Leaf area index response of four obsolete and four modern cotton cultivars to two nitrogen levels

The optimal nitrogen (N) rate for cotton (Gossypium hirsutum L.) production in the late 20th century is greater than it was in the middle of the century (112 versus 27 kg ha-1). Part of the reason for this difference is that modern cultivars exhibit a greater harvest index than obsolete cultivars. This greater harvest index helps to allow modern cultivars to utilize greater N rates. However, factors other than harvest index, such as the development of leaf area in response to N, may also play an important role. Therefore, the objective of this study was to characterize leaf area index (LAI) of four obsolete and four modern cultivars at a low and high fertilizer N level. Cotton was grown in the field for two years (1992 and 1993) with two locations each year. The locations were a Beulah fine sandy loam and a Dubbs silt loam. Two preplant fertilizer-N rates were used, a low (22 kg N ha-1) and a high (112 kg N ha-1). Leaf area index was determined at three stages in each year (early, mid, and late season). Yield was determined at maturity. Averaged across years, locations, and cultivars, late-season LAI increased from 2.32 at 22 kg N ha-1 to 3.15 at 112 kg N ha-1 by late season. In 1992, modern and obsolete cultivars had similar LAI responses to N at early and mid season but by late season, LAI of modern cultivars was greater under high N than the obsolete cultivars (3.53 versus 2.95). Lint yield of the four modern cultivars was 372 kg ha-1 greater than the four obsolete cultivars at 112 kg N ha-1 and 289 kg lint ha-1 greater at 22 kg N ha-1 in 1992. The LAI response to N level of the modern cultivars was similar to that of obsolete cultivars in 1993 at all three sampling dates. In 1993, the lint yield of modern cultivars was 238 kg ha-1 greater than obsolete cultivars under 112 kg N ha-1 and 182 kg lint ha-1 at 22 kg N ha-1. In summary, our results best support the hypothesis that the higher yield of modern cultivars at high fertilizer N is unrelated to their LAI.