Strategies for managing weeds in a wheat, red clover, vegetable crop rotation transitioning to organic production

Weed management is one of the biggest challenges faced by organic farmers. We investigated the use of two weed control strategies, critical period (CP) of competition and no seed threshold (NST), and the effect of soil amendments for farmers transitioning from conventional to organic production. Field experiments were conducted at the Ohio Agricultural Research and Development Center in Wooster, OH. In 2001, a 4-year rotation of wheat, clover, cabbage, and processing tomato was established in soil previously in a conventional corn/soybean/forage agronomic rotation. The experimental design was a split plot in a randomized complete block with 4 replications. Main plots were soil amendments (none, raw dairy manure, composted dairy manure). Amendments were applied in spring at the rate equivalent of 101 kg N/ha and incorporated prior to planting. Subplots were weed control strategies; NST, where seedling weeds were removed weekly for the whole season and no weeds permitted to mature seeds in the field, and CP, where plots were kept weed-free for the first 5 weeks of crop growth. Evaluations included emerged weed communities both in the field and seedbank. Time taken to hand-weed was documented and labor cost of using CP and NST weed management strategies calculated. Yield of tomato and cabbage were recorded. The NST reduced redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters (Chenopodium album L.) by 30--62% and 22--60% respectively in the rotational crops. This was corroborated by weed seed data from soil samples taken in the spring following use of CP and NST strategies the previous year. Some significant effects (P ≤ 0.05) were noted among the soil amendment on density of redroot pigweed and common lambsquarters, though no clear trends were observed. Achieving the NST required 33 to 92% more labor than did the CP. However, the cost of labor required for the NST was not greater than weed control costs typically experienced by organic farmers. In a greenhouse experiment, growth and seed production of redroot pigweed was reduced in field soil amended with livestock manure. Growth and seed production of the weed was greater in soils amended with compost. Rates of manure and compost were equivalent to 2x and 3x those applied to tomato and cabbage in the field experiments.