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Alfalfa seeding rates often too high

San Joaquin Valley alfalfa growers typically seed new fields at 25 or more pounds per acre to ensure a healthy stand, but when methods and conditions are correct, half as much could be sufficient, according to a Fresno County farm advisor.

Shannon Mueller, speaking at a recent field day at the Kearney Agricultural Center at Parlier, said a population of 10 to 20 plants per square foot a year after planting, the recommended range for high production, can be achieved with seeding rates of between 10 and 15 pounds under optimum conditions.

The traditional risk management strategy of high seed rates is being challenged by the added technology costs of Roundup Ready varieties, she said.

“For traditional varieties,” Mueller said, “the cost of added seed has been cheap insurance when you have anything less than ideal conditions at planting, but Roundup Ready varieties have a $3 per pound technology fee in addition to the cost of the seed.”

Moreover, the coating for these glyphosate resistant varieties means there is 8 percent less seed per pound than with raw seed, and 5 to 10 percent of the seed is nulls, or seed susceptible to the herbicide.

Seeding adjustments

Mueller added the seeding rate can be adjusted according to seed coating, climate, soil salinity, condition of the seed bed, and method of planting.

In a simplified comparison, she said 25 pounds of raw seed translates to nearly 130 seeds per square foot, and the same weight of conventional coated seed translates to about 86 seeds per square foot. In the Roundup Ready varieties the coating is thinner, and that means about 119 seeds per square foot.

“So, these are all well above the 20 to 50 seeds per square foot you want for stand establishment. Even with subtracting the nulls, you still have a reasonable number of potential plants.”

Research has shown survival of alfalfa plants to be 10 to 50 percent, and populations decline so that one year after planting, the population typically is 10 to 20 plants per square foot.

Mueller said coating came into vogue several years ago when shortages of seed led seedsmen to apply coatings to extend supplies. Three bags of raw seed became the equivalent of four bags of coated seed.

Although some coatings are inert materials, more often they include fertilizer, fungicides, or Rhizobial bacterium to promote germination to establish the same number of seedlings with fewer seeds.

Seed trials

She said her trials in 1988 compared raw and coated seed, both seeded at the 25-pound rate. Although in the first month the coated seed germinated more plants than the raw, by one year after planting, there was no significant difference between the number of raw and coated seed plants.

“Survival was greater with the coated seed even though we planted one-third fewer seeds with it. The value of seed treatments depends on the degree of disease in the field, and you need to look at it as a risk-management policy. In some years it will really pay to have a fungicide treatment on the seed, and in others there will be no advantage.”

Mueller said numerous studies in California and elsewhere prove that alfalfa finds its level of plant populations during the first year.

“If you have fewer seedlings that initially establish, more of those survive through the first year than when you have higher populations in the early stand. Lower seeding rates have a higher survival rate.”

Another speaker, Carol Frate, Tulare County farm advisor, said nematodes and many other issues have to be considered in deciding when to replant an alfalfa field. Many growers, lured by good prices, want to immediately replace poorly producing stands.

In general, she recommended, at least two — or better yet — three years of a rotational crop should be grown before returning to alfalfa. The chief reason, she said, is autotoxicity.

“After many years of debates there is now some general agreement that when alfalfa plants, especially the leafy parts, decompose in the soil, they release chemicals that have a negative impact on germination of new alfalfa plants.”

However, she added, there is research indicating that the toxicity may be overcome if the residue is worked in and the field is irrigated to complete breakdown of the material at least two to three weeks before planting for a new stand.

The second pitfall in replanting is the accumulation of harmful organisms. They are numerous species, from root knot and lesion nematodes to root-nibbling insects and bacteria and fungi that attack plant crowns. “Those will be significant harm in a new planting if you plant alfalfa again right away.”

The problem is a “black box,” she explained, because no one can tell just which organisms or how many of them are to blame for a stand decline. A new seedling faces a potentially very high population of organisms that will nibble its roots if not outright kill it.

Weeds reappear

And, Frate added, if weed seeds are in the soil and couldn't be controlled in the original field, they will likely show up again in the new alfalfa.

In an update on the California Department of Pesticide Regulation's ground water protection areas (GWPAs), Ron Vargas, Madera and Merced County farm advisor, said it remains to be seen what impact they will have on weed control in the more than 1 million acres of alfalfa in California.

GWPAs are locations vulnerable to pesticide contamination by leaching through the soil and/or runoff. Herbicides listed in the state regulations are atrazine, bentazon, bromacil, diuron, norflurazon, prometon, and simazine. These require permits when used for agriculture in GWPAs.

Diuron and norflurazon, Vargas said, are the only two on the list used in alfalfa. “The two issues that caused these to be on the list are: first, their ability to leach more readily into ground water and second, their persistence, or half-life.”

He said the impact on use of the two materials could be relatively minimal because in 2003, 197,000 acres of alfalfa were treated with diuron and 32,100 acres were treated with norflurazon.

By comparison, he added, the herbicide with the greatest use on alfalfa in California in 2003 was TR-10 (Treflan) on 307,000 acres.

Depending on which weeds growers need control in individual fields, alternatives to diuron and norflurazon are available, Vargas said.

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