad 7 provided proso millet control equal to application at 10 gpa with 1% v/v Quad 7. Accent applied at 2.5 gpa with 2% Quad 7 would require one-half the total amount of Quad 7 versus application at 10 gpa with 1% Quad 7 while achieving the same level of control. Weed control from Roundup Ultra increased as spray volume decreased from 10 to 5 to 2.5 gpa for reduced rates Roundup Ultra. Roundup Ultra is reported to have a full adjuvant load in the formulation, but the total
amount of surfactant applied per acre increased as spray volume decreased.
Efficacy of many POST herbicides can be maintained or improved when applied at 2.5 gpa, provided the total amount of adjuvant applied per acre remains unchanged. Low volume applications would save time and labor without any additional chemical costs.
Dr. Brad Ramsdale
NDSU Weed Science, Application Technology
EPA EXTENDS EXEMPTION ON HERBICIDE FOR DRY BEANS
The EPA has approved a ND request to extend the exemption on Raptor from June 30 to July 15 due to adverse weather conditions which has prevented many producers from applying Raptor during the original exemption period that ended June 30.
The exemption now allows a single ground or aerial application of Raptor at a rate of 4 ounces of product per acre through July 15, 2000. Applicators must follow all instructions, warnings and precautions on the product label and have a copy of the exemption label in their possession during application.
ISSUES REGARDING REGISTRATION OF ROUNDUP AS A PREHARVEST
AID IN DRY BEANS
Question 1: Has the GMO issue influenced the delay of registration of Roundup for preharvest application in dry beans?
Answer: No. The issue is the amount of glyphosate residue in the seed. EPA regulates the concentration of any pesticide in any grain, seed, or harvest product. EPA has set a tolerance for glyphosate in dry bean seed but U.S. studies conducted for registration exceeded tolerance by 2 to 3 times. Canadian residue studies showed much lower residues in bean seed and Monsanto is using the Canadian data for U.S. registration. U.S. residue trials in dry beans were conducted at 3 qt/A. However, current U. S. registration is seeking a maximum of 1 qt/A
resulting in less residue in dry bean seed.
This may be insignificant because many dry bean processors are establishing standards requiring a zero tolerance of pesticide residues in bean seed. So, even though a registration is likely growers that use Roundup as a preharvest application may unable to sell their dry bean seed.
Question 2: Is there zero tolerance now in dry bean seed with current registered pesticides?
Answer: Probably not. The manufacturer of each chemical used in dry beans could provide the level of pesticide residue in dry bean seed because that information is required prior to Section 3 registration. Possibly only a few herbicides have no residue in dry bean seed. An example is Clearfield (IMI resistant) sunflower. Sunflower lines are being developed that are resistant to Raptor and other Imi herbicides. American Cyanamid has not detected residues from Raptor in other crops registered so they expect none in Clearfield sunflower. Regardless, any pesticide labeled in dry beans has residues in dry bean seed equal to or less than the tolerance set by EPA.
Question 3: What kind of weed control can we expect from 1 qt/A of Roundup?
Answer: Experience from preharvest application in wheat suggests excellent control of most annual grass and broadleaf weeds (with some exceptions like nightshade and buckwheat) and quackgrass and Canada thistle. Some growers trying to reduce chemical cost may use lower rates than 1 qt/A and in many situations (not with Canada thistle and other perennial weeds) may achieve satisfactory control.
Question 4: How does the variability in dry bean maturity affect rate and use of Roundup?
Answer: This is not an issue with dry bean fields that dry down uniformly. However, this is a concern in fields with beans showing delayed maturity. As opposed to fully mature dry bean plants, dry beans with delayed maturity will continue to absorb and translocate herbicide to sinks or in the seed. Flowering and pod set in dry beans is a progressive process starting in the middle of the plant and progresses both up and down. Prior to complete maturity pods on dry beans may show a wide range in maturity and any immature pods can continue to translocate herbicide to the beans. This may cause high variability in herbicide concentration in dry bean seed. Monsanto will need to provide guidelines on the label when registration for Roundup is approved.
A common response of dry beans to stress is delayed maturity. Areas of water logged soil, sub-lethal herbicide drift, localized insect feeding, and herbicide carryover can cause delayed maturity. Growers scheduling Roundup application when most of the field has reached maturity may have areas of the field where bean plants are still green. Those green plants will translocate more Roundup to the seed and higher residues, perhaps greater than allowed, might result.
NDSU Extension Weed Specialist