Lower leaves of greenhouse-grown common cocklebur and velvetleaf were shaded to 5% of full light over a 12-d period while upper leaves remained exposed to full light to determine weed foliar and branching responses to partial shading similar to that encountered in soybean crops. Shading increased lower leaf senescence and specific leaf area, and decreased branch length and number of second-order leaves in both species compared to unshaded controls. Common cocklebur branched more extensively along the lower portion of its stem than velvetleaf under both shaded and unshaded conditions. Upper leaves of partially shaded velvetleaf were held in a more perpendicular position to the light source beginning 3 days after treatment (DAT) compared to upper leaves of unshaded plants. Shading of lower leaves caused an increase in upper (unshaded) leaf area beginning 3 and 6 DAT in velvetleaf and common cocklebur, respectively. Petiole length of upper leaves also increased in response to shading in both species. Total plant dry weight at 12 DAT was unaffected by shading in velvetleaf but was reduced 10% by shading in common cocklebur. While common cocklebur maintained greater lower shoot growth in the presence of shade than velvetleaf, there was a greater change in upper leaf angle by velvetleaf in response to shading than by common cocklebur. These results support previous field observations of apparent greater shade tolerance of common cocklebur compared to velvetleaf and indicate that both species have the ability to compensate for shading of lower leaves by altering upper shoot growth.

The growth of green foxtail (Setaria viridis L. Beauv. ♯3 SETVI) and yellow foxtail [S. lutescens (Weigel.) Hubb. ♯ SETLU] in full sunlight, under 55 and 73% shade and within a wheat (Triticum aestivum L. ‘Neepawa’) stand, was compared in field experiments conducted over 3 yr. Shade resulted in a proportionately greater reduction in tiller number of yellow foxtail than of green foxtail. Plants growing in full sunlight produced up to five times more tillers than those growing in the crop. Averaged over the 3 yr, the two shade treatments reduced dry-matter accumulation of both species by 40% or more. The dry weight of plants within the crop was only about one-eighth of that of plants grown in full sunlight. Under both shade treatments and in the crop, yellow foxtail was consistently taller than green foxtail. Nevertheless, no differences in dry weight occurred between species under either shade treatment or in the crop. The results indicate that neither of the two species is distinctly more shade tolerant than the other, nor better adapted to compete with wheat.