THE USE AND VALUE OF BACK-CROSSES 

 IN SMALL-GRAIN BREEDING 



Harry V. Harlan and AIerritt N. Pope 

 United States Department of Agriculture, Washington, D. C. 



BACK-CROSSES have long been 

 used in animal breeding to fix 

 desired characters in cattle, 

 horses, or other live stock. Despite 

 any bad effects of inbreeding, back- 

 crossing has been one of the best 

 methods of retaining desired variations 

 of conformation or other intangible 

 complex characters. 



So far as the writers know, back- 

 crosses have not been used in small 

 grains to secure definite types in the 

 progeny. They have been used to 

 study inheritance and incidentally have 

 entered into the complex hybrids made 

 more frequently by early plant breeders 

 than at present. They seem to have 

 been largely if not entirely neglected 

 in any definite breeding programs to 

 produce progeny of specific types, in 

 spite of the fact that most small grains 

 are self-fertilized and hence are im- 

 mune to the evil effects of inbreeding. 

 On the other hand there has been a 

 nearly universal assumption that de- 

 sired types are to be selected from the 

 F-' progeny of matings of suitable par- 

 ents. Recently there has been a wide- 

 spread feeling that large numbers 

 should be used. 



The writers feel that there is an 

 important place for back-crosses in 

 small grain breeding that is not now 

 fully appreciated. It must be acknowl- 

 edged at the same time that the barley 

 projects here discussed are not yet far 

 enough advanced to be entirely satis- 

 factory as examples. The results to 

 date, however, are so promising and 

 the method so plausible that others 

 working with small grains may find it 

 worth considering when breeding for 

 definite ends. 



Smooth-Awned Barley 



One of the barley projects now 

 under way is the production of varie- 

 ties with smooth awns. This is of 

 especial interest because both the large 

 generation and the back-cross methods 

 have been and are being tried. The 

 extensive co-operative breeding work 

 at the ]\Iinnesota Agricultural Experi- 

 ment Station was carried on by selec- 

 tions in the F- and later generations. 

 This work is being continued. At two 

 other experiment stations the produc- 

 tion of smooth-awned barleys is an 

 important project. The production of 

 high-yielding smooth-awned varieties is 

 of much importance. The rough awns 

 are objectionable both in harvesting the 

 crop and in feeding the straw. They 

 definitely limit the acreage of barley in 

 places where the acre yield of feed 

 from barley exceeds that of the crop 

 grown in preference. 



Several smooth-awned barleys have 

 been imported into the United States. 

 Most of these are black, six-rowed, 

 hulled varieties. Crosses with these 

 and the American commercial varieties 

 were first made by the senior author in 

 191 1 and 1912. In 1913 an F- popula- 

 tion was grown at St. Paul, Minn., in 

 co-operation with the ^Minnesota Agri- 

 cultural Experiment Station. In later 

 years other F= populations were grown 

 and a large number of segregates 

 tested. The crosses at St. Paul were 

 with barleys of the ^Manchuria type. 

 In the Western States, hybrids with 

 the barleys adapted to arid regions 

 were made. Although the stocks of 

 smooth-awned barleys available for 

 hybridization were inferior in yield to 

 Manchuria or the Western forms, out 



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