High temperatures put California alfalfa crop under the heat

July’s higher than normal temperatures translated into great difficulty for alfalfa growers to produce a high quality crop following a spring season filled with excessive rain and flooding in the Central Valley and Northern California, according to University of California at Davis Forage Specialist Dan Putnam. He said more normal, “downright pleasant,” temperatures returned in August, but the first six months of this year were the hottest across the nation since the federal government began keeping records in 1890.

“Hot weather causes rapid lignification of alfalfa stems and plant stress. Entomologists feel that the heat units may contribute to high summer worm populations including alfalfa caterpillar and armyworms,” said Putman in the California Forage Update newsletter released on Aug. 25 by the California Alfalfa & Forage Association (CAFA) and the Alfalfa and Forage Systems Workgroup of the University of California Cooperative Extension Service.

Putnam said about 50 all-time high-temperature records were broken during the month of July, and several cities in California set records for extended heat waves, including Fresno with six consecutive 110-plus-degree days, and Sacramento with 11 consecutive triple-digit days.

In the CAFA’s July newsletter, it was reported that supplies of high-test milk cow quality hay could be tight throughout 2006 despite a sizeable increase in alfalfa acreage. It’s not surprising given late spring rains that plagued Central Valley growers. The June acreage report from the National Agricultural Statistics Service (NASS) estimated a 60,000-acre increase in alfalfa acreage for California in 2006.

In an article by NASS Senior Agricultural Economist Seth Hoyt, he said a 60,000-acre increase would normally boost production substantially above the previous year. However, with a late start to the season in Central and Northern California plus low tonnage in some areas, alfalfa hay yields may well be down in 2006. At the end of July, NASS surveys were projecting a seasonal average yield of 6.7 tons per acre versus 6.9 tons last year.

“With the hot July weather, it will be interesting to see the yield survey for July thru August,” he noted. Currently, NASS is estimating California growers will produce 7.1 million tons of alfalfa in 2006 versus vs. 6.9 million in 2005, only a 3 percent increase despite the six percent acreage increase.

UC Davis Entomologist Larry Godfrey said in the Aug. 25 newsletter that worms, lepidopterous larvae, have been very common to severe in San Joaquin Valley hay fields for the past couple of months. Alfalfa caterpillars, the larval stage of the common yellow butterfly, are normally fairly abundant in alfalfa fields in the summer.

“This species is easy to kill with many insecticides,” Godfrey said. “Beet armyworms and western yellow-striped armyworms have also been very common with the species breakdown varying among fields and areas. Armyworm larvae are more difficult to kill and populations can persist through cutting and damage the regrowth.”

There are some differences in insecticide susceptibility between the two-armyworm species with western yellow-striped armyworms being less susceptible to some insecticides than the beet armyworm, he noted. Armyworm larvae leave the leaf veins intact where alfalfa caterpillars eat the entire leaflets.

Alfalfa can withstand considerable populations of worms before treatment is needed, 10 to 15 live larvae per 180-degree sweep with a sweep net, Godfrey said. The thresholds have been easily exceeded in some fields – numbers as high as 70 to 80 per sweep have been seen. Many of the worms hide during the heat of the day so look carefully when monitoring fields for worm pressure. Also watch populations on young re-growth, he said.

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