The Importance of Adjuvant Use with Alfalfa Herbicides

Transcription

1 The Importance of Adjuvant Use with Alfalfa Herbicides Mick Canevari, University of California Cooperative Extension San Joaquin County Introduction Postemergence foliar applied herbicides are used extensively in seedling and established alfalfa to control broadleaf and grass type weeds. Many of these herbicides require an adjuvant be added to the spray mixture to improve their performance. Today, the market place is inundated with more than 400 agricultural adjuvants made from organic substances to synthetic compounds and ranging in price from a few cents to hundreds of dollars per gallon. There is a lack of product knowledge and conflicting information available for adjuvants. More field tests are needed to help define the best adjuvant for the herbicide as to reduce herbicide rate and lower cost to the grower. Adjuvants are necessary because they: Increase spray deposition Reduce spray run off Enhance absorption & uptake Correct mixing problems Minimize evaporation Postemergence Herbicides Four postemergence herbicides were evaluated in alfalfa in 98-: Pursuit herbicide was registered in alfalfa in 97. It is a newly developed family of chemistry that translocates into the weed through the leaf and inhibits the development of amino acids. This class of herbicide has shown significant response with the use of adjuvants in the spray mixture. Reports from other researchers have also demonstrated improved weed control with the use of different adjuvants over the standard non-ionic surfactants (Figure 1). Pursuit effectiveness for weed control depends on the amount of active ingredient that can move into the plant as quickly as possible with the adjuvant that best complements this action. Figure 2 illustrates the effect of various adjuvants had with Pursuit to control Wild Radish and the cost per acre of each treatment. 2,4-DB or Butyrac (Figure 3) has long been a popular herbicide choice widely used in various alfalfa growing regions to control broadleaf weeds. This Phenoxy based herbicide acts as a plant growth hormone to kill weeds. In broadleaf weeds 2,4-DB is broken down to 2,4-D, a commonly known and widely used herbicide in cereal grain crops. When an adequate amount of phenoxy herbicide accumulates into the plant it causes a rapid regeneration of cells without division, resulting in abnormal growth, plant twisting and 01 Proceedings of the California Weed Science Society (Volume 53)

2 eventual collapse. Weed researchers have also noted the benefit of using non-ionic surfactants to improve weed control with the amine formulation of 2,4-DB. Figure 3 illustrates effect of adjuvants on Wild Radish control. Prism herbicide is the most recent addition to gain California alfalfa registration in September 98. Prism controls a wide range of grass weeds, including perennials and annual bluegrass at lower rates than herbicides of similar chemistry. Its systemic mode of action also requires it to move through the leaf tissue and be carried by the plant phloem to sites of action. Prism being a weak acid herbicide greatly benefits from the addition of an adjuvant for penetration and to modify the ph of the spray solution (Figure 4). Poast is the same chemistry family as Prism with the same mode of action. It has been used for many years in both seedling and established alfalfa for control of annual and perennial grasses. It has no effect on broadleaf plants and as in the case of other translocating herbicides, needs to penetrate the leaf surface quickly with maximum uptake for the best control. Poast was one of the first postemergence type herbicides to show improved performance when mixed with a crop oil concentrate adjuvant and ph modifier. The use of oil adjuvants and a ph adjustment helped increase weed kill by 40% (Figure 5). Adjuvants Four adjuvant types were used in these tests. It was difficult to select which adjuvants of the hundreds available to choose from that would fairly represent each adjuvant class. Ultimately the decision was based on convenience and commercial availability of the product. Methylated or Esterfied Seed Oils MSO/ is one of the newest additions of adjuvants used in California. Derived from plants of either soybean, cotton, sunflower, canola or corn, these oils often result in better weed control with certain herbicides compared to paraffin oils or surfactants. They have become popular with the newer herbicides in the Sulfonylurea and Imidazolinone (Pursuit) chemistry. MSO s have proven superior in warm temperatures, lower humidity and windy conditions that favor rapid evaporation of the spray solution. MSO use rates range between one pint and one quart per acre in most cases. Non-ionic surfactants (surface-active agents) or have long been used as the standard spreader and wetter adjuvant. They reduce the surface tension of a spray solution in order to allow a more intimate contact between spray droplet and leaf surface. They help the herbicide spread and penetrate through the leaf cuticle more efficiently. They are recommended for most foliar applied herbicides. Their use rate is measured on a volume basis between ¼% to ½% VV. Crop oil concentrate or adjuvants generally contain 80% non-phyto toxic paraffin based oil and % surfactant. s are compatible with oil soluble herbicides and the addition of the surfactant complements the spray solution to spread better, lower water surface tension and speed penetration into the leaf. The oil also reduces evaporation and crystallization of the spray solution under hot temperatures. The use rate of most products is one quart per acre. 01 Proceedings of the California Weed Science Society (Volume 53) 93

3 Organosilicone surfactants or silicones are the newest of adjuvants and rapidly gaining popularity with certain pesticides. Silicone surfactants reduce the surface tension of water based sprays far beyond that of and thus been given the nickname Super wetters and spreaders. In tests they have shown to be superior in coverage, particularly when in contact with highly pubescent or rough leaf surfaces difficult to reach with oil adjuvant alone. They have also shown faster penetration times through the cuticle and into stomata openings than oils. One hundred percent silicone surfactants are still open to debate for use with certain herbicides such as Roundup. The super wetting properties can be a disadvantage if the spray solution becomes too thin (higher rates) and runs off the plant before maximum absorption and chemical uptake is reached. Under hot temperature, silicones are subject to faster dissipation than oil based adjuvants reducing herbicides maximum uptake. The spray solution must fall between a ph of 6 to 8 for the silicone surfactants to remain stable and perform best. The use rates are measured on a volume basis and range between 1/8% to ¼% VV. Fertilizers used as adjuvants are increasing in interest. Ammonium sulfate, or Ureanitrate (UN-32) added to the spray solution can increase herbicide effectiveness. Researchers are still uncertain as to the precise interaction of the fertilizer and herbicide but suggest it reduces crystallization of herbicides, helps herbicides penetrate the leaf cuticle, and modifies the cell environment for herbicides such as Poast, Prism or Pursuit to work more effectively. Summary Adjuvant types showed a significant difference on weed control with Pursuit, Poast and Prism herbicides. 2,4-DB did not show a difference among adjuvants for weed control but a difference was noted for crop injury in seedling alfalfa. (Figure 6) Pursuit performed best when mixed with the oil adjuvants either the MSO or types. Improved activity was seen with the combined organosilicone and MSO product. The surfactant only equaled the oil adjuvants when a fertilizer was added into the solution; this treatment was also the most cost effective. The Organosilicone treatment alone was inconsistent in performance showing dramatic differences in weed control for the two years. Prism and Poast herbicides performed best in both years with the Oil adjuvants. There was little difference between the MSO and the although the trend favored the. The showed a 10%decline in control of Yellow foxtail compared to the Oils. Adding a fertilizer solution to the did increase weed control. The Organosilicone had a negative effect and reduced control by 35%. Conclusion Adjuvants play an important role in weed control with poast emergence foliar applied herbicides. They modify negative environmental effects unfavorable for optimum herbicide performance. They generally improve overall weed control and are particularly important in aiding the more difficult to control weed species. By improving efficacy a reduced rate of herbicide can often be utilized to help reduce cost. However; adjuvants alone will not substitute the importance of proper timing and spray coverage of weeds Proceedings of the California Weed Science Society (Volume 53)

4 Adding Ammonium based fertilizer solutions appears to be a positive component to the spray solution were it was tested. The minimal cost for these commercial grade fertilizers with widespread availability strongly supports their use whenever possible. The chart below highlights the adjuvant types best suited for the herbicides tested. Adjuvant Pursuit 2,4-DB Prism Poast MSO X X X X X X X Silicone + fertilizer X X X Figure 1. Figure 2. A d j u v a n t C o m p a r i s o n o f P u r s u i t H e r b i c i d e I n N o r t h D a k o t a Pursuit UAN + Micronutrients Petroleum oil + UAN MSO/ + UAN MSO + + UAN MSO + Organosilicone MSO + Drift retardant Adjuvant blend Pigweed % C o n t r o l Cocklebur Foxtail Richard Zollinger NDSU 97 C o s t o f C o n t r o l w i t h P u r s u i t H e r b i c i d e F o r W i l d R a d i s h MSO + OS + UN32 Rate.063 lb/a.094 lb/a Source: M. Canevari 98, 01 Proceedings of the California Weed Science Society (Volume 53)

5 Figure 3. C o s t o f C o n t r o l w i t h 2, 4 - D B H e r b i c i d e f o r W i l d R a d i s h Rate.75 lb/a 1.5 lb/a Source: M. Canevari 98, Figure 4. C o s t t o c o n t r o l Y e l l o w F o x t a i l w i t h P r i s m H e r b i c i d e MSO MSO + OS + UN32 Rate.125 lb/a.25 lb/a Source: M. Canevari Figure 5. Figure 6. C o s t t o c o n t r o l Y e l l o w F o x t a i l w i t h P o a s t H e r b i c i d e C r o p I n j u r y f r o m v a r i o u s a d j u v a n t s o n a l f a l f a h e r b i c i d e s Rate.28 lb/a.47 lb/a Source: M. Canevari % Alfalfa stunting 25 0 No phyto No phyto 2,4-DB Pursuit Prism Poast MSO Silicone +UN32 High rate + MSO Proceedings of the California Weed Science Society (Volume 53)