Source: Mark Bernards, Extenson Weeds Specialist and Lowell Sandell, Extension Educator, Weed Science, University of Nebraska 

Winter annual weeds will be regreening and beginning to grow vigorously in the next few weeks. Some fields will even turn purple by planting time. What is the impact of winter annual weeds and what should you think about in planning control programs?

Winter annual weeds begin germinating in early fall, and some species (especially marestail) continue to germinate until after planting. Depending on the species, they begin flowering in early to mid-April and most set seed and senesce by mid-June or earlier.

Control of winter annuals is relatively easy in the fall and early spring, but harder once they begin flowering as they are less susceptible to herbicides. In addition, the time between the start of flowering and when they have viable seeds can be relatively brief. Henbit, for example, produces mature seeds within two weeks of flowering. Plants can be flowering and shedding mature seeds at the same time.

Because winter annual weeds are small and their growth doesn’t overlap extensively with crop development, their impact on grain yield is sometimes overlooked. To examine this more closely we conducted research in 2007-2009 to examine how the timing of winter annual weed removal affected corn and soybean yield.

UNL Research: Delaying Weed Control Decreases Yield
Experiments were conducted in dryland fields at the South Central Agricultural Laboratory near Clay Center and at the Agronomy Farm in Lincoln. At both locations, corn and soybean were planted about May 15 each year. Crop yield from plots where winter annual weeds were removed in November of the previous year were compared to yield when weeds were removed the subsequent March, April, May or June.

In five of six site years, not controlling winter annual weeds prior to planting corn or soybean resulted in a greater than 5 percent yield loss. In four of six site years, that yield loss exceeded 10 percent. The critical dates for winter annual weed control to prevent yield loss ranged from April 15-May 5 for corn, and March 20-May 5 for soybean when corn and soybean were planted in mid-May.

One of our hypotheses when we started this research was that winter annual weeds would dry out the soil and thereby reduce crop growth. We measured soil moisture weekly each year, but did not measure any differences in soil moisture when winter annual weeds were controlled at planting or earlier. This suggests that the soil moisture used by winter annual weeds did not cause yield loss. Precipitation from March through May was relatively plentiful in 2007-2009 at the research sites, so that may explain the lack of differences. In years where early season precipitation is limited, water depletion by winter annual weeds may cause reduced growth and yield loss.

A second hypothesis was that winter annual weeds would sequester nitrogen in its biomass. We measured winter annual weed uptake of nitrogen in above-ground tissue of 4-13 lb/ac by April 15, and 21-33 lb/ac by May 15. Nitrogen sequestration by the winter annual weeds was likely a contributing factor to the yield loss.

In addition to sequestering nitrogen, winter annual weeds may harbor pests. For example, henbit and field pennycress are known hosts of soybean cyst nematode (SCN). SCN has been documented to overwinter and reproduce on the roots of henbit in Indiana and Illinois.

In areas where dense winter annual weed infestations occur on part or all of the field, we recommend controlling those weeds at least two weeks before planting to protect against yield loss. In addition, controlling winter annual weeds before mid-April will reduce the risk of weeds going to seed and increasing winter annual weed pressure in subsequent years.