Managing Herbicice Resistance In Ryegrass

By now, many tree fruit and nut producers in California’s Central Valley are familiar with glyphosate-resistant weed species. In the north Sacramento Valley, glyphosate resistant ryegrass is widespread. In the south Sacramento Valley and through much of the San Joaquin Valley, many horseweed and hairy fleabane populations are no longer controlled with formerly effective glyphosate applications.

With herbicide-resistant weeds, spontaneous changes or mutations in the DNA of an individual plant can lead to changes in plant biochemistry and result in loss of efficacy of a single herbicide or whole classes of herbicides for the new biotype. When a resistant biotype is first introduced into a field, whether from a new mutation or seed introduction from another area, it is usually a single plant or perhaps a few individuals and probably is not a noticeable weed problem. However, if those resistant plants produce seed and the same herbicide is used again (and again) the resistant biotype can become dominant in the population in just a few generations.

For most conventional fruit and nut producers, rotating herbicide modes of action is a critical first step in reducing problems with herbicide resistant weeds while ensuring the viability of currently available herbicides. However, whenever feasible, non-chemical weed control techniques like mowing, tillage, and hand-weeding should also be used to supplement chemical weed control tactics.

One postemergence herbicide that is becoming more important in tree fruit and nut crops is glufosinate (Rely, Bayer CropScience). Rely is a very good, broad-spectrum herbicide that can provide control of many common weeds, including several glyphosate-resistant species. However, similar-sounding chemical names and Rely marketing strategies have led to some confusion about the relative strengths of glyphosate and glufosinate.

The biggest difference in weed control efficacy of these two herbicides is related to the translocation, or systemic movement, of the active ingredient once it enters the plant. Glyphosate is generally very well translocated in susceptible plants while glufosinate translocation is much more limited. Because of the reduced levels of translocation, good coverage (adequate water volume, nozzle, and pressure selection) is much more critical for glufosinate compared to glyphosate.

On small broadleaf weeds, glufosinate and glyphosate often provide very similar levels of control as long as spray coverage is adequate. Because the growing point of grass weeds is below the soil surface, these weeds can be more difficult to fully control with glufosinate; good burndown of treated tissue is usually observed, but regrowth can occur. Similarly, in established perennial weeds, glufosinate is usually less effective than glyphosate due to regrowth after the initial burndown.

Although Rely does not have the same weed control properties as glyphosate, especially for grasses and perennial weeds, it is a very useful herbicide for reducing selection pressure for new glyphosate-resistant weed biotypes and for managing existing glyphosate-resistant populations. Glufosinate and glyphosate have completely different target enzymes in different biosynthetic pathways. Thus far, there are very few reports of resistance to glufosinate and no reports of glyphosate-resistant weeds also being resistant to glufosinate.

 

Tips On Ryegrass

Annual ryegrass is currently the main weed with resistance to glyphosate in the Sacramento Valley. Here are a few important points:

• Although ryegrass is a winter annual weed, under irrigated conditions it can germinate at any time of the year.

• For the past two years we’ve been studying mid-summer (July/August) germinating ryegrass. Some of these weeds are surviving the pre-harvest burndown herbicides and entering the early fall as older well tillered plants which are difficult to control.

• Early fall applications in 2010 of pre-emergent herbicides in one Sacramento Valley almond weed control trial gave erratic control of ryegrass when they were applied before the first postharvest irrigation. According to one knowledgeable pest control adviser, applying pre-emergent herbicides after the first postharvest irrigations often gives better ryegrass control.

When properly used as part of a well-planned weed management program, Rely can be an effective weed control and resistance management tool for walnut orchards. In particular, it is important to remember that Rely does not translocate as well as glyphosate and applications should be planned accordingly. Best season-long weed control and reduced selection of herbicide-resistant weed biotypes is likely to be obtained using combinations of preemergence herbicides, postemergence applications of glyphosate and glufosinate or other burndown materials, and non-chemical control tactics wherever possible.