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Clean fields, good stands best medicine for California alfalfa

Two major pests today in California alfalfa are weevils – the alfalfa weevil and the Egyptian alfalfa weevil Alfalfa weevils defoliate the alfalfa plant and in the worst case scenario can consume all of the plant’s green tissue.

With about 40 years of pest management experience under his belt, University of California IPM Specialist Pete Goodell still preaches the virtues of integrated pest management.

“To me, IPM is not a definition but a philosophical approach – a platform from which we launch solutions. I think prevention is the catapult from which we start,” Goodell told a crowd of 200 alfalfa growers, pest control advisers, and others gathered for an all-day pest management workshop held on the eve of the 2016 California Alfalfa &Forage Workshop in Reno, Nev. in late November.

He says every pest situation is different and there are no silver bullets or simple solutions. It’s important to weigh all related issues before taking action.

“New technology can come along but to depend on a single form of technology will only lead to more problems later.” He added, “Do your best before entering the ‘alligator swamp.’”

IPM is an ecosystem-based strategy based on long-term prevention, and a combination of management techniques – biological, cultural, and chemical controls – to manage pests in agricultural, urban, and wildland or natural areas.

He noted early work on the IPM concept was conducted decades ago on the spotted alfalfa aphid in alfalfa. The IPM specialist has been with the UC IPM program since its launch in 1981.

Goodell’s opening workshop comments was followed by speakers sharing their latest findings to achieve effective pest management in alfalfa.


Two major pests today in California alfalfa are weevils – the alfalfa weevil and the Egyptian alfalfa weevil (EAW), UC Extension entomologist Larry Godfrey told the crowd. Alfalfa weevil defoliates the alfalfa plant and in the worst case scenario consumes all of the plant’s green tissue.

While both weevils look exactly alike, they behave differently and have biological differences between the two.

“If you look at them you can’t tell them apart,” Godfrey noted.

The EAW prefers hot weather and is found largely in California’s Central Valley and low desert alfalfa fields. The intermountain and coastal regions are typically home for the alfalfa weevil.

While alfalfa weevil tends to hibernate under leaf litter and around fence rows it lays its eggs inside alfalfa stems. The pest’s ovipositor inserts the eggs inside the hollow stem, filling it with eggs which hatch about a month later.

The adult emerges around June, completing the one generation per year. Development occurs between 50 degrees to 87 degrees.

According to Godfrey, effective management techniques include insecticides, early harvest, and ‘sheeping off’ alfalfa fields. Resistant varieties are not available.

“If you can sheep off your fields, which doesn’t fit everywhere, it does a tremendous amount of good. It removes the stems with the eggs inside. It can help a lot.”

To get a good handle on the alfalfa weevil’s threshold in the field, Godfrey says use a standard sweep net and 180-degree sweeps.

On alfalfa weevil control, biological control organisms including parasitoids and microorganisms can work in some areas of the country but unfortunately not in California.

“Fungi (microorganisms) need lots of moisture to do their job. California is dry so they will not work well,” he says.

Many insecticides are registered for alfalfa weevil. He noted data which suggests pyrethroid products are not working well in Northern California’s Scott Valley.

“At least in Northern California, we definitely need some new options.”


UC Cooperative Extension Farm Advisor Kurt Hembree of Fresno County discussed weed management during alfalfa stand establishment, noting the importance of creating a uniform stand to help the field battle weeds over the life of the stand.

Hembree said, “Not getting control of weeds upfront will impact the first cutting, future cuttings, how well the stand fills in those areas, and whether the weeds will take over the field.”

He calls clean early stands the most important weed control tool available. Creating and filling in a stand without large gaps create more efficient weed control.

This can impact weed control more than an herbicide.

 “Trying to develop a stand upfront and really making an effort to make sure hay comes out of the ground properly and in a timely manner is really critical.”


In a good established stand, Hembree says identifying seedling weeds early and herbicide timing are important. Environmental conditions during and after herbicide applications can influence how the product impacts an alfalfa stand, sometimes more so in alfalfa than other crops.

He says Roundup herbicide remains the main product used in alfalfa – considered the ‘go to’ product for heavily weedy fields with difficult to eradicate perennial weeds, nutsedges, and grasses. Roundup mixes well with many other products but he stressed rotating active ingredients to help reduce resistance issues.

Select and apply herbicides based on weed sensitivities – the larger the weeds in alfalfa the harder they are to kill, he says.

Hembree says ideal planting dates in California are August and September in the Central Valley, and August and April to mid-May in the intermountain region.


UCCE Farm Advisor Emeritus Mick Canevari, San Joaquin County, discussed weed control in established alfalfa – basically second through sixth year stands. He said producing premium-quality hay should always be the grower’s goal.  

Since most alfalfa hay is fed to dairy cows, Canevari warned that some weeds literally “stink” and can contribute to an off flavor in milk, plus cause palatability issues, including the weed Mexican tea which has a “horrible smell.”

He said the top two poisonous weeds in alfalfa are groundsel and fiddleneck which contain alkaloids which are toxins for animals.

Canevari called chemical control in alfalfa “the last stand of defense for weeds.” A good non-chemical weed control option is crop rotation which reduces the seed bank. The lack of crop rotation is an invitation for existing diseases, weeds, or pests to infest newly seeded alfalfa fields.

Good alfalfa crop rotation options include grasses, wheat, barley, oats, safflower, and corn.

The key to a successful stand in irrigated fields, especially flood irrigated, is laser leveling before planting to prevent water from standing too long. Canevari says pooled water opens the door for phytophthora root rot and other anaerobic conditions which can kill the stand.


On creating a good seedbed, Canevari says some growers prefer to plant 40 pounds of seed per acre to help build good alfalfa stems and possibly mitigate problems during planting or bad weather.  

“When we have the right ratio of small clods and fine soil when planting seed we don’t need 40-50 pounds per acre,” he believes. “Some believe a grower needs 25 pounds of seed per acre, or 100 seeds per square foot. If we end up with a stand of 20-30 plants per square foot then we’re happy and have a successful stand.”

Canevari says it’s important to avoid soil compaction in the field. He urges growers to test the soil for the pH level. If soil amendments are needed, pre-plant is the best time to apply them.

“Your best form of weed control is a good stand of alfalfa – you can’t beat it.”

Other methods of weed control include chemical, mechanical, and animal. Mechanical is a great tool in the beginning of the year and the end of the season. He says animal grazing research on sheep grazing alfalfa in Tulare County suggests that sheep grazing does not damage the stand.

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