Heterosis and combining ability for grain yield and some agronomic traits were studied among thirty F1 grain sorghum crosses and their elven parents under three NPK levels during 2014 and 2015 seasons. Significant differences among genotypes were found for all studied traits, indicating wide genetic diversity. The interaction of genotypes with each of years and NPK levels were significant in most studied traits. The analysis of variance for combining ability revealed that the mean square due to entries, parents, parents vs. crosses, crosses, lines, testers, lines × testers turned up significant for all studied characters and suggesting that the experimental materials possessed considerable variability that both general and specific combining ability were involved in the genetic expression of these characters. The female line ICSB610 showed significant and negative general combining ability (GCA) effects for days to 50% heading and panicle length and positive for grain yield and plant height. It may be used to develop high yielding, early flowering, and tall hybrids with short panicles. For specific combining ability (SCA), effects, the crosses ICSA613 × ICSR89028 and ICSA20 × ICSR53 gave positive and highly significant SCA effects which indicated that these crosses can be considered desirable combiners. These crosses had also high grain yield per se and one of the parents with highest GCA effects. The observations on portioning of combining ability variance into additive and dominance variances indicated the role of both additive and dominance gene action. The magnitude of non-additive variance was higher than the additive variance by many folds for all studied traits.

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