Corn gluten meal as herbicide: A worthwhile investment?

The material below was first presented as a poster:

Michael Bomford, Anthony Silvernail, Akree Peterson, and Seth Detenber. 2006. Corn gluten meal as organic herbicide: A worthwhile investment for organic growers? Kentucky Academy of Science Meeting, Agicultural Sciences Section, November 10, 2006. Morehead, KY.

Abstract

Corn gluten meal (CGM) is a byproduct of corn (Zea mays L.) wet milling that contains several dipeptides that suppress seedling germination and root growth. It has been patented as a natural pre-emergent herbicide, and approved for use in certified organic production systems; commercial product labels recommend application rates of 50-100 g/m², at a retail cost of approximately 5-10 ¢/m². In 2005 and 2006 we applied CGM at 0, 50, 100, and 200 g/m² to organically-managed soil with a history of high weed pressure. CGM was also applied at 400 g/m² in 2005 only. To test the effect of incorporation we compared surface application of CGM to shallow incorporation at each application rate. All weeds were identified and counted six and three weeks after treatment in 2005 and 2006, respectively. Aboveground weed biomass was collected, dried, and weighed at the end of the 2005 study. Linear regression showed that CGM application reduced weed counts, relative to untreated soil, by approximately 18% per 100 g/m² increment (2005: n = 10, r²=0.63, P = 0.01; 2006: n = 24, r²=0.54, P = 0.003). Soil incorporation of CGM further reduced weed counts by approximately 14% in 2006 (P = 0.001), but this effect was not significant in 2005. CGM application did not affect total weed biomass; surviving weeds compensated for lower germination rates by growing larger. We conclude that CGM’s low herbicidal effect at recommended rates does not justify its high cost.

Introduction

Weed management is a major challenge for organic growers, who cannot use synthetic herbicides. Corn gluten meal (CGM) is a byproduct of corn (Zea mays L.) wet milling containing dipeptides that inhibit seedling germination and root growth (1). It has been patented as a natural pre-emergent herbicide, and is approved for use in certified organic systems if it is not derived from genetically modified corn (2, 3).
CGM can be purchased in bulk for <33¢ kg^-1 (<15¢ lb^-1) (4). Bagged as an organic herbicide, guaranteed to be free of genetically modified corn, it costs >$1.35 kg^-1 (>60¢ lb^-1) (5). The herbicide label recommends application at 100 g m^-2 (20 lbs per thousand sq ft). This represents a cost of $1,300 ha^-1 ($525 ac^-1) per treatment -- approximately the same as the cost of hand-weeding.

In preliminary tests of CGM applications to corn, we noticed improved weed control where the product was incorporated by shallow cultivation, rather than being left on the soil surface. We set out to determine the effect of CGM application rate and incorporation on weed germination in organically-managed soil with a history of strong weed pressure. We wished to determine whether the amendment was sufficiently effective to justify its high cost to organic growers.

Materials and Methods

Studies were conducted using soil from organically-managed land with a history of strong weed pressure at the KSU Research and Demonstration farm, near Frankfort, KY.

2005. A 1 x 20 m (3.3 x 66 ft) strip of freshly-cultivated land was divided into 20 square plots on 30 June, 2005 (Fig. 1). Each plot was treated with CGM or distiller’s mash (a corn-based byproduct of distillation) at one of five application rates (0, 50, 100, 200 or 400 g m^-2; 0, 10, 20, 40 or 80 lbs per thousand sq ft). One plot from each treatment combination was raked to shallowly incorporate the amendment. Treatments were completely randomized. A six week-old tomato (Lycopersicon esculentum Mill.) seedling was transplanted into the center of each plot. A drip irrigation line running through the center of each plot was used to maintain sufficient soil moisture for tomato growth. After six weeks without further intervention, all weeds were pulled, identified, and counted on 8-12 August, 2005. Aboveground portions were dried and weighed. 2006. On 21 June, 2006, freshly-cultivated soil was collected to fill 24 plastic trays (23 x 32 cm; 9 x 12 in.) to a depth of 5 cm (2 in.) (Fig. 2a). CGM was applied to the soil surface at one of four application rates (0, 50, 100, or 200 g m-2; 0, 10, 20 or 40 lbs per thousand sq ft), and either incorporated by stirring, or left on the surface (Fig. 2b). Trays were randomly arranged on greenhouse benches in three complete blocks. Trays were watered regularly for three weeks, and then all weeds were pulled, identified, and counted on 11 July, 2006 (Fig. 2c). Analysis. Plots treated with distiller’s mash in 2005 were omitted from the analysis. Least squares ANOVA was conducted for each year, with application rate and incorporation as model terms. Independent variables were weed density (both years) and aboveground weed biomass (2005 only).
	Figure 1. 2005 study site.

	Figure 2a. 2006 study: Soil collection.

	Figure 2b. 2006 study: Corn gluten meal application.

	Figure 2c. 2006 study: Weed counting and identification

Results

Weed density averaged 274 and 828 m^-2 in 2005 and 2006, respectively. The five predominant weed species were 1) goosegrass (Eleusine indica L. Gaertn.), 2) smooth pigweed (Amaranthus hybridus L.), 3) yellow foxtail (Setaria glauca L. Beauv.), 4) chickweed (Stellaria media L. Vill.), and 5) giant crabgrass (Digitaria sanguinalis L. Scop.) (Fig. 3).
CGM application reduced weed density in both years (Fig. 4), but had no effect on the aboveground weed biomass collected in 2005. Weed density was inversely correlated with aboveground biomass of individual weeds (r = -0.76, n = 20, P < 0.0001). Incorporation of CGM further reduced weed density in 2006, but had no significant effect in 2005.


Figure 3. Proportion of weed count and weight (top) accounted for by five dominant weeds (bottom).	Figure 4. Effect of CGM concentration on weed density in 2005 (dark green) and 2006 (olive). Open symbols and dotted lines denote soil incorporation.

Conclusions

CGM applied before weed germination reduces weed density. Application at the label rate (100 g m^-2) reduces weed density by approximately 30%, with high variability. Effectiveness increases with application rate over the range of rates tested.
Soil incorporation may enhance CGM effectiveness.
In the absence of additional control, the reduction in weed density afforded by CGM application does not necessarily translate into reduced weed biomass.
The degree of effectiveness of CGM does not justify its cost.

References

Unruh, J.; Christians, N. & Horner, H. (1997). Herbicidal effects of the dipeptide alaninyl-alanine on perennial ryegrass (Lolium perenne L.) seedlings. Crop Science 37: 208-212.
Bingaman, B. & Christians, N. (1995). Greenhouse screening of corn gluten meal as a natural control product for broadleaf and grass weeds. HortScience 30: 1256-1259.
Organic Materials Review Institute (2004). OMRI Generic Materials List. OMRI, Eugene, OR. 158 pp.
Steevens, Barry (2006). Byproduct feed price listing. University of Missouri Extension. Accessed at http://agebb.missouri.edu/dairy/byprod/bplist.asp, 10/25/2006.
Juftes, R. (2006). Organic Farming and Gardening Supplies: Fall/Winter Catalog. Seven Springs Farm, Check, VA. Accessed at http://www.7springsfarm.com/catalog.html, 10/25/2006.

Acknowledgements

Akree Peterson and Seth Detenber completed their work on this project as student interns with Kentucky State University's Research and Extension Apprenticeship Program. Evans-Allen funds were used for all other salaries, materials, and supplies.