Breeding 443 



grade females approach in merit and breeding the pure- 

 bred sires employed. This illustration makes plain the 

 important principle that the more nearly a herd approaches 

 in excellence the ideal, the more difficult does further im- 

 provement become. It is easy to improve a mediocre 

 herd, but extremely difficult to better or even maintain 

 a highly improved one. 



Up-grading is economical because the sire mates with 

 the entire female herd and his influence consequently ex- 

 tends to the entire pig crop. The influence of the sow, 

 on the other hand, is limited to a few. Due to the fact that 

 the increased cost of the pure-bred boar over the scrub 

 or grade boar is thus distributed among a very large num- 

 ber of pigs, the cost of the improvement resulting is ex- 

 tremely low. 



Cross-breeding. 



By cross-breeding is generally understood the mating 

 of two pure-bred individuals which belong to different 

 breeds, as the use of a Poland-China boar on Yorkshire 

 sows, or a Berkshire boar on Duroc-Jersey sows. The 

 mating of a pure-bred boar of one breed to grade sows of 

 another breed is also referred to as cross-breeding. Funda- 

 mentally, cross-breeding means the mating of individuals 

 of dissimilar type or breeding. In this sense it is correct 

 to speak of crossing one family or strain with another 

 family or strain of the same breed. 



Experiments and observations show that cross-breed- 

 ing usually has the effect of slightly increasing vigor and 

 feeding qualities. Since these qualities are fundamental 

 in the profitable production of market animals, the reason 

 for the practice of this system of mating is apparent. 

 Another possible advantage in the production of market 



