Effects of Gibberellic Acid and Sublethal Levels of Four Herbicides on the Cool-season Regrowth of Two Tropical Forage Grasses
- A. S. Whitney,
- R. E. Green and
- O. R. Younge
A series of experiments were conducted at two locations in Hawaii to evaluate the feasibility of increasing the cool-season growth of pasture grasses by means of chemical sprays.
In a preliminary trial from December to March, pangolagrass (Digitaria decumbens Stent) was treated with three levels of nitrogen (0, 56, and 112 kg/ha as (NH4)2SO4) and then with the following chemicals: ametryne, atrazine, simazine, and dicamba, each at 560 and 2240 g/ha; and gibberellic acid (GA) at 12 and 37 g/ha. Ametryne at 2240 g/ha reduced yields at the ll2-kg N/ha level only. No other significant effects were noted, but there was some indication of a small non-significant GA response.
Kikiyugrass (Pennisetum clandestinum Hocht. ex Chiov.) was treated at two locations during February with three levels of N (22, 56, and 168 kg/ha as (NH4)2SO4) and with simazine at 247 or 741 g/ha, GA at 62 g/ha, or simazine plus GA. Simazine did not affect yields except at the 168-kg/ha level of N at the 640-m elevation site; under this set of conditions, dry matter (DM) and crude protein (CP) yields were increased by 40% over the untreated control.
Much larger increases in DM and CP yields were associated with GA treatment; relative increases were highest, up to 9-fold, under low N levels, especially at the 900 m elevation site (av. soil temp. @ 5 cm ≤ 18 C). Ab. solute responses were highest, up to 2200 kg DM/ha, under high nitrogen levels and at the 640 m elevation. The GA treatment appeared to stimulate production by causing rapid increases in both photosynthetic area and stem tissue. The added stem tissue presumably provided additional carbohydrate “sink” capacity, thereby minimizing starch accumulation in the chloroplasts during daylight hours. Subsequent yields of GA-treated swards harvested a second time were less than controls at the 900 m elevation but greater than the controls at the 640 m elevation.
The limited response from the application of triazine compounds is in contrast to many previous reports. Our results indicate that they may be of only limited usefulness for tropical grasses, especially when the N is supplied in forms other than nitrate.
In contrast, the application of 62 g/ha of GA may be a useful adjunct to nitrogen fertilization for increasing forage production during periods of moderate temperature stress.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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