Planting corn - control ryegrass first

Mid-South corn growers are thinking about planting because planes flying burndown herbicide applications have been active in the Delta when the weather has been good during the past few weeks.

The burndown calls have also increased since the middle of January, and many of these questions have been about controlling glyphosate-resistant Italian ryegrass. Hopefully, this is a sign that more growers and consultants are aware of the severity of the ryegrass problem and are taking appropriate steps to manage it early, before it becomes too large to control.

Unfortunately, research plots cannot be sprayed with an airplane, and the weather in January delayed many of our treatments for glyphosate-resistant Italian ryegrass. We were able to spray a few treatments during the third week of January.

Based on research over the last two years at Stoneville, mid-January to mid-February is the optimum time to apply clethodim (Select Max or 2-pound generic formulation) for control of glyphosate-resistant Italian ryegrass.

Although these early burndown applications of clethodim generally work slowly (five to six weeks for optimum effect), making the application as early as possible has several benefits. Without fail, smaller weeds are easier to kill.

Today, most of the Italian ryegrass I have seen in the Mississippi Delta is still at a manageable size. However, I drove through northeast Louisiana earlier this week, and the Italian ryegrass in that area seems to have already entered the period of rapid growth that occurs in this species when temperatures begin to increase in late-winter.

In addition to Italian ryegrass still being relatively small right now, the “spring flush” has yet to begin. Little to no Italian ryegrass has emerged since mid-December in our monitoring plots scattered across the Delta.

Attempting to control fall-emerged and spring-emerged Italian ryegrass with one postemergence herbicide application is usually unsuccessful due to poor spray coverage. The fall-emerged plants are larger and their leaves will intercept the herbicide before it reaches the smaller spring-emerged plants.

Early burndown application

Making an early burndown application in January or early-February for Italian ryegrass allows time to determine how well the first application worked and flexibility in deciding how to control escapes.

An early burndown targeting Italian ryegrass is more critical in fields to be planted to corn than those that will be planted to other crops. The window for burndown herbicide application is shortened in corn because of the early planting dates.

Due to the competitive nature of Italian ryegrass, it is essential to control this weed prior to planting corn. Clethodim must be applied at least 30 days prior to planting corn. If the clethodim application for Italian ryegrass was not timely, then corn planting may have to be delayed to avoid crop injury.

Most people consider corn to be one of the more competitive crops. It usually emerges quickly, grows rapidly, and shades the middles in a much shorter time than cotton or soybeans.

A general rule is that if corn can be maintained weed-free for the first four to five weeks after emergence, then there will be no effects on yield due to weed competition; however, this rule applies to weeds that emerge at the same time or following corn. It does not apply to Italian ryegrass, which is usually 12 to 24 inches tall during corn planting season in the Mississippi Delta.

When corn is planted into standing Italian ryegrass, the aboveground parts of Italian ryegrass will reduce the light and space available to developing corn seedlings while ryegrass roots will limit the water and nutrients that can be taken up by corn.

Another motivation for controlling Italian ryegrass prior to planting is the lack of over-the-top herbicide options available in corn. Some ALS inhibitors (Accent, Resolve, Steadfast, and Stout) labeled in corn have activity on Italian ryegrass. Unfortunately, Italian ryegrass is resistant to ALS-inhibiting herbicides in 13 counties in the Mississippi Delta, rendering this chemistry ineffective for ryegrass control in many areas. Therefore, it is imperative to control all Italian ryegrass before corn is planted.

For corn, a spring burndown program for glyphosate-resistant Italian ryegrass should begin with clethodim (12 to 16 ounces of Select Max or 6 to 8 ounces of 2-pound clethodim formulation) applied not less than 30 days before planting.

Italian ryegrass that escapes the early burndown application of clethodim should be treated with paraquat (3 to 4 pints of Gramoxone Inteon or 2 to 2.67 pints of 3-pound paraquat) plus atrazine (1 quart) before corn emerges.

Italian ryegrass field day

Glyphosate-resistant Italian ryegrass continues to pose major problems for growers in many areas of Mississippi and across the Mid-South. On March 10, Mississippi State University will host a field day highlighting management programs for glyphosate-resistant Italian ryegrass. Registration will begin at 7:30 a.m. at the Capps Center on the campus of the Delta Research and Extension Center in Stoneville, Miss. A field tour of research plots is planned; however, the program will be moved indoors in the event of inclement weather.