Abstract Palmer amaranth ( Amaranthus palmeri S. Watson) is among the most economically damaging weeds in the United States and has evolved resistance to numerous herbicides, including glyphosate. This fast-growing species has expanded its range into different regions, including New York State, where glyphosate-resistant (GR) populations are now found. We evaluated how temperature modifies two components of weed risk, sensitivity to glyphosate and competitiveness against corn ( Zea mays L.), under growth chamber conditions. First, we performed a dose–response study testing glyphosate efficacy for a GR A. palmeri biotype from New York State and a glyphosate-susceptible (GS) biotype from Nebraska. Tests were conducted at low (25/15 C), medium (30/20 C), and high (35/25 C) day/night temperatures. The nontreated GS biotype produced greater biomass at low versus high temperatures. This biotype was highly sensitive to glyphosate across all temperatures. In contrast, biomass of the nontreated GR biotype did not vary with temperature. This biotype was less sensitive to glyphosate across all temperatures. Our second study tested whether temperature modifies the impacts of A. palmeri on corn biomass at four planting densities (zero, one, two, or four A. palmeri with one corn plant). We used the same three temperature regimes and the same two A. palmeri biotypes as the dose–response study. There was no evidence that the biotypes, at any planting density, were more competitive with corn under elevated temperatures. In fact, the biomass of A. palmeri decreased with increasing temperatures while corn biomass did not vary. In the competition study, the GS biotype from Nebraska had higher biomass than the GR biotype from New York. Overall, these results do not support the hypothesis that increasing temperatures would exacerbate the risk posed by the tested A. palmeri biotypes in corn. Instead, these two A. palmeri biotypes may be highly problematic under temperatures already occurring in New York State.
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