FIGURE 2-1 Index of farm productivity in the United States, 1948–1996.

aThe index of productivity was determined by dividing all input values (such as feed, seed, livestock purchases, and pesticides) by all output values (such as feed crops, poultry, and eggs). Input and output values are unit-free quantity indexes that measure change over time as weighted by price. They were determined with Fisher 's Ideal Index number procedure (also known as geometric mean of Laspayres and Paache indexes).

Source Data from Ball et al., 1997.

has been changes in planting practices facilitated by the availability of effective herbicides. Historically, for example, corn was planted in hills of three or more plants and, in many cases, in check rows, which allowed farmers to cultivate the corn in two directions for weed control. With the advent of effective herbicides, farmers switched from hill planting to drilled, narrow-row planting. The plant population increased from 10,000–12,000 plants per acre to 25,000–30,000 plants per acre. That led to the development of new high-yield hybrids that could tolerate the high population densities. Herbicides also allowed corn to be planted earlier in the growing season, and this resulted in a higher yield potential for the crop. Before herbicides, corn had to be planted later so that the first flushes of weeds could be killed with tillage. The development of soil-applied insecticides also allowed more farmers to grow maize for multiple years and increased productivity on an area-wide basis. Wheat production has

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