Standards of purity provide that no seeds of noxious weeds and not more than 0.05 percent of common weeds be present.

The production of hybrid seed of grain sorghum utilizes a cytoplasmic male-sterile female or seed parent and a restorer line as the male or pollen parent. Thus none of the heads in the female rows should produce pollen.

The rows require patrolling to remove any plants with heads showing fertility. Grain sorghum flowers are complete, and normally about 90 percent are self-fertilized.

Because detasseling, as with corn, is not possible in grain sorghum, a male-sterile method is required to produce hybrids of grain sorghum.

A genetic-sterile method was used to produce the first commercial seed of the hybrids distributed in 1956, but it was discontinued.

Hybrids of grain sorghum are of such recent origin that many seed-production practices still are not proved. Practices that were considered all right in 1961 very likely will be improved with increasing specialization in the production of more expensive seed. Longer experience may reveal that particular sections and techniques produce seed of better quality.

Locality is important. Northern areas, such as Nebraska, South Dakota, and Iowa and parts of Missouri and Kansas where johnsongrass is not common, have little difficulty in producing seed free from outcrosses to johnsongrass or sorghum almum types.

Where the growing season is short, as in Nebraska, Iowa, and South Dakota, however, the seed must be early in maturity and harvested early in the fall to avoid frost damage.

Irrigated land is the most suitable for seed production, because ample soil moisture insures uninterrupted growth, maximum yields, and a minimum of pollen production in the male-sterile parent. Furthermore, harvest may be hastened, and fields may mature more uniformly because irrigation can be stopped at the proper time.

Weathering and discoloration of sorghum seed and sprouting in the head are hazards, especially in the more humid areas. Germination in the head may occur during damp autumn weather. Production in regions of high risk of frost damage may require a harvest method in which the heads are removed from the plants and dried in large driers, much as the ears of corn are removed from the plants and dried.

Production of sorghum seed in the Southwest has advantages: A long, frost-free season; low humidity at harvesttime; high yields and large seeds; harvesting with combines; the need to dry only in certain seasons; nearness to the market; and low seed costs.

The greatest hazard in southern localities is outcrossing with johnsongrass, sorghum almum, or forage types. Care in selecting isolated fields in places with a minimum of offtype sorghums can insure pure seed.

Seed set is more difficult in sorghum seed than in corn. The sorghum plant is sensitive to changes in heat, moisture, and length of day.

Kafirs react differently from milos. Seed fields with good seed set usually give seed that is low in outcrosses.

The planting of 4 rows of the pollen parent alternating with 12 rows of the seed parent is most convenient mechanically, although the 2-and-6 combination is followed in southwestern Iowa.

With the 4-and-12 combination, a 4-row planter can be used where the parents are planted on different dates. A self-propelled combine can gather readily the four rows of the pollen parent before the seed crop is harvested.

The proper timing of planting parents with different flowering dates sometimes is difficult. Low frequency of outcrosses is expected when the parents nick at flowering and a large part of the female florets are pollinated in a few days.

Hybrids requiring different planting dates for the parents need increased isolation and careful attention at planting time. Extra pollen rows around the field may improve seed set, especially at the ends and on the side of prevailing winds.

When seed set is low and temperature and humidity are favorable, the female florets may remain fertile for a long period after the male-row pollen has been shed. The risk of outcrossing then is high. This risk is greatest with adequate rain and mild temperatures.

We need some method whereby we can end the period during which the stigmas in the female florets are receptive to pollen.

In roguing seed fields of grain sorghum to remove offtype or undesirable plants, it is necessary to pull up the entire plant instead of merely removing the heads, because late tillers and side branches may flower and produce seed. In irrigated fields or in humid areas, precautionary measures are necessary to prevent uprooted plants from again taking root in the moist soil.

Both pollen rows and seed rows are rogued carefully. The male requires special care to reduce the amounts of undesirable pollen in the field. In female rows, fertile and offtype plants must be removed before they shed pollen.

Offtype plants may differ from the majority in such traits as height, head type, color of anthers and plants, and presence or absence of awns.

Most dwarf types tend to produce mutants toward tallness in ratios of about 1 to 1,000 plants. They are removed as offtype. Plant breeders have started work to develop types without tall mutants.

Plants that show evidence of pollen fertility in the cytoplasmic-sterile parent are removed as the anthers emerge. Some pollen is shed, because the anthers must emerge before the plants can be identified. Frequent patrolling of the rows during pollinating time therefore is necessary to reduce self-pollination, which would produce non-hybrid seed, which yields less than hybrid seed and sometimes gives off-type plants in the farmers' fields.

Certification standards for tolerances on fieldwork vary. An example: The maximum tolerance is not more than 1 definitely offtype plant per 2 acres; not more than to doubtful offtype plants per acre; no head smut; not more than 1 head in 100 with kernel smut; and no uncontrolled areas of field bindweed, hoarycress, Russian-knapweed, or johnsongrass. If field inspection shows evidence of hedge bindweed, morning-glory, or velvetleaf, special measures must be taken in connection with inspection of the grain.

Most producers of hybrid seed have followed a practice of making special harvests from their seed fields during October to get representative samples of seed for planting in winter observation plots in Mexico, Florida, Jamaica, or southern Texas. Such winter plantings usually mature sufficiently to determine purity before the seed is to be sold. The seedsman thus can withhold undesirable lots.

MOST SEED of grain sorghum is combined at moisture levels of 13 percent and less. Drying therefore is not needed. Aeration may be needed in some areas if such combined seed is stored in bulk.

Some seed is harvested at 16- to 18-percent moisture and then dried to a maximum moisture content of 12 percent in batch or continuous-flow driers.

An important consideration in determining the maximum air temperature for drying seed is the time required for drying, because it determines the length of time the seed remains in contact with heated air.

A maximum air temperature of 95 is recommended for drying deep depths (3 to 6 feet) of seed of 16- to 18-percent moisture. In tests in Texas, an air temperature of 120 , with the seed exposed to the heated air for 2.5 hours, was not injurious to the germination of seed dried in thin columns 10 inches thick. A maximum air temperature of 110 is normally recommended.

Even though seed is cooled as a part of the heated-air drying operation, the temperature of the seed is usually above normal when it is placed in storage. It should be stored in bags therefore, or some provision should be made to cool seed after it is stored in bulk. A practical and economical method of doing this is with an aeration system that uses a motor-driven fan to move small amounts, one-tenth to 1 cubic foot of air per bushel a minute, of air through the stored seed.

Another procedure is to harvest heads at moisture levels of 25 percent in the grain, dry with air at a temperature of 110 , and thresh with a reduced number of concaves in the thresher.

Sizing and cleaning seed of grain sorghum requires less equipment than for corn, because the small seed does not require sizing. The necessary operations include scalping, to remove large grains; screening out the extremely small grains; and screening and air cleaning to remove weed seeds. The clean product is given fungicide protection (with thiram or captan) and then weighed and packaged in 50-pound bags, ready for delivery.

JOHN M. AIRY is the production manager of Pioneer Hi-Bred Corn Co., Des Moines, Iowa. He has had experience in producing seed of hybrid corn and sorghum and has conducted research on many problems.

L. A. TATUM is Chief of the Cereal Crops Research Branch, Crops Research Division, Agricultural Research Service, Beltsville, Md. He has done research with cereal crops.

J. W. SORENSON, JR., is professor of agricultural engineering at Texas Agricultural and Mechanical College, College Station, Tex. He has conducted research on engineering problems encountered in harvesting, drying, and storing field crops.