Abstract

Weeds are a limitation to cocoa production, and herbicide use in cocoa cultivation is known to lower the cost of weed management and support better plant growth. As a result, glyphosate was recommended for use in cocoa cultivation following a two-year field trial at the Cocoa Research Institute of Ghana. Recent informal debates among scientists and farmers, however, point towards a disagreement regarding the effect of glyphosate on young cocoa. Consequently, an experiment was conducted from 2011 to 2013 at the Cocoa Research Institute of Ghana to re-evaluate the suitability of glyphosate (Isopropylamine) for weed management and its effects on growth and yield of young cocoa. Five rates of glyphosate viz: (i) 1,920 g active ingredient a.i. ha-1 using polyjet nozzle; (ii) 1,920 g a.i. ha-1 using very low volume (vlv) nozzle; (iii) 960 g a.i. ha-1 vlv; (iv) 720 g a.i. ha-1 vlv; (v) 480 g a.i. ha-1 vlv; and (vi) manual weeding were tested in a randomized complete block design with four replicates in the field. Effects of treatments on weeds, cocoa growth and yield were recorded for three years. In a gauze house study, cowpea and maize were used to determine residual effects of glyphosate. Emergence, survival and dry matter accumulation by these plants were recorded. Results from the gauze house study showed that glyphosate did not exhibit residual soil activity at these rates. Glyphosate at 960 g ha-1 significantly increased yield of three year old cocoa compared to the other rates and manual weeding. The 1,920 g ha-1 rates significantly reduced the initial yield of 3 year old cocoa compared to the other glyphosate rates. Cost analysis showed that glyphosate at 960 g a.i. ha-1 was Ghȼ 136.00 (11%) and Ghȼ 1,784.00 (61%) cheaper than the 1,920 g a.i. ha-1 rates and manual weeding respectively. It was therefore concluded that glyphosate can be applied at 960 g a.i. ha-1 (equivalent to 2.0 l ha-1 in 120 l of water) for effective weed management without significant adverse effects on growth and yield of young cocoa.

Key words: Weeds, glyphosate, cocoa, growth, bean yield and cost.