As many of us enter into the growing season many folks require glyphosate to control unwanted vegetation. A number of times I have seen posts and comments from people regarding what they feel was poor weed control by their chemical application. The majority of the information below is taken from the Pioneer Seed Company (PSC) website. While the information on the PSC website is directed towards those applying glyphosate, it is also relevant to any herbicide that is taken into a plant via photosynthesizing tissue. There is lots to consider when reading the information below, but the cost of chemical and our time as habitat managers is valuable.
Environmental Conditions
Foliar absorption of herbicides occurs in a liquid phase only; once a water droplet has dried on the leaf surface and herbicides have crystallized little to no additional absorption occurs. Therefore, any environmental condition speeding the drying of spray droplets on a leaf surface will reduce absorption. Low humidity and high winds can greatly reduce drying time, thereby allowing little time for absorption to occur. Conversely, high humidity with little wind slows the rate of drying and lengthens absorption time. Rainfall shortly after (< ½ hour) glyphosate application can wash spray droplets from the leaf surface. A foliar application should be "rain fast" once droplets have dried on the leaf surface.
Temperature, soil moisture, and solar radiation that optimize plant growth facilitate absorption and translocation of glyphosate. When photosynthetic rates are high photoassimilate produced in leaf epidermal cells is rapidly loaded into the phloem, other organic molecules like glyphosate are similarly loaded, and both are quickly translocated to the internal portions of the plant. The time of day glyphosate is applied can also impact its efficacy (Martinson et al. 2005). Applications made between 9 a.m. and 6 p.m. tend to maximize glyphosate activity. Short-lived temperature spikes (> 90 ºF) can also enhance absorption by reducing cuticle viscosity and allowing easier passage of foliar-applied herbicides.
Tolerant Species
Although glyphosate is considered a non-selective herbicide there are a number of broadleaf weeds common to the U.S. Midwest that are somewhat tolerant. Reduced susceptibility to glyphosate or tolerance is inherent in a weed species or population and does not indicate a genetic change as resistance does. Look at publications released by your in state land grant universities and extension services for local plant populations that may be problematic to control with glyphosate alone.
Adjuvants and Use Rates
Glyphosate is the common name given to the chemical compound N-(phosphonomethyl) glycine; the molecule is a weak acid (parent acid) which can be formulated as any number of salts. Currently manufacturers of glyphosate formulate it as an isopropylamine, ammonium, or potassium salt. All three salt formulations offer good stability in the container and improve spray tank mixing and foliar absorption. Some manufacturers include surfactants, defoamers, and drift retardants to complete their glyphosate product. Products including surfactants are often said to be "fully loaded" and usually don’t require the addition of a non-ionic surfactant (NIS), however, manufacturers are not required to provide that information so the product label should be consulted.
For maximum field performance glyphosate applications should be made with ammonium sulfate and an NIS (Hartzler et al. 2006). If a glyphosate product label specifies the addition of an NIS, ensure it contains at least 80% active ingredient (a.i.) and typical use rates are 0.25% by volume. Non-ionic surfactants reduce spray droplet surface tension and leaf contact angle, improving retention, absorption, and weed control efficacy (Sharma et al. 2004). The use of ammonium sulfate is recommended by most product manufacturers; it should be added to the spray solution before glyphosate at 8.5 to 17 lb per 100 gallons of water.
Ammonium sulfate reduces the antagonistic effect of hard water on glyphosate. Water is considered "hard" when it contains various salts such as Ca2+, Mg2+, Fe2+, Na+, and Zn2+. Some of these salts are found in great abundance in rural water supplies and readily bind with glyphosate reducing its solubility, absorption, and field performance (Stahlman and Philips, 1979; Nalewaja et al. 1996). The sulfate anion in ammonium sulfate binds with the salts in hard water and precipitates them out of solution, reducing the antagonistic effect.
Glyphosate products also vary in their parent acid concentrations. For example; Roundup WeatherMax® contains 4.5 lb a.e. per gallon, while Gly-4® contains 3 lb. The standard glyphosate application rate is 0.75 lb a.e. per acre; determine the product rate by using 0.75 as the numerator and the parent acid concentration as the denominator to determine gallons of product per acre (0.75/parent acid concentration = gallons/acre * 128 = oz/acre). Rates should be adjusted for weed height by using the standard rate for weeds < 6 inches tall, and increasing the rate by 50 and 100% for weeds between 6 and 12 inches and > 12 inches in height.
Recommended spray volumes differ by glyphosate product label; minimum spray volumes range from 3 to 5 gallons per acre and maxima from 20 to 40. Research indicates that glyphosate performance improves with decreasing spray volume to rates as low as 2.5 gallons per acre (Ramsdale et al. 2003). Reduced spray volumes decrease the likelihood of antagonism with hard water and increase glyphosate concentration per droplet. Since foliar-applied herbicides move by simple diffusion, maintaining a high concentration gradient improves absorption. Ultra low carrier volumes may provide insufficient spray coverage in dense weed/crop canopies, however, and the orifice size of spray tips necessary for such volumes are easily plugged. Carrier volumes of 10 to 15 gallons per acre are probably a good range for sufficient performance under a diversity of field conditions.
I will talk more about herbicide usage in the coming weeks as I report on the conversion of some of our cool season grass hay ground here at Maple Hill Farms to a highly diverse forb mixture we are planting to replace the basically unused cool season grass areas by wildlife.
