Studies were conducted in a stale field production system in 2012 and 2013 to determine the effect of herbicide timing on Palmer amaranth control and 'Covington' sweetpotato yield and quality. Treatments consisted of flumioxazin at 72, 90, or 109 g ai ha(-1) applied 45 d before transplanting (DBT) or 1 DBT, or sequentially the same rate at 45 DBT followed by (fb) 1 DBT; flumioxazin 109 g ha(-1) applied 1 DBT fb S-metolachlor (800 g ai ha(-1)) at 0, 6 (+/- 1), or 10 d after treatment (DAT); flumioxazin at 72, 90, or 109 g ha(-1) plus clomazone (630 g ai ha(-1)) applied 45 DBT fb S-metolachlor (800 g ha(-1)) applied 10 DAT; and fomesafen alone at 280 g ai ha(-1) applied 45 DBT. Nontreated weed-free and weedy controls were included for comparison. Flumioxazin application time had a significant effect on Palmer amaranth control and sweetpotato yields, and the effect of flumioxazin rate was not significant. Treatments consisting of sequential application of flumioxazin 45 DBT fb 1 DBT or flumioxazin plus clomazone 45 DBT fb S-metolachlor 10 DAT provided the maximum Palmer amaranth control and sweetpotato yields (jumbo, No. 1, jumbo plus No. 1, marketable) among all treatments. Delayed flumioxazin application timings until 1 DBT allowed Palmer amaranth emergence on stale seedbeds and resulted only in 65, 62, 48, and 17 control at 14, 32, 68, and 109 DAT, respectively. POST transplant S-metolachlor applications following flumioxazin 1 DBT did not improve Palmer amaranth control, because the majority of Palmer amaranth emerged prior to S-metolachlor application. A control program consisting of flumioxazin 109 g ha(-1) plus clomazone 630 g ha(-1) at 45 DBT fb S-metolachlor 800 g ha(-1) at 0 to 10 DAT provides an effective herbicide program for Palmer amaranth control in stale seedbed production systems in North Carolina sweetpotato.
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