Corn gluten meal (CGM) has been screened for efficacy as a natural product herbicide and fertilizer in turf on the same plot since 1991. The study is being conducted at the Iowa State University Research Station north of Ames, IA in an area of 'Parade' Kentucky bluegrass. The soil in this experimental area is a Nicollet (fine-loamy, mixed, mesic Aquic Hapludoll) with an organic matter content of 4.6%, a pH of 6.45, 3 ppm P, and 144 ppm K.

Individual experimental plots are 5 x 5 ft and there are 5 treatments with 3 replications. The experimental design is a randomized complete block. Corn gluten meal is applied each year to the same plots at 0, 20, 40, 60, 80, 100, and 120 lbs/1000 ft² (Table 1). Because corn gluten meal is 10% N, these rates are equivalent to 0, 2, 4, 6, 8, 10, and 12 lb N/1000 ft². The CGM is applied each year in a single early-spring preemergence application using 'shaker dispensers'. The materials are watered-in with the irrigation system. Supplemental irrigation is used to provide adequate moisture to maintain the grass in good growing condition. In 2001, applications were made on April 26.

Turf quality was monitored from May 8 through September 13 (Tables 1 and 2). It was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst turf quality. Weed populations were measured by either counting the number of plants or estimating the percentage area coverage per individual plot. Crabgrass infestations were determined by counting the number of plants per individual plot. Plants in the 1 to 3 leaf stage were found in June 12 but the growth rate was very slow because of the high temperatures and low rainfall. The crabgrass plants were large enough to count by July 27 and subsequent data were taken on August 9, August 16, August 28, and September 13. Dandelion populations were assessed by counting the number of plants per individual plot. Clover populations were determined by estimating the percentage area of each plot covered by clover. Dandelion and cover data were taken from May 8 through September 13 (Tables 4 and 6).

Data were analyzed with the Statistical Analysis System (SAS , Version 6.12) and the Analysis of Variance (ANOVA) procedure. Effects of CGM on turf quality and weed control were examined using Fisher’s Least Significant Difference (LSD) means comparison tests. To better express weed control, weed population data were converted to percentage reductions as compared to the untreated controls.

At spring greenup in May, there were differences between treated and untreated turf and by mid May, the differences were more pronounced (Table 1). Quality differences between CGM treated turf and untreated turf were significantly different for the entire season but on some dates, the quality of turf treated at various CGM levels was not different from the untreated control. By mid to late August, turf quality deteriorated because of low rainfall amounts. Some rainfall improved turf quality by early September.

Treatment with CGM resulted in numeric reductions in crabgrass populations for the entire season. The differences were not statistically significant because there were large variations in counts among the replications for the treatments (Tables 2 and 3). There was no crabgrass in turf treated at 40 lb/1000 ft² and greater for the entire season (Table 2). In 2002, crabgrass control was equal to or better than in previous years at all CGM levels except at 20 lb/1000 ft² (Table 8).

Dandelion counts were significantly reduced by CGM for the entire season as compared to the untreated controls (Tables 4 and 5). Mean reductions were at least 92.3% for CGM at 40 lb/1000 ft² and higher and greater than 96% for CGM at 60 lb/1000 ft² and higher. In 2002, CGM at 20 and 40 lb/1000 ft² reduced dandelion populations more than in 1997, 1998, 1999, and 2000 (Table 9). Dandelion control at the other CGM rates was equal to or greater than in previous years.

Percentage clover cover was significantly reduced in turf treated with CGM as compared with the untreated controls for the entire 2002 season (Tables 6 and 7). Mean reductions in clover cover were > 77% as compared with the untreated controls in turf treated with CGM at all levels except 20 and 40 lb/1000 ft². Clover control in 2001 was similar to that in previous years except at 40 lb/1000 ft² (Table 10). Control at this CGM level was not as good as in 1994, 1996, 1997, 1998, 1999, and 2000.

Table 1. Visual quality¹ of Kentucky bluegrass treated in the 1991 Corn Gluten Meal Weed Control Study.