The authors of the study, a team from the U.S. Department of Agriculture's Agricultural Research Service (ARS), looked at weather data from Beltsville, Maryland spanning 18 years. They wanted to identify which meteorological phenomena had the most effect on the growth and proliferation of weeds and whether this effect was direct or indirect.
What they uncovered was that precipitation had a significant and positive impact on crop competitiveness, but only if it occurred during late vegetative or early crop growth, and an adverse effect on weed cover.
The team also identified three cultural practices that improved the competitiveness of crops while hampering weeds, albeit to a lesser degree than precipitation.
First, the researchers recommend carefully using a rotary hoe. This equipment delays and, in many instances, prevents the emergence of weeds from the soil. Hoeing needs to be done before the weeds grow a taproot. Once they have begun to sprout from the soil, it will have been too late for this method.
A rotary hoe also loosens compacted or crusted soil, making it easier for crops to emerge. The approach is particularly great for plants with large seeds, such as corn and soybeans, as they are usually planted deep enough not to be damaged by the hoe. They also tend to grow roots quickly to anchor themselves firmly to the soil.
Depending on weather and moisture conditions, hoeing can be performed five to seven days after planting. If the weather remains cool after planting, hoeing can be performed a little later, but never after the crops have begun to emerge from the soil.
Second, the team identified delayed planting as a viable approach to weed management. Research published by the Cambridge University Press compared the effects of planting early and late in the critical period for weed control (CPWC), the phase in the crop growth cycle when weed interference causes heavy yield losses. It was found that delayed planting resulted in lower yield loss compared to planting early in the CPWC. (Related: Organic farming found to eliminate plant parasites longer than conventional chemical pesticides.)
Third, the authors recommended diverse crop rotation as a means to improve crop competitiveness, cause variance in weed populations, and improve soil health. In crop rotation, a succession of crops is planted in the same soil over several years. This is in contrast to planting just one type of crop continuously.
The effects of crop rotation, particularly in terms of yield, are well-noted. Data from the USDA indicate that corn yields increase by seven percent when rotated with soybean. When rotated with alfalfa and grass hay, corn yields can rise by up to 15 percent.
The reasons behind the benefits offered by crop rotation aren't fully understood, but experts believe the following may be involved:
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