Previous research in Texas has found a widespread incidence of high residual nitrate concentrations in soil profiles in all cotton production regions throughout the state. One reason for the excess nitrate may be over fertilization with nitrogen (N). Currently, Texas AgriLife Extension Service recommends 50 lbs available N/bale of lint. These recommendations were based on older conventional varieties.
Anecdotal evidence suggests that current genetically-modified (GMO) cotton varieties may be more N efficient than conventional varieties used a few years ago, thereby requiring less N to produce each bale of lint. Thus, if commonly used GMO cottons actually require less N, then current recommendations over apply N. Over applying N results in an increased susceptibility to insects, increased use of plant growth regulators, increased defoliation rates, and a delayed harvest at the end of the season.
Field studies were conducted in College Station and Corpus Christi, Texas to determine the nitrogen requirements of commonly used GMO cotton varieties so that fertilization guidelines may be modified to result in more accurate and effective nitrogen recommendations. The studies evaluated four cotton varieties and five nitrogen rates under center pivot irrigation. The experimental design of these studies was a split plot design. Main plots were the cotton varieties and subplots consisted of the nitrogen rates. Prior to nitrogen applications, soil samples were taken to a depth of five feet to determine residual soil nitrate. Plant height and total nodes were measured at pin head square, peak bloom, and at cutout (NAWF=5). Percent canopy closure was measured at peak bloom. Beginning at first bloom, nodes above white flower counts were taken on weekly basis to determine number of days to cutout. At harvest, lint yield and fiber quality were evaluated. Nitrogen concentration of cottonseed was measured allowing calculation of pounds seed N/acre.
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