GENETIC IMPROVEMENT OF BEEF CATTLE

GENETIC IMPROVEMENT OF BEEF CATTLE ADAPTATION IN AMERICA
W. Hohenbokena
, T. Jenkinsb
, J. Pollakc
, D. Bullockd
and S. Radakoviche

Introduction
Management systems and environmentsf differ widely for beef cattle populations across
the United States. A typical animal occupies several environments during its lifetime,
each presenting a unique set of challenges. No animal or breed maximizes the
conversion of input to salable product across all environments, nor is the genetic makeup
of any animal or breed optimally suited to the challenges encountered in any one
environment. To a certain degree, therefore, all beef cattle in America are less than
optimally adapted. The opportunity exists to improve profitability of beef cattle
production and to maintain integrity of cattle production environments through programs
designed to achieve balanced genetic potential for adaptation, production and product
quality within specific environments.
With financial support from the USDA Agricultural Research Service and the Beef
Improvement Federation and under the auspices of the National Beef Cattle Evaluation
Consortium, concerned geneticists and cattle producers met in March, 2004g
to define
adaptation in beef cattle, characterize important stressors in major production
environments, and identify opportunities to improve adaptation through genetic means.
Results were presented to the beef cattle industry in a symposium in October, 2004.
Participants and registrants agreed that the problem was critically important to profitable
and sustainable beef cattle production and that new programs should be designed to foster
the genetic improvement of adaptation of beef cattle in America. The goal of this
document is to present those conclusions to a wider audience of stakeholders.
Why are American beef cattle less than optimally adapted?
Response mechanisms to environmental challenges have been evolving in cattle
populations for millions of years. Adaptation has been successful, and populations
capable of sustained production now exist throughout most inhabited regions of the
world. Why, then, are American beef cattle less than optimally adapted? There are
several reasons.

a
Professor Emeritus of Animal Science, Virginia Polytechnic Institute and State University.
b
Research Animal Scientist, U. S. Meat Animal Research Center, Clay Center, NB. c
Professor of Animal Breeding, Cornell University and Executive Director, National Beef Cattle
Evaluation Consortium.
d
Associate Professor of Animal Science and Extension Beef Cattle Specialist, University of Kentucky. e
Radakovich Cattle Company, Earlham, IA.
f
Italicized words within regular text are defined in the Glossary.
g
Facilities provided by the Noble Foundation, Ardmore, OK.
2
Prior to domestication, cattle had a demanding but uncomplicated job description; they
had first to survive and then to reproduce. To accomplish these goals, they evolved
anatomical, physiological, immunological and behavioral mechanisms appropriate to
conditions in Eurasia, their center of origin. Thousands of bovine generations hence,
domestic descendents in contemporary America face vastly different parasites, diseases,
stresses and nutritional challenges. It is not surprising that a gene pool conferring
adaptation to past and distant environments confers less than optimum adaptation
to current and, indeed, to future conditions.
Cattle were domesticated in western Asia some 10,000 years ago. Cattle and cattle
production technologies subsequently migrated outward from centers of domestication,
eventually to colonize much of Europe, Africa and Asia. With an estimated initial
migration rate of six miles per decadea1
, natural selection could easily accommodate
adaptation of cattle to their newly encountered environments. During recent times,
however, the speed of migration has accelerated (air freight can transport animals,
gametes and embryos throughout the world in a matter of hours). Beef cattle
management systems are changing more rapidly as well, typically in the direction of
greater intensification. Compared to only a few decades ago, for example, cows now
produce their first calf at two rather than three years of age, animals are maintained at a
higher density per unit of land area, and cattle are fed to market on higher energy diets.
In many instances, management systems and environments are changing more
rapidly than animal populations can adapt to such changes through natural
selection2
.
Domestication created opportunities for the formation of and differentiation among many
locally adapted cattle populations. Our ancestors lived in a society of small tribes at that
time, with limited material and cultural exchange among groups3
. The role of cattle was
determined by the needs of each tribe- milk and meat production, power generation, the
accumulation of wealth and religious or cultural iconography, for example. Tribal
definition of value thus imposed a new ‘environmental’ challenge on cattle populations,
that of fulfilling an economic role. Phenotypic selection was applied4
, as animals more
successful in meeting the community standard of value were allowed to reproduce while
less successful individuals were not. Planned matings and natural selection exerted by
local environmental challenges also promoted the creation of populations well adapted to
local requirements. As social organization gradually evolved from tribes to communities,
communities to villages, villages to cities, cities to states and states to nations,
interactions among human populations increased5
, and the isolation of local cattle
populations diminished. When allele frequencies and gene combinations favorable to
production in a local environment were disrupted through exchange of breeding animals,
adaptation to specific environments declined. National and international trade in
breeding animals, gametes and embryos now allows an animal to produce offspring in
environments very different from the one to which that individual is adapted. While
providing many benefits to efficient livestock production, movement of genes into new
environments also can reduce adaptation of a resident herd to its unique conditions
and challenges.

a
Numerical footnotes correspond to literature citations listed at the end of the document.
3
An idea whose time has come back
Beef cattle geneticists in the American South and West concluded in the 1970s that
“genetic adaptation to local environments is important in commercial beef cattle
production”6
. Furthermore, “indiscriminate distribution of breeding stock (or their
semen) to different environments” should be avoided until something is known of the
adaptive merit of that stock. They advised that animals be performance testeda
under
environmental conditions similar to those that their progeny were likely to encounter.
Evidence supporting these recommendations was provided by their classical experiment
to investigate genotype by environment interaction. They started with two genotypes, a
line of Hereford cattle selected in and adapted to Montana and another Hereford line
selected in and adapted to Florida. These states also constituted the production
environments; half of each herd was transferred to the other location, where production
of the cows and their descendants was monitored over an 11-year span. Genotype by
environment interaction would occur if the production difference between cows of
Montana versus Florida origin differed depending upon the location in which they were
compared. Such was the case. At Miles City, Montana, the Montana cows and their
descendents exceeded Florida cows and their descendants by an average of 14 pounds in
calf production per year. In Brooksville, Florida, average annual calf production of
Florida cows and their descendants was 84 pounds greater than that of Montana cows
and their descendants! As might have been expected, cows from each origin were most
productive in the environment to which they were adapted.
Gradual response to mild selection to increase production traits, as occurred during most
of the history of the co-dependence between cattle and man, generally does not detract
from an animal’s ability to survive and reproduce. In fact, selection to increase sustained
annual production selects automatically for traits important to adaptation. In recent
decades, however, the application of refined knowledge of inheritance, improved
information technology and advanced reproductive techniques has allowed dramatic
increases in selection intensity and selection response. Rapid response to intense
selection for increased product (as opposed to increased sustained production) can
sequester resources formerly utilized to support reproduction and survival. Rapidly
increased genetic potential for production may be achieved, therefore, at the
expense of decreased genetic merit for adaptation.
Hidden costs of selection
Among domestic food animals, broiler chickens are the poster species for rapid rate of
response to selection. They are highly prolific and turn generations rapidly, allowing for
a high intensity of selection. Furthermore, commercial poultry breeding companies have
clear, consistent objectives, most prominently to increase growth rate, feed conversion
efficiency and breast meat yield. Selection responses in these traits have not been
without cost. Undesirable correlated selection responses include reduced fertility of
broiler breeders and increased severity and incidence of ascites, sudden death syndrome,
distortion of long bones and tibial dyschrondroplasia throughout the life cycle7
. In a

a
Words in regular print within italicized text are defined in the Glossary.
4
similar manner, progeny testing and artificial insemination have fostered rapid response
to selection for increased milk yield in dairy cattle, for which undesirable correlated
responses are poor rebreeding performance and increased incidence of metabolic
imbalan

Results (English) 3:

在美国
W hohenbokena
肉牛适应性遗传改良，T. jenkinsb
，J. pollakc
，D. bullockd
和美国radakoviche

介绍
管理系统和environmentsf大相径庭肉牛群体在
美国。一个典型的动物在它的一生中占有几个环境，每一个呈现出独特的挑战。没有动物或品种最大限度地提高输入转化为可销售的产品在所有的环境中，也不是遗传
任何动物或品种最适合在任何一个
环境中遇到的挑战。因此，在一定程度上，美国的所有肉牛都不到最佳的适应。提高肉牛养殖收益的机会生产和维护牛生产环境的完整性，通过程序设计，以实现平衡的遗传潜力，适应，生产和产品质量在特定的环境中，有关遗传学家和养牛者遇见三月，2004g
定义
肉牛改编，描述主要生产
环境重要的压力源，并识别改进机会通过遗传的方式适应。
结果在十月的一个座谈会，对肉牛产业的2004。
参与者和注册人同意的问题是非常重要的和可持续的盈利
肉牛生产和新项目的设计应以培养
在美国肉牛适应的遗传改良。本文件的目标是提出这些结论的利益相关者的更广泛的观众。为什么美国牛肉牛不到最佳的适应？