574 ERNST J. SCHREINER 



frequencies of resistance genes to a high level in all varieties. Three 

 cycles produced essentially borer-resistant varieties. The general proce- 

 dure for this work was to self-pollinate individuals in a segregating 

 population, evaluate the Si lines, and intercross a selected sample of 

 these Si lines to provide the new base population. 



Paterniani (1967) has used a modified ear-to-row selection procedure 

 in maize (selection among and within half-sib families) that he concludes 

 is superior to recurrent selection for general combining ability. Open- 

 pollinated progenies were evaluated in replicated field trials, and the 

 selected half-sib families were subjected to mass selection within 

 families to provide the half-sibs for testing in the next generation. 

 Yield improvement of 13.6 percent per cycle compared to the original 

 population was obtained in three cycles of selection. 



From the theoretical standpoint, the Monte Carlo computer simulation 

 by Cress (1967) on reciprocal recurrent selection and modifications in a 

 bisexual organism with two allels at each of 40 independently segregating 

 loci may be of interest. He concluded that two points seem essential to 

 all recurrent selection systems in order that both maximum genetic poten- 

 tial and rapid progress are possible: 



"(1) All genetic material entered into a long term program 

 of selection with progeny testing should be combined into one 

 synthetic population. Any subsequent populations required 

 would be obtained by sampling this synthetic. This procedure 

 reduces the problem of multiple alleles and can only increase 

 not reduce the genetic potential." 



"(2) One generation of selfing (or inbreeding) should 

 precede the test cross, where the real time lapse measured 

 against rate of progress shows this to be more efficient." 



For improvement of perennial forage crops, the usual procedure is 

 phenotypic selection of clones for polycross tests for general combining 

 ability. Syn-0 parents for each cycle of selection and intercrossing 

 are then selected on the basis of the performance of the polycross 

 progenies . 



LARGE-SCALE PROGENY TESTING 



Large-scale progeny tests are feasible for blister rust resistance. 

 Bingham (1968) has shown that mixtures of 10 or more pollens can be used 

 to obtain relatively reliable estimates of general combining ability and 

 breeding value of blister rust-resistant plus trees. In 95 out of 100 

 cases, the results fell within ±3 percent of those obtained by using 4 

 individual tester crosses. This level of accuracy is adequate for large- 

 scale practical testing, but Bingham also noted that prevention of patchy 

 inoculation of test plots, as well as an increase in the number of progeny 

 test plots, would further improve accuracy. 



