Overseeded bermudagrass spring transition response to mowing height, nitrogen rate, sulfonylurea herbicide, and allelopathy

Hybrid bermudagrass [Cynodon transvaalensis Burtt-Davey x C. dactylon (L.) Pers.] is the preferred turf for golf courses and athletic fields across the southern United States because of its dark green color, fine texture, good wear and drought tolerance, and excellent recuperative rate. Despite its many attributes, bermudagrass goes dormant and turns an unsightly brown color when soil temperatures fall below 10-13oC (50-55oF). Perennial ryegrass (Lolium perenne L.) is often overseeded into bermudagrass in the fall to sustain acceptable turf quality through the spring months. However, perennial ryegrass aggressively competes with the bermudagrass for resources in the spring and potentially releases disruptive allelochemicals into the environment. This competition and inhibition complicate the spring transition, impair the bermudagrass base, and result in unacceptable turf quality. Field studies were conducted to determine the best treatment(s) to optimize the spring transition and ensure continuous acceptable turf quality. Growth chamber studies were conducted to investigate potential ryegrass inhibition/allelopathy on common bermudagrass seedlings.The field experiment utilized an overseeded soccer field to monitor the spring transition under different mowing heights, fertility rates, and trifloxysulfuron applications. Two mowing heights (1.2 and 2.5 cm), two nitrogen rates [18 kg N ha-1 week-1 (low) and 36 kg ha-1 week-1 (high)], and three herbicide rates and application dates [trifloxysulfuron at 0.005 (low) and 0.017 kg ai ha-1 (high) April low and high May and untreated] were examined. Turf responses measured were turf quality (TQ), percent perennial ryegrass (PRG), shoot dry weight (SDW), root dry weight (RDW), bermudagrass shoot counts (BSC), and percent bermudagrass (PBG). Trifloxysulfuron (TFS) had a significant effect both years on all measured responses except 2006 RDW. In 2006, untreated and 2.5 cm low April TFS treatments sustained acceptable turf quality while all other treatments experienced unacceptable quality for a minimum of 2 weeks. However, both 2.5 cm low April TFS and control treatments possessed unacceptable PRG at study's end, 30 June 2006. In 2007, untreated and 1.2 cm low May TFS treatments at both fertility levels maintained acceptable (>7) quality for the duration of the study. All TFS treatments had 0 PRG and 100 BSC and PBG ratings at study's end, 1 July 2007.The first growth chamber experiment utilized pots seeded with bermudagrass that received irrigation water contaminated with various concentrations of perennial ryegrass roots or shoots. The potential allelopahic/inhibitory effects of two sources of contaminant (perennial ryegrass roots or shoots) and four amendment rates (0, 5, 10, 20 g L-1) on bermudagrass germination and growth (root length density, root mass density, specific root length, root ash weight, dry shoot weight, bermudagrass shoot number, and bermudagrass tiller number) were examined. No bermudagrass inhibition or yield reductions were observed for any of the ryegrass irrigation solutions.The second growth chamber experiment utilized pots that contained bermudagrass seeded in soil amended with various rates of perennial ryegrass root or shoot. The potential allelopathic/inhibitory effects of two sources of amendment (perennial ryegrass roots or shoots) and four amendment rates (0, 2, 12, 23% per 25g soil) on bermudagrass germination and growth (root length density, root mass density, specific root length, root ash weight, dry shoot weight, bermudagrass shoot number, and bermudagrass tiller number) were examined. The highest concentration of ryegrass shoots per mix (23%) reduced bermudagrass shoots, tillers, shoot weight and ash weight.In conclusion, cultural practices must be coupled with herbicides that aid spring transition to achieve a complete, timely spring transition in Clemson, SC. Under normal spring and summer conditions, best overall transition in Clemson, SC followed 1.2 cm mowing height, 36 kg N ha-1 week-1 and 0.005 kg ai ha-1 mid-May trifloxysulfuron treatments.Perennial ryegrass shoots amended into the soil inhibited bermudagrass seedling emergence and subsequent growth. Reductions in germination, size, and weight of bermudagrass seed and seedlings are evidence of allelopathic/inhibitory effects by perennial ryegrass (Inderjit and Keating, 1999 Rice, 1974). Therefore, allelochemicals are potentially leaking from severed and decaying perennial ryegrass shoots and inhibiting/altering bermudagrass growth and development.