Synopsis Ten hybrids and 10 inbreds were planted on 5 different dates in 1956 and their maturities, based on silking dates, were calculated in heat units by 15 different methods. The present method of calculating degree days, daily mean minus 50° F., was improved by correcting for temperatures below the minimum for growth, 50° F., and above the optimum for growth, 86° F. The number of heat units required for silking, designated as effective degrees, remained relatively constant for crops with different planting dates, while calendar days varied widely.

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Two hundred F2 families from each of two crosses, ‘DeOro’ ✕ ‘Dillman’ and ‘Linore’ ✕ ‘B‐5128,’ were studied in the F3 and F4 generations for reaction to low temperatures at two locations. Each generation was grown in a different year. During the first season, the plants were subjected to severe low temperatures, as low as —23C (—9F) at Chillicothe, Texas, but snow cover protected the plants so that cold injury was mild to moderate. The second season, temperatures dropped as low as —17C (2F) at Denton, Texas, without snow cover, and cold injury was moderate to severe. A cold hardiness index (CHI) was calculated from the degree of injury and the percentage of plants injured in each row. Dominance deviations could not be detected in the F3 or F4 generations. Due to the seasonal differences, genetic variances of families in the F4 were often higher than variances in the F3. Heritability estimates of CHI calculated from components of variance were .35 in the F3 and .59 in the F4 of the second cross. Heritability was estimated also by the regression of F4 on F3 which was .75 ± .15 for the first cross, and .56 ± .04 for the second cross. The regression estimates were unbiased by genotype‐ year and genotype‐location interactions. The expected genetic advance in F4, by selecting the top 5% in F3, was 12 CHI units in both crosses. The observed gain in F4 was the same as predicted in the first cross, but it was much less than expected in the second cross. Estimates of genetic variance in the F4 increased as the season progressed indicating that selection near the end of the season would be much more efficient than selection after the first period of freezing. Growth habit appeared to be closely associated with cold hardiness. Cold hardy lines all formed a rosette of branches during the winter contrasted to the upright growth of the cold sensitive lines