This investigation was carried out at the Experimental Farm, Fac. of Agric.,
Kafr EI-Sheikh, Tanta Univ. during the two seasons 1998 and 1999 to evaluate five
single cross of hybrids corn (Zea mays L.) namely, S.C. 10, S.C. 122, S.C. 123, S.C.
124 and S.C. 125 and three, three-ways cross namely TW.C. 310, T.W.C. 320 and
T.W.C. 321 under different levels of plant densities of 20 (0₁), 25(02) and 30 (03)
thousand plants/fed., and nitrogen fertilization (80, 120 and 160 kg/fed) to estimate
the phenotypic and genotypic stability parameters for grain yield under different         environments.                                                                                    . The analysis of variance for each experiment separately relative to 1998 or
1999 seasons was conducted then the combined data of both seasons was also done
for the grain yield and yield components. The data were analyzed by Eberhart and
Russell (1966), Tai (1971) and Pinthus (1973) procedures to estimate phenotypic and
genotypic stability parameters for grain yield. The results could be summarized as follows:. 1)          For plant densities the number of days to 5C Yo silking as well as plant and ear
heights were increased as the plant density increased, in both seasons and
combined data, number of ears/plant was not significantly affected by increasing
plant densities, ear length, ear diameter, number of rows/ear, number of
kernels/row and 100-grain weight decrease as plant density increased. The
highest grain yield was obtained with 25,000 thousand plant/fed. 2)          Increasing nitrogen rate from 80 to 160 kg N/fed. lead to delaying silking,
increasing plant and ear heights, ear length, ear diameter, number of rows/ear,
number of kernels/row and 100 grain weight. The highest average of grain yield
was obtained with 120 kg. N.lfed. 3)          The highest yielder variety was S.C. 10 (34.94 ard.lfed.) followed by S.C. 124
(33.69 ard.lfed.) and S.C. 122 (31.61 ard.lfed.) while, the least yielder varieties
were S.C. 123 (28.45 ard.lfed.) and T.W.C. 320 (27.94 ard.lfed.). 4)          The crosses S.C.123, S.C.124 and TW.C. 321 exhibited the lowest number of
days to 50% silking toward earliness. 5)          The hybrid varieties S.C. 122 and S.C. 124 would be the most stable varieties
with respect to grain yield because they have values of "bi" around unity with
small and insignificant deviations from regression. 6)           The analysis of genotypic stability using Tai's method (1971) showed that both
linear and deviation from linearity were highly significant. The distribution of
varieties according to their values of Ui and i,i showed that all varieties were
unstable under studied environments. S.C. 124 was the nearest variety to the
average stability. T.W.C. 320 was the worst with respect to its average and its U1
and f.i. 7)          The varieties S.C. 122 and S.C. 124 had high values for coefficient of
determination 0.95 and 0.92, respectively.