No abstract

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 eff ects, mustard biomass production, and weed suppression during mustard growth and aft er incorporation. Th e 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 fl owering. 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. Th e total number of weed seedlings emerging aft er mustard incorporation was not signifi cantly reduced, but there was a species-specifi c response, with a decrease in common lambsquarters (Chenopodium album L.) and grass emergence. Th e results permit a location-specifi c 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.

Enhanced NH4 nutrition causes thicker roots (higher g/m root length) than NO 3 nutrition. This study examined the underlying anatomy of Nform altered root morphology. Corn (B73xLH51) was cultured in complete nutrient solutions containing NO 3 -or NH 4 -N. Plants were harvested four times between 13 and 20 days after germination. Root lengths were measured and photomicrographs taken of primary and nodal root cross-sections. Plants grew better under the NH 4 regime. This was probably due to controlling acidity in the pH 6.0 to 6.5 range using 2.0 mM MES. The primary root system was 16% thicker under NH 4 -N. No difference due to N form in the cross-sectional area of the primary axis base was detected, suggesting that NH 4 -N caused thicker laterals. Cross sectional areas of the first tier nodal roots were 27%-and 2nd tier roots 10.5%-greater under NH 4 -N. Changes in the stelar and cortical regions were proportional in 1st tier nodal roots, but the cortex accounted for most of the NH 4 -induced thickening in 2nd tier nodal roots. The number of late metaxylem elements was not affected by N form. Possible implications of these morphological and anatomical differences are discussed.