Weed problem appears to be the most deleterious factor causing between 25 and 60% reduction in potential yield of cowpea. Field trials were therefore conducted to study the effect of inter-row spacing and period of weed interference on growth and yield of cowpea (Vigna unguiculata (L) Walp) at the Teaching and Research Farm of the Federal University of Agriculture, Abeokuta (07° 15′; 03° 25′ E) in South Western Nigeria during the early and late wet seasons of 2009. The experiment consisted of eight main plots of weed interference which included initial weed removal for 3, 6, 9, and 12 weeks after sowing (WAS) and subsequently weed -infested until harvest as well as initial weed infestation for corresponding periods and thereafter kept weed free until harvest. There were also sub-plot treatments of three inter-row spacing of 60, 75, and 90 cm. All treatments in different combinations were laid out in a split-plot design with three replications. In both trials, the use of inter-row spacing of 60 cm resulted in significant reduction in weed growth as evident in lower weed dry matter production and subsequent higher cowpea pod and grain yields than those of 75 and 90 cm inter-row spacing. Initial weed infestation of up to 3 WAS did not have any adverse effect on crop growth and cowpea grain yields provided the weeds were subsequently removed. On the other hand, cowpea grain yield loss was not significantly averted by keeping the crop weed free for only 3 WAS without subsequent weed removal. In this study, initial weed-infestation for 6 WAS and beyond significantly depressed various crop growth parameter and cowpea grain yield compared with the crop kept weed free throughout its life cycle. In order to obtain optimum yields similar to that of the weed free cowpea field, it was required to keep the crop weed free for 6 WAS and beyond. However, frequent weeding beyond 9 weeks after sowing did not improve cowpea yield significantly and as a matter of fact it may even result in reduction of cowpea grain yield due to mechanical damage of hoe weeding. The practical implication of this finding is that early weeding starting from 3 WAS is very crucial for cowpea production while the critical period of weed removal for optimum yield in cowpea is between 3 and 9 WAS in the forest-savannah transitional zone of south Western Nigeria.

Field studies were conducted in 2017 at Concord in northeast Nebraska to evaluate how timing of weed removal and application of pre-emergence (PRE) herbicides influences the growth of soybean. The studies were laid out in a split-plot arrangement of 14 treatments (2 herbicide regimes and 7 weed removal timings) with four replicates. The 2 herbicide regimes were: No PRE and PRE application of a premix of sulfentrazone plus imazethapyr (140 plus 28 g ai ha-1). The 7 weed removal timings were: V1, V3, V6, R2 and R5 soybean growth stages, as well as weed free and weedy season long. Soybean growth parameters (leaf area and dry matter) were collected at R6 growth stage. Delayed timing of weed removal significantly influenced the soybean growth by reducing leaf area and dry matter. Without the application of PRE herbicides, a 5% reduction in soybean leaf area and dry matter occurred when weed removal was delayed until 100 growing degree days after emergence (GDD; equivalent to V1 soybean stage or 10 days after emergence [DAE]). Meanwhile, the use of PRE herbicide prevented the same level of reduction until 382 GDD (V5 soybean stage; 34 DAE). The practical implication is that the use of PRE herbicides protected soybean growth and delayed the need for post-emergence application of herbicides by three weeks after soybean emergence.

Widespread use of dicamba-based herbicides such as Clarity® (dicamba diglycolamine salt, 480 g l-1), Engenia® (dicamba N,N-Bis-[3-aminopropyl] methylamine salt, 600 g l-1) and XtendiMax® (dicamba diglycolamine salt, 350 g l-1)with Vapor-Grip Technology for weed control in dicamba-tolerant (DT) crops have resulted in UN-intended drifts, partly due to windy and common temperature inversions in many parts of United States. It is unclear if the dicamba-based herbicides made of different formulations or technologies have differential impact on sensitive soybeans including a conventional variety. Thus, field studies were conducted in 2016 and 2017 to evaluate the relative sensitivity of a conventional soybean to micro-rates of three dicamba-based herbicide products (Clarity®, Engenia® and XtendiMax® ) applied at second trifoliate (V2), seventh trifoliate/beginning of flowering (V7/R1), and full flowering (R2) stages of soybean. The dicamba micro-rates were 0, 0.56, 1.12, 5.6, 11.2, and 56 g ae ha-1; equivalent to 0, 1/1000, 1/500, 1/100, 1/50, 1/10 of the standard rate (560 g ae ha-1) respectively. The experimental design was a randomized complete block design in a split-split-plot arrangement with 4 replications. There was no significant difference in visual injury, growth or yield response of the conventional soybean to the three dicamba herbicides. The dicamba micro rates caused 40-80% visual injury and 0-97% yield loss depending on the growth stage of application. The estimated effective doses (ED values) suggested that conventional soybeans exposed to dicamba micro-rates at V7/R1 growth stage were more sensitive than those exposed at V2 and R2 growth stages. Based on the ED values, about 0.1% of dicamba standard rate was enough to cause 10% soybean yield loss when applied at V7/R1 stage; while about 1% of dicamba standard rate was required to cause the same level of yield loss when applied at V2 or R2 stage. By implication, dicamba drift on sensitive soybean plants should be avoided to prevent yield loss.