Winter annual weeds, such as field pennycress and henbit, have grown into a serious management concern in no-till corn and soybean for some growers. Managing these weeds before planting no-till corn can be particularly important to managing soil temperature, allelopathy, cutworms, and grain yield according to previous studies.

Something that has been less emphasized is the use of soil nitrogen (N) by winter annual weeds and the effect of this on corn yield when N is limiting.

Corn growers may choose to delay the first application of herbicide until near the date of corn planting to limit the number of passes they need to make across the field and avoid the cost of additional applications.

However, winter annual weeds complete most of their vegetative growth and N uptake in early spring, so delayed herbicide applications may lead to additional N use by these plants and affect N mineralization of weed biomass during the growing season.

Reporting in the July-August issue of Agronomy Journal, researchers from Kansas State University evaluated the effects of delayed herbicide applications to control winter annual weeds on N availability and grain yield in no-till corn following soybean at 14 sites in eastern Kansas. The study took place in 2010 and 2011, and all sites had naturally occurring populations of winter annual weeds.

All combinations of three herbicide application dates (November-March, April, and May) and five N rates (0, 15, 30, 60, and 120 pounds N per acre) were used and the effect on corn plant population, soil nitrate-N, early corn N uptake, corn ear leaf chlorophyll meter readings at silking, and grain yield were recorded. Chlorophyll meter readings are significantly correlated with N concentration in corn tissue and therefore used as a tool for assessing N status in corn ear leaves.

The results showed that delaying herbicide application until spring—when winter annual weeds are actively growing and taking up N—can reduce available N for the subsequent corn crop. Across sites and years, the average N uptake by winter annual weeds was 16 pounds per acre by May.

Across sites, years, and N fertilizer rates, delaying weed control until near the time of corn planting reduced corn plant populations by 526 plants per acre, early corn N uptake by 24%, corn ear leaf chlorophyll meter readings at silking by 3.4%, and grain yield by 7.6 bushels per acre, compared to the earliest herbicide application in March or November.

Nathan Mueller, the lead author of the research, says that “Even when using average N rates for this region, corn growers can avoid reductions in N supply and increase corn yields by controlling winter annual weeds as early as possible before planting, including fall herbicide applications.”

In their study, for example, the researchers found that when herbicide applications were delayed until April and May, an additional 15 to 30 pounds N per acre was needed to achieve similar yields to those obtained when weeds were controlled on the earliest application date.

“If corn growers get in a pinch due to adverse weather conditions and don’t control winter annual weeds until planting, extra N could be applied as starter or side-dressed later to compensate,” Mueller says.

However, this study also suggests that yield limitations from factors other than N may be associated with delayed herbicide applications. So it’s best to target no-till fields with winter annual weed pressure for fall or early, pre-plant herbicide applications to decrease the probability of reduced corn yield.

View the abstract of the paper: Winter Annual Weed Management and Nitrogen Rate Effects on Corn Yield.