Four crops, corn, soybeans, wheat and hay occupy 80% of the planted crop acreage in the US. Corn is the nations largest crop. In 1998, corn was harvested on 73 million acres which produced 9.7 billion bushels. In 1995, approximately 18%. of the crop was used for food, seed and industrial uses; 59% was used for animal feed; and 23% was exported.
Acreage
The area in corn in the United States has varied since 1900 from a high of 116 million acres in 1917 to a low of 64 million acres in 1969. The area planted to corn in 1999 was 78 million acres and 71 million was harvested.
Yields
Along with the marked variations in corn acreage have come dramatic increases in yields of grain, and changes in production inputs and practices. Corn grain yields have increased from an average of 29 bushels per acre in 1900 to 134 bushels per acre in 1998 (Figure 1), an average gain of about 1.0 bu/ac/yr. But since 1940, the average annual rate of gain has been about 1.8 bu/ac/yr.
Genetic Advances
From 1940 to 1980, the biologic technology factor of genetic improvement contributed about 59% of the unit area production of corn (Cardwell, 1982). Prior to about 1930 open pollinated corn varieties were used exclusively. At the present time hybrid seed is used on essentially all land planted to corn. Genetic improvement has not only contributed to yield increases but also to other desirable plant components such as resistance to lodging and tolerance to increased plant populations, insects, and diseases. More recently, value added traits have been introduced such as high oil content.
Fertilizers
At the present time, fertilizers are applied to over 98% of the corn acreage. Prior to about World War II, inorganic commercial fertilizers were not commonly used for corn or, if so, were used sparingly, and primarily were phosphate and potash. After the war use of fertilizers increased dramatically. Technologies and production plants for nitrogen, developed during the war, were used to reduce the cost and increase availability of manufactured nitrogen fertilizer. Total fertilizer use for all crops increased from about 1 million tons in 1940 to 21 million tons (nutrients) in 1993 (Figure 2). The increase in use of fertilizer was dramatic until about 1976 and then started to level off. Since about 1985, fertilizer use has been stable. Phosphate and potash fertilizers were introduced in about 1940 and increased at a constant rate until the early 1980's and then remained steady after that time (ERS, 1994). Use of nitrogen fertilizers began a sharp increase in about 1960 and has remained constant since the mid 1980's. About 30% of the nutrients applied to all crops came from manure in 1985 (ERS, 1994). It has been estimated that about 30 to 50% of the increase in yields since 1940 have been due to the increased use of commercial fertilizers (Nelson, 1990).
Tillage and Management
Until the last decade or so the standard tillage practice for corn was use of the moldboard plow for primary tillage followed by several secondary tillages and mechanical cultivation after the crop was up. Now about two-thirds of the crop is planted without use of the moldboard plow (Allmaras et al., 1997), and mechanical cultivation is limited to one, or no operations.
Corn populations have increased from an average of about 12000 to 14000 plants per acre prior to 1940 to over 27000 plants per acre in the Cornbelt, with top producers using over 30000 plants per acre. Row widths have been reduced from the standard of 40-42 inches to 30 inches or less. Corn planting times have progressively become earlier in the last several decades with the availability of herbicides for early season weed control and improved seed quality and low soil temperature tolerance.
Prior to 1940, corn was usually grown in rotation with small grains and a close grown sod crop (e.g. alfalfa, clover, grasses). Traditional rotations were abandoned during the 1960's and 1970's. In 1993, 82% of the corn was grown in a monoculture or alternated with soybeans (Allmaras et al., 1997).
Pesticides
Modern herbicides were introduced in the late 1940's and now are used on over 97% of the corn acreage (ERs, 1996). Total and per acre use of pesticides for all crops reached their peak in the early 1980's and has since declined (ERs, 1994). Corn accounted for 55% of the 410 millions pounds of herbicides used in 1986. Atrazine, and atrazine containing mixtures, were applied to 70% of the treated acreage in 1996.
Insecticide usage on corn increased markedly during the 1960's as a result of the development of organophosphate insecticides and the concurrent rapid switch to monoculture corn production resulting in an increase in crop losses due to corn root worm. Insecticide usage on corn peaked in 1975 and has continued to decline as a result of changes in cultural practices and the use of integrated pest management (IPM) practices. Corn accounted for 44% of all insecticides used on crops in 1986. Average pounds per acre of total pesticides for corn has recently declined (ERs, 1994), but this decline reflects a change in chemistry allowing much lower use rates of new generation pesticides rather than decrease in acres treated.
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http://citv.unl.edu/cornpro/html/history/history.htmlas retrieved on Sep 30, 2005 04:50:49 GMT. Cache source: 15mar2007
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Literature Cited
NOTE: This is Google's cache of
http://citv.unl.edu/cornpro/html/literature.htmlas retrieved on Sep 30, 2005 05:29:02 GMT. Cache source: 15mar2007
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