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 4.2%, a pH of 6.75, 17 ppm P, and 103 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 2000, applications were made on April 18.

Turf quality was monitored from April 26 through August 24 (Table 1). Visual turf quality was assessed using a 9 to 1 scale with 9 = best, 6 = lowest acceptable, and 1 = worst turf quality. Additional quality data were taken on October 27 because above normal temperatures and moderate rainfall resulted in a 'greenup' of the turf.

Weed populations were measured by either counting the number of plants or estimating the percentage cover per individual plot. Crabgrass plants in the 1- to 3-leaf stage were found in late June. Crabgrass count data represent the number of plants per individual plot. The crabgrass plants were large enough by July 14 to count the number per plot. Subsequent counts were made on July 25, August 2, August 8, August 16, August 24, and September 12 (Table 2).

The predominate broadleaf species were dandelion and clover. Dandelion infestations were determined by counting the number of plants per individual plot. Clover populations were estimated by assessing the percentage area of each plot covered by clover. Dandelion and clover cover data were taken on May 2, June 2, June 15, June 22, June 30, July 7, July 14, July 25, August 2, August 8, August 16, August 24, and September 12. These numbers were converted to represent the percentage reductions as compared with the untreated controls (Tables 3 and 4).

Data were analyzed with the Statistical Analysis System (SAS, Version 6.12) and the Analysis of Variance (ANOVA) procedure. Effects of CGM on bluegrass quality and weed control were examined using Fisher’s Least Significant Difference (LSD) means comparison tests.

Spring 2000 was quite warm and dry with above-average temperatures and below-average rainfall. At spring greenup in April, there were slight quality differences between treated and untreated turf but by mid-May, the differences were quite distinct (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-July, the quality of all treated and untreated turf began to deteriorate because of the lack of rainfall. Some rainfall improved turf quality by late August. Late season data from October 27 show a definite CGM effect in turf treated with CGM at 60 lb/1000 ft² and above.

Treatment with CGM resulted in numeric reductions in crabgrass populations for the entire season but the differences were not statistically significant (Table 2). Mean crabgrass reductions for the entire season were „ 98% for all CGM rates except 20 lb/1000 ft² (Table 2). In 1997 through 1999, crabgrass counts were higher in turf treated with 20 lb/1000 ft² than in untreated turf. In 2000, crabgrass counts were higher in untreated turf and CGM at 20 lb/1000 ft² provided a 63.3% reduction in crabgrass (Table 5). Reductions in crabgrass counts for 2000 were higher than in 1997, 1998, and 1999 at all other CGM levels.

Dandelion counts were significantly reduced by CGM at all levels except 20 lb/1000 ft² as compared with the untreated control (Table 3). Mean reductions were at least 89.5% for CGM at 40 lb/1000 ft² and higher and greater than 96% for CGM at 60 lb/1000 ft² and higher. In 2000, CGM at 20 and 40 lb/1000 ft² reduced dandelion populations more than in 1997, 1998, and 1999. Dandelion control at the other CGM rates was similar to the levels in 1999 (Table 6).

Percentage clover cover was significantly reduced in turf treated with CGM as compared with the untreated controls for the entire season except July 14 (Table 4). Mean reductions in clover cover were > 79% as compared with the untreated controls in turf treated with CGM at all levels except 20 lb (Table 7). Clover control in 2000 was similar to that in previous years except at 20 lb/1000 ft². At this rate, the level of clover control was higher than in 1998 and 1999.

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