Weed population dynamics in land removed from the conservation reserve program

A field study was established in southern Iowa in 1994 to study seasonal an d longterm weed population dynamics on land being brought back into product ion after 8 yr as part of the conservation reserve program (CRP). The study was a split-plot design with four replications; two tillage regimes, two c rop rotations, and three herbicide application methods were used. Even thou gh the tillage regime did not influence individual weed population density throughout the study, the no-till (NT) regime had more weeds compared to co nventional tillage (CT). However, when weeds were grouped into categories, tillage influenced broadleaf weeds in 1994 and 1996 and total weeds in 1995 . Plots under the NT regime had an average of 46 broadleaf weeds m(-2) comp ared to 27 in CT in 1994, with Amaranthus rudis Sauer (common waterhemp) be ing the most prevalent. NT had a total of 186 weeds m(-2) compared to 125 m (-2) weeds in CT in 1995; however, in 1996, CT plots had 184 weeds m(-2) co mpared to 121 m(-2) in the NT regime. Except for broadleaf weeds in 1994, c rop rotation did not influence the number of weeds, and herbicide applicati on methods had the greatest effect on weed populations. Overall, weed popul ations were greater in 1997, 1996, and 1995 than in 1994 for all herbicide application methods. The no-herbicide treatment had the highest number of w eeds throughout the study The total number of weeds in band and broadcast t reatments averaged 41 and 26 m(-2) in 1994; 96 and 24 m(-2) in 1995; 96 and 12 m(-2) in 1996; and 109 and 95 m(-2) in 1997. The use of broadcast herbi cides in NT should be recommended for land coming out of CRP. Regardless of the herbicide application method or crop rotation, CT plots had better yie lds for both Zea mays L. (corn) and Glycine max L. (soybean). Glycine max h ad a better stand compared to Z. mays in the first year, indicating that a rotation starring with G. max might be preferred in the land coming out of CRP. Nomenclature: Amaranthus rudis Sauer AMATA, common waterhemp; Zea mays L., corn; Glycine max L., soybean; Andropogon gerardii Vitman, big blue stem; B romus inermis Leyss, smooth bromegrass; Melilotus officinalis Lam, yellow s weetclover.