Annual ryegrass (Lolium multiflorum) is the most damaging weed in soft red winter wheat in Northeast Texas. It was originally introduced to this region as a forage plant for grazing but has become a noxious weed. Annual losses from this pest run in the millions of dollars. When the sulfonylurea (SU) herbicides (Amber, Glean) were developed in the mid-1980s they provided excellent control of this weed. However, over time, efficacy of the SUs has decreased, causing the weed to reemerge as a difficult problem for area wheat producers.

Hoelon, an ACCase mode of action herbicide, replaced Aber and Glean in the late 1980s, but its efficacy has been declining over the last few years.  Axial XL, another ACCase herbicide, was introduced in 2008 and has been the most effective herbicide for control of annual ryegrass in recent years. Although Axial XL is still providing good ryegrass control in some regional wheat fields, we are beginning to see reduced control with this product in many fields across the region.

Over the past two years, we have had good success controlling resistant ryegrass with a two-step herbicide program of Axiom (delayed pre-emergence to spike) followed by Axial XL (2- to 3- tiller ryegrass). Two additional herbicides have recently been labeled for a delayed pre-emergence to early post emergence application, Zidua and Anthem Flex. Both contain the active ingredient pyroxasulfone, and have been effective in controlling resistant ryegrass in research trials.

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The IPM approach

Wheat can be successfully grown in fields that are infested with herbicide resistant annual ryegrass.  Each of the following suggestions will improve the chances of success in infested fields.  When all of these tactics are employed together in an IPM approach, the odds of producing a profitable wheat crop are greatly enhanced.

Cultural control

Annual ryegrass populations (both resistant and susceptible) in cropland can be greatly reduced by using cultural and mechanical means in combination with chemical control techniques.

Crop rotation

The most effective management technique to control herbicide resistant annual ryegrass populations is crop rotation.  Where resistant ryegrass in observed following a timely Axial XL application, the best option is to rotate that field to another crop for one or two years.  The best rotation option appears to be corn, followed by grain sorghum, cotton, and soybeans.  Sunflowers may be an additional option but more research needs to be done to ascertain the agronomic viability of sunflowers in this region. 

Crop rotation enables growers to attack their ryegrass populations from several angles.  First, rotation to summer annual crops allows growers to reduce ryegrass populations that emerge over the fall and winter months with tillage and/or non -selective herbicides.  In addition, the introduction of other classes of herbicides into the rotation crop program will further suppress any ryegrass “escapes” from the fall-winter management program.

Sometimes crop rotation is not a good option.  Soils with the worst ryegrass infestations in the region are the Crockett and Wilson silt loams.   The Crockett series consist of deep, moderately well drained, loamy soils on uplands.  They have slopes ranging from 1 to 5 percent.  In contrast, The Wilson series consists of deep, poorly drained soils.  Wilsons are relatively flat, with slopes in the zero to 1 percent range.  Both soil types are often referred to as “greyland” soils.

These soils are best suited to grow wheat and cotton.  Grain sorghum has some problems with lodging when grown on these soils, but its drought tolerance would be advantageous.  They are too droughty to produce dependable yields of corn and soybeans.  Cotton is not a good short term option for most producers in the region as it requires specialized harvesting equipment.  These considerations have made wheat the crop of choice for these greyland soils.  

Plant certified seed

Plant certified seed or at least clean your bin run seed prior to planting. These are relatively inexpensive ways to keep your ryegrass infestations confined to specific fields, and minimize its spread to non-infested fields.  Over the past 30 years, we have seen many fields that became infested with ryegrass from “bin run” wheat seed that had not been cleaned prior to planting.   

Clean harvesting equipment

Harvest and tillage equipment should be cleaned before moving from infested fields to minimize the spread of ryegrass seed.  It would be best to harvest the ryegrass infested fields last.

Plant ryegrass infested fields late

Ryegrass seed begins to germinate in the fall as soil temperatures cool, and rainfall returns to the region.  But not all ryegrass is created equal.  Improved ryegrass varieties developed and introduced annually for forage production germinate more uniformly than the native ryegrass (often referred to as “feral” ryegrass) that has evolved from the earliest plantings in the region. Feral ryegrass populations exhibit greater dormancy and have plants that germinate later in the growing season.  We used Gulf ryegrass for comparison because the feral ryegrass populations we see today likely evolved from the Gulf introductions of the 1950s.  The following table illustrates this phenomenon.
 

Since the highest percentage of feral ryegrass seedlings emerged late in this study, it stands to reason that late tillage will destroy more of these seedlings.

Consider planting into a stale seedbed

Some local producers have been planting wheat in a stale seedbed with good success.  They prepare the seedbed early in the fall, and allow the ryegrass seed in the germination zone to emerge.  Just prior to planting, they spray the field with glyphosate (i.e. Roundup), and then plant the wheat seed.  Spraying after planting but before the wheat has emerged is another option.

This practice fits well into a resistant ryegrass management system, when the wheat is planted as late as possible to reduce the ryegrass germination prior to the onset of cold weather.  Leaving the soil undisturbed will minimize the movement of additional ryegrass seed into the germination zone.  Local ryegrass populations are not as susceptible to glyphosate as they used to be, but it is still effective on seedling ryegrass.

Use row placed phosphate fertilizer

Research has consistently shown that row placed phosphate fertilizer is one of the best management practices for wheat, as well as other row crops.  One pound of phosphate in the row is roughly equivalent to two pounds, broadcast.  This is one of the few inputs that can be reduced with no penalty in yield, and it is particularly attractive when phosphate prices are high. 

This practice is also advantageous in a resistant ryegrass management program.  Row placed phosphate is readily accessible to seedling wheat, and it enables the plants to rapidly gain a competitive advantage over seedling ryegrass emerging between the rows.  Vigorous, well-nourished seedling wheat plants are very competitive with seedling ryegrass plants.  This early advantage to wheat is critical, as ryegrass becomes more competitive as the growing season progresses.

Plant an earlier maturing variety

Late maturing varieties are generally developed in more northern climates, and their progression is determined more by day length than temperature.  This “safety mechanism” prevents the plants from jointing too early in mild winters and succumbing to a late freeze in the spring. These varieties spread by tillering in the fall and early winter but do not produce very much early forage.  They are designed to spend more of their life span in colder weather, and often under snow cover.

In contrast, earlier maturing varieties are developed in southern breeding programs and their development is often controlled more by temperature than day length.  They generally produce more fall and early winter forage, which is desired by livestock producers.  It is this same early forage characteristic that makes these varieties attractive in a resistant ryegrass management program. The table below lists the commonly grown varieties in the region, and their relative maturities.

Chemical control: two-step herbicide program

In addition to Axiom, we now have two additional herbicides labeled to control resistant ryegrass in winter wheat, Zidua and Anthem Flex.  Both contain the active ingredient, pyroxasulfone, and are very effective in controlling herbicide resistant annual ryegrass. None of these products should be sprayed on unsprouted wheat seed, but all of these materials can be sprayed as a delayed pre-emergence (after the wheat is sprouted but before it emerges) for best results.  Later applications (up to 1-2 leaf ryegrass) can also be effective, but control is not quite as consistent as the delayed pre-emergence applications.  Of the three products, Axiom is probably the best choice as a stand-alone on 1- to 2-leaf ryegrass, as it contains a low rate of metribuzin, which will provide some burn down activity on the emerged ryegrass.  We have had some promising results tank mixing metribuzin with Zidua and Anthem Flex to enhance the postemergence ryegrass control, but do not have enough data to recommend these tank mixes in 2014.

Suggested use rates of these products as a delayed pre-emergence application on the silt and clay loam soils of northeast Texas are as follows.  Always read the label before applying any of these materials. 

Herbicide                     Rate/Acre

Axiom                          6 oz.

Zidua                           2 oz.

Anthem Flex                3 to 3.5 oz.

When used according to label specifications, Axiom, Zidua, and Anthem Flex should provide 80 percent to 95 percent control of resistant ryegrass.  Axial XL can then be used to control the remaining ryegrass plants that “escape” the early treatments.  Axial XL should be applied to ryegrass when it is in the two- to three-tiller stage of development, which generally occurs in early to mid-January in northeast Texas. 

Growers will likely see a few ryegrass plants survive this two- step treatment.  However, the ryegrass escapes will be suppressed by the wheat crop, and are not visible until after the wheat is headed.  Our research has shown these late emerging ryegrass plants are not competitive and do not significantly reduce grain yields. 

The following varieties have shown good tolerance to Axiom and pyroxasulfone:

USG 3555, USG 3295, USG 3251, Syngenta Magnolia, Syngenta Coker 9553, Syngenta Oakes, Syngenta Jackpot, Terral TV 8525, Terral TV 8861, Pioneer 25R30, Pioneer 25R40, and Pioneer 25R47.

A. Braley, Research Technician, Texas A&M University-Commerce;  R. Sutton, Assistant Research Scientist, Texas A&M AgriLife Research;   S. Stewart, Research Technician, Texas A&M University-Commerce,  D. Reid, Agronomy Professor (retired), Texas A&M University-Commerce