Historical US cotton yields (USDA, 1998) are presented in Fig. 8.12 along with changes in [CO2] (Friedli et al., 1986; Keeling and Whorf, 1998). There has not been any attempt to relate long-term yield responses to temperatures because temperature data are much more complex. Cotton yields prior to 1940 were approximately 200 kg ha-1 and remarkably stable from year to year. Yields began to increase by about 1940 and reached 800 kg ha-1 in the mid-1990s. Numerous changes in technology occurred during that period. These include development of improved varieties, improved weed and insect control, increased use of fertilizers, and irrigation of sizable acreages. The number of acres grown decreased dramatically from the mid 1930s to the early 1950s. As acreage declined, yields began to improve, probably because the lowest-producing land was removed from production. Low-cost fertilizer became more widely available in the 1950s and crop production research increased
dramatically after World War II, resulting in many changes in technology that improved cotton yields.
During this period, atmospheric [CO2] also increased dramatically due to the burning of fossil fuels and other human activity; from 306 |mmol mol-1 in 1930 to 364 |mmol mol-1 in 1997 (Fig. 8.12). From the responses of cotton growth and other physiological processes to increasing atmospheric [CO2], it appears reasonable to estimate that yields probably increased by 19% because of [CO2] increases during that period. Results from the previous section suggest that cotton yields will increase about 60% when [CO2] increases by 300 |mmol mol-1 above today's ambient [CO2], provided average daily canopy temperatures do not exceed about 30°C. If it is reasonable to assume that the yield response to CO2 has been linear below today's ambient concentration, and that an approximately 60 | mol mol-1 increase in [CO2] has occurred since 1940, this may have caused cotton yields to increase by about 12%. However, the relative changes in canopy photosynthesis are larger below than above 350 |mmol CO2 mol-1 (Fig. 8.12) and so it is likely that the growth response has been greater below than it will be above 350 |mmol mol-1. Therefore, the historical cotton yield increase due to increasing [CO2] may have been larger than 12%. Experiments at subambient [CO2] have been conducted by Allen et al. (1987) on soybean and by Mayeux et al. (1997) on wheat. These authors estimated that the yields of soybean and wheat have increased by about 13% and 54%, respectively, due to the increase in atmospheric [CO2] since pre-industrial times (about 1850).
A number of studies have been conducted to determine the influence of genetic improvement on cotton yields. The most common approach is to use modern cultural practices to relate cultivar yields from a particular year backward to the year of the cultivar's release. Results vary by region, the years the comparisons were made, and the cultivars selected for comparison. The yield improvements range from 5.6 to 11.5 kg ha-1 year-1 (Meredith et al., 1997, and references cited therein). Assuming that all cotton acreage currently being cultivated is planted in the most modern varieties, it may be calculated that about 50% of the yield increase may be attributable to improved varieties during the period from 1938 to 1993. This leaves considerable yield increases that may be attributed to other causes, some of which are fertilizer use, improved tillage practices, pest control and land selection. Meredith et al. (1997) also compared cultivars grown using the N fertility practices recommended in 1940 and 1993. They found a 27% improvement because of N use efficiency among modern cultivars compared with the obsolete cultivars. The change in N use resulted in a 10% increase in yields.
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