Previous crop and herbicide timing application effects on weed population growth rate

A well-designed crop rotation can create an unstable environment that disrupts weed population growth rates. In combination with effective herbicide programs, growers may maintain weed populations at levels below competitive and economic thresholds. The objectives of the present study were to evaluate how the preceding rotational crop determines the response of weed populations to in-season postemergence herbicide programs and the weed population density of the following crop season. The first-year crop treatments were corn (Zea mays L.), cotton (Gossypium hirsutum L.), peanut (Arachis hypogaea L.), grain sorghum [Sorghum bicolor (L.) Moench.], and soybean [Glycine max (L.) Merr.]. In the second year, all plots were planted with cotton, and herbicide treatments were single applications 2 or 6 weeks after planting (WAP), two sequential applications 2 and 4 or 4 and 6 WAP, three sequential applications 2, 4, and 6 WAP, and a weedy control without herbicides was included. In the absence of herbicides, corn had the lowest population growth rates for broadleaf weeds (λ = 0.8) while peanut and grain sorghum had the highest (λ = 1.7 and 1.3, respectively). The results indicated that herbicide applications focused exclusively on preventing yield loss may not be sufficient to ensure weed population reductions. Thus, the observed population growth rates (λ = 2 for grassy weeds and λ = 1.26 for broadleaved weeds) indicated that weed issues would continue increasing, despite meeting yield goals. Considering population growth rates when assessing weed management strategies is key to determining the sustainability of the crop production operation.