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Weed economic thresholds: Maybe not

Developed primarily by entomologists for insect control, it is also a standard in weed management. However, longtime University of California weed scientists Robert Norris is convinced the philosophy doesn’t have a place in weed science. He even goes so far as to allow that said weed science made a “major tactical error” in adopting the principle.

Norris said in tolerating any weeds — certainly beyond a seed setting stage — only compounds the problem and makes the farmer increasingly dependent on herbicides. And, it also invites resistance to ongoing herbicide use.

Norris told the Western Society of Weed Science recently in Coeur d’Alene, Idaho that current economic weed threshold levels only explode seed production into the thousands and even millions and the problem never goes away.

Weed tolerance levels should be zero, he believes. That is not a popular position in today’s politically correct world of integrated pest management. Nevertheless, Norris believe weed IPM is locking into perpetual herbicide use.

Growers, he said, want to whittle down their weed problems each season. However, they are only compounding it with today’s economic thresholds. Weed control should be a science of managing population dynamics rather than a single season economic practice.

Where weeds are allowed to set seed, a seed bank is created. This seed bank must be factored in into any economic assessment of weed problems, but he does not believe there has been enough research in weed seed-setting dynamics.

Norris is an expert in barnyard grass, a major weed in sugar beets, tomatoes and other California crops. Over the years the seed production capability from a single plant has been anecdotally and scientifically estimated at from 8,500 to 40,000. However, in 1992 Norris determined that that number is actually more than 700,000.

Controlling weeds after they have set seed in numbers like that lock farmers into a seed bank problem that may never go away. For example, velvetleaf weed seed have remained viable in soil for 17 years before it emerged.

To demonstrate what he believes is the fallacy of using insect pest economic threshold philosophy in vegetation management, he said there are only a few insects where one insect’s offspring can total 1,000. Most weed species, however, are at least that high and it is not uncommon to find the ratio of one plant producing 10,000 seed. In some cases the ratio can be one to one million seeds per plant.

The other flaw in translating an economic thresholds idea from insects to weeds is that with bugs, there is generally more than one generation per year. With weeds there is only one generation per season. Although some insect pests overwinter, most do not. Just the opposite is true with weeds. If weeds are allowed to go to seed, the population can explode from one year to the next.

Norris recommends the “wildfire paradigm” be adopted for weed control. “Don’t let the fire get started and if it does start, put it out when it is small.” One way to put it out early is by rouging fields of single weeds, but Norris said most farmers do not consider a minor infestation worth the effort. One or two weeds could lead to a perpetual problem.

It takes a long-term commitment to achieve zero weed tolerance. It may cost more initially and force growers to use hand labor more,” said Norris, “but it may not be that much more costly,” he added after three years of a long-term study on the economics of zero tolerance.

“As populations go down, your hand hoeing costs will go down,” he said.

The respected weed control specialist who joined the UC Davis staff in 1967 said there are growers who are successfully practicing zero tolerance. One of those is the J.G. Boswell Co., California’s largest cotton producer, based in Corcoran, Calif.

Norris said since the mid-1950s Boswell’s weed management philosophy has been no weed sets seed.

“They laughed at me when I asked them if they had economic weed threshold level” said Norris, recounting a visit to the giant Kings County farm where a weed patch discovered in a newly harvested grain field generated an immediate call to the farm shop for a tractor to come and disk it under before seed could set.

He cited a vegetable producer from Gonzales who has not applied a herbicide to a spinach field since he fumigated it with methyl bromide seven years ago. He plants a cover crop and spends $70 to $80 per acre hand-hoeing it out, but has not spent a dime in seven years for herbicides.

Economic thresholds may be politically correct, said Norris, but he is convinced they are not an ecologically sound method of reducing weed pressure on crops.

He admits few agree with him, but more than three decades of weed control research in California has convinced Norris that he is not a Don Quixote flailing at windmills.

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