438 



E. J. WELLHAUSEN 



yields are based on an average of four replications in each of 20 experiments 

 in two different localities and are highly significant statistically. 



The double topcross hybrid H-305 yielded 20 per cent more than Celaya 

 and was equal to Celaya in maturity. Hybrid H-301 yielded only 8 per cent 

 more than Celaya but was about ten days earlier. In normal years this is a 

 decided advantage in adaptation to the variable rainy season of the Bajio. 

 The major portion of the 2000 hectares planted in 1948 for hybrid seed pro- 

 duction in the Bajio was used for the production of these two double top- 

 cross hybrids. According to a formula presented by Mangelsdorf (1939), the 

 gain in yield of these two double topcrosses in F2 would be about half of 

 what they showed in Fi over Celaya. The results of an experiment set up to 

 measure the difference in yield between the Fi and F2 generations of eight 

 different double topcrosses made up as described above are given in Table 



TABLE 27.8 



COMPARISON OF Fi AND F, GENERATION YIELDS OF EIGHT 

 DOUBLE TOPCROSSES IN PER CENT OF CELAYA 



27.8. The data are based on an average of four replications in each of two 

 locations. 



According to the data presented in Table 27.8, the differences in yield 

 between the Fi and F2 generations were statistically significant only in the 

 double topcrosses 3 and 8. In topcross 8 the difference was in favor of the Fo 

 generation. Some of the double topcrosses undoubtedly held a certain ad- 

 vantage in yield over Celaya in the F2, but further data are needed before 

 accurate conclusions can be drawn with respect to the comparative yield 

 capacities of the Fi and F2 generations of double topcrosses. 



Although some improvement was achieved over Celaya by means of the 

 multiple topcross method, the direct recombination of lines from Celaya has 

 given better results. Celaya definitely offers the best breeding material for 

 Zone 3. Crosses between Celaya and other races in Zone 3 such as Conico 

 Norteno and Tabloncillo have been disappointing. The race Conico Nor- 

 teiio introduces some earliness, but it also contributes susceptibility to root 

 rot and ear-rots. The race Tabloncillo of western Mexico behaves in a 

 similar manner. In addition, it introduces certain undesirable ear char- 

 acters. 



