California Herbicide Resistance – 6 Questions On How It Compares To South, Midwest

As a weed scientist, Dr. Lynn M. Sosnoskie has a fairly unique perspective in terms of herbicide resistance. She spent much of her early career at the University of Georgia, dealing with glyphosate-resistant Palmer pigweed when the weed first gained attention as a persistent pest. Later, she moved to the West Coast, with a position at the University of California Davis where she worked with glyphosate resistant hairy fleabane and junglerice.



She is now a University of California agronomy and weed science advisor in the state’s upper San Joaquin Valley.

Weed resistance is an important consideration for growers in California but perhaps in different ways as compared to the rest of the country. In the following 6 questions, we asked her for an overview of the state’s issues and how they may or may not compare to resistance trends on the eastern side of the Rocky Mountains.

#1. To what extent do you find herbicide resistance in California, particularly in the state’s important Central Valley region?

Currently in California, 30 unique cases of herbicide resistance have been confirmed. The most frequently encountered resistances have been to the ALS inhibiting herbicides and glyphosate, with 7 occurrences each. Resistances to the ALS herbicides were, in fact, some of the first to be identified in California and initially occurred in species common to rice production systems in the Sacramento Valley.

Weeds with resistance to glyphosate have been found in both crop and non-crop areas. By crops, I’m including both field crops and permanent crops – orchards and vineyards. Non-crop areas include roadsides but also along the state’s extensive network of irrigation canals.

The development of resistance is a function of our over-reliance on one or a few active ingredients.

#2. Are Californian’s detecting species with multiple resistance?

Yes. So far, 5 weed species have populations with documented resistance to up to 4 herbicide sites of action. Those 5 species are late watergrass, barnyardgrass, hairy fleabane, horseweed and Italian ryegrass. While late watergrass can be a significant concern for some California growers, the other species are more widely distributed. People in the rest of the country regularly contend with them as well.

#3. Obviously, you left out Palmer pigweed – the main player in the South and, increasingly, in the Midwest. Is it a factor in California to any degree?

We do have resistant Palmer populations. The thing with Palmer is that we don’t really know how big the problem is. It doesn’t appear to be at the levels people deal with in the Southeast, Midsouth or Midwest.



In theory, our diverse crop rotations and more frequent use of tillage and cultivation have probably kept it at bay. One of my goals in 2019 is to do some surveys to better describe the geographic distribution and severity of resistance in Palmer amaranth in California.

#4. What kind of recommendations are being made in terms of preventing herbicide resistance or at least restricting its spread?

First and foremost, we need to closely scout after herbicide applications and carefully record herbicide performance. Those records provide the best way to quickly identify repeated instances of weed control failure.

Applicators should ensure that their equipment is properly calibrated and that they are applying effective herbicides at appropriate rates to manage the target species.

Whenever possible, diversify herbicides to reduce chemical selection pressures that result from the repeated use of a single herbicide or site of action. If allowable, incorporate physical and cultural weed control practices into a vegetation management plan.

We’re emphasizing the need to control unwanted plants when they are small and never allow escapes to set seed. Cleanliness, too – preventing seeds of herbicide-resistant weed species from moving between infested and non-infested sites. Just as important, we need to be diligent in managing weeds on roadsides, canal banks, fence lines and field margins. Problems can start there and then move into fields.

#5. During your time in Georgia, you worked on a variety of research projects, particularly in terms of resistant Palmer pigweed. That included effects of pollen movement, seed longevity in the soil, plus the use of rolled rye cover crops to suppress seedling emergence. To what extent does that kind of research in the South and Midwest apply to how Californians might approach herbicide resistance, especially among Palmer populations?

I think that this research really highlights how quickly this Palmer amaranth can become established and how difficult it can be to manage once resistance develops. For example, female Palmer amaranth plants can produce hundreds of thousands of seed each; consequently, a small infestation can become a large one in a very short period of time.

As another example,Palmer amaranth can grow very rapidly and seedlings can quickly exceed the treatments heights listed on herbicide labels that are recommended to achieve effective control. I’m hoping that the work I and other scientists accomplished will help others to develop management strategies to effectively and economically control this species before populations become over large.



#6. You were in the Southeast on the forefront of severe herbicide resistance issues there – again, primarily with Palmer pigweed. Are you seeing any parallels between California’s situation now and what you observed in the South early on – or, will California’s resistant weed issues trend far differently?

I’m very early into my tenure here in the Central Valley, so I can’t really make too many comparisons between the Southeastern US and California. The agronomic production environments are very different, and I think that really affects how quickly resistance can develop. It is important for us to recognize the work that has been done in other states when developing management plans in California.

However, it is important that we also develop data that is specific to California Palmer amaranth populations and crop environments to most effectively control this weed.