ROTATIONAL CROSSBREEDING AND HETEROSIS 377 



Contrary to expectations, our three-line crosses have not given us as much 

 increased performance over the two-line crosses as I expected on the basis of 

 the results with breed crosses and theoretical expectations. I do not know 

 the cause, but I am inclined to believe that it is due to inadequate sampling, 

 and that as our samples become larger the advantages of the three-breed 

 continuous cross will become more pronounced. 



FARM APPLICATIONS 



How does this work out on farms? The records on one of the large farms 

 with which I am working show that their percentage of survival from the 

 purified lines (230 litters) is 75 per cent, whereas their survival from the 

 crosses of lines (248 litters) is 92 per cent under the same conditions. The 

 survival of crossbred pigs out of crossbred sows is 91 per cent, but the cross- 

 bred gilts weaned an average of 9.1 pigs, to 8.3 for the first cross pigs and 7.2 

 for the purified lines. 



This discussion would not be complete without reference to hybrid boars 

 and how they are entering the picture. I have not seen sufl&cient data to al- 

 low me to appraise properly the advantages and disadvantages of the so- 

 called hybrid boar, but he does seem to be proving popular with a number of 

 farmers. If, then, the hybrid boar is here to stay, what is his place in rota- 

 tional breeding? In my opinion, it will not change the basic situation ma- 

 terially, except that at least six inbred lines will be needed to produce the 

 boars for the rotational crossing of the production of commercial stock. In 

 this case, we will then use hybrid boar of lines 1 and 2, the following year 

 hybrid boar of lines 3 and 4, the next year hybrid boar of lines 5 and 6, and 

 then we will go back to 1-2, to 3-4, and to 5-6 in rotation. 



In thinking about rotational crossing, we need to keep in mind that it is 

 merely a procedure whereby we are able to maintain our breeding females 

 (and perhaps our breeding males), as well as the offspring, in a relatively 

 permanent hybrid state. It in no way affects the basic concepts of hybridiza- 

 tion. It is just a means of utilizing hybridization, and if at some future date our 

 methods of production change, as for instance the general development of 

 so-called pig hatcheries, then we may well find some other method of cross- 

 breeding better suited to the swine industry. 



