of indigenous stocks to increase resistance 
to environmental stress and thereby perhaps 
greater resistance to disease and pests. 
In accepting this approach, we should rec- 
ognize a high negative correlation between 
metabolic heat production and physiological 
tolerance--often used synonymously with heat 
tolerance. But there is a high positive relation- 
ship between the level of body temperature, 
metabolic heat production, and milk yield or 
growth rate. Hence, we have two conditions-- 
one of physiological tolerance and the other 
of productive tolerance, If there is a negative 
relationship between physiological tolerance 
and productive performance, then certainly 
some minimum for productivity would be re- 
quired before attempts are made to select 
for low body temperature. Perhaps the same 
phenomena could be applied to selection for 
disease resistance. In some environments it 
may have an inverse relationship to the direc- 
tion animal breeders wish to go on produc- 
tivity. Present evidence suggests attention to 
nutrition, management, and disease control is 
more worthwhile than selection for resistance 
to environmental stress untilthere is aclearer 
understanding of the physiological mechanisms 
involved in adaptability. 
Sterility 
Losses from reproductive failure constitute 
the greater loss among females in farm live- 
stock. There is little evidence of genetic 
factors as a major cause, but Casida (1961) 
suggests that antibody production could be a 
possible factor, Thus, some cause of sterility 
may be indirectly related to disease conditions 
and should be considered along with attention 
to disease problems. 
Hormones and Drugs 
Increased use of drugs and hormones may 
solve, add, or create a whole host of new 
problems in disease control. There is already 
some evidence in the work of Landauer (1957) 
which has shown that treatment with insulin 
will produce the defect "'rumplessness" in 
chickens. 
153 
SUMMARY 
The foregoing discussion does not pose a very 
bright future for the animal breeder in con- 
trolling disease through genetics. Disease and 
parasite problems can be controlled largely 
by slaughter, vaccination, and a high level of 
sanitation. These solutions may not always be 
practical or feasible. The animal breeder 
must, therefore, ask himself how far he should 
go in eliminating diseases. Present genetically 
caused abnormalities can be controlled by 
recognizing the mode of inheritance and em- 
ploying systematic breeding programs. 
The recognized offenders of abnormalities 
that occur under certain environmental con- 
ditions can be eliminated; therefore, such 
conditions can be kept under fairly good 
control, 
In the case of pathogenic diseases, most 
animals have a general resistance to one or 
more pathogenic organisms, but until research 
provides (1) a better understanding of the 
disease and its relation to other economic 
traits, (2) more precise means of classifying 
animals as to resistance, and (3) satisfactory 
identification methods that can be used early 
in life, selection for resistance is not war- 
ranted. A system of selection for these dis- 
eases would also be expensive, and there is 
no assurance that changes in organisms would 
not be greater or more rapid than genetic 
progress can be made in farm livestock. It 
appears that animal breeder should work 
closely with veterinarians and others in de- 
veloping systems of sanitation and induced 
immunity to supplement the emphasis already 
applied through selection for maximum per- 
formance, 
The era of breeding animals suited to existing 
natural climatic conditions seems passing; 
therefore, attention must be given to ways 
and means of controlling the environment in 
such a way as to make it optimum for max- 
imum productive efficiency. If this trend is 
accepted, then techniques of management will 
probably be among the most fruitful approaches 
to lowering the costs of farm livestock losses 
caused by disease and pests. Resolving the best 
methods for this will require the teamwork 
of engineers, physiologists, nutritionists, and 
others, 
