Vegetable crop emergence and weed control following amendment with different Brassicaceae seed meals

Brassicaceae seed meals produced through the oil extraction process release biologically active glucosinolate secondary products and may be useful as a part of biological weed control systems. Before meal can be used most efficiently, recommendations for suitable planting dates that maximize weed control but reduce crop injury must be determined. Our objectives were to determine the impact of 1 and 3% (w/w) meal applications of Brassica napus L. (canola), Brassica juncea L. (oriental mustard) and Sinapis alba L. (yellow mustard) on crop emergence and weed biomass in a growth chamber and field study. Results from the growth chamber experiment indicated that lettuce emergence was reduced by at least 75% when planted into 3% S. alba-amended soil earlier than 5 weeks after meal application. After 5 weeks, emergence was not different among treatments. Crop emergence was not reduced by any meal treatment as compared to the no-meal treatment in year I of the field study. In year 2, crop emergence in each 1.2-m row was inhibited by all meal treatments and ranged from 16 plants in the 3% B. juncea treatment to 81 plants in the no-meal treatment. The difference between emergence results in year I and year 2 is likely due to differing climatic conditions early in the season prior to irrigation, and the method of irrigation used. Redroot pigweed (Amaranthus retroflexus L.) biomass was 72-93% lower in 1% B. juncea and 3% treatments relative to the no-meal control in the first weed harvest of year 1. These same treatments had 87-99% less common lambsquarters (Chenopodium album L.) biomass. By the second weed harvest, redroot pigweed biomass in meal treatments (0.02-1.6 g m(-2)) was not different from that in the no-meal treatment (0.97 g m(-2)). Redroot pigweed biomass in 3% B. juncea plots was reduced by 74% relative to the no-meal treatment in the first harvest of year 2. This treatment also reduced common chickweed [Stellaria media (L.) Vill.] biomass by 99% relative to the 1% meal treatments. While pigweed biomass was reduced by 3% B. juncea in the early part of the season, by the second harvest this same treatment had the greatest pigweed biomass. Despite significant variability between years, 3% B. juncea did provide early season weed control in both years. Repeated meal applications, however, may be necessary to control late season weeds. Inhibition of crop emergence appears to be highly dependent on the amount and distribution of water and needs to be further studied in field settings.