Extension Agent, Field Crops
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The Willamette Valley of western Oregon is a premier
seed production region, especially for clovers and cool-season
grasses. Three things contribute to Oregon growers' ability to
produce seed of the highest quality: expertise, climate, and
infrastructure. The expertise comes from long experience. Many
seed growers operate family farms that have been producing seed
for four generations. In addition to over 32,000 acres of clover seed, over 480,000 acres of grass seed are also grown in the Willamette Valley. A large industry infrastructure has developed, including transportation for selling seed across the United States and internationally. Many seed companies, with state-of-the-art seed cleaning plants, are located in the area.
An important part of the seed industry infrastructure
includes the Seed Certification program handled by Oregon State
University. A blue Certified Seed tag guarantees that the seed
is the genetically pure variety listed on the tag and that it
has met high quality standards. A field in the certification program
field is inspected at least once in the seedling stage and again
each year prior to harvest. The seed must be sampled and tested
through the OSU Seed Laboratory.
The clover seeds grown in Oregon are primarily
red, crimson, arrowleaf, and white or Ladino. Red, crimson, and
arrowleaf clover seed is grown mostly in the north valley (Washington,
Yamhill, and Polk counties). White clover seed is grown in the
south valley (Linn County). About 2,000 acres of red, crimson,
and arrowleaf clovers for seed are grown under irrigation in other
areas of Oregon.
Red Clover
Red clover is a perennial. In the Willamette Valley,
it is commonly planted in the spring, but may also be planted
in August or September, especially if the grower can irrigate.
In late May the plants start to bloom, but growers cut the fields
for forage around this time. The purpose is to delay the bloom
so that all the flowers come at the same time. Most growers sell
the forage as green chop to dairies. Even if a grower does not
sell the forage, he will clip the field, because a uniform bloom
is important for seed production. Seed is harvested in August
and September.
Wild bumble bees are better pollinators of red clover than honey bees. Honey bees, with a shorter proboscis, can't reach the nectar in the long flowers of red clover, so they will go to other flowers if available. Since bumble bees nest in the ground, it is to a grower's advantage to leave some wild areas near the field for habitat. Honeybees can still help seed yield, and many growers contract with beekeepers to place hives in their fields. White, crimson, and arrowleaf clovers all require the honeybee hives for adequate pollination.
Aphids are a problem in red clover that usually requires treatment with insecticide once a season, before the bloom to avoid killing bees. Aphids may reduce seed yields, but a worse problem is created by the sticky honeydew aphids produce. Seeds covered with honeydew are difficult to harvest and clean.
A red clover stand usually lasts for only one or two seed crops. The reason is the clover root borer (Hylastinus obscurus). This beetle is the limiting factor in red clover production in the Willamette Valley. The adult lays eggs in the crown of the plant. The larvae mine out channels in the roots, which then become a site for infection with root rot fungus (Fusarium solani). Plants are weakened, and the seed yield drops in the second year. A third year of seed production is not economical. There is no control for clover root borer other than rotation.
When June rainfall is lower than usual, the seed yield of red clover can be improved by irrigation at early flowering. Normally a healthy first-year stand with little root borer damage can produce a maximum yield with no irrigation.
Improved varieties of red clover yield less
seed than the common type (Medium Red), so forage producers should
expect seed of improved varieties to cost more. The right improved
variety, however, can yield more forage and be more than worth
the extra cost of seed.
White Clover
White
clover is a perennial that spreads by stolons. Since the flowers
are produced on the stolons, the trick to getting a good seed
yield is reducing leaf growth so that the plants produce more
stolons. This is the opposite of what a pasture producer tries
to do. Seed growers plant in rows up to 20 inches wide to allow
room for stolons to grow. They deliberately stress the plants
by grazing them heavily with sheep after harvest in the fall and
again for about a month in the spring. Achieving the right amount
of stress on the plants is a matter of trial and error. Each year
is different, and each variety is different. For this reason,
white clover is notorious for variable seed yields. Seed is harvested
in August.
White clover seed in the south Willamette Valley is grown primarily for the soil quality benefit it provides to the grass seed crop following it. Some growers rotate a field out of white clover every two years, and some keep a stand for up to four years. To open the stand and make it last longer, some growers are experimenting with applying herbicide using a shrouded sprayer to re-make rows. This equipment has coulters on each side of the sprayed strips to slice the stolons and prevent herbicide uptake by the strip of clover the grower wants to leave. This can work well, but a drift of herbicide during the application will cause a lot of damage.
Crimson Clover
The hills of the north Willamette Valley are beautiful
in mid-May, when the crimson clover fields bloom scarlet red.
Crimson clover is an annual, planted in late September or early
October.
There is risk to having clover plants too small going into the winter because frost heaving on some soils can break the tiny roots and kill the stand. There is also a risk to having them too big. Early-planted crimson clover with lush foliage is more susceptible to Sclerotinia crown rot and wilt (Sclerotinia trifoliorum), which can suddenly defoliate a field in the winter. A well-established stand can re-grow, but yield is lost.
Flowers open from the bottom of the flower head to the top. Growers hope for cool, cloudy days during the bloom period, so the flowers will open more slowly, giving bees plenty of time to visit each one. If the weather is hot and sunny, all the blooms open at once. Then not every flower gets pollinated, and the seed yield is lower.
Crimson clover
is harvested in late June and early July. As with the other clovers,
it's swathed at night, when dew is on the plants, to reduce seed
shatter. It's allowed to dry in the swath for about a week, then
harvested with a combine using a belt pick-up header.
Arrowleaf Clover
Arrowleaf clover is an annual, similar to crimson clover in it's requirements.
Like crimson, it's planted in the fall, but harvested later, in August and early
September. Also like crimson, the flowers bloom from bottom to top, and the
flower head will continue to grow taller and produce more seed as long as growing
conditions are good.
Seed Cleaning
Seed cleaning is an art. Each seed lot from the
field contains a different set of weed seeds and different amounts
of dirt and damaged seed. An operator must know how to adjust
the equipment to clean each lot perfectly, while losing a minimum
of good seed. 
The first piece of equipment in
the cleaning process is the air-screen cleaner. It uses a set
of sloped, vibrating screens to remove particles that are a different
size from the clover seed. The top screen removes the larger particles
by letting the clover seed fall through, and the lower screens
sift out the smaller seeds and dirt. As the seed moves across
the screens, air blows through to remove light dirt and chaff.
This equipment can be high volume, with two or three sets of screens
stacked above one another.
Another piece of equipment commonly used is the
indent cylinder. Seed is fed through the center of the rotating
cylinder, and centrifugal force pushes it into the round indents
in the cylinder. Seeds, such as grass seeds, that don't fit the
indent shape are separated out.
After good seed has been separated from poor seed
and dirt by size and shape, it's further separated by weight,
using the gravity deck. As the sloping platform vibrates and air
blows through, the lighter poor seed moves to one side and the
heavier good seed moves to other. At the front edge, seed falling
off the different sides of the platform is separated.
Two pieces of equipment used only
for clover are the velvet rolls and the magnetic cleaner. Velvet
rolls remove weed seeds with rough surfaces. As the seed passes
down the groove between two rollers rotating in opposite directions,
the rough-edged seeds catch on the velvet and are lifted out,
while the smooth clover seed slides through. The magnetic cleaner
is used for special weed problems and cracked clover seed. The
seed is wetted slightly and mixed with powdered iron. It then
passes over a magnetic drum. The iron lodged in the cracks and
on the sticky surfaces of weed seeds causes these seeds to stick
to the drum and be removed.
Samples are examined at each stage of the cleaning
process so that the equipment can be adjusted if necessary.
Minor Crop Issues
One of the biggest difficulties clover seed growers face is the fact that clovers are minor crops. Chemical companies are not willing to invest the money in research required to obtain pesticide registrations for minor crops because they don't see much potential profit. Growers therefore have limited options for pest control.
It's critical for a seed field to be kept
free of weeds, because a weed could become a problem in a new
area if its' seeds were planted with the crop seed. For this reason,
keeping fields clean is a costly part of seed production. Growers
use the full arsenal of registered herbicides, but they may also
need to control weeds using hand labor.
Clover growers, through the Oregon Clover Commission, have negotiated
with some chemical companies to support 24c or Special Local Needs
(SLN) registrations, with the research funded by the growers themselves.
There are several current SLN pesticide registrations in Oregon
for clover grown for seed. Many of the labels specify that treated
foliage and seed screenings cannot be fed to livestock. The Oregon
Clover Commission is currently funding efficacy and residue trials
for possible registration of more herbicides.