Effect of herbicides on black pigweed and sesbania pea, and yields of five grain sorghum cultivars in central Queensland

Abstract

Six experiments were conducted in central Queensland to compare the efficacy of some post-emergence herbicides and mixtures in controlling black pigweed (Trianthema portulacastrum) and sesbania pea (Sesbania cannabina). The herbicides tested were atrazine, 2,4-D, dicamba, picloram plus 2,4-D, and fluroxypyr and mixtures of atrazine with 2,4-D, dicamba, picloram plus 2,4-D, fluroxypyr or tridiphane. In addition, 4 experiments were conducted to assess the tolerance of 5 sorghum cultivars (Sorghum bicolor) to some of these individual herbicides and atrazine mixtures. Small black pigweed and sesbania pea (< 10 cm diameter) were controlled with atrazine at 1.0 kg a.i./ ha, while for larger black pigweed (up to 15 cm diameter) atrazine at 2.25 kg/ha and atrazine mixtures were effective and for sesbania pea (up to 12 cm high) atrazine at 2.25 kg/ha, picloram plus 2,4-D at 35 + 140 g a.i./ha, fluroxypyr at 0.3 kg a.i./ha and atrazine mixtures were effective. In general, control of both weeds by mixtures with atrazine at 1.0 kg/ha was as effective as atrazine at 2.25 kg/ha alone. In the tolerance experiments the treatments were applied at 18-20 days after planting when the number of sorghum leaves was 4-6. Spraying with 2,4-D, dicamba, MCPA, picloram plus 2,4-D and atrazine mixtures with 2,4-D, dicamba and picloram plus 2,4-D consistently caused injury symptoms, delayed flowering and sometimes reduced grain yield. However, the susceptibility of sorghum to these treatments varied with seasons and cultivars. Overall, yield reductions were less when 2,4-D, dicamba and MCPA were applied at lower rates in the atrazine mixtures than when applied alone. All sorghum cultivars were tolerant of atrazine at 4.5 kg/ha. For effective control of both weeds, for crop safety and for minimum atrazine residues after harvest, we recommend that the weeds black pigweed and sesbania pea be sprayed when less than 10 cm in diameter or height, respectively, with atrazine at 1.0 kg/ha.