Sudan Grass. 21

mixture of Johnson grass in the Sudan grass seed sown for hay pro- duction north of the thirty-eighth parallel is no great disadvantage. South of this line, where Johnson grass is likely to become a pest, care must be used to see that the Sudan grass seed is free from Johnson grass seed.

The following suggestions may be remembered with advantage by the farmer in the Southern States: A farmer may grow sufficient seed for his own plantings and thus be assured of its purity ; when he finds it necessary to buy Sudan grass seed and his land is free from Johnson grass, he should purchase only that grown outside the Johnson grass region or from responsible growers in the South who can g uarantee the purity of the seed. If the Sudan grass is to be

FIG. 10.—Seeds of Johnson grass (left) and Sudan grass (right) showing their

comparative size and their similarity in shape.

seeded on land already containing Johnson grass, the presence of seed of the latter is a matter of small importance.

The presence of even a small number of Johnson grass seeds can be detected by a properly trained seed analyst. A method for their identification has been formulated by F. H. Hillman, of the United States Department of Agriculture, and is described fully in Depart- ment Bulletin 406.

A point to be remembered in the production of Sudan grass seed is that the plant hybridizes very freely with sorghum, especially with the sweet sorghums. In dry regions where the pollen is car-

ried for considerable distances by the wind, a Sudan grass field intended for a seed crop should be 60 to 80 rods from any sorghum. Another source of cross-pollination exists in the volunteer plants of sorghum sometimes found in fields that were planted to sorghum the previous year. To avoid such sources of trouble, fields that have been growing other crops than sorghum should be chosen for the Sudan grass seed crop.

To make sure of pure seed, roguing the field at least once a year must be resorted to, and the rogues should be removed before a