ABSTRACT
Maize is very sensitive to early weed competition due to a slow early growth rate and wide row spacing (Cerrudo et al., 2012) and is often infested by weeds that are highly competitive for water, nutrients and light causing significant yield losses (Sattin et al., 1992). The extent of yield losses caused by weed competition depends on several factors such as weed density, weed species growth rate, weed size, weed canopy shape, uniformity of weed distribution, weed emergence date relative to the crop, crop
competitiveness and density, soil type and crop fertilization (Eleftherohorinos, 2014). When weeds and crop co-exist in the early growth stages of the crop, there is no direct resource competition as plants are small and widely spaced with little requirements for nutrients and water. However, experiments demonstrated that plant competition is triggered initially by the red to far-red light ratio (R:FR) reflected by neighbouring plants well before direct competition starts (Sattin et al., 1994). Once a plant is physiologically triggered into a shade avoidance response, these plants do so at a physiological cost, which may constrain plant development and possibly reduce reproductive fitness (Liu et al., 2009). At some point, depending on weed species, direct interference between the maize crop and weeds begins for both above and below ground plant organs, resulting in resource limitations (Aldrich and Kremer, 1997; Rajcan and Swanton, 2001). The critical period for weed control (CPWC) is defined as the period in the crop growth cycle during which weeds must be controlled to prevent crop yield losses (Zimdahl, 1988). Several studies have determined the effects of weed competition on yield loss and the critical period during which maize is mostly affected by competition with weeds (Zanin et al., 1994; Norsworthy and Oliveira, 2004; Otto et al., 2009; Cerrudo et al., 2012; Page et al., 2012; Knezevic and Datta, 2015). For instance, Page et al. (2012) reported that grain yield loss increased as weed control was delayed reaching a maximum of 65%, and that the beginning of the CPWC occurred, on average, between the third-and fifth-leaf stages and ended at the ten-leaf stage.
