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When beef cattle (Bos taurus) are fed grain of maize (Zea mays L.) in which the concentration of phosphorus (P) exceeds the animal’s need for this element, the excess P is excreted in the feces. Spreading this manure on cropland increases the potential for P pollution of surface waters by run-off. Experiments were undertaken to determine the relative magnitudes of genotypic and nongenotypic variances of P concentration in maize grain (P-Gr) to assess the ability to select maize genotypes in which this trait more closely matches the dietary need of beef cattle. Genetic variability was found in a population developed from a cross of Illinois High Protein (IHP) x Illinois Low Protein (ILP). Because of few low P-Gr segregates, the IHPxILP population was not considered a good breeding source for this trait. Nongenetic sources of variance were significant but small compared with genotypic variances. Broad-sense heritability (H) for P-Gr among S1 family means in the IHP x ILP population was estimated at 0.82. This high value suggested that this trait would respond to selection. A comparison of mean values of S1 families selected on the basis of 2-yr data suggested that the loss in efficiency resulting from selecting on 1-yr data would be only approximately 5%.