Crop rotation and herbicide program effects on Palmer amaranth and common ragweed population growth rate

The success of weed management decisions must be assessed not only in the short-term within season but also in the long-term over several seasons. This study investigated the effects of crop rotation and herbicide program structure on the population growth rates of Palmer amaranth (Amaranthus palmeri S. Watson) and common ragweed (Ambrosia artemisiifolia L.). A field experiment was conducted over a 3-year period in North Carolina to compare cotton (Gossypium hirsutum L.)–sweetpotato [Ipomoea batatas (L.) Lam.]–soybean [Glycine max (L.) Merr.], cotton–peanut (Arachis hypogaea L.)–soybean, cotton–tobacco (Nicotiana tabacum L.)–soybean, and cotton–soybean–soybean rotations and preemergence and postemergence herbicide application timings. Results showed that preemergence herbicide application in the soybean phase of the rotation reduced Palmer amaranth populations 79%. However, the preemergence herbicides were only effective at reducing weed populations for the current season, not beyond. Common ragweed population growth rate was highest after the first 2 years (λ = 1.63) of the cotton–tobacco–soybean rotation. Preemergence herbicides were effective in reducing common ragweed populations, particularly in rotations with cotton–sweetpotato and cotton–peanut. Soybean yields were similar across rotations ranging from 62 bu/ac to 68 bu/ac. Annual use of preemergence herbicides was essential to reduce Palmer amaranth populations. For common ragweed, the effectiveness of preemergence herbicides to mitigate population growth was reduced when poorly competitive crops were part of the rotation.