CHAPTER XXXII 
HYBRIDIZATION IN ANIMAL BREEDING 
Within the past century the tendency in practical breeding operations 
has been toward the upbuilding of different breeds by a process of rigid 
selection. Naturally this method of breeding has looked upon any 
suggestion for the introduction of foreign blood with intolerance, an 
intolerance which is shown particularly in the rigid requirements of 
registration laid down by all breeders’ associations. But the work in 
plant breeding in particular has demonstrated that occasionally hybrids 
have acommercial value in themselves aside from their usefulness as a 
source for the establishment of new varieties. Accordingly within the 
past decade there has been a growing tendency to investigate more 
closely the question of hybridization in animals, both in its scientific and 
practical aspects. 
Grading.—In practical animal breeding grading refers to the method of 
improving a herd of animals of indifferent blood by the use of pure-bred 
sires. In the United States this practice has been very common on western 
ranches where the common scrub stock of the range has been graded up 
largely by the use of Hereford bulls, and bulls of other beef breeds. The 
practice is very common in horse breeding throughout the entire nation, 
for a large proportion of the stallions which stand for public service are 
pure-bred animals. In horse-breeding, however, it is regretably too often 
true that the pure-bred sires used in successive generations are not of the 
same breed, consequently the term grading is to be applied to this sort 
of breeding with some reservations. Grading is a practice universally 
commended. Aside from providing a market for a large number of 
pure-bred sires which would otherwise be sent to the shambles, a desid- 
eratum which may account for some of the warm advocacy it has 
received from breeders of pure-bred livestock, it does actually lead to 
notable improvement when practised intelligently, for pure-bred livestock 
which has been selected for many generations for particular utilitarian 
purposes is on the whole very much superior to scrub stock. Moreover, 
since the proportion of pure-bred animals is very low, it is a positive fact 
that any considerable improvement must depend upon some method of 
raising the general level of the great number of inferior animals. 
Perhaps in no line of livestock production are results so readily com- 
parable as in dairy cattle. Here performance is becoming more and 
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