Scientist Pinpoints When Weeds Are Most Meddlesome

Based on field studies he has conducted near Urbana, Ill., since 2004, ARS ecologist Marty Williams has identified specific timeframes during the sweet corn growing season when competition from weeds will inflict yield losses. Moreover, this so-called critical period for weed control is influenced by the sweet corn planting date, notes Williams, who works in ARS' Invasive Weed Management Research Unit at Urbana.

In Illinois, one of several Corn Belt states, growers plant sweet corn between April and early July, depending on the market they're targeting. May and June are peak months of planting for both fresh and processing markets.

A few years ago, Williams decided to determine the critical weed-control period and, for good measure, find out whether planting dates affect a crop’s susceptibility to weeds. His experimental design called for planting sweet corn in either early May or late June and allowing weeds to grow to various heights (e.g., ankle, knee or shoulder high) before killing them off.

In other plots, Williams let the weeds—including common lambsquarters, redroot pigweed and green foxtail—grow throughout the corn's growing season, about 83 days. This enabled him to measure and compare each weed treatment's effect on the crop's yield of marketable ears.

In general, the May-planted crop suffered the greatest yield losses—up to 85 percent, versus 15 percent for June-planted corn. Moreover, the May corn's critical weed-control period began earlier, on day 18, when the corn had reached its four-leaf stage.

For the June-planted corn, the critical weed-control period didn't begin until day 53, which was beyond the 12-leaf stage and only a few weeks before harvest. Williams attributed the difference to sweet corn’s excellent growth in late-season conditions, which gave the crop an edge over weeds.

For sweet corn growers in the north-central United States, planting in June or later could mean savings on herbicides, tillage or time spent hand-pulling weeds, such as might be practiced by organic farmers.

ARS is the U.S. Department of Agriculture's chief scientific research agency.