Kevin R. Keel, James E. Altland, Charles H. Gilliam, Glenn R. Wehtje, Tim L. Grey, Gary J. Keever, and Donald J. Eakes

In nursery crop production, herbicides are typically broadcast over the top of the containers three or four times annually. Broadcast application results in up to 86% herbicide loss depending on plant size, habit, and container spacing. Recent reports have found elevated herbicide levels in nursery runoff water, suggesting that non-target herbicide losses may be a source of contamination for local drinking water supplies and surrounding bodies of water. Therefore, alternatives to broadcast application are needed to reduce potential problems with herbicides in nursery runoff water.

Several approaches have been evaluated to reduce herbicide loss from container-grown nursery crops, including containment ponds. This study concluded that herbicides did not accumulate in either pond sediment or water due to herbicide degradation in the containment pond. Another approach to reducing non-target herbicide loss is slow release tablets impregnated with herbicide; however, low water solubility resulted in insufficient weed control. This limitation can be partially overcome by adding a surfactant to the herbicide. More recently, several controlled-release fertilizers (CRFs) coated with Ronstar were evaluated for weed control. This technique used about 80% less herbicide than traditional broadcast applications. Weed control differed among the Ronstar-coated CRFs, suggesting that characteristics of the fertilizer influence herbicide activity.

The objective of this research was to determine Ronstar leaching rates from Ronstar- coated fertilizers and to evaluate factors affecting leaching rates. Additionally, we sought to determine if applying either a sticker or an oil to Ronstar-coated Osmocote would enhance uniformity of Ronstar leaching.

METHODS
EXPERIMENT 1. Five commonly used CRFs were evaluated: Meister 24-4-7 (Helena Chemical Co., Memphis, Tennessee), Nursery Special 12-6-6 (Pursell Industries, Sylacauga, Alabama), Polyon 24-4-12 (Pursell Industries), Osmocote 17-7-12 (Scotts Co., Marysville, Ohio), and Nutricote 20-7-10 (Florikan E.S.A. Corp., Sarasoto, Florida). Glass beads, 0.16 inches (4mm) in diameter were also coated with Ronstar to serve as a nonabsorbent control. Fertilizers were coated with commercially-formulated Ronstar (Ronstar 50WP) supplemented with sufficient radioactive Ronstar to facilitate detection. An aqueous solution of 5.0 mg ai/ml was prepared using both formulated and radioactive Ronstar. This solution [0.0164 ounces (492 microliters)] was applied to 20 grams (0.044 pounds) of each fertilizer, and allowed to air dry for 48 hours. The resulting concentration of Ronstar to fertilizer was 0.12mg ai/g.

Twenty grams (0.044 lb) of each Ronstar-coated fertilizer was placed into separatory funnels and 0.7 ounces (20 ml) of water was added, slightly covering the fertilizer. After 30 minutes, water was allowed to drain into a 4.2 ounce (125 ml) flask for 10 minutes. Leachate volume was measured and 0.03 ounce (1 ml) subsamples were assayed for radioactivity using liquid scintillation spectrometry. This procedure was repeated daily for 14 days. The amount of Ronstar in each leaching was determined by multiplying the amount of radioactivity in the 0.03 ounce (1 ml) subsample by the volume of leachate collected. The entire experiment was repeated twice. The number of leaching events required to remove 70 to 80% of the total applied Ronstar was determined (see table).

EXPERIMENT 2. Osmocote 17-7-12 was coated with Ronstar as previously described. The Ronstar-coated Osmocote was then coated with 0.0067 ounces (200 microliters) of either Complex (sticker; Riverside/Terra Corp. Sioux City, Iowa), Plex (sticker; Riverside/Terra Corp.), Prime Oil (Riverside/Terra Corp.), or Intac (Loveland Industries Inc., Greely, Colorado). Ronstar-coated Osmocote alone and Ronstar-coated Polyon alone were also included as control treatments. The 14-day leaching and Ronstar detection procedures were as previously described.

RESULTS
RONSTAR RELEASE RATES, EXPERIMENT 1. After one leaching, radioactive Ronstar recovered from glass beads and Nutricote exceeded 50% of the total Ronstar applied. Meister and Osmocote released 44% and 35%, respectively. Nursery Special (30%) and Polyon (22%) released the lowest percentages with the first leaching. With the third leaching, 18% of total applied Ronstar was recovered from Ronstar-coated Polyon while less than 10% was recovered from the glass beads or the other fertilizers. After three leaching events, 70 - 80% of the total Ronstar applied during the study was recovered from Meister, Osmocote, and Nutricote fertilizers, while 56% was recovered from Polyon. After the fifth leaching, Ronstar-coated Polyon consistently leached the highest level of Ronstar. For example, total Ronstar recovered at the seventh leaching event was less than 2.0% from Meister, Nutricote or Osmocote compared to 5.l% for Polyon. Polyon required about three times the number of leaching events to remove 70% of the applied Ronstar compared to Nutricote and Osmocote fertilizer (see table). The number of leaching events required to remove 70% of the Ronstar from Nursery Special was similar to Polyon. This would be expected since Nursery Special contains Polyon prils. Polyon and Nursery Special also required the most leaching events to remove 90% of the applied Ronstar (see table). In previous work, Ronstar-coated Polyon and Nursery Special fertilizers were more effective in controlling weeds than Osmocote. This enhanced weed control obtained in the field likely resulted from the extended release of herbicide from Polyon.

RONSTAR LEACHING RATES, EXPERIMENT 2. The control treatments of Ronstar-coated Osmocote and Polyon alone resulted in leaching patterns similar to those in the first experiment. In the first leaching, 85% of the applied Ronstar was recovered from Osmocote, while only 24% was leached from Polyon. After the third leaching, Ronstar residue on Osmocote was essentially depleted as evident by less than 1% recovery from the remaining 11 leaching events. Ronstar recovery from Polyon ranged from 20-25% during each of the first three leaching events and was consistently higher than Osmocote throughout the duration of the study except on day 1. When Plex was added to the Ronstar-coated Osmocote fertilizer, Ronstar recovery was similar to that for Ronstar-coated Osmocote alone. However, Prime Oil plus Ronstar-coated Osmocote fertilizer reduced Ronstar recovery compared to Ronstar-coated Osmocote alone. With the first leaching, 16% of the applied Ronstar was recovered from the Prime Oil plus Ronstar-coated Osmocote followed by recovery rates between 7 and 10% through the sixth leaching event. About 5% was recovered in the seventh through eleventh leaching events. When Intac or Complex was coated onto Ronstar-coated Osmocote, Ronstar recovery was similar to the pattern obtained with Prime Oil.

SURFACE CHARACTERISTICS OF FERTILIZERS. Polyon surface area was 23% greater than the fertilizer with the next largest surface area. The greater surface area of Polyon may contribute to its slow release and superior weed control properties. When these smaller prils are spread evenly over the container medium surface, a more even herbicide distribution is obtained relative to a fertilizer with larger prils. This conclusion is supported by previous work with herbicide tablets that showed definite circular rings of weed control around the tablets. Finally, assuming equal volumes of water from daily irrigation, greater fertilizer surface area may aid in retaining the herbicide against possible leaching, assuming adsorption is related to physical characteristics.

The surface of Polyon underwent apparent surface erosion over the 14 leaching events. Initial roughness of Polyon changed to a smoother more uniform appearance after leaching (data not shown). This suggests loss of the fertilizer coating, which would result in herbicide release. The surface structure of the other fertilizers appeared similar before and after the 14 leachings.

Superior weed control obtained with Ronstar-coated Polyon in previous work may be attributed to its ability to release the herbicide over a longer period of time compared to the other control release fertilizers tested. This improved weed control may result from either more uniform distribution over the substrate surface, or erosion of Polyon's surface. The addition of Intac, Prime Oil, or Complex to Ronstar-coated Osmocote altered the Ronstar recovery rate to a rate similar to that obtained with Ronstar-coated Polyon. Thus, superior weed control obtained with Ronstar-coated Polyon in previous work should be available with Ronstar-coated Osmocote with the addition of one of the successful additives. These data may provide future options to the nursery industry for reducing non-target herbicide loss while maintaining effective weed control.