Crop Performance, Weed Community and Soil Changes in an Irrigated Organic Transition Field in the Southwestern USA.

The National Organic Program (NOP) and local certifiers normally require three years of transition before a field can become organically certified.  During the transition period, crop fields can be managed to both produce an agricultural product and to provide ecosystem services that lead to improved organic crop production.  Some issues that are particularly relevant in the organic transition period include weed suppression and building up of soil quality. Such improvements in agroecosystem health can be accomplished, in part, with winter cover crops.  The objective of this study was to assess the impacts of different cover crops on weed communities, soil quality and the yield of a summer vegetable crop produced in an irrigated, organic transition field. A trial was established in Las Cruces, NM, to assess the impacts of selected winter cover crops including rye (Secale cereal L.), wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) or oats (Avena sativa L.), planted during each of two years following summer green manure of sesbania (Sesbania exaltalta). These treatments were compared with a control plot with no winter cover crop treatment (winter fallow) and sesbania planted in the summer. After two years of transition, there were no significant differences among the treatments for selected soil quality indicators including, the wet aggregate stability, permanganate oxidizable carbon, mean weight diameter of the dry aggregates and soil chemical measurements. There were significant differences in the dry biomass of the winter cereals, with rye having significantly lower biomass than the other winter covers. All the cover crop treatments led to significant weed suppression compared with the control treatment and there were few differences in weed community composition and structure among cover crop treatments. The yields of the summer vegetable, which was Swiss chard (Beta vulgaris var. cicla), planted into strip tilled, non-bedded plots were not significantly different between the control and the barley, rye, and oats plots. Swiss chard planted into the wheat plots yielded significantly less than the control. This study will continue until the end of the organic transition period, 2012-2015.