Grinnells eliminated first calf heifers from 
their analyses because they had a high re- 
sistance as compared with other cows. 
(3) Before deciding to include mastitis resist- 
ance in a selection program, we must know 
how it relates to other economic traits. For 
example, some have stressed the importance 
of the teat sphincter as a barrier to udder 
infections. McEwan and Cooper (1947) found 
a higher frequency of mastitis among fast 
milking cows than among slow milking cows. 
They suggested breeding for slow milking 
cows as a possible means of controlling 
mastitis. These findings have been verified 
by Dodd and Neave (1951), but they point out 
that differences in milking rate cannot ac- 
count for the large differences between herds 
in frequency of mastitis. 
By contract, present emphasis on selection 
for high production and culling cows for 
economic reasons gives some weight to 
mastitis resistance. O'Donovan et al. (1960) 
reported a depression of 10 percent in the 
lactation milk yield of infected cows. Other 
reports indicate depressions ranging from 
5 to 24 percent, depending somewhat on the 
definition of mastitis used. If it is safe to 
assume a relationship between susceptibility 
and severity of mastitis, there is automatic 
selection against mastitis susceptibility when 
we choose those individuals that actually pro- 
duce the most milk. 
A study of DHIA records by Asdell (1951) 
reveals that about 22 percent of the cows 
on test are removed from herds each year. 
Almost half of these are culled because of 
low production and udder trouble. The fact 
that 2.5 percent (about 40,000 cows) of the 
population is culled annually because of udder 
trouble indicates a rather strong rejection 
of mastitis-susceptible cows. 
(4) Since most of the possible genetic prog- 
ress would come through selection and proving 
of sires, a uniform national program of scoring 
and officially recording a large segment of 
the cow population would be required. 
Thus, until research provides a better 
understanding of the disease and its relation 
to other economically inherited traits, we 
have no basis for advocating drastic changes 
in our concept of breeding better dairy cattle 
than through selection for production. 
151 
Other Diseases 
The problems of breeding for the control 
of mastitis have been discussed at length 
because similar situations presently exist 
with most pathogenic diseases in farm live- 
stock. For example, (1) in swine--atrophic 
rhinitis (Koch et al., 1958; Schulz, 1962), 
brucellosis (Cameron et al., 1943), and pig 
dysentery (Englehart, 1949); (2) in sheep-- 
Escherichia coli infection (Fredeen, 1963), 
scrapie (Dickinson and Young, 1964; Draper 
and Parry, 1962; Pattison, 1964), and fleece 
rot (Dunlop and Hayman, 1958); (3) in poultry-- 
avian leucosis (Siegmund, 1961), Rous sarcoma 
virus (Waters and Burmester, 1961; Crittenden 
et al, 1965), Salmonella pullorum (Hutt, 1949), 
fowl typhoid (Lambert and Knox, 1932), and 
poultry coccidosis (Champion, 1954; and (4) 
other pathogenic conditions too numerous for 
discussion at this time. 
It should be pointed out that although scrapie 
has been included as a pathogenic condition, 
there are two schools of thought on its origin-- 
one as a genetic abnormality supported by 
Draper and Parry and the other pathogenic 
postulated by Dickinson and Young. 
There are other diseases caused by patho- 
genic organisms, which take heavy tolls in live- 
stock productive performance, but for which 
there is little evidence of genetic resistance. 
Such a disease is hog cholera, Still others, such 
as foot and mouth disease, have supporters as 
being more prevalent among European-type 
cattle than zebu stock, but such observations 
have been largely onnewly introduced animals. 
Comparisons between zebu stock and cattle 
from European countries, where the disease 
has been prevalent for a long time, will be 
needed for an adequate test. 
RESISTANCE TO PARASITES AND 
PESTS 
The term "parasite,'' as used here, applies 
to forms of animal life that have been adapted 
to live on or in farm livestock, and ''pest'' 
refers to those insects that prey on livestock. 
Protozoa Diseases 
Some examples of protozoa diseases for which 
genetic immunity has been claimed are tick 
