Inter-row cultivation timing effects on waterhemp (Amaranthus tuberculatus) control and sugarbeet yield and quality

The invasion of waterhemp into northern sugarbeet growing regions has prompted producers to re-integrate inter-row cultivation into weed management programs, as no currently registered herbicides can control glyphosate-resistant waterhemp POST in crop. Inter-row cultivation was a common weed control practice in sugarbeet until the release of glyphosate-resistant sugarbeet cultivars in 2008 made the use of inter-row cultivation unnecessary. In the late 2010s, producers began again to use inter-row cultivation to remove weeds that glyphosate did not control, but producers need information on the effectiveness and safety of inter-row cultivation when used with soil-residual herbicide programs. Efficacy and tolerance field experiments were conducted in Minnesota and North Dakota from 2017 to 2019. Results from the efficacy experiment demonstrated that cultivation improved waterhemp control 11% and 12%, 14 and 28 d after treatment, respectively. Waterhemp response to cultivation was dependent on crop canopy and precipitation after cultivation. Cultivation had minimal effect on waterhemp density in three environments, but at one environment, near Galchutt, ND in 2019, waterhemp density increased 600% and 196%, 14 and 28 d after treatment, respectively. Climate data indicated that in 2019 Galchutt, ND received 105 mm of precipitation in the 14 d following cultivation and had an open crop canopy that probably contributed to further weed emergence. Results from the tolerance experiment demonstrated that root yield and recoverable sucrose were not affected by cultivation timing or number of cultivations. In one environment, cultivating reduced sucrose content by 0.8% regardless of date or cultivation number, but no differences were found in four environments. Damage/destruction of leaf tissue from in-season cultivation is probably responsible for the reduction in sucrose content. Results indicate that cultivation can be a valuable tool to control weeds that herbicide cannot, but excessive rainfall and open crop canopy following cultivation can create an environment conducive to further weed emergence. Nomenclature: Glyphosate; waterhemp; Amaranthus tuberculatus (Moq.) J.D. Sauer; sugarbeet; Beta vulgaris L.