Corn gluten meal (CGM) is being screened for efficacy as a natural product herbicide in turf. This long-term study was begun in 1995 at the Iowa State University Horticulture Research Station north of Ames, IA. The experimental area is established 'Ram 1' Kentucky bluegrass. The soil is a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) with an organic matter content of 3.9%, a pH of 7.2, 4.5 ppm P, and 107 ppm K. Prior to treatment in 1995, the percentage broadleaf weed cover within the study perimeter exceeded 50%.

Individual experimental plots are 10 x 10 ft with three replications. The experiment is arranged in a randomized complete block design. Each year corn gluten meal is applied to the same plots at a yearly rate of 40 lb CGM/1000 ft² (equivalent to 4 lb N/1000 ft²) using four different regimes of single and split applications for a total of five treatments (Table 1). Four applications of 10 lb/1000 ft², split applications of 20 lb/1000 ft², an initial application of 30 lb plus a sequential of 10 lb/1000 ft², and a single application of 40 lb/1000 ft² are included with an untreated control.

Initial applications of all treatments for 1999 were made on April 30 before crabgrass germination. The second application of treatment 2 was made on June 7, the third on July 21, and the final on August 26. Sequential applications of treatments 3 and 4 were made on July 21.

The experimental plot was checked for phytotoxicity after each treatment. Turf quality data were taken weekly from April 28 through September 24. Visual quality was measured using a 9 to 1 scale with 9 = best and 6 = lowest acceptable, and 1 = worst quality (Table 1 and 2).

Crabgrass was first observed on July 27. Population data were recorded on August 3, August 11, August 19, August 24, August 31, September 7, and September 14 (Table 3).

Broadleaf herbicide was applied to this study in Fall 1999 in error. Data for broadleaf weed populations, therefore, are probably not accurate reflections of levels of control provided by corn gluten meal. Clover infestations were estimated by determining the percentage of each individual plot covered by clover. Data were taken on July 1, August 31, September 7, and September 14 (Table 7). Dandelion populations were measured on these same dates by counting the number of plants per plot (Table 5).

Data were analyzed with the Statistical Analysis System (SAS, Version 6.12) and the Analysis of Variance (ANOVA) procedure. Means comparisons were made with Fisher’s Least Significant Difference test (LSD). Crabgrass, clover, and dandelion population data were converted to represent percentage reductions as compared with the untreated controls (Table 4, 6, and 8).

There were no phytotoxic symptoms detected on the treated bluegrass. Visual turf quality was significantly better in bluegrass treated with CGM than in the untreated control from April 28 through August 3 except on June 2 (Tables 1 and 2). After August 3, there were significant differences in quality between treated and untreated turf but the quality of grass treated with 40 lb CGM in the spring deteriorated and was not different than the untreated control on August 11 and August 31. Mean visual quality for the entire season was better for bluegrass treated with CGM than the untreated grass.

Crabgrass populations were lower in turf treated with CGM than in untreated turf but the counts were not statistically different (Table 3). Crabgrass numbers were low in the untreated controls. Broadleaf weed species were well established when the crabgrass was emerging especially in the untreated controls and the competition from the broadleaves and the mature turf probably prevented the establishment of large crabgrass populations within the untreated plots. Because of large differences in crabgrass populations among the treatments in each replication, reductions in crabgrass populations were not statistically different at the 0.05 level on any of the collection dates (Tables 3 and 4). Populations were, however, reduced by as much as 97.9% in CGM treated turf as compared with untreated turf. Bluegrass treated with split applications of 20 lb CGM reduced crabgrass counts by at least 92.0% from August 3 through September 14.

In 1999, crabgrass control was much better at all CGM levels than in 1995 through 1998 (Tables 9 and 10). In contrast to previous years, crabgrass populations in the untreated controls were greater than in treated grass. Split applications of 20 lb CGM provided 95% crabgrass reductions in 1999 as compared to 45, 33, 50, and 86% in 1995, 1996, 1997, and 1998, respectively. A single 40 lb CGM application did not reduce crabgrass numbers in 1996 and 1997 and provided only 54 and 53% reductions in 1995 and 1998, respectively. In 1999, the 40 lb application reduced crabgrass numbers by 90%.

Corn gluten meal significantly reduced dandelion numbers as compared with the untreated controls on all data collection dates (Table 5). Mean crabgrass reductions were similar for all CGM treatments and ranged from 68.9 to 84.8% (Table 6).

Clover populations were very low in all treated and untreated turf (Table 7). This is probably due to the application of broadleaf herbicide in Fall 1998. Mean clover cover for the untreated controls was only 5.7%. All CGM treatments provided similar levels of clover cover reductions according to the mean reductions and all were statistically different from the untreated controls (Table 8).

Because this study was mistakenly treated with 2,4-D in late fall 1998, comparisons of dandelion and clover data with those of previous years may not be valid. Dandelion control in 1999 was better than in previous years especially in grass treated with the 20 lb CGM split treatments (Table 11). Clover control was much better in 1999 than in past years but the percentage clover cover in the entire study area including the untreated controls was much lower.

(April 28 - July 14).

Table 2. Visual quality¹ of Kentucky bluegrass treated with corn gluten meal in the 1995 Corn Gluten Meal Rate Weed Control Study

(July 21 - September 24).

NS = means are not significantly different at the 0.05 level

Table 3. Crabgrass counts¹ in Kentucky bluegrass treated with corn gluten meal (CGM) in the 1995 Corn Gluten Meal Rate Weed Control Study.

NS = means are not significantly different at the 0.05 level.

Table 4. Crabgrass counts¹ in Kentucky bluegrass treated with corn gluten meal (CGM) in the 1995 Corn Gluten Meal Rate Weed Control Study.

NS = means are not significantly different at the 0.05 level.

Table 6. Dandelion count reductions¹ in Kentucky bluegrass treated with corn gluten meal in the 1995 Corn Gluten Meal Rate Weed Control Study.

Table 7. Percentage clover cover¹ in Kentucky bluegrass treated with corn gluten meal in the 1995 Corn Gluten Meal Rate Weed Control Study.

NS = means are not significantly different at the 0.05 level.

Table 8. Percentage reductions in clover cover¹ in Kentucky bluegrass treated with corn gluten meal in the 1995 Corn Gluten Meal Rate Weed Control Study.

NS = means are not significantly different at the 0.05 level.

Table 9. Crabgrass counts¹ in Kentucky bluegrass treated in the 1995 Corn Gluten Meal Rate Weed Control Study for 1996 through 1999.

NS = means are not significantly different at the 0.05 level.

Table 10. Percentage crabgrass count reductions¹ in Kentucky bluegrass treated in the 1995 Corn Gluten Meal Rate Weed Control Study for 1995 through 1999.

NS = means are not significantly different at the 0.05 level.

Table 11. Percentage dandelion count reductions¹ in Kentucky bluegrass treated in the 1995 Corn Gluten Meal Rate Weed Control Study for 1996 through 1999.

Table 12. Percentage clover cover reductions¹ in Kentucky bluegrass treated in the 1995 Corn Gluten Meal Rate Weed Control Study for 1996 through 1999.