Agronomic assessment of gibberellic acid and cytokinin plant growth regulators with nitrogen fertiliser application for increasing dry matter production and reducing the environmental footprint

Abstract

Abstract Plant growth regulators (PGRs) are increasingly being used in agriculture for a variety of purposes. In pasture systems, the use of PGRs is relatively new and the effects of its application on plant productivity and physiology are poorly understood. A mowing trial was established in Waikato in early spring 2012. Key objectives from this study were: to determine effects of gibberellic acid (GA3) and cytokinin (CPPU) application with and without nitrogen (N) fertiliser application on pasture production, N concentration (%N), root biomass and potential reduction in N leaching from grazed pastures. Treatments (with five replicates) were: control, urea (40 kg N/ha), GA3 at two rates +/- urea, and CPPU at three rates +/- urea. Implications for grazed pasture and N leaching were modelled for a case study dairy farm. Application of GA3 alone showed a significant (P<0.05) rapid increase in dry matter (DM) production compared with the control between 5 and 29 days after application. The DM yields from GA3 alone were similar to those with urea-N alone and the effects of applying GA3 and urea-N together were additive. Application of CPPU showed no significant effect on pasture production. There were no treatment effects from either PGR on root biomass. The %N in herbage was significantly lower in the GA3 treated plots than the control, which would reduce urinary-N excretion under grazing. Preliminary modelling of data for a grazed pasture showed a potential reduction in annual urine-N leaching of 4-29%. Keywords: Gibberellic acid (GA3), cytokinin (CPPU), dry matter yield, total N, root biomass