Bureau of Plant Industry—No. 184 (1911)
The Production of Vegetable Seeds: Sweet Corn and Garden Peas and Beans

CROSS-POLLINATION.

The location and character of soil are of less importance in the production of seed corn from which the best results may be expected than that the fields be so situated as to avoid as far as possible liability to mixture through the pollen. This is a far more common cause of inferior quality than is generally supposed. Seed growers object to growing Black Mexican corn because they say it crosses so freely with other sorts. There is no evidence, however, that this variety crosses more readily than others, but when crosses of Black Mexican and other varieties do occur the effects are more readily seen. Instances are known where Moore's Concord was evidently crossed with Black Mexican corn growing nearly two miles away; but in another case, where the Black Mexican and the Moore's Concord varieties were planted side by side on the same day, there was no indication of mixture beyond the sixth row from the dividing line.

Corn pollen in abundance has been obtained from the plumage of a blackbird, which must have flown between 4 and 6 miles from the last field of corn visited, and doubtless others of the large flock to which the bird belonged carried equal quantities of pollen, more or less of which they must have scattered in the field of seed sweet corn in which they alighted.

Difference in season of maturity is not always a protection against crossing. The writer knows of a case in which there was clear evidence of mixture in both directions between Extra Early Red Cory and Stowell's Evergreen corn growing side by side, although the Cory was planted some days before the Evergreen, but in another instance there was no sign of mixture between the Cory and the Black Mexican varieties planted side by side on the same day, all of the silk and the tassels of the Cory being ripe and dry before even the earliest tassel appeared on the Black Mexican, and in this case there were no late-blooming suckers on the Cory to furnish pollen for the earliest Black Mexican plants.

In another instance, where two sorts of corn were separated by a thick grove of tall trees about 4 rods wide, there was no sign of mixture, but just beyond the grove, where the lots of corn were separated by an equal width of grass land, there was abundant evidence of crossing.

In another case, where two sorts were separated by a thick osage hedge not over 12 feet high, there was no indication of crossing within 8 or 10 rods of the hedge, while beyond that distance there were many crosses.

Experience shows that neither a distance less than several miles nor any varietial difference can be relied upon as a certain protection against a mixture of pollen. In most farming regions it is impracticable to locate a field of seed corn so as to guarantee that there shall be no mixture through pollen, though much can be done to lessen the probability of a mixture. How this may be best accomplished is a different problem in each case. Usually the most practical way is to plant each lot of seed as far as possible from any other corn, and also to have as much difference as possible in the dates of ripening of the seed corn and of the corn in the nearest field.

Fortunately, the effect of crossing in corn is rarely masked for a number of generations, as it often is in leguminous plants. It frequently shows so plainly in the grain which is the immediate result of the cross that much of the hybrid corn can be removed by careful sorting before shelling, and it is well to throw out the whole ear rather than to pick out the mixed grain, as is the common practice, because crossing does not always change the appearance of the grain the first season, and there is a strong probability that on an ear on which crossed grains are visible there are other crossed grains which show no external sign of mixture.