S-metolachlor and rainfall effects on sweetpotato (Ipomoea batatas L. [Lam]) growth and development

• Sweetpotato response to S-metolachlor with or without rainfall was rainfall was investigated.
• S-metolachlor effect on growth and development of sweetpotato was rate dependent.
• Rainfall 24 h after S-metolachlor application worsen the effect on storage roots.
• Storage roots were the most sensitive to S-metolachlor among sweetpotato plant parts.
• Sweetpotato storage root biomass was inversely proportional to S-metolachlor rate.

The herbicide S-metolachlor is used to control or suppress annual grasses, nutsedges and several broadleaf weeds in sweetpotato. However, a decline in storage root quality is suspected when an excessive rainfall occurs within 24 h after application. A sunlit, controlled environment experiment was conducted to investigate sweetpotato response to five levels of S-metolachlor (0.00, 0.86, 1.72, 2.58 and 3.44 kg ha−1), and two levels of simulated rainfall (0 and 38 mm at 51 mm h−1) immediately after application. Sweetpotato slips were transplanted into white polyvinyl chloride pots filled with sandy loam soil. S-metolachlor treatments were applied to slips and a simulated rainfall treatment delivered immediately after transplanting and herbicide treatment. All pots were transferred to sunlit growth chambers that were maintained at 30/22 °C, day/night temperatures and ambient carbon dioxide concentration (400 μL L−1) for 60 days. An evapotranspiration-based irrigation system was used to supply water and nutrients. Plant biomass components and quality of storage roots were recorded 60 days after transplanting. There was no difference between rainfall treatments across S-metolachlor rates for vine lengths, leaf numbers and leaf area. These parameters, however, declined linearly and significantly with increase in S-metolachlor concentration. Total storage root weight declined linearly with increased S-metolachlor concentration; the decline was steeper with simulated rainfall. Yield of marketable storage roots declined by 18 and 31% in the absence of rainfall and 55 and 79% in the presence of rainfall with S-metolachlor at minimum (0.86 kg ha−1) and maximum (1.43 kg ha−1) recommended label rates, respectively, used to control weeds. Yield reduction was directly proportional to the rate of S-metolachlor applied in the absence or presence of rainfall; 77 and 123 g fresh weight kg−1 ha−1S-metolachlor for no-rainfall and rainfall treatment, respectively. These results can be used to improve management decisions to optimize yield under field conditions as well as to mitigate risk of injury that could be associated with the use of S-metolachlor in sweetpotato production systems.