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Effects of Gibberellic Acid on the Cool Season Regworth of Two Tropical Forage Grasses¹

Although the tropical grasses are very productive in Hawaii and elsewhere, their production at subtropical latitudes or elevations is often limited by seasonally low-temperatures. The feasibility of overcoming low-temperature effects by applying gibberellic acid (GA₃) was therefore evaluated over a 4-year period on swards of kikuyugrass (Pennisetum clandestinum Hochst. ex. Chiov.) at two sites and pangola digitgrass (Digitaria decumbens Stent) at one site.

Significant responses to 62 g GA/ha were obtained in all experiments over the range of N-fertilization tested (68–180 kg N/ha). Over a 4-year period, the average response of kikuyugrass to GA under a clipping regime was 1260 kg dry matter (DM)/ha. The average response to ammonium sulfate (at N rates 68 kg/ha) was 15 kg DM/kg N. Average pangola responses were 1450 kg DM/ha from GA treatment and 10.5 kg DM/kg N. In some cases, the GA responses could not be duplicated by higher levels of N fertilization.

GA applied on the date of clipping was as effective as GA applied 2 weeks later, but under some conditions, GA was less effective in increasing (DM) production once regrowth entered the boom portion of the regrowth curve. Growth retardation subsequent to the GA response occurred in clipped plots but not in plots which were left undipped or were defoliated by 2 weeks of grazing.

GA treatment appears to be feasible for increasing production of kikuyugrass and pangola during the cool season. The responses were enhanced by a moderate level of N-fertilization, and the full-season responses from single applications of GA were better under grazing than under clipping. The GA-treated forage was stemmier and thus less completely consumed by grazing animals, however.

Key Words: Pennisetum clandestinum • Digitaria decumbens • Nitrogen fertilization • Kikuyugrass • Pangola digitgrass

² Associate agronomist, Hawaii Agric. Exp. Stn., Kula, HI 96790.

Received for publication May 15, 1975.