Use of Plant Growth-Promoting Rhizobacteria in Tall Fescue and Bermudagrass Forage Systems

Abstract

A 2-yr study was conducted evaluating plant growth-promoting rhizobacteria (PGPR) as an alternative N source for ‘Russell’ bermudagrass (Cynodon dactylon) and ‘KY 31’ tall fescue (Lolium arundinaceum) at two locations in Alabama. Fourteen, 3-m2 plots were treated with High N (19 kg N ha-1), Low N (10 kg N ha-1), Accomplish LM® (AMS), AMS + Low N (AMS + Fert), DH 44 (PGPR strain), Blend 20 (PGPR blend), and a negative control. Forage samples were taken every 4 weeks with a 0.1-m2 quadrat then analyzed for neutral detergent fiber (NDF), acid detergent fiber (ADF), crude protein (CP), total digestible nutrients (TDN), and dry matter (DM) using near infrared spectroscopy. Fertilizer applications were performed at the beginning of the growing season and then 30 d later. There was a treatment × harvest interaction on forage biomass and all nutritive value parameters, excluding CP, for both forage species (P < 0.0001). Across the growing season, High N had the greatest forage biomass compared to the control for both forage species. DH 44 had the lowest ADF in both forage species. For both forages, PGPR treated plots produced biomass and maintained forage nutritive value similar to that of commercial N fertilizer. In experiment 2, the PGPR strains making up Blend 20 and DH 44 were incubated with Sterling Blue, 2,4-D Amine, and Prowl H2O® and evaluated for survival at 0, 24, 48, and 72 h after inoculation. There was an effect of PGPR, herbicide, PGPR × herbicide, and PGPR × herbicide × hour interactions on the PGPR concentration of the flasks after incubation with the herbicides (P ≤ 0.05). Regardless of herbicide, both AP 7 and AP 18 had greater CFU counts than AP 282 and DH 44 at all time points (P ≤ 0.05). DH 44 with all three herbicides had the lowest CFU ml-1 at all time points (P ≤ 0.05). AP 7 mixed with Sterling Blue at 48 and 72 h were significantly greater (P ≤ 0.05) than all other interactions, excluding AP 7 with Sterling Blue at 24 h as well as AP 18 with Prowl H2O® at 72 h. A field demonstration was done using ii botanical compositions to analyze the efficiency of each PGPR strain with all three herbicides, including controls (n = 2). There was an herbicide (P < 0.0020), PGPR × herbicide (P < 0.0002), and PGPR × herbicide × days after treatment (DAT) (P < 0.0101) interaction effect on the botanical composition. Sterling Blue and 2,4-D had the greatest percentage of weeds averaged across all DAT (74 vs. 76 %, respectively). The DH 44 control at all DAT had the lowest percentage of weeds (0 DAT: 59; 14: 49; 28 DAT: 53 %, respectively). As a result of these studies, DH 44 showed promising results as a bioherbicide in the field demonstration. More studies need to be conducted to determine the mechanistic ability of PGPR to act as a biofertilizer in forage-based systems.