Comparison of root- and shoot-acting growth retardants on a grass/clover sward

The effects of six grass growth retardants, maleic hydrazide (±2,4-D), mefluidide, paclobutrazol, EL500 and a mixture of mefluidide with paclobutrazol on a grass/clover sward were compared over a 3-year period. Maleic hydrazide, which is the most commonly used retardant, gave poor levels of growth control except when used with 2,4-D. Greatest growth suppression, a 38% reduction in dry matter production from May to July, was obtained from a mixture of mefluidide and paclobutrazol. Differences were observed between the effects of the shoot-acting retardants, maleic hydrazide and mefluidide and the root-acting retardants, paclobutrazol and EL500. Shoot-acting retardants were less effective in wet weather and lasted for a shorter period than root-acting retardants. They also increased compensatory growth after a period of growth suppression, and increased the proportion of clover in the sward.