Two field experiments were conducted during summer seasons of 2011 and 2012 to study the effect of irrigation regimes at different growth stages on seed, oil and protein yields of soybean cultivars. The interrelationships among seed yield ha^-1 and its attributes through simple correlation and stepwise regression analysis were evaluated. The results indicated that the percentages of oil and protein in the seeds were significantly affected by water regimes and caused a decrease in oil percentage and increase in protein percentage. In addition, both oil and protein output per unit area was significantly reduced, as water regimes increased. The results showed that skipping irrigation during vegetative, flowering and pod filling stages had no effect on economic yield and save 14% from total irrigation costs of soybean production, but skipping two or three irrigations during vegetative, flowering and pod filling stages had effect on economic yield of soybean production under the environmental conditions of Giza region. Results  revealed that Giza 111 proved to be the best cultivar followed by Giza 21 among the six cultivars included in the test under different water regime. Simple correlation analysis indicated that seed yield ha^-1 was positively correlated with number of branches plant^-1, number of pods plant^-1, 1000-seed weight and seed yield plant^-1. Stepwise regression procedure indicated that number of branches plant^-1, number of pods plant^-1 and number of seeds pod^-1, were the most important characters affecting seed yield ha^-1. Combined analysis of variance (ANOVA) showed that effect of irrigation regimes, cultivars and irrigation regimes × cultivars on seed, oil and protein yield were significant.

Key words: Soybean, skipping irrigations, cultivars, seed yield, oil yield, protein yield, correlation, regression.