Alfalfa Seed Production in the Western United States

 

Shannon Mueller,

 Agronomy Farm Advisor, UC Cooperative Extension, Fresno County

 

 

Introduction

Contaminated seed is the likely source for most reported sprout associated outbreaks.

 

A number of field and vegetable crop seeds are sprouted for human consumption, such as alfalfa, clover, onion, radish, broccoli, lentils, and mung beans.  Seed for sprouting is produced throughout the world, but the major suppliers are in the US, Canada, and Australia.  Crop production practices vary with the type of crop and the area where it is grown.  Laws and regulations governing seed production practices are specific to the state or country.  The buyer needs to know where and how the seed was produced and handled in order to minimize the risk of food safety problems.

 

The following information will help make you aware of the questions you need to ask your seed grower, conditioner, and/or supplier.  It may also help you understand some of the constraints seed growers face in producing seed for sprouting.

 

U.S. Seed Production

Approximately 80 million pounds of alfalfa seed are produced each year in the United States.  85% of that is produced in five western states - CA, ID, OR, WA, and NV.  The balance is from AZ, UT, MT, WY and other states.  The primary market for that seed is planting stock to produce forages to support the livestock industry in the US and throughout the world.  Only a small fraction of the seed produced is used for sprouting.  If the seed grower is not aware that the seed is to be used for human consumption, he/she may unknowingly employ production practices that create a potential health risk.

 

Dormancy

Varieties of seed are classified by their dormancy characteristics.  Most sprouters prefer seed of non-dormant varieties as compared to seed of dormant varieties because non-dormant seeds germinate quickly and uniformly, producing a high yield of sprouts.  Non-dormant seed is produced under climatic conditions similar to those in the Western U.S, especially the Central and Southern regions of California.  Semi-dormant seed is produced in areas with environments similar to those in parts of the Pacific Northwest and Midwestern states.

 

Sources of Contamination

Sources of contamination are present in the field and in the harvest equipment as well as at the conditioning, seed supplying, and/or sprouting facility.  Some sources of contamination are fairly easy to control, while others are difficult if not impossible to control.  Sources of contamination include:

·        Water

·        Animal Waste or Manure (Wild or Domestic Animals) and Insects

·        Poor Equipment Sanitation

·        Personal Hygiene

 

Contamination may be sporadic and at low levels, but once pathogens are present in the seed lot, they are likely to survive for extended periods of time under normal seed storage conditions, allowing the opportunity for cross contamination and major spread if seed lots are mixed.  Therefore, it is important to reduce the risk of contamination as much as possible at each step in the production, conditioning, and distribution process.

 

 

Typical Seed Production Practices

Field Preparation and Stand Establishment

Plants for seed are grown in typical agricultural environments.  Crops are established at many times during the year.  Most typically, planting dates are in the spring or fall.  Crops may be planted in rows on beds, or may be flat planted.  The planting configuration depends on soil type, slope, cultural practices, and irrigation system design.  Seeds are planted into soil where they germinate, grow, bloom, are pollinated, and set more seed to complete the cycle.  Some seed crops are grown as annuals (i.e. mung beans) which are harvested one time only, and some are grown as perennials (i.e. alfalfa seed) which are harvested once or twice a year for 2 or more years.

 

Spring Clipping / Grazing

For alfalfa seed, and other perennial crops, the season begins when the forage that has grown during the winter or early spring is removed in an operation called clipback.  If there is enough forage present, and previous chemical use doesn’t restrict it, this clipping can actually be a hay harvest.  In some areas, sheep are brought into the seed fields to remove this winter growth.  If the seed from the field is destined for the sprout market, sheep should not be allowed to graze.

 

Fertilization

Based upon soil test results or plant tissue testing, growers use commercial fertilizers and/or manures to maintain the fertility of the soil for maximum crop production.  The application of animal manure should be avoided during the production of a seed crop for sprouting.

 

Irrigation

All crops require water for maximum production.  In some areas, water supplied through rainfall is supplemented by water delivered via surface canals or water pumped from deep wells.  Many growers use a combination of canal water and well water to irrigate their crop.  It is applied to the surface of the soil by flood, furrow, or sprinkler irrigation systems.  If the seed is to enter the sprout market, all precautions should be made to prevent contamination of the water source.

 

Crop Protection

During the production season, the crop must be protected from a number of insect, vertebrate, and weed pests to maximize seed yield and quality.  Pests present in seed fields include, but are not limited to, insects, rodents, birds, reptiles, deer, sheep, cattle, coyotes, dogs, and cats.  A certain background level of contamination must be accepted, as it is not possible to prevent wild animals from flying over or running through agricultural fields.  However, some sources of contamination are possible to control by restricting grazing animals for example.

 

From stand establishment through the final cleaning/conditioning process, the goal is to eliminate weeds, or at least prevent them from going to seed.  In the field, growers use herbicides, mechanical cultivation, and weeding crews to remove weeds.  During conditioning, a variety of screening and separation techniques are used to remove foreign seeds.  A lot of the alfalfa seed can be lost in the process, so it is more efficient and economic to control weeds in the field.

 

There are a number of insect pests that have an impact on the yield or quality of alfalfa seed.  Growers monitor most pest and beneficial insect population using sweep nets.  Lygus are the most difficult insects to control.  They are a problem throughout the season feeding on the flower buds, bloom, and developing seeds within the pod.  There are a limited number of pest control options, most of which don't provide economic control.  Chemical control is the most effective, but many materials used to control lygus restrict the use of the seed for sprouting.

 

Pollination

Cross-pollination is required in order to produce high quality seed of many commercial crops.  Honey bees, leafcutter bees, or alkali bees are typically used to pollinate the crop.

 

 

Harvesting

In the field, the seed is protected by the seedpod, but once harvest begins, the seed is exposed to a substantial amount of dirt and debris and localized contamination may be spread throughout the seed lot.  Mechanical damage is also an important issue.  Any damage to the seed coat caused by harvest equipment could aggravate contamination by making removal of pathogenic microorganisms during subsequent steps more difficult.

 

Cutting or Swathing

Once the majority of the seed is mature, the crop is cut at the base of the plant and laid in long rows or swaths in the field to air dry.  Any green seed that remains on the plant will continue to ripen (mature) in the windrow.  In many areas, when seed is grown for commercial planting stock, it is desiccated using chemicals that dry the standing crop in preparation for harvest.  Air-drying requires more time and is riskier than chemical desiccation, but desiccants restrict the use of the seed to planting stock only.

 

Combining

A standard combine is used to pick up the crop in the windrow and thresh the seed from the pod.  The equipment must be carefully adjusted to separate the often very small seed from a large amount of plant material without damaging it.  The equipment should also be adjusted to minimize soil in the harvest.

 

Transport

Seed is augured out of the combine into bins or a truck for transport to the conditioning facility.  If the seed is destined for the sprout market, it should be transported in a sanitary trucking container avoiding contamination from soil, manure, animal and rodent urine, etc.  Ideally, the seed should be protected with a tarp during storage and shipping.

 

 

Conditioning

Conditioning is the process of cleaning the seed to remove weed seeds, debris and dirt.  Equipment such as an air screen cleaner, specific gravity separator, velvet rolls, disc and cylinder separators, and/or a spiral separator may be used.  As with the harvest process, conditioning has the potential to spread localized contamination throughout the seed lot.  It is important to limit sources of contamination and minimize the risk of cross-contamination in the conditioning facility.  Seed, cleaning equipment, storage bins, and supplies (such as shipping bags) should be protected from contamination by rodents, birds, insects, and reptiles.  Equipment must be cleaned between lots to minimize the chance of cross-contamination from other infected seed lots.  Usually cleaning is accomplished by blowing out the equipment with compressed air.

 

The equipment used to clean the seed may cause damage in the process.  The damage may be inadvertent, or it may be purposeful (scarification).  In California, seed is not typically scarified, but in other parts of the US and the world, the practice is more common.  Scarification is the scratching of the seed coat to improve the percentage and uniformity of germination of a seed lot with a high hard seed percentage.  A number of different techniques are employed to scarify seed.  If seed is scarified, and then comes in contact with a pathogen, the pathogen may lodge in cracks in the seed coat and be more difficult to remove than from an intact seed coat.  However, the scarification process itself does not introduce contamination any more so than other pieces of equipment in the harvest or conditioning process.  Most suppliers do not scarify seed or distribute large quantities of scarified seed.

 

The conditioner may test individual seed lots for purity and germination, but screening for the presence of pathogens is not typical.

 

 

Storage

Once the seed is cleaned, it is stored in the warehouse in bulk, bins, or bags prior to distribution.  During this time, seed should be protected from contamination by animals, insects, or agricultural or industrial wastes and chemicals.  This means seed should be stored in closed, impermeable containers, positioned in such a way that contact with contaminants is minimized and regular inspection is possible.  Visual inspection of the seed and the area between and around storage containers allows monitoring of pest problems and timely control if necessary.

 

 

Bagging

Most seed is distributed to the supplier and to the sprouter in 50-pound bags.  When packaged for distribution, a tag may be attached to each bag specifying any or all of the following information: supplier information, lot number, seed type, germination and purity, date, seed company, and/or country of origin.  The lot number is important in maintaining the identity of the seed and, if properly recorded, facilitates tracking the seed from the grower’s field to the conditioner, supplier, and sprouter.  The lot number may be found on a tag attached to the bag, or it may be printed directly on the bag.

 

The bag provides the first line of defense against contamination of the seed.  Clean bags should always be used if the seed is destined for sprouting.  Solid bags will minimize possible secondary infestation as compared to open weave bags.

 

 

Seed Supplier / Distributor

Ideally, the seed supplier will only work with reliable and reputable seed producers and conditioners.  Seed suppliers must know the source of the seed and require certification that chemicals restricting use of the seed for sprouts were not used, animal waste was not applied to the field, and any other stipulations desired.  The supplier may utilize contracts with growers or conditioners that specify production practices to reduce the possibility of contamination.

 

Initial Inspection

The seed supplier should inspect all bags upon delivery for evidence of contamination.  Visually examine bags and pallets for signs of possible contamination including insects, water stains, and presence of rodent or bird droppings, weeds, or foreign material.  It is strongly recommended that all bags and pallets also be inspected using a black light.  Bags of seed that have been contaminated with rodent urine will glow when viewed using a black light.  Bags with evidence of contamination should not be used.  Stored seed and surrounding areas should be inspected frequently to insure that pests are not present.

 

Maintain Identity of Seed Lots

The seed supplier should retain or place tags on bags with a lot number that can be traced back to the original source of the seed.  Records of performance or pathogen testing are maintained by lot number.

 

The definition of a “lot” is important.  A lot may be defined as all of the seed from a specific field, or a truckload.  It may also be defined as all the seed of a given variety from a grower’s farm.  Some handlers may mix various quantities of seed and assign a lot number to that container.  The lot number is important in maintaining the identity of the seed and, if properly recorded, facilitates tracking the seed.  Assigning a lot number to the smallest, discrete unit is desirable in limiting the amount of seed that might need to be recalled in the event of a problem.

 

Suppliers should make it a practice not to mix seed lots to upgrade the seed or increase the size of the lot.  Mixing lots may spread contamination and definitely hampers traceback.  Some seed from other countries may be mixed prior to arrival at the distributor.  The supplier is taking a risk by not knowing the precise origin of that seed.

 

Sampling and Testing the Seed Supply

In light of recent food safety concerns with sprouts, it is strongly recommended that seed suppliers establish a sampling plan and certification program for seed distributed to sprouters.  Suppliers should test samples from desired seed lots prior to committing to purchase.  The most rigorous testing involves both a visual inspection and a laboratory test for human pathogens such as Escherichia coli 0157, Salmonella , and often Listeria monocytogenes  If the seed sample passes the initial testing phase, the entire lot may be purchased and retested using the same procedures.

 

Sampling

Various sampling procedures can be used to ensure a representative sample is collected (i.e. sampling 5 bags and every 10^th bag in the seed lot or sampling 15% of the bags to obtain a 1 pound sample from 10,000 pounds of seed).  All seed should be held in quarantine until the results of testing show no contamination.  While a negative result does not guarantee the absence of pathogens, a positive result would allow a producer to avoid using seed lots that have been shown to contain pathogens that may cause disease in humans.

 

Testing

The visual test uses magnification to look for evidence of rodent droppings, excreta, fungus, molds, or a large number of cracked and broken seeds.  Samples should also be evaluated under black light for signs of urine.  In addition to testing the seed itself for the presence of human pathogens, a sample may be sprouted to allow testing of both the runoff water and the sprouts for the presence of the same pathogens.  Tests for pesticide residues are not routinely conducted.  Results of seed testing should be maintained by lot number.

 

Seed suppliers recommend that sprouters also test seed delivered to them.  In seed, it is difficult to detect the presence of pathogens that may cause illness in humans, so the more testing that is conducted, the better the chances of detecting a problem if it exists.  Seed distributed to sprouters should carry a label warning that the seed may contain microorganisms that may cause illness in humanss and should be sanitized prior to use.  The major seed suppliers in the US require that the sprouters they distribute to sign a contract that they will sanitize the seed prior to sprouting.  Sprouters should always treat seed as if it contains pathogens and handle it in such a way that limits cross contamination.

 

It is NOT recommended that seed suppliers sanitize seed before distribution.  Sanitizing seed early in the handling process provides no guarantee that the seed will still be clean when it reaches the sprouter.  The closer the seed is to the actual sprouting process at the time it is sanitized, the better.  However, this recommendation may change if irradiation becomes the standard practice for sanitizing seed.  The equipment is costly and it would not be economically feasible for individual sprouters to purchase.  In that case, the seed suppliers will probably irradiate the seed prior to distribution.

 

Unfortunately, rejected seed often finds a buyer somewhere.  Less conscientious sprouters may be willing to take risks associated with buying “cheap seed”.  All sprouters need to establish safeguards that prevent contaminated seed from entering the food chain.  Individual sprouters need to develop a relationship with the seed supplier and insist upon receipt of clean seed from a reliable source, recognizing that they may have to pay slightly more for the seed.  Dealing with disreputable seed suppliers is risky business.

 

 

Goals / Recommendations

Dedicated Seed Source

Regulating seed production practices can reduce the risk of contamination.  Seeds for sprout production should be grown under conditions that minimize exposure to microorganisms that cause disease in humans.  Growers, conditioners, and seed suppliers should only sell seed for sprouting that conforms to specific requirements.  They should be prepared to sign an affidavit that the seed was grown without chemicals that restrict its use for sprouting, and that animal manure was not applied to the production field.  Certifying that attention was paid to cleanliness of the water source and insect, weed, and rodent control is also important.  Good agricultural practices should be systematically implemented to reduce the potential for microbial contamination of seeds for sprout production.

 

Maintain Identity of the Seed

All seed should be identified with a lot number traceable back to the field in which it was grown.  Certificates of origin should state where the seed was grown, not where it was cleaned.  A seed agreement detailing growing/harvesting/conditioning requirements and the results of any testing should accompany each lot of seed.

 

 

Conclusion

Little information is available on how and when seeds become contaminated with bacterial pathogens.  Possible sources of contamination include fecal contamination, including contaminated agricultural water, use of inadequately treated manure as a fertilizer, location of fields near animal facilities, access by feral animals, poor sanitation of equipment, and inadequate agricultural worker hygiene.  Once present, pathogens are easily spread throughout a seed lot during harvest and conditioning.  Prevention or intervention is possible at many of the steps between seed production and sprouting.  Increasing awareness of the potential for contamination from the field through the sprouting facility and addressing concerns regarding food safety will improve handling of seeds and sprouts and reduce the risk of foodborne disease.