Rebecca Champagne1, Eric Gallandt2, and John Navazio3
1PhD Candidate, Ecology and Environmental Sciences, 2Professor, University of Maine, and 3Plant Breeder, Johnny’s Selected Seeds
Interim report: February 19, 2021
Take-home points:
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In the greenhouse experiment, no differences in root and shoot morphology were detected between cultivars at one true leaf. At two and four true leaves, cultivars in the “top” category had greater shoot height, area, and mass compared to cultivars in the “middle” and “bottom” categories.
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Cultivars in the “top” category had higher crop mortality from tine harrows than finger weeders.
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Total hand weeding time (h ac-1) was higher for the hand weeding treatment across all cultivars. Finger weeding resulted in lower hand weeding time for the “top” category, while tine harrowing resulted in similar hand weeding time across cultivars.
Problem
Weed management in organic carrot (Daucus carota L.) remains an issue due to the crop’s slow emergence and growth, leading to a size disadvantage compared to the ambient weed community (Colquhoun et al., 2017). Post-emergence physical weed control (PWC) is critical due to the poor competitiveness of carrots throughout their early stages of growth, but the small size of carrots can mean high weed efficacy comes at the cost of crop injury. Cultivar selection is a method that could improve PWC selectivity and result in greater weed control efficacy by selecting cultivars which exhibit higher competitive ability. Cultivars selected on the basis of large shoots have shown an inverse relationship to weed biomass due to improved crop competitiveness (Colquhoun et al., 2017). Carrot root length and area are also traits that could help increase PWC selectivity due to greater soil adherence and anchorage in the soil (Fogelberg and Gustavsson, 1998).
Approach
In this study we evaluated several carrot cultivars, selected to represent a spectrum of carrot root and shoot growth morphology, in both a greenhouse and field trial. Cultivars were divided into “top”, “middle”, and “bottom” categories based on their known growth morphology (Table 1). Objectives included: (1) analysis of carrot early growth traits (Fig. 1a); (2) recording injury resulting from PWC with select tools (Fig. 1b); and (3) analysis to determine if early growth traits correlate with crop injury from PWC to serve as a proxy for cultivar “cultivation tolerance”.
Figure 1. a) carrot shoot scans using WinRHIZO software to determine shoot area, and b) carrots in a tine harrow treatment plot at the Roger’s Farm.
Table 1. Selected carrot cultivars and respective suppliers for greenhouse and field trials.
| Cultivar | Cultivar Category | Supplier | Greenhouse Trial | Field Trial |
|---|---|---|---|---|
| Bolero | Top | Johnny’s Selected Seeds | Y | Y |
| Red Cored Chantenay | Top | John Navazio/Johnny’s Selected Seeds | Y | |
| Yellowstone | Top | Johnny’s Selected Seeds | Y | Y |
| Dragon | Middle | John Navazio/ Johnny’s Selected Seeds | Y | Y |
| Napoli | Middle | Johnny’s Selected Seeds | Y | |
| SFF | Middle | Phil Simon | Y | Y |
| Mokum | Bottom | Johnny’s Selected Seeds | Y | |
| NB 8483 (NB1) | Bottom | Phil Simon | Y | Y |
| NB 8524 (NB2) | Bottom | Phil Simon | Y | Y |
Results
While no early growth characteristic differences were detected at one true leaf in the greenhouse trial, cultivars in the “top” category had greater shoot height, area, and mass at two and four true leaves compared to other categories (p <0.0001; data not shown). Interestingly, this cultivar category also had greater mortality from tine harrows in the field trial, as tine harrows would come into contact with carrot shoots to a greater degree than finger weeders (p =0.012; Fig. 2). Marketable total crop fresh weight was similar between cultivars in the “top” and “middle” categories, with the exception of ‘Dragon’ (p <0.0001; Table 2). Cultivars in the “bottom” category had comparable or lower marketable total fresh weight (Table 2). Cultivars in the “bottom” category have smaller roots and shoots at two and four true leaves and are likely more susceptible to damage from PWC.
Figure 2. Mean (±SE) crop mortality (%), separated by cultivar category and tool. Different letters indicate significant differences between means at α=0.05.
Table 2. Mean (±SE) marketable versus unmarketable crop yield response variables. Data is averaged across treatments. Different letters represent significant differences at α=0.05.
| Cultivar | Marketable Total Fresh Wt. | Marketable Root Fresh Wt. | Unmarketable Total Fresh Wt. | Unmarketable Root Fresh Wt. |
|---|---|---|---|---|
| --------g m-2-------- | --------g m-2-------- | --------g m-2-------- | --------g m-2-------- | |
| Bolero | 1586±145 b | 1205±112 ab | 160±21 | 110±14 |
| Yellowstone | 1419±166 bc | 823±95 c | 252±44 | 139±26 |
| Dragon | 2275±178 a | 1580±122 a | 265±31 | 175±21 |
| SFF | 1058±115 bc | 865±94 bc | 253±46 | 204±37 |
| NB1 | 1023±68 c | 828±57 bc | 162±20 | 126±16 |
| NB2 | 445±39 d | 341±31 d | 248±28 | 186±22 |
Next Steps
Correlation analysis to determine relationships between carrot early growth characteristics and field trial variables, such as crop mortality, and marketable and unmarketable yield, will be used to develop criteria for carrot cultivar “cultivation tolerance”. Results will be shared with plant breeders and farmers to help improve organic carrot breeding and production.
Both the greenhouse and field experiments will be conducted again in 2021 with the same cultivars to provide insight on temporal responses.
Literature Cited
Colquhoun, J.B., Rittmeyer, R.A. and Heider, D.J. (2017). Tolerance and suppression of weeds varies among carrot varieties. Weed Technology. 31:897-902.
Fogelberg, F. and Gustavsson, A.M.D. (1998). Resistance against uprooting in carrots (Daucus carota) and annual weeds: a basis for selective mechanical weed control. Weed Research. 38:183-190.