Foliar application of fertilizers and pesticides is a pretty common technique in turfgrass management. All sorts of things can affect the effectiveness of a foliar spray, including nozzle type, spray volume, adjuvant and droplet size, and the impact of many of these has been well studied. However, the retention of foliar spray on the turfgrass leaf surface has not been well studied. Because of this missing information, researchers at the University of Illinois set out to examine two different characteristics of foliar application: foliar retention and foliar retention efficiency. Foliar retention is simply the amount of the spray that stays on the leaf, while retention efficiency is the ratio of the volume that stays on the leaf divided by the volume applied.
To examine foliar retention and retention efficiency, cores of creeping bentgrass — L-93, maintained at 0.5 inch (1.3 cm) — were harvested and taken to the greenhouse. A yellow food dye (tartrazine) was used as a tracer, allowing quantifiable measurement of foliar retention. Using this tracer to measure the retention of foliar sprays, a series of experiments were conducted. These experiments were: 1) different spray volumes (10 to 120 gallons/acre; 95 to 1,125 liters/hectare) with either flat-fan or air-induction nozzles; 2) simulated dew volume (100, 200 or 400 gallons/acre; 950, 1,900 or 3,800 liters/hectare), estimated from real-life dew collection, followed by application of a nonionic surfactant; and 3) one rate of dew (208 gallons/acre; 1,950 liters/hectare), followed by adjuvant treatments of either a nonionic surfactant or methylated seed oil at 0.25% or 0.75% v/v (spray volumes of 190 or 750 liters/hectare; 20 or 80 gallons/acre). All these various treatments were applied, cores air-dried for one hour, green tissue removed and then analyzed for the tracer (via spectrophotometer).
Foliar retention of the spray increased as spray volume increased, indicating that, regardless of spray volume, most of the foliar application stayed on the leaf. This showed that the bentgrass could capture increasing spray volume. The retention efficiency did decrease as spray volume increased, and at a spray volume of 80 gallons/acre (750 liters/hectare), retention on the leaf was 85% of the spray applied, while the rest of the spray was found in the thatch/mat layer. Best retention efficiency was at the lowest spray volume, with 98% of the spray retained on the leaf (10 gallons/acre; 95 liters/hectare). Nozzle type did not affect foliar retention, but the flat-fan nozzles provided higher spray uniformity.
The addition of adjuvants/surfactants increased retention efficiency by 90% to 94% of the foliar spray applied, regardless of spray volume (up to 120 gallons/acre; 1,125 liters/hectare). The type of adjuvant had no effect. Thus, using adjuvants increased foliar retention efficiency. And what about the presence of dew? When dew was heavy (400 gallons/acre; 3,800 liters/hectare), retention of foliar spray was reduced by 11%, indicating that runoff of the foliar spray could be more likely when dew is heavy. When dew was not as heavy, it did not affect retention of the foliar spray.
So, from this work, what are some recommendations to optimize retention of a foliar spray on a turfgrass leaf? First, best foliar retention efficiency (keeping more of your spray on the leaf) is achieved at lower spray volumes. Second, the addition of adjuvants can improve the foliar retention, regardless of spray volume. And third, the presence of heavy dew can reduce retention efficiency of foliar spray.
Source: Zhang, P., and B. Branham. 2019. Measurement of foliar spray retention on creeping bentgrass. Weed Technology 33:827-832.
Editor’s note: Read all of Beth Guertal’s recent Verdure columns.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 21-year member of GCSAA.