Degree Type


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Degree Name

Doctor of Philosophy




Critical %N is defined as the concentration associated with maximum yield with respect to N. Objectives of this research were to determine: (1) the extent to which various maize single crosses differ in their critical %N; (2) the constancy of these values in different crop seasons; (3) the relationship between the critical %N of single crosses and their parental lines; and (4) the N use efficiency of different single crosses. The study involved N-rate experiments in 4 site-years, and commonly used genotypes selected for a wide range in %N in the grain. Maximum yields and critical %N values were predicted from the separate regressions of yield and of %N on the quadratic functions of N fertilizer rates;Maize single crosses differed significantly in critical %N, and ranged from an average of 1.33 to 1.71. Critical %N values for different site-years were highly correlated, as shown by coefficients ranging from 0.83 to 0.90 and averaging 0.87 for 21 single crosses used in 3 site-years. The critical %N and associated %P values were highly correlated, with r-values of 0.76 and 0.83 in two experiments;The critical %N values of the single cross progeny were highly correlated with the mean critical %N values of the parental inbreds, indicating that an estimate of the critical %N of single crosses could be obtained from a knowledge of the critical %N values of their parental inbreds;Maize single crosses differed in their efficiency of N use. Hybrids containing B73 (a low-N inbred) produced the most grain with no applied N and with sufficient applied N to maximize yields. However, based on yield response per unit of applied N, hybrids containing B14A (a high-N inbred) were at least as efficient. The B73 hybrids contained more total N than B14A hybrids at the no-N level, but the reverse was true with sufficient N applied to maximize yields. Hybrids containing B14A were the most efficient of the 3 groups of hybrids studied in recovery of the N applied to maximize yields;A knowledge of the critical %N in the grain of maize will be useful in diagnosing N sufficiency, in characterizing hybrids for protein feeding value, and in improving hybrids for protein content.



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Cloyce Gene Coffman



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148 pages