March 28, 2008 

Weed-and-feed - the combined application of a herbicide and nitrogen fertilizer application on winter wheat in the early spring - is a popular practice. A producer is afforded many advantages when using this practice.

Figure 1. Winter wheat sprayed in the early spring showing fertilizer damage (left) and eight days later after recovering from most of the damage.

Advantages

• Combining the operations is economical, requiring only one trip through the field rather than two. Other economies can be realized by being able to choose the necessary herbicides to match existing weed pressure and being able to fine-tune the amount of fertilizer to match current crop needs, nitrogen fertilizer prices, and the expected price for the harvested crop.
• Fall nitrogen fertilizer application encourages excess fall growth 1) which can deplete soil moisture necessary for winter survival and 2) invite disease problems. The onset, build-up, and overwintering of several diseases are encouraged by vigorous fall growth of well fertilized wheat.
• A spring nitrogen fertilizer application it is less likely to lose nitrogen. Fall-applied nitrogen can be lost through leaching. Spring application leaves less time for the applied N to be subject to conditions leading to losses.

Potential Injury

The leaf burn from nitrogen application is often aggravated by the co-application of herbicides. Producers can be alarmed at the sight of a lush green wheat field turned yellow by a weed-and-feed nitrogen application. They may question whether the fertilizer application will pay off at harvest and whether the leaf burn injury is great enough to decrease yields.

The injury data in Table 1 were collected from a study conducted in 2007 in southwest Nebraska at sites near McCook and North Platte at UNL's West Central REC Dryland Research Farm. Herbicide treatments were applied to Jagalene winter wheat in combination with 28-0-0 (urea ammonium nitrate). Injury such as this, and even greater, is common when combining fertilizer and herbicide treatments. Injury tends to increase as fertilizer and /or herbicide rates increase. Herbicide products containing dicamba tend to produce slightly more crop injury. Dicamba also can cause the wheat plant to assume a "sleepy" appearance, with the leaf blades held less erect. (Note that in both locations all plots, including untreated plots, include injury from an extended period where temperatures dropped below freezing and snowfall occurred.) The injury usually disappears or is hidden by new wheat growth as the crop continues growing (Figure 1).

While injury symptoms are fairly common, the effect on grain yields is less predictable. Data collected in our 2007 studies showed a yield decrease with most cases (Table 2). However, in this study the two locations had ample fertility in the fall before wheat was sown. These studies represent an undesirable scenario where every additional component of the weed-and-feed treatment caused slightly more injury and yield loss. It is believed that in fields of lower fertility, yield increases would result from the fertilizer application.

Reducing Potential Injury

Using the following steps can help you reduce potential yield losses from a weed-and-feed operation:

• Spray early when there is less foliage to intercept spray solution. More fertilizer reaches the soil immediately where it can do the most good. Also, the herbicide will make better contact with emerged weeds if the wheat canopy isn't covering them.
• Using lower herbicide and fertilizer rates when practical will result in less injury to the wheat crop, reducing the chance of yield loss. Also, injury can be reduced by using more water for the carrier.
• While weed-and-feed is a great operation, always consider separate operations; matching spray herbicide operations to when most weeds have emerged and fertilizer operations to when the crop can most benefit. The nitrogen fertilizer should be applied as soon as field conditions permit in the spring. Do not apply fertilizer on frozen soil.
• Separate operations also allow the use of streaming nozzles to apply the fertilizer. More fertilizer can be directed to the soil, with less coverage of the plant with fertilizer solution, reducing how much of the plant that experiences leaf burn. Herbicides can't be applied with streaming nozzles.
• Don't plant early. As with fall fertilizer applications, if the wheat crop is not planted early, a producer can avoid excessive fall wheat growth. This excessive growth depletes soil moisture and encourages wheat diseases.

Robert Klein
Extension Crops Specialist
West Central REC, North Platte