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observed in the double cross unexpectedly exceeded the yield decline observed in some ofthe single crosses. Also once again, the yield of an OPV planted as a check was alwayssuperior to advanced generations of hybrids. But a noteworthy new result of the secondtrial was the finding that yields do not necessarily continue to decline after the secondgeneration; in the case of the single cross H-34, yield actually increased in the F3 and F4generations (Figure 3).In another on-station trial conducted in a tropical lowland region of Mexico, RamírezVallejo et al. (1986) evaluated the effects of seed recycling on the performance of threedouble-cross hybrids (H-503, H-507, H-510). With all three hybrids, F1 plants yielded betterthan advanced-generation progeny and better than a check OPV, V-522 (Table 17). After fivecycles of inbreeding, the yield decline averaged 16%. Virtually the entire yield declineoccurred between the F1 and F2 generations; an equilibrium was reached in subsequentgenerations, as no statistically significant changes in yield were observed between the F2and F5/F6 generations. Ramírez Vallejo et al. (1986) emphasize, however, that no selectionpressure was applied during the trial, and they speculate that had selection pressure beenapplied (e.g., to simulate farmers’ selection practices), then the advanced-generationprogeny might well have outyielded the F2 plants.Yield (kg/ha)10,0009,0008,0007,0006,0005,0004,0003,0002,0001,0000, y, y, y, y, y, y,,,,,,-25%-5%,,,,,,,,,,,,,,,y ,, , y yCommercial seed (F1)Second-generation seed (F2)Third-generation seed (F3), y, y, y, y, y,,,,,,,,,,, y, y, y, y, yH-34 H-30 V-23(single cross) (double cross) (improved OPV)Figure 2. Effects of seed recycling on yields oftwo commercial maize hybrids, Mexico.Source: Espinosa Calderón et al. (1990)-42%Yield (kg/ha)12,000,, yy10,0008,0006,0004,000,,Second-generation seed (F2),,, yy,, yy,, yy,, yy,, yy,,,,,,, yy,,y,y,,y,,,,,,-51%Commercial seed (F1)+20%+10%-27%,,,,,,,,,,,,Third-generation seed (F3)Fourth-generation seed (F4),, yy,, yy ,,,, yy ,,, yy ,,,, yy, y, y, y, y2,000,, ,,,, ,,yy 0,,yH-34 , H-36 ,, ,, yyH-68 , , yH-33 V-23(single cross) (single cross) (single cross) (double cross) (improved OPV)Figure 3. Effects of seed recycling on yields of selectedmaize hybrids, Mexico.Source: Espinosa Calderón et al. (1993)-6%-29%Table 17. Yield decline in advanced generations of hybrids, Mexico41Generation of inbreedingF1 F1’ F2’ F3’ F4’ F5’H-503 4,194 3,577 3,213 2,586 3,097 3,160H-507 3,964 3,393 3,014 2,937 3,122 3,092H-510 4,239 3,473 2,781 3,302 2,710 3,216Mean 4,132 3,481 3,002 2,942 2,976 3,156Inbreeding depression (%) n.a. 16 27 29 28 24Source: Ramírez Vallejo et al. (1986).