The buckhorn seed has a mucilaginous surface, which becomes sticky when dampened and will pick up finely ground sawdust. Thus a larger, rough-surfaced, less dense seed unit is formed. Smooth legume seed will not take on any of the sawdust.

A buckhorn seed mixture and a sufficient amount of water and sawdust are mixed in an auger or continuous-type paddle stirrer. The enlarged buckhorn seeds then can usually be removed with an air-screen cleaner. The specific gravity or the velvet roll machines may be used to complete the separation if necessary.

The timothy bumper mill separates seeds on the basis of a difference in shape. It is a special machine developed to remove weed seeds.

The timothy seed mixture is fed onto the lower corner of the upper end of a small rectangular metal plate, which is inclined slightly in both directions but more in the lengthwise direction.

A back-and-forth movement of the plate, parallel with its short side, is smooth in one direction but is stopped with a bump in the other. Round timothy seeds roll downhill lengthwise of the plate between bumps and fall off the end. Irregular-shaped seeds are gradually bumped uphill across the plate and fall off the side.

In the commercial machines, many plates are suspended in two identical frames, which are bumped together simultaneously. With proper adjustment of feed rate, bump intensity, and plate inclination, the timothy bumper mill will separate alsike clover, Canada thistle, sorrel, ryegrass, quack-grass, and other seeds from timothy.

The vibrator separator is a newer machine that makes a division of seed on the basis of a difference in shape and surface texture.

As in the timothy bumper mill, the seed mixture is fed near the center of the upper end of a rectangular deck inclined both sideways and endwise. The deck is activated by an electromagnetic vibrator. Flat or rough seeds climb to the high side of the deck. Round and smoother seeds roll, tumble, and slide to the low side.

The endwise tilt induces both seed fractions to travel to the discharge end, where they drop off and are kept separate by dividers.

The tilt of the deck may be adjusted both sideways and endwise to provide a wide range of deck inclinations. Decks can have various textures, ranging from smooth metal to rough sandpaper, depending on the seed components that are being separated. The intensity of vibration can be regulated by a rheostat controller in the electrical circuit. To increase capacity, multiple decks may be mounted in a rigid frame and powered by a single vibrator. A properly adjusted vibrator separator removes seeds of curly dock from crimson clover, dogfennel and hedgemustard from timothy, rippleseed plantain and ergot from bentgrass, dodder and lambsquarters from carrot seeds, and sweet vernal grass from ryegrass.

The color separator separates the seeds on the basis of a difference in color or brightness.

One type of machine picks up the seeds on a series of suction fingers and carries them past a photo tube, where they are judged for color or brightness and ejected into separating containers one at a time.

Color separators are practical for larger seeds, like those of beans and peas. They have not been used for smaller seeds because of the low capacity involved in scanning the particles individually.

MANY FACTORS should be considered in designing a seed-cleaning plant. Handling and cleaning the seeds should be possible without mixing or damaging and with a minimum of equipment, personnel, and time.

The seed separators, elevators, conveyors, and storage bins should be arranged so seeds can flow continuously from beginning to end, yet flexible enough to bypass a machine or return part for recleaning.

Other factors to be considered are kinds of crop seeds to be cleaned, kinds of contaminating crop and weed seeds,volume of seed to be handled, method of handling (bulk or sacks), type of conveying system (pneumatic or mechanical), and location of shipping and receiving facilities.

The commonest plant layout is the multistory design, in which machines are placed in a vertical processing line and seed flows by gravity from one to another. This arrangement requires a high, reinforced structure and many workers to operate equipment on the several floors.

Single-story plants are increasing in popularity because of the emphasis on reduced cost of structures, less labor, varietal purity, and the need for special processing of certain crops.

There is no all-purpose machine that will remove all the objectionable material from the different kinds of seed.

Each unit in a cleaning sequence is employed to make a specific separation. The machines used and their arrangement in the processing line depend on the crop being cleaned; the condition of the crop (partly or completely hulled); the type of other materials present (rocks, sticks, stems, leaves, dirt, weed seed); the size of the contaminating material compared to the crop; and the proportions of each in the mixture.

When a seed lot to be cleaned arrives at a processing plant, a sample is first analyzed to determine which machines to use and the best flow pattern through the plant. Combined seed ordinarily requires one or more precleaning treatments.

If the seed lot includes a high percentage of large trash, green leaves, green weed seeds, or insects, it is first rough cleaned on a scalper. If a large percentage of the seed is unhulled, it is often necessary to hull the entire lot.

A lot containing hard seed may be scarified during the precleaning sequence or some time after cleaning is complete, depending on its ability to maintain viability after processing. Many grains and most grasses are given a debearding or deawning treatment with either the hammermill or debearder. Many native grasses require special treatment to remove troublesome appendages.

After precleaning, most of the remaining contaminants are removed with an air-screen cleaner. Some lots, with a minimum of excess material, may be finished by this machine, but most will need further processing with finishing machines to make first quality seed. This unit is also capable of accurate sizing for thickness and width with the selection of proper screens.

The finish cleaning may be accomplished by many machine arrangements, but an indent disk or cylinder usually is next in line. These units separate weed seeds that are different in length and also may be used to divide the lot according to size.

The specific gravity separator will next split off light and heavy contaminants with most of the cut being usable seed. The "middlings" or mixture fractions may be rerun on another gravity separator or a different type of machine for further cleaning.

Further processing will depend upon the characteristics of the specific contaminant remaining. Rough weeds are removed by the velvet roll; seeds of irregular shape by the spiral or inclined draper; and seeds with rough or mucilaginous surfaces by the magnetic or buckhorn machine.

The separator selected generally is the one that will remove the most contaminating materials with the smallest loss of crop.

Manufacturers have developed outstanding seed-separating equipment, but the knowledge and skill of the operators remain most important.

LEONARD M. KLEIN is an agricultural engineer, Harvesting and Farm Processing Research Branch, Agricultural Engineering Research Division, Agricultural Research Service, Corvallis, Oreg.

JAMES HENDERSON is sales manager of A. T. Ferrell & Co., Saginaw, Mich.

ABRAHAM D. STOESZ is head plant materials technician, Plant Technology Division, Soil Conservation Service.