Detasseling

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Detasseling is the act of removing the pollen-producing tassel from a corn (maize) plant and placing it on the ground. Detasseling is done to cross-breed, or hybridize, two different varieties of corn. Fields of corn that will be detasseled are planted with two varieties of corn. By removing the tassels from all plants of one variety, all the grain growing on those plants will be fertilized by the other variety's tassels. In addition to being more physically uniform, hybrid corn produces dramatically higher yields than corn produced by open pollination. With modern seed corn the varieties to hybridize are carefully selected so that the new variety will exhibit specific traits found in the parent plants. The detasseling process typically involves the use of specialized machines and human labor.

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[edit] Machine detasseling

Nearly all detasseling is done in two steps; the field will firth be detasseled by machine and then detasseled manually. Machine detasseling is itself typically a two step process. Initially a detasseling machine called a "cutter" will go through the rows of corn to be detasseled and cut off the top portion of the plant. This is done to make the field more uniform so that a "puller" machine can come through the corn field a few days later and pull the tassel out of the plant by catching it between two rollers moving at a high rate of speed. This will remove a majority of the tassels.

Detasseling machines can typically only remove from 60 to 90 percent of the tassels in a seed corn field. This is far less than the 99.5 percent that need to be removed to produce the uniformity of seed desired by farmers. The main problems for the machines are that they are unable to adapt quickly to height differences in plants and they throw tassels into the air where they can become lodged in other corn plants and inadvertently allow pollination. It is desirable that the pulled tassel ends up on the ground to prevent this problem.

[edit] Manual detasseling

Whether or not a field of seed corn is initially detasseled by machines, eventually people are employed to detassel the plants that the machines missed and to remove any tassels that the machines left in the leaves of other corn plants. This is done either by having "detasselers" walk through the corn field removing the tassels or by having detasselers ride though the corn field on a detasseler carrier. From eight to twelve detasselers are usually carried by each machine and these machines are typically employed when the corn is too tall to be detasseled from ground level.

Detasseling work is usually performed by teens; as such, it serves as a typical rite of passage in rural areas of the Corn Belt. (Byron 2002)(Gustafson 2003) For many teens in these areas it is their first job. Detasseling typically begins in July and lasts an average of two to three weeks. Exact starting dates depend on the specific area of the country and the growing conditions of any given year. In addition to employing a large teenage workforce, some areas of the country employ migrant workers as detasselers.

[edit] Seed corn fields

Seed corn fields are planted in a repetitive pattern known as a "panel", "block" or "set" depending on the area of the country. There are two main planting patterns for these panels. A panel may be planted in a 6:2 pattern where six "female" rows, the rows to be detasseled, are followed by two "male" rows, the rows that will be used to pollinate the detasseled rows. Panels are also commonly planted in a 4:1 ratio with four female rows followed by a single male row. Other, less common, patterns are also used including 4:2 and 4:1:6:1. In all cases the pattern is continued throughout the corn field. (Smith 2004, pp. 584)

All or portions of seed corn fields may be surrounded by extra rows of male plants if it is likely that wind may carry foreign pollen into the seed corn field. These extra rows are called "buffer" or "isolation" rows depending on the area of the country.

[edit] History

[edit] Early 20th century

Detasseling was used in the late 1800's and early 1900's in the "ear-row" method of corn breeding. In this method alternating rows of corn are detasseled and the seed from the detasseled rows is saved for planting the following season. However, ear-row breeding did not result in large yield increases and was largely abandoned after a few years.(Wallace 1925, pp. 223)

Around 1910 experimental corn breeders became excited by the possibility of improving corn yields by crossing two high yielding varieties. Again, this was accomplished by planting the varieties in alternating rows and detasseling one of the varieties. This method of seed production also proved disappointing and was also abandoned.(Wallace 1925, pp. 224)

However, from this early work in cross breeding, developed the modern hybridization process where one inbred line of corn is crossed with another. In 1908, George Harrison Shull described heterosis, also known as hybrid vigor. Heterosis describes the tendency of the progeny of a specific cross to outperform both parents. In 1917 a process was developed that would make this hybridization commercially viable. In 1933 less than 1% of the corn produced in the United States way produced from hybrid seed; by 1944 over 83% was. (Copeland 1995, pp. 236) This hybrid seed is produced by crossing two inbred lines by planting a row of one inbred variety followed by several rows of a second variety. The tassels of the second variety were removed by hand so that the second variety could be pollinated by the first.

[edit] Late 20th century

Hybrid corn was detassled manually until the mid 1950's when a cytoplasm was discovered that would cause one of the inbred lines to be male sterile while the hybridized seed corn it produced would regain male fertility. This gene allowed seed corn companies to greatly reduce their labor costs by producing seed corn without the need for manual detasseling. By the mid 1960's nearly all seed corn was produced with this gene. (Basra 1999, pp. 52)

This situation was changed in 1971 with an outbreak of the fungus Southern Corn Leaf Blight. The cytoplasm used to produce male sterility was highly susceptible to this fungus. At the time approximately 90% of hybrid corn used in the United States contained this gene. {{Harvard citation|Smith|2004|pp=601} About 15% of the corn crop was lost to infection and for the next few years male sterility was abandoned and nearly all seed corn was again detasseled manually. (Copeland 1995, pp. 238)

In the mid 1970's machines were developed to help reduce the large labor costs associated with manual detasseling and as a response to a shrinking rural teen labor force. In the 1980's male sterile varieties were reintroduced that were not susceptible to Southern Corn Leaf Blight, however the reliance on a single sterile variety seen in the 1960's has not been repeated. (Basra 1999, pp. 51-52)

Today corn hybridization is accomplished by a combination of machine and manual detasseling as well as male-sterile genes.

[edit] References