Seed Potato Certification

Potato is one of a number of vegetatively propagated crops. Potato tubers that either are planted whole or have been mechanically cut are the propagating material. The process of vegetative propagation can cause unique problems in maintaining varietal purity and in the management of "seed-borne" diseases. Disease pathogens present in the propagative material will, with all probability, be transmitted to the progeny. Additionally, the cut surfaces of seed potato tubers are large wounds that can act as infection courts for pathogenic organisms from other sources.

In an effort to provide the commercial potato industry of North America with potato seedstocks that are varietally pure and relatively free of disease-causing organisms: an elaborate system evolved, that of seed potato certification. The process of seed certification has changed a great deal during the past twenty years. Technological advances enabling the rapid multiplication of seedstocks under laboratory and greenhouse conditions as well as sophisticated and sensitive pathogen testing techniques have revolutionized the seed potato industry.

Concept of Seed Potato Certification

Seed potato certification in North America was first discussed during the First Annual Meeting of the Potato Association of America in 1914. By 1920, 12 states and all Canadian provinces were engaged in seed potato certification. In Canada, seed certification is under the general control of the federal government. In the United States, however, seed potato certification is the responsibility of either a land-grant university, a state department of agriculture or a grower (crop improvement) association. As a result, a great deal of diversity exists among states in the rules and regulations that govern the certification process.

The certification agency is generally responsible for conducting all required inspections, be they field, storage or at shipping point. Since participation in seed certification is voluntary, the responsibility to carry out all recommendations and to follow seed certification regulations rests solely with the grower. Interaction and communication between the seed certification agency, the seed potato grower and the commercial potato industry they serve is critical.

In the beginning, the primary purpose of seed potato certification was to provide reasonable assurances of varietal purity. However, that rapidly changed in order to meet the demand of reducing the incidence of seed-borne, virus-caused diseases. Viruses were responsible for the phenomenon known at the time as the "running out" of varieties. Today, in addition to assuring varietal purity, the establishment of disease levels within fields entered into the certification process requires a major effort on the part of certification agencies.

In the past, tubers from individual plants or hills that appeared to be visually free of disease problems were saved for replanting. These hill selections were frequently planted together as a "tuber-unit." In a tuber-unit, a tuber from a hill is cut into seed pieces and planted sequentially in a unit. This unit is followed by the remainder of the tubers from the hill which are also planted as units. If a disease problem appeared in any plant of a tuber-unit, the entire unit would be destroyed (Fig. 9). This procedure was used for several decades as a means of producing and multiplying seed stocks that were relatively free of major disease problems.
 

FIGURE 9A. Tuber unit consisting of four seed pieces. The four seed pieces are planted as a unit in the field. A sizeable space separates units within the field.			FIGURE 9B. Tuber unit plants in the field showing healthy and diseased units. The diseased units can be identified and eliminated easily.

A number of disease-causing pathogens, however, can remain latent or symptomless within a seed tuber and go visually undetected during the seed certification and inspection process. There have been instances when the disease problem was not detected until the seed lot was purchased by a commercial grower, resulting in severe economic losses. This resulted in the development and adoption of new laboratory testing and seed stock multiplication techniques that have dramatically affected the quality of certified seed potatoes. Today, nearly all certified potato seed stocks originate from meristem tissue-culture plantlets produced under laboratory conditions.

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Meristem Tissue-Culture Seed Stock Development

The advent of tissue culture, in which plants are grown in artificial media under sterile conditions in the laboratory, has revolutionized nuclear seed stock development. Most certification agencies currently operate tissue culture laboratories that produce the initial stocks of pathogen-free planting material. A number of private companies throughout the United States also produce meristem-derived, pathogen-free seed. These companies either market their seed stocks locally or on a national scale.

The tissue culture procedure involves the removal of the small growing point or meristem, approximately the size of a flake of black pepper, from a tuber sprout or stem of a potato plant. The meristem is placed in a test tube or other vessel with media containing all of the necessary macro- and micronutrients, carbohydrates, growth regulators and salts required for growth and development into a plantlet. Once the plantlet is growing, it is ready for nodal cutting and pathogen testing (Fig. 10).
 

FIGURE 10. Tissue culture plantlets in test tubers. The plantlets have to grow in an environment designed to produce disease-free planting stock.

A nodal cutting from a tissue culture plantlet is a stem segment containing an axillary growing point and a leaf. Since a tissue culture plantlet has its own leaves, it is capable of producing its own food. Therefore, the nodal cutting is placed on a different medium that promotes root and shoot development. The nodal cutting process will be repeated many times in order to increase and obtain the number of tissue culture plantlets needed for minituber production.

During the initial nodal cutting process, pieces of the plantlet are retained for laboratory testing for the presence of disease-causing pathogens. Each plantlet of each variety is exhaustively tested for the pathogens causing soft rot, blackleg, ring rot and spindle tuber. In addition, each plantlet is tested for potato leaf roll virus, potato virus Y. potato virus X, potato virus S. potato virus A, and potato virus M. At this stage, 100% of all tissue culture seed stock is tested for disease-causing pathogens. During latter stages of seed stock development, seed certification agencies test 0.5-25% of the plants.

Once the laboratory has produced the desired number of plantlets for each variety, they are ready for tuber production. These plantlets can be planted outdoors directly into the field if great care is taken but are most commonly planted into beds in a greenhouse or screenhouse for minituber production. Under the controlled conditions of a greenhouse or screenhouse, the plantlets can be carefully cultivated and monitored. Several months, after planting, minitubers can be harvested and stored until the following growing season. Minitubers range in size from 1.3 - 5.1 cm (0.5 - 2.0 in) and are priced accordingly (Fig. 11). Microtubers, tubers produced in tissue culture medium, are also being marketed for their use in the production of disease-free seedstocks. At the current time, microtubers appear better suited for planting in greenhouses and screenhouses rather than in the field due to their extremely small size.
 

FIGURE 11. Mini tubers derived from greenhouse production of tissue culture plants. Mini tubers range in size from 1.3 to 5.1 cm.

Minitubers or tissue culture plantlets planted into the field are the initial source of certified seed potato lots. These lots will be multiplied and increased until a sufficient quantity is available for commercial use. During the increase process, the seed lots are subjected to visual field inspections and further disease testing. The number and intensity of which is greatly dependent upon the certifying agency in which the seed is being produced. However, all states and Canadian provinces have either a voluntary or mandatory limited generation system, depending on the seed production area.

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Limited Generation Seed Production

During potato production, seed or commercial, the plant is constantly exposed to sources of contamination by disease-causing pathogens. The probability of a seed tuber or seed lot becoming contaminated with pathogenic organisms increases every year the seed lot is in production. To minimize this, seed certification agencies have enacted regulations that basically restrict or limit the number of years the seed lot can be eligible for the seed certification process. This system is referred to as limited generation. Limited generation systems are handled differently in each seed production area. Additionally, the name of the seed class or number of the seed lot generation varies considerably among seed certification agencies.

Seed lots are limited in the number of years that they can be produced in the field after the tissue-culture derived material has left the laboratory or greenhouse. This varies from five to nine years, depending upon the seed production area (Table 13). Seed certification agencies also differ in what term is used to describe the generation of the seed lot. Much of this variation is influenced by whether or not the tissue culture plantlets or minitubers were produced on a state or provincially operated farm or on individual seed grower farms. Maine, New York, Wisconsin and Canada all operate seed farms. In general, seed produced from these farms does not receive a generation number until it leaves the farm and is grown by individual seed potato mowers. Some seed certification agencies also have specific criteria relating to disease tolerances and other regulations for each successive field planting. Since this can be very confusing, even to seed certification personnel, commercial growers are encouraged to contact the certification agency responsible for seed certification in the production area in question.

Table 13. Limited generation certified seed potatoes: field planting equivalency.¹
 

		Term used by Agency for seed potatoes harvested from field planting number2
Agency	13	2	3	4	5	6	7	8
Alaska California Colorado Idaho Maine Michigan Minnisota Montana Nebraska New York No. Dakota Oregon Utah Washington Wisconsin Canada	G1 N G1 N (MPBF)* N N N N (UF)* N N G1 N (U of W)* PE	G2 G1 G2 G1 G1 G1 G1 G1 G1 G1 G2 G1 E1	G3 G2 G3 G2 G2 G2 G2 G2 FU1 G2 G2 G3 G2 FG2 E2	G4 G3 G4 G3 G1 G3 G3 G3 G3 FU2 G3 G3 G4 G3 FG2 E3	G5 F G5 G4 G2 G4 G4 G4 G4 FU3 G4 G4 G5 G4 FG3 F	G6 C G6 G5 G3 G5 G5 -- G5 F G5 G5 G6 -- FG4 C	-- -- -- G6 G4 -- -- -- -- -- -- -- -- -- -- --	-- -- -- -- G5 -- -- -- -- -- -- -- -- -- -- --

*MPBF=Maine Potato Board Farm, UF=Uihlein Farm, U of W=University of Wisconsin Farm

¹The purpose of this table is to express equivalency of terms used by various certification agencies for seed potatoes harvested from a series of successive field plantings. For specific criteria relating to disease tolerances and other requirements, the reader is referred to the certification regulations of the agency in question.

²C=certified, E=elite, F=foundation, N=nuclear, U=Uihlein, PE=pre-elite, G=Generation.

³The first field planting utilizes laboratory tested stocks which may be tissue cultured plantlets, greenhouse produced minitubers, stem cuttings or line selections. Contact agencies for details as to types of stocks planted in their programs.

Maximizing Potato Seed Performance

Factors Affecting Seed Performance

The productivity of seed is dependent on many factors. A primary factor is seed quality, which. is covered in the Certified Seed section of this handbook. Other factors include: seed tuber size and age, storage, handling, preplant warming, type (whole or cut), seed piece size, sprout status, disease control and seed placement. The factor listed last is the primary reason for improper seed spacing, contrary to the common belief that most stand problems are the result of seed piece decay. The cause of imprecise planting is a complex of seed piece size and shape (a function of seed cutting) and planter operation.

Seed Age

Both chronological and physiological age of potato seed affects productivity. Chronological age is the time between time of harvest and time of planting. In general, the chronological age of seed planted for the fall crop is six to eight months. The physiological age of seed is a combination of the chronological age and the time interval between planting and harvest of the seed crop and storage temperature. Young seed tubers are generally the product of planting late (short season) and storage at temperatures between 38 and 40°F (3.3-4.4°C) after initial curing. They are sprout free at time of planting. Tubers that do not fit the above description are considered old. Physiologically old seed emerges sooner and produces more stems and tubers per hill than younger seed tubers. This results in a higher early yield than young seed, which makes it more desirable for areas with a short growing season and for the production of crops produced for early markets. In long season areas, however, young seed will produce higher yields of larger tubers than old seed.

Very old seed is undesirable in any area. Seed pieces of very old seed often produce tubers without above ground plants or produce tubers shortly after plant emergence. Generally these tubers are all that are produced and are smaller than the original seed piece.

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Storage

Desired storage temperature for seed potatoes after initial curing is 38-40°F (3.3-4.4°C). Storage temperature above 40°F (4.4°C) may promote premature sprouting of seed potatoes, particularly of varieties with a naturally short rest period. Quality may also be low if seed tubers are stored too cold. When tubers of susceptible varieties are exposed for long periods (20 weeks or longer) to a temperature near 32°F (0°C), mahogany browning, an internal reddish brown discoloration, may be found in irregular patches occurring anywhere in the tuber. The boundaries of the discolored areas are irregular and indefinite. This phenomenon differs from freezing injury when tubers are actually subjected to freezing temperatures. Tuber flesh with freezing injury usually becomes soft and watery when placed in a warm environment. Either of these conditions impairs seed quality.

Handling and Pre-Plant Warming

Recent research shows that damage (bruising) to seed potato tubers following storage reduces yield potential by as much as one-third. Handling seed tubers with low damage levels can be accomplished by implementing the same concepts that reduce tuber damage during harvest. These concepts are covered in detail in the section on Harvesting the Potato Crop.

Tubers of seed potatoes should be warmed to 50-60°F (10-15.6°C) before handling and cutting. Some 7-14 days before planting the seed should be removed from cold storage. Tubers taken from 40°F (4.4°C) storage should never be planted directly. Tubers go through an initial "sweat" period (condensation of moisture on the surface) when changed from a cold to a warm environment and this can contribute to seed piece decay, particularly with cut seed.

Type and Size of Seed

Some growers prefer to plant uncut, small, seed tubers. The use of small, uncut seed tubers reduces the hazard of spreading disease in the cutting operation and helps insure a better stand of plants, particularly under adverse field conditions. The desired size range for planting whole tubers without any cutting is 1 1/2 to 2 ounces (Fig. 12). Some growers are reluctant to use small whole tubers because these may be the progeny of virus-infected plants. Use of certified seed which has met winter test standards reduces this concern. Planting uncut seed tubers frequently results in a heavier set of tubers than that obtained for the same variety when cut seed pieces of the same size are planted.
 

FIGURE 12. Small whole size seed exhibiting apical downance and a large seed tuber suitable for cutting into four seed pieces. Seed size should range from 1.5 to 3.0 oz. (43-85G).

Seed pieces, either freshly cut and planted or properly healed before planting, can be just as productive and healthy as whole small seed tubers. Seed tuber size will have an effect on the productivity of the subsequently cut seed pieces. Large seed tubers (greater than 9 oz.) give a higher percentage of seed pieces without eyes (blind seed pieces) than smaller tubers. This can be a substantial problem in varieties that have uneven eye distribution. Small seed pieces (less than 1.5 oz.) produce fewer stems (one to two/hill) than do large ones. Excessively large seed pieces can produce too many stems per hill increasing competition for light, nutrients and water. In this case, many small tubers are produced. Very large seed pieces are not well suited for proper planter operation. The desired seed piece size from the stand point of productivity and planter performance is over 1.5 oz. (43 g) and under 3.0 oz. (85 g).

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Cutting Seed Potatoes

Several types of mechanical seed cutters are available, all of which do an effective job if managed properly. Machines are available specifically for cutting round-tuber varieties or long-tuber varieties. Some machines can be effective in cutting both tuber types. Best results are obtained only if seed tubers are graded to a uniform size before cutting. Seed cutters should be kept in good repair, the knives sharp and properly adjusted. Sanitation during the seed cutting and handling operation is essential to reduce disease spread. Every machine should be thoroughly cleaned and disinfected often and no less than between the cutting of each separate seedlots. It is desirable to thoroughly clean and sanitize the equipment at least on a daily basis, even when seedlot changes do not occur, particularly if large seed tuber lots are being cut. Mechanical cutters rarely eliminate the need for some human labor to remove the undersize pieces and hand-cut the excessively large seed pieces.

Blocky seed pieces are desired. Cutting of long-tuber varieties with a machine designed or adjusted for round varieties will result in long slender seed pieces with too much cut surface area. A large cut surface provides a greater hazard for seed piece decay if environmental conditions after cutting are favorable for decay organisms whether it be in storage or in the soil. The desired seed piece is 1 1/2 to 3 oz. (43-86 g) with one to three active eyes. Table 8 gives the average weight of the seed pieces in a 12-pound (5.4 kg) sample. Growers should periodically take samples and determine the average seed piece size. When evaluating the sample, the number of chips, seed pieces without eyes, and other non-usable seed pieces should also be noted.

The distribution of seed piece sizes should also be evaluated. A distribution which has most of the seed pieces within the 1.5 to 3.0 oz. (43-8g) will result in the best planter operation and will give the best economic return. Seed pieces less than one ounce should be discarded with a "sliver eliminator" and the cutter should be adjusted to minimize seed pieces over 3.5 oz. (99 g).

Soil conditions at planting are frequently favorable for suberization or healing of the cut surface so that freshly-cut seed pieces can be planted directly after cutting. This requires that the cutting operation and the planting operations be synchronized to avoid holding unplanted cut seed pieces an extended length of time. It is safe to cut seed tubers some time ahead of planting if storage conditions promote healing of the cut surfaces. Wound healing is best accomplished by: 1) holding the cut seed pieces 3 to 5 days at temperatures of 55-65°F (12.8-18.4°C), 2) maintaining a relative humidity of at least 85 percent, and 3) providing good ventilation. Failure to provide any of these conditions can lead to seed piece decay. Seed pieces which are blocky and well healed are equally as sound as whole seed tubers.

Cut seed pieces should not be exposed to hot sun or wind for even a short time or they will severely shrivel and may decay. Covering loads of cut seed pieces with a canvas during transport is recommended. If planting is delayed, the best place to hold cut seed pieces is in a potato storage with good air circulation, high humidity, and temperature control.

Seed potatoes should never be put into used fertilizer bags, bulk boxes, or other containers where fertilizer or pesticide residues are present, as damage to the seed may result. Storages used for receiving and/or holding seed tubers should not have been used to store commercial potatoes treated with the sprout inhibiting chemical CIPC.

Sprout Status

The most productive sprouts produced by a potato tuber are those that begin to grow first. Sometimes it is necessary to remove these initial sprouts because planting is delayed and excessive sprouting has occurred. If these large sprouts are not removed they can interfere with the planting operation. Unless this is the case desprouting should be avoided, because it results in reduced plant vigor and can spread disease. If sprout removal is necessary, it should be done only once as repeated sprout removal will increase the age of the seed thereby adversely affecting seed performance.

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Seed Treatment For Disease Control

The need for seed piece treatment varies from region to region. Numerous studies have been performed to evaluate seed piece treatment. Current seed treatment materials only affect disease organisms present on the surface of tubers. They do not prevent transmission of virus or growth of bacteria. Most seed treatments are applied as a powder or dust, and there may be some benefit from the drying action on the fresh-cut surface. These materials also provide some protection from invasion of the seed pieces by soil inhabiting organisms and thus may result in a reduction of seed piece decay following planting. Seed treatment is not to be considered a cure for poor seed handling and for a poor seed environment, either before or after planting.

Planting The Crop

Time of Planting

Planting time varies considerably from region to region depending on local climatic conditions and intended market use. The major portion of potatoes grown in the northern tier of U.S. and bordering provinces in Canada are planted from April to early June. In the southern states planting occurs from November to February and in the states in the middle zone of the U.S., potatoes are planted in March and April. In general, potatoes should not be planted when the soil is below 45°F (7°C) or above 70°F (21°C). At time of planting, soils should be moist but not excessively wet. In irrigated desert areas, it is common to pre-irrigate prior to planting. Planting in cold, wet or hot, dry soils increases the potential for seed piece decay. Within reasonable limits, the early establishment of a crop results in greater yield and dry matter potential. In regions where the growing season is longer than is required for full development of the varieties grown, planting dates are selected to provide the crop with the most favorable environment during the critical period of tuberization, proper maturity for the desired market, and economic yields.

The variety grown, fertilization program, availability of irrigation, soil type, and market outlet are major factors in determining the desired between and within row spacing of potatoes. Row widths generally range from 30-36 inches (76-91 cm), although both wider and narrower rows are used. Spacings within the row are influenced most by the intended market and the variety grown. Seed pieces of varieties with the tendency to set a small number of tubers and/or develop oversize tubers should be spaced at 7-9 inches (18-23 cm) apart within the row. Closer spacings may apply where irrigation is available or where moisture is not expected to be limiting. Varieties which generally set a high number of tubers should be spaced 11-14 inches (28-46 cm) apart in the row. The use of close in row spacing, may help reduce losses from hollow heart and growth cracks, and result in improved appearance and an increase in the number of marketable tubers.

The market outlet is a major determining factor in selecting the proper spacing. Potato processors who produce frozen French fried products desire large tubers, within limits, and often pay a premium for a high percentage of tubers over 10 ounces (284 g). Production of large tubers is enhanced by wide spacing. Seed buyers, however, prefer tubers under 3 1/4 inches (8 cm) diameter or 10 ounces (284 g). Whole B-size tubers often demand a premium.

Depth of Planting

A general practice in the northern regions of the U.S. is to plant 2 to 3 inches (5-8 cm) deep (measured from top of seed piece to ground level) with a total soil cover of only 3 to 4 inches (8-10 cm) from seed piece to the top of the covering ridge to encourage rapid emergence. This results in strong vigorous plants which are resistant to attacks by blackleg, rhizoctonia, and seed piece decay. In certain locations, growers "drag off" the planting ridge by removing the tops of the ridges before emergence as a weed control measure as well as to enhance emergence. When planting ridges are low and effective pre-emergence herbicides are used this practice is not needed. Ridging or hilling, as the plant develops, is usually practiced to enhance stolon development, prevent tuber greening, and facilitate harvest. Ridges or hills are sometimes built up gradually in two or more tillage operations or it may be done in one. Planting depth and soil cover vary from area to area because of differing soil, moisture and climatic conditions and established practices. In some low and flat areas in humid regions, it may be desirable to place the seed at a level above that between the rows. This is done to protect the seed pieces from excess water which may stand on the surface because of poor or slow drainage. In areas where moisture may be limiting, seed pieces are usually planted from 3-5 inches (8-13 cm) deep to make sure they are covered at all times with moist soil. In some areas, seed pieces are planted six inches (15 cm) deep in preformed beds. Regardless of depth at planting, the seed piece is usually about 6-8 inches (15-20 cm) below the top of the bed or hill.

Four general types of potato planters are most frequently used: the automatic picker; the cup type; the assisted feed; and the tuber-unit planter (Fig. 13). The picker planter is the most popular and operates on the principle of forcing nail-like picks into the seed piece to carry it out of the filling reservoir and then dropping it into an open seed furrow. The punctures made in the seed piece can result in the spread of disease and can provide an entry for decay organisms. The picks must be straight, sharp, and properly spaced to obtain desired seed piece spacing. Pick length and arrangement should be adjusted to the seed size to prevent skips or multiple plantings.

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FIGURE 13. Semi-mounted potato planters are used widely. Potato planters come in two-, four-, six-, or eight-row units. Picker-arm and cup-type planters are the most popular.

The cup-type planter uses a cup device instead of the pick to convey the seed to the soil. Seed pieces are not punctured with this type of planter. The size of seed pieces must be nearly uniform to obtain optimum spacing. These planters are well adapted for use with whole small seed tubers.

Assist feed and tuber unit planters are used in some seed potato production farms. These planters are relatively slow compared to new commercial equipment. Their use, however, enables seed producers to grow high quality seed tubers by being able to identify and eliminate all plants produced by disease infected tubers. A small percentage of the potato acreage is planted with this type of equipment.

Seed spacing and uniformity of spacing is a function of: properly maintained and operated planters, seed piece size and shape, seed depth in the planter seed reservoir and planter speed. Planters need to be in excellent operating condition before the planting season begins. Planter mechanisms should be inspected regularly throughout the planting season and the equipment should be properly cleaned and stored after planting is through.

Seed piece size and shape greatly influences planter performance regardless of the type or make of machine used. Use of blocky cut seed pieces and/or uniformly sized whole seed enable the planter to more nearly achieve uniform, accurate spacing. Picker length and arrangement or cup diameter and chain vibration should be adjusted for the type and size of seed being planted.

The depth of seed in the picker or cup reservoir should be at a level that all picks or cups leaving the reservoir should have seed pieces on them. Generally the reservoirs should be approximately three-quarters full. Having the reservoir too full or not full enough results in poor and erratic plant spacing.

Planter speed also affects seed piece distribution. Planting too fast or too slow will give undesirable results. The optimal speed is a function of planter operation and seed piece size and shape. There is some evidence that acceptable seed piece placement is generally achieved at speeds between 2.5 and 3.0 miles per hour.

There is considerable variation between planters, seed lots and field conditions. Regular checks of seed distribution are required to achieve the desired spacing. Seed piece placement by each planter unit should be inspected routinely. Two or more 25-foot sections of row planted by each unit should be dug up or exposed by straightening or taking off the covering disks. The number of seed pieces in the measured area should average out to the desired spacing. Precise placement of seed at the desired spacing is currently achievable but requires considerable management input. Some research data suggest that precise placement is not necessary for optimal yield due to plant compensation. Every effort should be made, however, to have the distribution as uniform as possible by altering planter speed and making appropriate mechanical adjustments.

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Amount of Seed to Plant

The amount of seed needed to plant an acre varies according to seed piece size, row, and plant spacing. Table 14 shows the seed amount required for several spacings and seed piece sizes. Growers should strive for plant populations that are optimum for their area, growing conditions, variety, and market.
 

Table 14. Seed potatoes required to plant an acre at different spacing with seed pieces of various weights.
 

	Seed required per acre when seedpieces weigh an average of---
Spacing of rows and of seedpieces within rows	1 1/2 oz (43 g)>	1 3/4 oz (50 g)	2 oz (57 g)
		cwt of (kg)
Rows 30 in (0.76m) apart 6 in (15.2 cm) 8 in (20.3 cm) 10 in (25.4 cm) 12 in (30.5 cm) 14 in (35.6 cm)	32.6 (14.8) 24.5 (11.1) 19.6 (8.9) 16.3 (7.4) 14.0 (6.4)	38.0 (17.2) 28.6 (13.0) 22.8 (10.3) 19.0 (8.6) 16.4 (7.4)	43.6 (19.8) 32.6 (14.8) 26.1 (11.8) 21.8 (9.9) 18.7 (8.5)
Rows 32 in (0.81 m) apart 6 in (15.2 cm) 8 in (20.3 cm) 10 in (25.4 cm) 12 in (30.5 cm) 14 in (35.6 cm)	30.7 (13.9) 22.9 (10.4) 18.4 (8.3) 15.4 (7.0) 13.1 (5.9)	35.9 (16.3) 26.8 (12.2) 21.4 (9.7) 17.9 (8.1) 15.2 (6.9)	40.9 (18.6) 30.6 (13.9) 24.5 (11.1) 20.5 (9.3) 17.5 (7.9)
Rows 34 in (0.86 m) apart 6 in (15.2 cm) 8 in (20.3 cm) 10 in (25.4 cm) 12 in (30.5 cm) 14 in (35.6 cm)	28.8 (13.1) 21.6 (9.8) 17.3 (7.8) 14.4 (6.5) 12.4 (5.6)	33.6 (15.2) 25.2 (11.4) 20.2 (9.2) 16.8 (7.6) 14.4 (6.5)	38.4 (17.4) 28.8 (13.1) 23.0 (10.4) 19.2 (8.7) 16.5 (7.5)
Rows 36 in (0.91 m) apart 6 in (15.2 cm) 8 in (20.3 cm) 10 in (25.4 cm) 12 in (30.5 cm) 14 in (35.6 cm)	27.1 (12.3) 20.4 (9.3) 16.3 (7.4) 13.5 (6.1) 11.6 (5.3)	31.7 (14.4) 23.8 (10.8) 19.0 (8.6) 15.8 (7.2) 13.6 (6.2)	36.1 (16.4) 27.2 (12.3) 21.8 (9.9) 18.1 (8.2) 15.5 (7.0)

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