380 Inbreeding, Selection, and Heterosis 



proximately 50 plants in each are grown in rows, one progeny 

 to a row. The plants are so spaced that each has equal and 

 ample growing space. During the second year the plants are 

 allowed to set seed and about the middle of August are scored 

 for seed-setting capacity. The method of scoring is by means 

 of an arbitrary" scale from to 5, based on the density of the 

 pods. A plant with no pods is and those in which the pods are 

 densely crowded are 5, whereas the various intermediate condi- 

 tions are 1, 2, 3, and 4. In this second year any decidedly poor 

 plants or progenies are removed from the field. 



During the third year of the cycle, all the plants that were 

 not discarded during the second year are again scored for den- 

 sity of pods. The scores of the second and third years are then 

 combined, and all the plants are removed from the field except 

 about 100 of the best plants. In the fourth year all plants 

 except about 80 of the best may be removed. These 80 plants are 

 allowed to set seed in the field. Since such seed is produced 

 under conditions of open pollination, it. comes from intercrossing 

 among the highly selected plants which are, of course, isolated 

 from all other stands of alfalfa. The seed is harvested at the 

 end of the summer and used to establish 80 new progenies, one 

 from each plant, in a new four-year cycle the following year. 



The results of this method have been highly successful. In a 

 three-year test at Edmonton, a stock which had been selected by 

 this method for ten years was compared with three ordinary, 

 unimproved varieties and one selfed line. As Table 23 shows, 

 the strain that was improved by maternal-line selection was 

 decidedly better than any of the other four varieties in seed 

 production. This high seed production w^as not procured at 

 the expense of hay-yielding ability, for the average annual 

 yields for all five types were nearly the same. 



A good example of the use of the phenotype as a criterion for 

 selecting individuals for reproduction is afforded by the poultry 

 industry. When breeding for egg production, the number of eggs 

 a hen produces is the phenotype of that hen. It seems to be 

 common among poultrymen to consider that the breeding value 

 of a hen can be determined by the number of eggs she lays 

 during her first laying year and to assume that a hen will be a 

 good breeder if she lays 200 or more eggs during that first year. 

 AVhen actual experiments were carried out by testing the egg 



