Few environmentally friendly products for weed control have been developed. Herbicidal soaps and biological agents are alternatives to synthetic products, but they have some serious drawbacks that impede their effectiveness. Some corn-based products have been patented as herbicides. Corn gluten hydrolysate has some potential as a herbicide, but large quantities are not commercially available.

Concern over the use of synthetic pesticides has led to increased research on the use of naturally occurring substitutes. Although much progress has been made in the development of environmentally friendly products for insect and disease control, the development of natural products for weed control has been more limited.

Herbicidal soaps
Herbicidal soaps have met with some success for weed control in turf. These materials are not technically natural products, but they have been accepted as substitutes by those who reject standard turf herbicides. (Two herbicidal soaps are Safer's Topgun Weed Killer from Woodstream Corp., Scarborough, Ontario, Canada, and Concern Fast-Acting Weed Killer from IntAgra Inc., Bloomington, Minn.) These materials work by disrupting the outer cuticle of the leaf's surface, dehydrating and eventually killing the target plant. Herbicidal soaps are effective, but they do not provide selective control. They kill all plants that they come in contact with, and the surface of each targeted weed must be spot-treated. A second limitation is that they kill only by contact and do not translocate into underground reproductive structures.

Living organisms
Another approach to controlling weeds is the use of biological agents, such as viruses, bacteria and fungi that selectively kill the target weed without killing the turf. This concept has merit. The biggest drawback is the difficulty in identifying organisms that have both the efficacy and the survivability for commercial use. In general, the organisms that have been identified are highly selective and control only one of the many types of weeds that infest turf.

The organism that has received the greatest attention in the turf industry is Xanthomonas campestris pv. Poannua (marketed as XPO by Eco Soil Systems of San Diego, Calif.), a bacterium that has been observed to selectively control annual bluegrass (Poa annua). The main limitation of the organism is that it is difficult to keep alive for use in field applications. It also appears to work much better on annual types of P. annua than on the more abundant perennial types. Research continues on this organism, and other strains may prove to be effective on perennial types of P. annua.

Graduate student Melissa McDade applies corn gluten hydrolysate spray to Kentucky bluegrass turf.

Corn-based extracts
The idea of using corn-based materials as natural herbicides began in the mid-1980s with a project on the use of cornmeal as a growing medium for a fungal organism that was to be incorporated into a newly established golf course green (3). Cornmeal was observed to have an inhibitory effect on the germination of grasses.

Further work demonstrated that there was a naturally occurring compound in the protein fraction of corn, the corn gluten meal (CGM), which had an inhibitory effect on the root formation of germinating seeds. The CGM contains 10 percent nitrogen by weight and makes an excellent fertilizer for plants that have a well-established root system. It is a pre-emergence material only and has no post-emergence effects on weeds that are already established.

U.S. Patent No. 5,030,268 on this concept was issued in 1991 and was later reissued with broader claims under No. Re. 34,594 (2). Corn gluten meal is marketed nationwide for annual weed control under the names Bio-Weed, Concern Weed Prevention Plus, Corn Weed Blocker, DynaWeed, Earth Friendly, Organic Weed Stopper Plus, ProPac, Safe Earth Natural Weed Control, Secure 'N Safe, Suppressa, Turf Minder, WeedSTOP and WOW! www.gluten.iastate.edu.

Inhibiting root formation
The next stage of the work was to search for the chemical or chemicals in the CGM that are responsible for the inhibition of root formation in germinating plants. To do this, graduate student Dianna Liu looked at several ways of extracting the active component and concentrating it in a water-soluble form for further study. She found that material developed by Grain Processing Corp. of Muscatine, Iowa, called corn gluten hydrolosate provided a concentrated source of the active components in a water-soluble form that could be further studied to identify its chemistry (7,11).

In further studies, Liu found that five individual dipeptides (combinations of two amino acids) were responsible for root inhibition. They included glutaminyl-glutamine, glycinyl-alanine, alaninyl-glutamine, alaninyl-asparagine and alaninyl-alanine. The idea of using these naturally occurring dipeptides as substitutes for synthetic pre-emergence herbicides was submitted to the U.S. Patent office in 1993, and the patent was issued on March 1,1994 as Patent No. 5,290,757 titled "Preemergence Weed Control Using Dipeptides From Corn Gluten Hydrolysate" (5). Later work has also shown that there is a pentipeptide (a combination of five amino acids) that also has activity (8).

Graduate student Bryan Unruh took the work further by studying the effects of alaninyl-alanine on the root cells of grasses (13,14). He found that the dipeptide had the ability to disrupt cell division at the root tip of germinating seedlings.

An untreated perennial ryegrass (Lolium perenne) root tip (magnification 20X)

A natural herbicide

CGM hydrolysate was tested for use as a natural herbicide. CGM hydrolysate has the advantage of being sprayable, whereas CGM must be applied in granular form (10). In the laboratory, corn gluten hydrolysate proved to be as much as four times more active than corn gluten meal on target weeds. It also showed activity on a variety of monocotyledonous and dicotyledonous species (9). Hydrolyzed soybean and wheat proteins were also found to contain the root-inhibiting compounds.

A patent application titled "Preemergence Weed Control Using Plant Protein Hydrolysate," which included the corn, soybean and wheat gluten hydrolysates, was submitted to the patent office and issued as patent No. 5,290,749 (4). International patent applications on the use of the hydrolysates, the dipeptides and the pentapeptide have also been made in several countries. To date, patents have been issued in Australia and Canada and are pending in Europe (6).

Current research centers on the development of corn gluten hydrolysate as a commercial product. This effort has been slow because large quantities of this material are not commercially available and it presently must be prepared in the laboratory. In addition, although the hydrolysate is much more active in laboratory tests than the CGM, it loses that edge in the field and is roughly equivalent to the CGM on a pound-per-pound basis. This is likely due to its high water solubility and to accelerated rates of microbial breakdown of the active components.

Graduate student Melissa McDade studied two methods of stablizing the corn gluten hydrolysate recently. The first method was to combine it with a soybean oil that is used to encapsulate commercial pesticides to improve their efficacy (1). The second method involved combining the hydrolysate with humic acid. The hydrolysate reduced crabgrass infestation in Kentucky bluegrass turf by up to 69 percent in 1998 and by 93 percent in 1999, but the addition of the soybean oil and the humic acid did not improve activity of the material (12).

The future
Where the research goes from here depends on the availability of the hydrolysates. The product was originally developed as a protein source for human consumption. It is not being produced for that purpose at this time, and developing production facilities to produce the material for herbicidal use alone will likely be cost-prohibitive. More work is also needed on improving the efficacy of the hydrolysates for field use. Although the soybean oil and humic acid did not increase effectiveness of the material in the field, there are other methods, such as encapsulation that may be effective.

The dipeptides may also have potential as natural herbicides. Availability of product and cost of production are again the main limitations that have slowed progress on this idea.

The future of natural products for pest control in turf depends on a number of factors, most of them centered on public demand and government regulation. Generally, natural products require a great deal of knowledge on the part of the user concerning the characteristics and life cycle of the target pest, and they can be difficult to use. They are also usually more expensive and less effective than synthetic pesticides. Therefore, widespread use of natural products is likely only in situations where synthetic materials are not an option.

Acknowledgments

Funding for this research was provided by the Iowa State University Research Foundation.

Literature cited

1. . Cantwell, J., and G. Kapusta. 1984. Evaluation of soybean oil as a carrier for preemergence soybean herbicides applied with rotary nozzles. North Central Weed Control Conference Proceedings 39:58.
2. Christians, N.E. 1991. Preemergence weed control using corn gluten meal. U.S. Patent 5,030,268. (Re-issued with broader claim in April, 1994, Re. 34,594).
3. . Christians, N.E. 1993. The use of corn gluten meal as a natural preemergence weed control in turf. International Turfgrass Society Research Journal 7:284-290.
4. Christians, N.E., J.T. Garbutt and D. Liu. 1994a. Preemergence weed control using plant protein hydrolysate. U.S. Patent No. 5,290,749.
5. Christians, N.E., J.T. Garbutt and D. Liu. 1994b. Preemergence weed control using dipeptides from corn gluten hydrolysate. U.S. Patent No. 5,290,757.
6. . Christians, N.E., J.T. Garbutt and D.L. Liu. 1994c. Root-inhibiting compounds as growth-regulating compounds and preemergence herbicides. Filed in Australia, Canada, Japan, Denmark, France, Germany, Italy, Sweden, Switzerland and the United Kingdom. Inter. Patent App. No. PCT/US94/08513. (Canadian Patent No. 2,144,321 Preemergence Weed Control Using Natural Herbicides. Granted July 1999).
7. Liu, D.L.-Y., and N.E. Christians. 1994. Isolation and identification of root-inhibiting compounds from corn gluten hydrolysate. Journal of Plant Growth Regulation 13:227-230.
8. Liu, D.L., and N.E. Christians. 1996. Bioactivity of a pentapeptide isolated from corn gluten hydrolysate on Lolium perenne L. Journal of Plant Growth Regulation 15:13-17.
9. Liu, D.L., and N.E. Christians. 1997a. Inhibitory activity of corn gluten hydrolysate on monocotyledonous and dicotyledonous species. HortScience 32:243-245.
10. Liu, D.L., and N. E. Christians. 1997b. The use of hydrolyzed corn gluten meal as a natural preemergence weed control in turf. International Turfgrass Society Research Journal 8:1043-1050.
11. Liu, D.L.-Y., N.E. Christians and J.T. Garbutt. 1994. Herbicidal activity of hydrolyzed corn gluten meal on three grass species under controlled environments. Journal of Plant Growth Regulation 13:221-226.
12. McDade, M.C., and N.E. Christians. 2001. Corn gluten hydrolysate for crabgrass (Digitaria spp.) control in turf. International Turfgrass Society Research Journal 9(2):1026-1029.
13. Unruh, J.B., N.E. Christians and H.T. Horner. 1997b. Herbicidal effects of the dipeptide alaninyl-alanine on perennial ryegrass (Lolium perenne L.) seedlings. Crop Science 37:208-221.
14. Unruh, J.B., N.E. Christians and H.T. Horner. 1997a. Mitotic and ultrastructural changes in root meristems of grass seedlings treated with alaninyl-alanine. Crop Science 37:1870-1874.

Nick Christians, Ph.D. , is a professor in the department of horticulture at Iowa State University in Ames.