In a series of field experiments, we evaluated the impact of four in-row mechanical weeding tools on carrots (Daucus carrota) and two surrogate weed species: yellow mustard (Sinapis alba), and German millet (Setaria italic). Finger weeders (F), torsion weeders (T), flextine harrows (X), and hilling discs (HD) were applied to carrots and weeds individually and in combination (tool ‘stacking’) when the carrots had 1 true leaf, approximately 25 days after planting. Tools combinations gave greater weed control and selectivity than single tools. The F+HD combination particularly, resulted in the greatest selectivity and reduction in the time required for subsequent hand weeding. No tool effects were detected on carrot quality or yield. However, across all tools, lower final carrot densities were associated with lower yields. The finger weeder controlled more weeds as soil moisture increased, or when more soil was moved into the carrot row. Whereas the torsion weeder did not appear to kill weeds through hilling and its efficacy was greater under relatively dry soil conditions. In many cases, yellow mustard was more susceptible to death by burial than German millet. One approach to improving the selectivity of mechanical cultivation tools in carrots is to identify varieties that are most tolerant to those tools. Identification of such “cultivation-tolerant” varieties and their associated traits may also be helpful for breeding efforts aimed at reducing costs associated with weed control. Field trials were conducted on sandy soils in central Michigan to compare cultivar response to each of four types of in-row weeding tools (finger weeder, torsion weeder, hilling discs, flextine harrow). Tools were applied to eight different cultivars of commercially available carrots at three sites in 2016 and 2017. Characterization of root vs shoot partitioning for each cultivar was evaluated based on the area of fresh root and shoot tissue at the time of cultivation. Differences in the survival rate of carrot cultivars were observed for the torsion weeder at all three sites and for the flextine harrow in one of three sites, but not for the finger weeder or hilling discs. At the time of cultivation, carrot cultivars varied in their root size at all three sites, but varied in shoot size at only 1 of 3 sites. Over all sites, there was a positive relationship between carrot shoot size and tolerance to the finger weeder, and a positive relationship between carrot root size and tolerance to the torsion weeder, although both relationships were only marginally significant (p=0.095; p=0.061). These results demonstrate that commercially available carrot cultivars vary in their tolerance to cultivation tools, and suggest that early partitioning to root tissue confers tolerance to tools that uproot (torsion weeder), while early partitioning to shoot tissue confers tolerance to tools that bury (finger weeder). A relationship was also observed between cultivar seed size and plant size at the cotyledon and 1st true-leaf stage, suggesting that screening of carrot cultivars or seed-lots for large seed size may be a useful strategy for improving carrot tolerance to cultivation tools, thereby improving the selectivity of those tools.
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