A study was undertaken to determine whether the relative leaf stages of common annual weeds and crops could serve as a reliable indicator of the time of weed emergence. Ten annual broadleaved and grass weeds were sown at successive intervals in field corn and soybean at Harrow, ON, Canada, in 1997, 1998, and 1999. All weeds emerging at a particular crop leaf stage were assigned to a cohort. Leaf numbers of the crop and different weed cohorts were recorded at 2- to 3-d intervals up to the eight-leaf stage of corn and the fourth trifoliate of soybean. For each weed species, categorical data analysis revealed a high degree of association between the leaf stage of a crop and the leaf number expected for an individual weed of a given cohort. For example, by the third trifoliate of soybean, most of the weeds emerging with the crop (VE cohort) had 8 to 10 leaves, whereas weeds in the V1, V2, and V3 cohorts averaged about seven, four, and two leaves, respectively. Year to year variation in the correspondence between crop and weed leaf numbers generally was small once variation due to time of weed emergence was removed, with one exception. Dry surface soil conditions during emergence of the VE cohort in corn in 1999 resulted in delayed leaf appearance of many weeds with respect to the crop. The relationship between weed and crop leaf stages can provide information for management decisions in two ways: (1) it indicates the relative time of weed emergence in assessing the need for control, and (2) it indicates the crop stage at which scouting for particular weed leaf stages should occur.
Nomenclature: Corn, Zea mays L. ‘Pioneer 3573’; soybean, Glycine max (L.) Merr. ‘NK 2492’.