Incomplete block designs for genetic testing: some practical considerations

Full-sib progeny and clonal trials to evaluate family and clone performance were simulated on a test site with various patterns of environmental varia tion to examine the effectiveness of randomized incomplete block designs (I CBs) under different design parameters (constant block size, variable block size, block shape and orientation, and family size). Simulations showed th at blocks of fixed size 5-20 were effective in removing most site variation in a test with 120 full-sib families of three seedlings per family. Slight variations in block size did not affect the precision of mean estimations much when blocks were considered random. Square blocks were more effective than either column or row blocks in removing both patchy variation and grad ients in one direction. With blocks of fixed size 10, increasing family siz e from 3 to 12 (at the expense of family number, which dropped from 120 to 30) reduced the average variance of family mean contrasts by 75%; most of t his reduction (> 50%) was achieved at 6 seedlings per family. The differenc es in precision of mean estimation between full-sib progeny and clonal tria ls were small with respect to the design parameters and patterns of site va riation examined.