Abstract
BACKGROUND: In the semi-arid United States, where relative humidity (RH) in summer ranges from 25% to 35%, suboptimal glufosinate efficacy is a recurrent issue, but higher early morning RH (60-80%) may increase its efficacy. This study aimed to optimize glufosinate efficacy under low RH conditions by (1) assessing the impact of droplet size, (2) determining the minimum duration of high post-application RH required for managing Bassia scoparia and Chenopodium album, and (3) quantifying absorption in response to post-application RH duration. RESULTS: A glasshouse study evaluated eight RH regimes, two nozzles (fine versus coarse droplets), and two glufosinate-susceptible populations for each species. Glufosinate-treated plants (0.6 kg a.i. ha(-1) + 20 g L(-1) ammonium sulfate) were exposed to elevated RH (≥ 60%) for intervals ranging from 0.5-h to 21-days and ambient RH (25-35%) for 21 days. Under ambient RH, 15-19% of B. scoparia plants survived, whereas none survived exposure to ≥ 0.5 h at elevated RH. Chenopodium album was controlled with 16 h and 21 days of elevated RH, while 60-90% survival was observed under < 16 h of elevated RH. A complementary laboratory study using liquid chromatography-mass spectrometry demonstrated increased glufosinate absorption with ≥ 0.5 h post-application exposure to elevated RH levels in both species. CONCLUSION: Relative humidity mediates glufosinate efficacy and absorption; however, the effects are species-specific. In B. scoparia, increased absorption coincided with improved glufosinate efficacy in elevated RH but not in C. album. Therefore, early morning glufosinate application is recommended for effective B. scoparia control, but it may not ensure satisfactory control of C. album. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.