Yield and Botanical Composition of Rhizoma Peanut‐Grass Swards Treated with Herbicides

Abstract

Weeds are an increasing problem in rhizoma peanut (RP) (Arachis glabrata Benth.), a warm‐season perennial forage legume. The objective of this field study was to measure the effect of glyphosate [N‐(phosphonomethyl)glycine] at 1.12, 2.24, or 3.36 kg a.i. ha−1 and triclopyr {[(3,5,6‐trichloro‐2‐pyridinyl)oxy]acetic acid} at 0.56, 1.12, or 1.68 kg a.i. ha−1 applied in the summer on dry matter (DM) yield and botanical composition of weed‐infested RP–grass swards at 2 and 4 mo after application. Mexican‐tea (Chenopodium ambrosioides L.) and cogongrass [Imperata cylindrica (L.) P. Beauv.] were the most common weeds. Glyphosate, at all rates, reduced Mexican‐tea DM 2 mo after application in both years. However, substantial recovery of existing MT plants was observed 4 mo after application at all but the high rate. Glyphosate had no effect on cogongrass or other grasses in 1995 or 1996. In both years, rhizoma peanut DM declined as the rate of glyphosate increased. Some recovery of RP was noted at the low (1.12 kg ha−1) rate of glyphosate by 4 mo after application. Edible DM (RP + other grasses) was reduced due to glyphosate treatment only at the high glyphosate rate. In both years, triclopyr was effective in reducing Mexican‐tea DM 2 mo after application with limited recovery of treated plants 4 mo after application. Cogongrass and other grasses increased in the triclopyr treatments in both years, possibly due to reduced competition from Mexican‐tea. Rhizoma peanut DM decreased as the rate of triclopyr increased in 1995 at 2 and 4 mo after application, but this effect was observed only at 2 mo after application in 1996. Triclopyr application had little effect on edible DM, but this was a consequence of the substitution effect of other grasses for RP. Both triclopyr and glyphosate can be useful in weed‐infested RP stands, but glyphosate at the rates tested was not as effective as triclopyr in controlling Mexican‐tea.