Drought stress and weed competition are two of the most important threats to corn production in the northeastern United States. Both pressures have the potential to worsen under climate change. In a 2-yr field study in Ithaca, NY, we tested the effects of drought and burcucumber, an increasingly problematic annual vine, on silage corn. Burcucumber seedlings were transplanted into corn rows at densities of 0, 0.5, 2, and 3 plants m−2 and a drought treatment was later imposed with rainout shelters constructed from steel frames and high-clarity plastic. Available soil moisture was lower in drought plots (47% ± 1% in 2018 and 52% ± 2% in 2019) than no-drought plots (69% ± 1% in 2018 and 68% ± 1% in 2019). Burcucumber planting density (P = 0.008) reduced fresh silage yield. Drought also reduced fresh silage yield (P < 0.001) with a drought-by-year interaction (P = 0.007): drought reduced fresh weight by 29% in 2018 (48,000 ± 2,000 kg ha−1 to 34,000 ± 3,000 kg ha−1) and by 9% in 2019 (38,000 ± 3,000 kg ha−1 to 34,000 ± 3,000 kg ha−1). Burcucumber planting density and drought did not interact. Overall, our findings indicate that drought and competition from burcucumber may have additive effects on silage corn in New York State. Regardless of water availability, active weed management is required to prevent yield losses due to burcucumber. Yield losses may be similar or greater in grain corn and might increase under climate change.

The introduction of herbicide-resistant crops and postemergence herbicides with a wide action spectrum shifted the research focus from how to when crops should be treated. To maximize net return of herbicide applications, the evolution of weed–crop competition over time must be considered and its effects quantified. A model for predicting the yield trend in relation to weed removal time, considering emergence dynamics and density, was tested on data from glyphosate-resistant soybean grown in cropping systems in Italy and Argentina. Despite an ample variation of weed emergence dynamics and weed load in the four trials, the model satisfactorily predicted yield loss evolution. The estimated optimum time for weed control (OTWC) varied from about 18 d after soybean emergence in Argentina to 20 to 23 d in Italy, with time windows for spraying ranging from 14 to 28 d. Within these limits a single glyphosate application ensures good weed control at low cost and avoids side effects like the more probable unfavorable weed flora evolution with double applications and the presence of residues in grains. Despite the apparent simplicity of weed control based on nonselective herbicides, the study outlines that many variables have to be considered to optimize weed management, particularly for the time evolution of the infestation and, subsequently, a proper timing of herbicide application.