Author
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ARANGO, JESUS - UNIV. OF NEBRASKA-LINCOLN |
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Van Vleck, Lloyd |
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Submitted to: Genetics and Molecular Research
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/8/2002 Publication Date: N/A Citation: N/A Interpretive Summary: This review covers general aspects of growth with emphasis on size and efficiency of beef cows. Approaches to evaluate adult growth, including traditional growth equations as well as covariance functions and random regression models are discussed. The role of cow-size on production efficiency has been treated extensively, with optimum size depending on the production system. Diversity of environments and management practices implies careful choice of cross must be practiced for each situation to maximize efficiency. The many breeds representing broad biological diversity for economic traits allow flexibility for matching genotypes to specific situations. To optimize body size, Dickerson (1970, 1978) selection for mature size best adapted to the environment, breeding system and market factors of the area of production and focus primarily on improvement of functional components of performance such as reproduction and growth. Size-related traits which are longitudinal data have been proposed for selection programs. Statistic methods are available such as traditional growth curves, covariance functions and random regression models. Understanding of genetic and environmental factors that affect size-related traits is needed to implement effective selection programs. Estimates of genetic and phenotypic parameters of these size-related variables are needed. Genetic progress for adult size may be limited by late expression and measurement. Use of records taken early in life would decrease generation interval and would seem reasonable given reports of high genetic correlations among weights at different ages. Earlier maturing traits, such as height, measured at early ages might be used in multiple trait approaches to select indirectly for optimal adult weight. Technical Abstract: Beef cattle vary widely in body size but optimal size depends on production system. Selection has placed emphasis on growth. Increased mature size of cattle may not be advantageous. Research has indicated no direct relationship between size and efficiency if nutrient requirements for each biological type managed for maximum reproduction and growth. Most important components that determine efficiency are milk production and mature weight. Most genetic evaluation programs report EPD for milking ability but only one does for mature size. Importance of body size to efficiency has led to including traits associated with size in selection programs. Expression of size can be represented by a set of size-age points that gradually change until maturity. These points are highly correlated. The challenge is how to condense those points into few parameters with biological meaning. Approaches range from simplest repeatability to full multivariate models. Repeatability model considers growth at different stages as the same genetic trait with constant parameters. At other extreme, each age-size point is considered a different trait. Alternatives include traditional "growth functions", which explain growth trajectory using a few parameters defined by deterministic equation. New are covariance functions and random regression models. Such infinite-dimensional models represent phenotype of an individual as a continuous function of time which is not restricted to redefined growth equations which offers flexibility for data from many ages. This review covers general aspects of growth with emphasis on size and efficiency of beef cows. Approaches to evaluate adult growth, including traditional growth equations, covariance functions and random regression models are discussed. |
