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Short‐season cover cropping can be an important weed management tool. To optimize the use of mustard Sinapis alba L. and Brassica juncea (L.) Czern. in the Great Lakes region, we assessed planting time effects, mustard biomass production, and weed suppression during mustard growth and after incorporation. The study was conducted in Illinois, Michigan, and New York for spring and fall from 2010 to 2012. Mustard was sown every ∼10 d from mid‐March to early June for spring plantings and from early August to mid‐September for fall plantings. Spring mustard biomass, weed density, community composition, and dry biomass were collected at mustard flowering. Fall mustard biomass, weed density, and dry biomass were collected at season end. Spring mustard biomass ranged from <0.5 to 4 t ha −1 . Early fall biomass ranged from 3 to 5.5 t ha −1 , and was related to growing degree days (GDD) according to a logistic function. Weed biomass during mustard growth was reduced by at least 50% in 9 of 10 site‐years (90%) for fall‐planted mustard but only 15 of 31 site‐years (48%) in spring plantings. Weed suppression was independent of mustard biomass. The total number of weed seedlings emerging after mustard incorporation was not significantly reduced, but there was a species‐specific response, with a decrease in common lambsquarters ( Chenopodium album L.) and grass emergence. The results permit a location‐specific recommendation to plant mustard cover crops 13 to 23 August in the southern Great Lakes Region, and no later than 1 to 10 September for adequate biomass production.
Björkman et al. (Fri,) studied this question.