Do cover crops suppress weeds in the U.S. Southeast? A meta-analysis

Abstract

Cover crops (CCs) have shown great potential for suppressing annual weeds within agronomic cropping systems across the US. However, the weed suppressive potential of CCs may be moderated by environmental and management factors that are specific to certain geographic areas and their associated characteristics. This may be particularly true within the Southeast (SE) region of the US where higher mean annual temperature and precipitation generate favorable conditions for both cover crop (CC) and weed growth. To understand the effects of this regional context on CCs and weeds, a meta-analysis examining paired comparisons of weed biomass and/or weed density under CC and bare ground conditions from studies conducted within the SE was conducted. Data were identified and extracted from 28 journal articles in which weed biomass and/or weed density were measured and cash crop yield data if they were provided. Fourteen studies provided 142 comparisons for weed biomass, 23 studies provided 139 comparisons for weed density, and 22 studies, pooled over both weed response variables, provided 144 comparisons for cash crop yield. CCs had a negative effect on weed density (p = 0.0016), but no effect on either weed biomass (p = 0.16) or cash crop yield (p = 0.88). The mean relative reduction in weed density under CCs was 44 percent. Subsequent analyses indicated that CC biomass was the key factor associated with this reduction. Weed density suppression was linearly related to CC biomass; a 50 percent decrease in weed density was associated with 6.6 Mg ha-1 of CC biomass. Edaphic, geographic, and other management factors had no bearing on this suppressive effect. This highlights the importance of generating adequate CC biomass if weed suppression is the primary objective of cover crop use, and the potential for CCs to reduce weed density over diverse soil, climate, and farm management conditions.