Tactical use of gibberellic acid and nitrogen fertiliser to improve production of perennial ryegrass and white clover swards

Abstract

The timing and frequency of exogenous gibberellic acid (GA) application was explored in a cut-and-carry small plot experiment using irrigated Canterbury swards. GA applications (8 g GA/ha/application) replaced urea applications (25 kg N/ha/application) over multiple harvests in late winter, early spring and autumn. Timing and number (1, 2 or 3 applications) of GA application occurred in combinations to test the effect of early (August and February), delayed (September and March) or late (October and April) application of GA. Annual herbage yield was 14.6, 18.3 and 18.3±0.40 t DM/ha/y for the three controls (0, 250 kg N/ha/y and 250 kg N/ha/y + 80g GA/ha/y respectively). Clover content varied with treatment and time of year and was more abundant in low N treatments. There was a positive relationship between N fertiliser rate and annual herbage yield. There was no relationship between increasing GA rate and annual yield, though there was evidence of annual yield improvements, associated with frequent GA application under low (100 kg N/ha/y) or moderate (150 kg N/ha/y) N fertiliser. In late winter, delaying a single application of GA into spring improved N use efficiency (P<0.01). Tactical use of GA, in place of N fertiliser, can be used to maintain herbage yield.