I 
Ch. 9 — Advances in Reproductive Biology and Their Effects on Animal Improvement • 169 
Table 30.— Heritability Estimates of Some 
Economically Important Traits 
Trait Heritability 
Calving interval (fertility) 10% 
Birth weight 40 
Weaning weight 30 
Cow maternal ability 40 
Feedlot gain 45 
Pasture gain 30 
Efficiency of gain 40 
Final feedlot weight 60 
Conformation score: 
Weaning 25 
Slaughter 40 
Carcass traits: 
Carcass grade 40 
Ribeyearea 70 
Tenderness 60 
Fat thickness 45 
Retail product (percent) 30 
Retail product (pounds) 65 
Susceptibility to cancer eye 30 
SOURCE. Larry V CundiM and Keiin E. Gregory, Beet Cattle Breeding. USOA. 
Agriculture Information Bulletin No 286. revised November t977, p 9 
may also have economic value,* but they are 
much harder to measure. 
The e.xtent to which important economic or 
performance traits are genetically determined 
and heritable \ aries from trait to trait and from 
animal to animal. (See table 30.) Heritability is 
defined as the percentage of the difference 
among animals in performance traits passed 
from parent to offspring*— e.g., bulls and 
heifers with superior weight at weaning might 
average 5 pounds (lb) more than their herd- 
mates. Because weaning weight has an average 
heritability estimate of 30 percent, the offspring 
of these top performing animals can be ex- 
pected to average 1.5 lb heavier at weaning than 
their contemporaries (0.30 x 5 = 1.5). This 
improvement can normally be expected to be 
permanent and cumulative as it is passed on to 
the next generation. The improvement accumu- 
lates like compound interest in a savings ac- 
count; gains made in each generation are com- 
pounded on the gains of previous generations. 
^Michael I. Lerner and H. P. Donald, \todern Developments in 
Animal Breeding (.\eu York; .Vcademic Press. 1966). 
'Heritability and genetic association are important in decisions 
about individual matings. Most breeding programs are concerned 
with spreading genetic gain rapidly throughout a population 
(herd, flock): thus two other refinements for selection enter the 
picture — generation inter\ al. and selection differential. 
hike laud, e(|uipment, and cash, breeding 
stock represents capital available to the com- 
mercial farmer. Bt'cause all in|)uts must be used 
efficiently, modern herd or flock managers can- 
not afford to leave reiiroduction to chance 
mating in the pen or on the range. These pres- 
sures for efficient production have been de- 
scribed as follow s:-* 
\\ here dairymen are judged by the luimher of 
cows milked in an houi'. there is no place for the 
slow milking cow or the man who will patiently 
milk her out. T here is no place for the time-con- 
suming hurdle flock of shee[), for the small flock 
of chickens maintained under e.xtensive condi- 
tions, or for the sow that must he watched 
while she farrows. By degrees all classes of 
stock are being subjected to .selection w'hich 
favors animals that need a minimum of individ- 
ual attention. 
T he scientific basis for modern breeding has 
dev eloped slow ly over the last century. Applied 
genetics— one jiart of today’s programs— has 
helped modernize livestock and poultry breed- 
ing bv elaborating on the variation of continu- 
ously distributed traits in a population; carrying 
over vv hat was known about rapidly reproduc- 
ing laboratory species, like fruit flies or mice, to 
the much slower reproduction of large farm 
animals; and developing the statistical tech- 
niques for predicting breeding values or merit 
and analyzing breeding programs.® 
Two examples show the powder of breeding 
tools and the increased efficiency and produc- 
tivity of today’s breeders’ stocks. 
• Over the past 30 years, the average milk 
yield of cows in the United States has more 
than doubled. At the same time, the num- 
ber of dairy cows in the United States has 
been reduced by more than 50 percent. 
(See figure 31.) Of this increase in output 
and efficiency, more than one-fourth can 
be attributed to permanent genetic change 
for at least one breed (Holsteins) partici- 
pating in the Dairy Herd Improvement Pro- 
gram. (See figure 32.) 
• Poultry production in the United States has 
become the most intensive industry among 
■•Ibid., p. 20. 
=Ibid.. p. 126. 
