Weed control in organic onion production is often difficult and expensive,
requiring numerous cultivations and extensive hand weeding. Onion safety and
weed control with mustard seed meal (MSM) derived from Sinapis
alba was evaluated in greenhouse and field trials. MSM applied
at 110, 220, and 440 g m−2 severely injured onions and reduced
onion stand by 25% or more when applied from planting to the one-leaf stage
of onions in greenhouse trials. MSM derived from mustard cultivars ‘IdaGold’
and ‘AC Pennant’ reduced plant dry weight of redroot pigweed with an
effective dose that provided 90% weed control (ED90) of 14.5 and
3.2 g m−2, respectively, in greenhouse trials, whereas the
ED90 of MSM from a low-glucosinolate cultivar ‘00RN29D10’ was
128 g m−2, suggesting that glucosinolate content and ionic
thiocyanate (SCN−) production contribute to phytotoxicity of MSM.
In field trials, weed emergence, onion injury, and onion yield were recorded
following single or three sequential applications of MSM from 1.1 to 4.5 MT
ha−1 beginning at the two-leaf stage of onions in 2008, 2009,
and 2010. By 8 wk after treatment (WAT), onion injury following MSM
sequential applications was 10% or less in all 3 yr. Combined over 2008 and
2009, 48 and 68% fewer weeds emerged 3 WAT with MSM at 2.2 and 4.5 MT
ha−1, respectively. In 2010, MSM at 2.2 and 4.5 MT
ha−1 reduced the number of weeds emerged 4 WAT by 91 and 76%,
respectively. MSM treatment did not significantly affect onion yield or size
in 2008 and 2009, but in 2010 onion total yield was reduced by 29% by three
sequential applications of MSM at 2.2 MT ha−1. MSM has potential
to be used as a weed-suppressive amendment in organic production systems,
but the risk of crop injury is substantial.