Weed populations present in the three GM maize hybrids and the conventional isohybrid control were documented in Experimental Phase (2010–2012) and Pilot Phase (2012–2013) studies. The weed inventories included species adapted to the different regions and are typical of agricultural fields (Agundis Mata and Concepción Rodríguez 1978; Rosales Robles and Sánchez de la Cruz 2010). Only those weeds of agronomic importance are reported here. Twenty-five different weed species were documented in the Experimental Phase studies; the five most common species of agronomic interest were Amaranthus palmeri, Chenopodium album, Convolvulus arvensis, Echinochloa colona, and Portulaca oleracea. In the Pilot Phase studies, 48 different weed species were documented; the 13 species of most agronomic importance were Amaranthus palmeri, Chamaesyce maculate, Chenopodium spp., Convolvulus arvensis, Cyperus esculentus, Echinochloa colona, Helianthus annuus, Leptochloa filiformis, Malva parviflora, Physalis spp., Portulaca oleracea, Solanum elaeagnifolium, and Sorghum halepense. Overall, Sinaloa, Tamaulipas, and Comarca Lagunera showed the highest numbers of species, with 30, 20, and 13 different weed species, respectively.