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Determination of the critical weed-free period in carrots grown on muck and mineral soils

Project Code: PRR07-260

Project Lead

Clarence Swanton - University of Guelph


To determine the critical period during carrot growth when the crop must be free of weeds and educate growers in reducing the use of herbicides based on recommendations resulting from this study

Summary of Results

Carrots are poor competitors with weeds and uncontrolled weeds may result in complete crop failure. Producers rely on a combination of mechanical methods and herbicides for weed control which includes the use of high doses of older residual chemicals. Carrot growers typically have a zero tolerance to weeds and in the absence of scientifically and economically based weed management guidelines, will attempt season-long weed eradication. This projects aimed at determining the critical weed-free period in carrots defined as the period after crop emergence when weeds must be controlled to prevent unacceptable yield loss. Knowledge of the critical weed-free period in carrots will enable growers to make informed weed management decisions based on economic costs and environmental benefits, that is controlling weeds only when these impact crop yield.

The study was conducted by University of Guelph scientists, in collaboration with AAFC researchers, in 2007 and 2008 at three carrot growing regions of southern Ontario: on organic soil of Bradford Marsh area, and mineral soils at Ridgetown and Simcoe research stations. Common commercial carrot cultivars (Enterprise, Pursuit, Fontana) on hill and flat cropping systems were used to compare yield and weed biomass in the carrot crops under weed-free all season, weed-free for increasing durations of time (to the 2, 4, 6, 8, 10, and 12 leaf stage of carrot development), weedy all season, and standard commercial treatment regimens.

Seeding date, the severity of weed infestation and the duration of weed emergence were found to influence the duration of the critical weed-free period in carrots. The critical weed-free period was relatively long and extended up to 1067 GDD (growing degree days), or approximately 89 days after planting, until carrots were at the 12 leaf stage, when carrots were seeded early and when weed infestation was high. In contrast, the critical weed-free period was short and lasted for a minimum of 414 GDD, or approximately 33 days after planting, until carrots were at the 4 leaf stage, when carrots were seeded later and weed infestation was moderate to low.

The key outcome from this study is that the maximum duration that the carrots must be free of weeds, without compromising yield, is until the 12 leaf stage of crop growth. Based on this finding, growers are recommended to scout fields for weeds until carrots are at the 12 leaf stage to protect the yield potential of the carrot crop. This critical period may occur over a shorter duration when carrots are seeded later in the season and weed infestation is moderate to low. It is important to consider the information gathered through scouting in making field-specific weed management decisions. Recommendations on the window for essential weed control in carrot crops will be incorporated into provincial weed management guidelines to help carrot growers optimize yields and profitability while reducing the economic and environmental consequences of unnecessary herbicide use.

For more information about this project or integrated weed management, please contact Dr. Clarence J. Swanton.

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