Vigilant Seed Bank Reduction for Weed Control

Vigilant Seed Bank Reduction: Whatever it takes, don’t let weeds set seed.

By Patrick Cavanaugh, Farm News Director

 

For the past 15 years, Robert Norris, professor emeritus and vegetable crops weed specialist, UC Davis Department of Plant Sciences, has continued to attend Weed Day each year at UC Davis and to contribute weed photography for CalPhotos, a UC Berkeley Digital Library Project photo database of world-wide plants, animals, landscapes, and other natural history subjects developed to provide a testbed of digital images for computer science researchers to study digital image retrieval techniques. Norris was involved with initiating the Plant Protection and Pest Management Graduate Program at UC Davis.

 

“I’ve been a botanist since I was 14 years old,” Norris said, “and I still have a lot of passion regarding weed control.” Norris has a strong and steady philosophy on weed control and it all comes down to seeds. “The last 25 years of my work, I looked at population dynamics of weeds, like seed longevity in the soil and what we call the size of the seed bank also known as the seed production by weeds. That’s really where I spent most of my time.

Field Bindweed
Field Bindweed

 

“I found that most people have a very poor idea of how many seeds are produced by a weed. This led me to question some of our current management philosophies; namely, the one that comes out of entomology—the use of thresholds (or how many weeds need to be present before treating them),” noted Norris. “I felt that for weed science, thresholds were not the way to go, and my position has been vindicated by the problems we’ve run into using thresholds.”

 

Norris offered the example, “Barnyard grasses are probably one of our most serious summer grass weeds. A small plant can produce 100,000 seeds; while a big plant, well over a million. I can remember going put in a tomato field years ago and looking at one barnyard grass plant. Because I had been working with it, I can say that plant probably put out 50,000 seeds. If you spread those seeds around an acre, that’s enough to give you serious yield loss the next year,” Norris explained. “Again, that’s one plant, spread out over an acre. Obviously its seeds wouldn’t spread over an acre [on their own], but with our tillage equipment we would move it around quite a bit.”

 

“My bottom line for about 30 years now is: Don’t let the weeds set seed. Whatever it takes, don’t let them set seed,” Norris said. If you follow that philosophy, Norris said after a while you drive the seed bank down.

 

“Many people don’t realize this, but some of our really big growers got on to it a long time ago. One farming operation I worked with for years, J. G. Boswell Co., with most of its land in Kings County. “I knew the manager in the late ’50s, into the ’70s. He now is retired now, but he came to this conclusion himself back in the late ’50s,” Norris said. “I haven’t been on Boswell’s property now for 20 years, because I retired. However, if you go down there, you will not see a weed problem, at least not like most growers.”

 

“The difficulty really is, in order to carry out this philosophy, you need to use hand labor for weed management and it is becoming less and less easy to find,” explained Norris. “Most weed management is done on a one-year one-crop basis; whereas, the type of management we’re talking about where we’re really thinking seed bank dynamics, has to be done over multiple years. Another big problem that I still see is if you miss one year, you can undo 5 to 10 years of what you’ve just been doing, because of this high seed output,” he said.


NEVER LET ‘EM SET SEED, by Robert Norris, Weed Science Society of America.


 

Jasieniuk on Weed Evolution

Tracking Herbicide Resistance in Weed Evolution  

By Emily McKay Johnson, Associate Editor

 

Marie Jasieniuk, professor and weed scientist at UC Davis College of Agricultural and Environmental Sciences, discussed her groundbreaking research, “I work on the population genetics and evolution of agricultural weeds and invasive plants,” she said. “We use molecular tools to look at the origins and spread of weeds. We also use molecular tools and genetic studies to understand the evolution of herbicide resistance in weeds,” Jasieniuk added, “to be able to propose management approaches that reduce the likelihood of further evolution and spread of resistant weeds.”

UC Davis Annual Weed Day 2016
UC Davis Annual Weed Day 2016

Jasieniuk and her team identify the origins of invasive plants, and determine how they were introduced. “We study how they were introduced, how they have spread and whether they have been introduced multiple times. Again, if we understand how they’re spreading, we can do something to try to stop the spread,” she said.

Italian rye grass, a weed Jasieniuk is currently studying, is problematic because it is resistant to Roundup, a popularly used weed and grass killer by growers. “UC ANR Cooperative Extension specialist, now emeritus, Tom Lanini, and I sampled over 100 locations of Italian rye grass and tested them for resistance to Roundup ten years ago,” she said about the project, funded by USDA. “Last year, we re-sampled all of those sites, and we’re re-testing to see if there’s been an increase or a decrease or no change in resistance to glyphosate, to Roundup,” she said.

Roundup isn’t the only weed and grass killer available on the market. “We’re looking at resistance to three other herbicides,” she said. Working with growers to determine the most efficacious weed treatments that also reduce the likelihood of wood resistance to herbicides,” Jasieniuk explained, “We interview growers about their herbicide use, non-chemical approaches, and integrated management techniques to identify management practices that correlate highly with low or no resistance,” she explained.

Resistance management is found to be more effective with a rotation of various herbicides. “What you want to do is rotate different types of herbicides with different modes of action,” Jasieniuk said. “Perhaps do tank mixes and incorporate non-chemical approaches as well,” she added.

Eliminating weeds can be as simple as disking and digging them out with a shovel when there are only a few. “I think, in many cases, this would have done a lot to prevent new weeds from coming in and certainly resistant weeds from spreading,” she noted.