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As California dairying expands: Weed spectrum change seen with manure use

The rapid expansion of the dairy industry in the San Joaquin Valley has been a good news/bad news scenario. The dairies are bringing new sources of income for alfalfa, corn and forage producers.

However, it is also resulting in a proliferation of weeds with the disposal of manure, particularly in small grains.

Non-composted manure is now frequently used as an expensive fertilizer to many fields. The result is the weed seed bank has grown exponentially and substantially shifted the weed spectrum.

“In the past few years, we've seen an explosion of weeds in small grain fields due to the use of non-composted dairy manure,” says Steve Wright, Tulare County University of California Cooperative Extension Farm Advisor. “Mustards are still dominant, but now we've got stinging nettle that has just exploded. Another one is chickweed. That never used to be much of a problem, but now, suddenly it is. Shepard's purse is yet another one that is not exactly a huge problem, but needs to be addressed.”

There are approximately 750,000 acres of small grain crops in California, according to Wright. About two-thirds of that is grown for silage — largely to service the burgeoning dairy industry in the state. Wheat comprises about 530,000 of those acres, followed by triticale, barley, oats and mixed forages.

Controlling weeds in these crops presents challenges including crop injury, off-site movement of herbicides, plant-back restrictions and other concerns. Older chemistries such as 2,4-D are still a standard, providing the most effective, least cost prohibitive control. However, crop injury is always an issue. MCPA causes less crop injury, however it also less effective on target weeds. It is easier on oats and can be tank-mixed with nitrogen.

For fiddleneck control, university trials have shown results with Buctril, Dicamba plus MCPA, Shark plus MCPA, 2,4-D and dicamba. Off-site movement of Shark is a concern, according to Wright.

“You have to be careful with it,” Wright says. “There have been some problems with Shark applications in the past that moved as far as a half-mile off-site and damaged almonds. It also has a plant-back restriction of 30 days for wheat, barley, sorghum, cotton and corn.”

Grass control

For grass control in wheat and barley, there are options, but care should still be exercised when choosing a material. There are resistance issues with Avenge, according to Wright. Hoelon is an option but should be avoided if soils are wet and cold due to its potential to cause crop injury under those conditions. Treflan gives suppression of wild oats and canarygrass. Researchers are looking at Fargo for future wild oats control.

Puma and Osprey are effective on wild oats and canarygrass, while Osprey picks up other weed species such as annual bluegrass, mustards, cheeseweed, chickweed, etc.

In some cases, herbicide labels seem to be open to interpretation, according to Wright. One notable example is Osprey's plant-back restriction to corn. At least in Tulare County, it's not an issue that is being “bird-dogged” by the regulators. However, it's not a policy that has exactly been put in writing either.

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