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2 S h ark ey, Diane Marie FACTORS AFFECTING THE SALE PRICE OF CENTRAL PERFORMANCE TESTED BEEF BULLS AND THE ESTIMATION OF GENETIC AND ENVIRONMENTAL TRENDS The Ohio State University Ph.D University Microfilms International 300 N. Zeeb Road, Ann Arbor, Ml 48106

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4 FACTORS AFFECTING THE SALE PRICE OF CENTRAL PERFORMANCE TESTED BEEF BULLS AND THE ESTIMATION OF GENETIC AND ENVIRONMENTAL TRENDS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Diane Marie Sharkey, B.S., M, S. * * * * * The Ohio State University 1982 Reading Committee: Approved By W. R. Harvey K. M. Irvin t?.. Adviser R. R. Reed Department of Animal Scienc

5 ACKNOWLEDGNENTS I would like to express ray sincere appreciation and gratitude to the following people who have provided guidence and encouragement during my graduate program and in the preparation of this dissertation: To Dr. W. R. Harvey, my adviser, for his continuous support, assistance and patience during the statistical analysis for this research and during the organization and writing of this manuscript. Grateful acknowledgment is extended to the other members of my graduate committee, Dr. K. M. Irvin and Dr. R. R. Reed for their suggestions and guidence. Finally, I wish to thank my family and fiance for their continual support and encouragement of all my academic endeavors.

6 VITA October 27, Born - Staten Island, New York B.S., Moravian College, Bethlehem, Pennsylvania Research Associate, Department of Animal Science, The Ohio State University, Columbus, Ohio M. S., Department of Animal Science, The Ohio State University, Columbus, Ohio PUBLICATIONS Sharkey, D. M., L. A. Swiger, W. R. Harvey and K. M. Irvin Indirect response of gain, feed utilization and carcass traits to selection for leanness in Yorkshire swine. Ohio Swine Research and Industry Report, Animal Science Series FIELDS OF STUDY Major Field: Animal Breeding Studies in Animal Breeding and Population Genetics. Professors F. R. Allaire, J. B. Griffing, W. R. Harvey and S. S. Y. Young Studies in Statistical Methods. Professors R. V. Skavaril, W. R. Harvey, P. R. Henderlong, T. A. Wilke, G. Mack, E. Carter and A. M. Dean Studies in Mathematical Genetics. W. R. Harvey Professor Studies in Physiology. Professors W. G. Venzke, ^T. L. Ludwick and H. C. Hines' Studies in Ruminant and Nonruminant Nutrition. Professors J. D.. Latshaw and W. J. Tyznik iii

7 TABLE OF CONTENTS ACKNOWLEDGMENTS... Page ii V I T A... iii LIST OF T A B L E S... vi LIST OF F I G U R E S... ix Chapter I. I N T R O D U C T I O N... *... 1 II. REVIEW OF LITERATURE... 4 Concept of Performance Testing.. 4 Traits Measured Birth W e i g h t Weaning Weight Age of D a m Initial Age and Weight Traits Measured at the End of the T e s t Length of the Test Period and Relationships of Traits Measured. 26 Sale P r i c e Measurement of Time Trends III. MATERIALS AND METHODS Source and Description of Data.. 48 Analysis of the D a t a Sale Data A n a l y s i s Analysis of Genetic and Environmental Trends IV. RESULTS AND D I S C U S S I O N Sale Data Analysis Simple Correlations Among Performance Traits A n g u s Polled Hereford S i m m e n t a l C h a r o l a i s iv

8 TABLE OF CONTENTS (Continued) Chapter IV. (Continued) Page Relationship of Performance Traits to Sale P r i c e A n g u s Polled Hereford S i m m e n t a l C h a r o l a i s Discussion of Sale Analysis Time Trend Analysis of Growth Performance Traits Herd E f f e c t s Fixed Environmental Effects Estimation of the Phenotypic T r e n d Estimation of the Genetic and Environmental Trends V. SUMMARY BIBLIOGRAPHY v

9 LIST OF TABLES Table Page 1. Number of Bulls Tested and Sold By Breed and Test Descriptions and Abbreviations Overall Means of Bulls Tested for all Traits by B r e e d Least-Squares Means of Bulls Tested for all Traits by B r e e d Overall Means of Bulls Sold for all Traits by B r e e d Simple Correlations Among Performance Traits (Excluding Frame Score), Sale Price and Percentile Rank of Price for Angus Bulls S o l d Simple Correlations Among Performance Traits (Including Frame Score), Sale Price and Percentile Rank of Price for Angus Bulls Sold Simple Correlations Among Performance Traits (Excluding Frame Score), Sale Price and Percentile Rank of Price for Polled Hereford Bulls S o l d Simple Correlations Among Performance Traits (Including Frame Score), Sale Price and Percentile Rank of Price for Polled Hereford Bulls S o l d Simple Correlations Among Performance Traits (Excluding Frame Score), Sale Price and Percentile Rank of Price for Simmental Bulls S o l d vi

10 LIST OF TABLES (Continued) Table Page 11. Simple Correlations Among Performance Traits (including Frame Score), Sale Price and Percentile Rank of Price for Simmental Bulls S o l d Simple Correlations Among Performance Traits (Excluding Frame Score), Sale Price and Percentile Rank of Price for Charolais Bulls S o l d Simple Correlations Among Performance Traits (Including Frame Score), Sale Price and Percentile Rank of Price for Charolais Bulls S o l d Partial Regressions for Estimating the Influence of Independent Variables on Sale Price (Angus Bulls) Partial Regressions for Estimating the Influence of Independent Variables on Percentile Rank of Price (Angus Bulls) Partial Regressions for Estimating the Influence of Independent Variables on Sale Price (Polled Hereford Bulls) Partial Regressions for Estimating the Influence of Independent Variables on Percentile Rank of Price (Polled Hereford Bulls) Partial Regressions for Estimating the Influence of Independent Variables on Sale Price (Simmental Bulls) Partial Regressions for Estimating the Influence of Independent Variables on Percentile Rank of Price (Simmental Bulls) Partial Regressions for Estimating the Influence of Independent Variables on Sale Price (Charolais Bulls) vii

11 LIST OF TABLES (Continued) Table Page 21. Partial Regressions for Estimating the Influence of Independent Variables on Percentle Rank of Price (Charolais Bulls) Fraction of Year Sum of Squares Accounted for by the Individual Degrees of the Polynomial Regression of Year Constants on Years in Angus Bulls T e s t e d Fraction of Year Sum of Squares Accounted for by the Individual Degrees of the Polynomial Regression of Year Constants on Years in Polled Hereford Bulls Tested Fraction of Year Sum of Squares Accounted for by the Individual Degrees of the Polynomial Regression of Year Constants on Years in Simmental Bulls Tested Fraction of Year Sum of Squares Accounted for by the Individual Degrees of the Polynomial Regression of Year Constants on Years in Charolais Bulls Tested Linear Regressions for Actual Phenotypic Trends and Estimated Genetic and Environmental Trends for Angus Bulls Tested Linear Regressions for Actual Phenotypic Trends and Estimated Genetic and Environmental Trends for Polled Hereford Bulls Tested Linear Regressions for Actual Phenotypic Trends and Estimated Genetic and Environmental Trends for Simmental Bulls Tested Linear Regressions for Actual Phenotypic Trends and Estimated Genetic and Environmental Trends for Charolais Bulls Tested viii

12 LIST OF FIGURES Figure 1. Phenotypic (- ), Environmental and Genetic ( ) Trends for Adjusted 205 Day Weight and Final Weight in Angus Bulls Tested Phenotypic ( ), Environmental ( ) and Genetic (... ) Trends for Final Age and Weight Per Day of Age in Angus Bulls Tested.. 3. Phenotypic ( -),Environmental and Genetic ( ) Trends for Average Daily Gain and Yearling Weight in Angus Bulls Tested Phenotypic ( ),Environmental ) and Genetic ( * ) Trends for Growth Index and Fat Thickness in Angus Bulls Tested Phenotypic ( ),Environmental ) and Genetic ( ) Trends for Total Gain and Preweaning Average Daily Gain in Angus Bulls Tested Phenotypic ( ),Environmental ( * ) and Genetic ( ) Trends for Pretest Average Daily Gain and Lifetime Average Daily Gain in Angus Bulls Tested Phenotypic ( ), Environmental ( - ) and Genetic Trends for Adjusted 205 Day Weight and Final Weight in Polled Hereford Bulls Tested Phenotypic (-" 1 ), Environmental ( -) and Genetic ( «) Trends for Final Age and Weight Per Day of Age in Polled Hereford Bulls Tested Phenotypic ( ), Environmental (- ) and Genetic ( ) Trends for Average Daily Gain and Yearling Weight in Polled Hereford Bulls Tested... ix Page

13 LIST OF FIGURES (Continued) Figure Page 10. Phenotypic ( ), Environmental ( ) and Genetic Trends for Growth Index and Fat Thickness in Polled Hereford Bulls T e s t e d Phenotypic ( ),Environmental and Genetic ( ) Trends for Total Gain and Preweaning Average Daily Gain in Polled Hereford Bulls Tested Phenotypic ( ), Environmental ( ) and Genetic ( «...) Trends for Pretest Average Daily Gain and Lifetime Average Daily Gain in Polled Hereford Bulls Tested Phenotypic ( ), Environmental and Genetic (. «-), Trends for Adjusted 205 Day Weight and Final Weight in Simmental Bulls T e s t e d Phenotypic ( ),Environmental and Genetic ( *«.) Trends for Final Age and Weight Per Day of Age in Simmental. Bulls T e s t e d Phenotypic ( " ),. Environmental ( ) and Genetic ( * ) Trends for Average Daily Gain and Yearling Weight in Simmental Bulls T e s t e d Phenotypic (..), Environmental ( ) and Genetic (. ) Trends for Growth Index and Fat Thickness in Simmental Bulls Tested Phenotypic ( ), Environmental ( -) and Genetic.. ) Trends for Total Gain and Preweaning Average Daily Gain in Simmental Bulls Tested Phenotypic ( ), Environmental and Genetic ( ) Trends for Pretest Average Daily Gain and Lifetime Average Daily Gain in Simmental Bulls Tested x

14 LIST OF FIGURES (Continued) Figure 19. Phenotypic Environmental and Genetic (.. ) Trends for Adjusted 205 Day Weight and Final Weight in Charolais Bulls Tested Phenotypic (" ),Environmental (r ) and Genetic Trends for Final Age and Weight Per Day of Age in Charolais Bulls T e s t e d Phenotypic ( ),Environmental and Genetic Trends for Average Daily Gain and Yearling Weight in Charolais Bulls Tested Phenotypic (" ),Environmental ( ) and Genetic (... ) Trends for Growth Index and Fat Thickness in Charolais Bulls Tested. 23. Phenotypic, Environmental (- ) and Genetic ( Trends for Total Gain and Preweaning Average Daily Gain in Charolais Bulls Tested Phenotypic ( ), Environmental ( ) and Genetic 0 ) Trends for Pretest Average Daily Gain and Lifetime Average Daily Gain in Charolais Bulls Tested... Page xi

15 FACTORS AFFECTING THE SALE PRICE OF CENTRAL PERFORMANCE TESTED BEEF BULLS AND THE ESTIMATION OF GENETIC AND ENVIRONMENTAL TRENDS By Diane Marie Sharkey, Ph.D. The Ohio State University, 1982 Professor Walter R. Harvey, Adviser Performance records of Angus, Polled Hereford, Simmental and Charolais bulls tested at the Ohio Bull Test Station from 1977 through 1982 were- evaluated to determine the extent that sale price was influenced by growth traits. Data from bulls finishing the 140 day test and sold in an auction sale were analyzed to determine the major criteria used by purchasers of these bulls as indicated by sale price. Traits available to the buyer in the sale catalogue were considered as independent variables in the analysis for each breed (adjusted 205 day weight, final test weight, test average daily gain, yearling weight, fat thickness, a growth index calculated as the average of the breed group ratio for average daily gain and yearling weight, weight per day of age and frame score) while sale price and percentile rank of price calculated within test and breed, were the dependent variables. Overall, buyers placed m ost emphasis on final weight, growth index, weight 1

16 2 per day of age and frame (when considered in the analysis). Data from all bulls entered in the test were evaluated to partition the phenotypic trends of each growth performance trait into estimated genetic and environmental portions using maximum likelihood techniques/ which adjusts for selection and culling practiced in the population. The phenotypic trend was estimated by regressing the within herd year constants on years. A comparison of performance records of paternal half-sibs in adjacent years estimated-the environmental trend plus one-half the genetic trend. The genetic trend was estimated as twice the difference between the regression of progeny performance on time and the pooled intrasire regression of sire progeny performance on time. This method of estimating genetic and environmental trends seemed to over-estimate environmental trends, which were practically identical to the observed phenotypic trends, and under-estimate genetic trends, which were essentially zero.

17 CHAPTER I INTRODUCTION The most important and often most difficult decision for a beef cattle producer is the one concerning the purchase of a bull. The buyer must consider specific qualities of the bull that may assist in the improvement of the productivity of his herd. There is real opportunity for improvement since the bull provides one half of the genetic material for a specified calf crop and since selection intensity can be more easily applied to bulls. Sire evaluation is important because most breeders have relatively small herds, a long calving season, manage calves differently and usually select replacement bulls from outside their own herd. The procedures of central performance testing of beef bulls are designed to provide a uniform postweaning environment for bulls and to obtain measures of their growth. The purpose of these procedures is to identify genetically superior individuals. However, little use has been made of data collected in central bull tests to determine the influence such tests have on the beef cattle population. Studies that have been reported which determined the extent that performance measured traits influence the sale

18 price of beef bulls in the late 1950's and early 1960's indicated that measures of test performance were of little importance to potential buyers (Rutherford et a l., 1966; Marlowe and Marrow, 1967; Marlowe, 1969). Some indication of an increasing awareness of the importance of performance records on the part of breeders was found in the late 1960's (DuBose et al., 1968). It is now necessary to conduct an investigation of the influence recorded performance data have on the beef industry and compare findings with previous results. There are many breeds of beef bulls represented at a central test station. Some breeds are faster growing and mature earlier than others. The relationships among many economic characteristics have been reported in numerous papers. However, there has not been a study to calculate correlations among traits for different breeds of beef cattle maintained under similar environments and managements. This study will use data from Charolais, Angus, Simmental and Polled Hereford bulls to determine correlation values between performance measured traits and sale price and observe differences between breeds. Bulls delivered to central test stations to start test usually vary widely in ages and weights. These conditions might affect their subsequent performance on test. It is desirable to adjust performance measures of individuals for these different sources of variation.

19 3 If the effects of initial age and weight on subsequent performance are important, then entrance requirements must be able to eliminate or adjust for these variables. The time trends that bulls accepted for test follow may give an indication of the current state of the beef cattle population. The direction of time trends for various traits may identify established tendencies of the beef cattle population and may reveal situations needing further improvement. However, time or phenotypic trends do not necessarily represent genetic changes in the population. Therefore, the genetic and environmental portions of the total trend will be identified and separated. This study was made in order to: (1) Obtain relationships of test performance traits and sale price within breed groups of bulls. (2) To determine the relative emphasis buyers of purebred performance tested bulls place on performance traits. (3) Determine genetic and environmental time trends of central test station performance data of beef bulls over a six year period.

20 CHAPTER II REVIEW OF LITERATURE Concept of Performance Testing Performance testing can be defined as the systematic collection of comparative production information for use in decision-making to improve efficiency and profitability of beef production (USDA, Beef Improvement, 1981). For performance testing to be useful to the cattleman, differences in performance among cattle must be utilized in decisions about his choice of sire. The most useful performance records for management, selection and promotion decisions will vary among and between breeders and commercial producers. Central livestock performance testing stations were established in the United States due to the need for comparative record information collected by an agent independent of the owners and because of the problem of estimating the genetic differences which exist between animals from different herds. The first central bull testing station in the United States was at Balmorhea, Texas in the early 1940's, but it wasn't until 20 years later that the presence of central testing stations influenced production testing of beef bulls (Baker, 1967).

21 Since the advent of central testing of bulls there have been substantial improvements in the performance of bulls measured at these stations. This is often taken as evidence of the effectiveness of the testing scheme and is assumed to be due to genetic changes in the breed. An advantage of performance testing is that an animal can be evaluated at an earlier age than would be possible with progeny testing. Therefore, the generation interval is reduced when performance tested individuals are selected for breeding. A disadvantage of performance testing is that it is unsuitable for traits not measurable in the live animal or which have low heritabilities (Preston and Willis, 1974). Performance or production.testing consists of measuring traits in the live animal. To improve a particular trait in a herd it must therefore be measurable. Traits such as weaning weight, feedlot gain, yearling weight and weight per day of age can easily be measured. Traits which can be measured based on an individual's performance will result in a faster rate of genetic improvement if attention is paid to the individual's record and if selection of superior animals is based on these records. The reason for recording various production traits is to evaluate differences between animals in order to make

22 effective comparisons (Gregory et al., 1961). The measurements taken should provide a basis for comparing individual animals on all heritable economically important traits. Heritability estimates of various traits involved in improved production have been reported. These estimates indicate the rate of progress which can be made by selecting for a particular trait. Performance testing to measure traits pertaining to a beef animal's economic usefulness has proven to be of chief importance in identifying superior animals and has gained broad acceptence with the beef industry (Pell and Thayne, 1978). Central testing stations provide a method of obtaining comparative record information of a limited number of economically important traits on bulls at one location which were previously reared in different locations in order to estimate differences which exist between individuals and herds. Genetically superior individuals can be more readily identified when the animals are maintained under a similar management system and their performance records are adjusted for known environmental differences (USDA, Beef Improvement, 1981). The stations help aquaint breeders with good performance testing techniques and the effectiveness of record of performance programs. They also provide a source of bulls which have been tested under comparable conditions.

23 The results from a central performance test are valuable since all animals of a given sex and age are given equal opportunity to perform through uniform feeding and management. During the test period records of economically important traits on all animals are systematically maintained and are adjusted for known sources of environmental variation such as age of dam or age of calf. The results obtained from a performance, test are used in selecting replacements and in eliminating poor producers. Bulls enter a central test from diverse backgrounds. There is usually little knowledge or control over their pretest environment. Consequently, the extent to which central testing procedures can accurately portray important genetic differences between animals depends on the success in providing a uniform test environment for the animals and the extent to which previous environmental effects carry over into the test period (Chapman, 1967). According to Chapman (1967), the success of central testing also depends on whether the responses of genetically similar animals under test conditions conforms with the differences that would have been exhibited under farm conditions. Test stations provide a uniform environment for measuring growth and other production traits in order to detect superior individuals and assist producers in

24 recognizing and selecting breeding stock which can improve the performance and quality of their beef herd. Central tests thus allow prospective buyers to compare the performance of bulls between herds of various producers more accurately than would be possible with on-the-farm testing. An advantage to the purchaser in evaluating bulls from a central test station is that the nutritional and other environmental conditions have been the same for all the bulls for the entire test period. However, pretest differences will not be accounted for during the test. The objective in the development and evaluation of measurement procedures is to be able accurately to rank individuals on the basis of their genetic worth. The trait must be measured under conditions where important environmental effects can be physically controlled or properly adjusted for in order to obtain the highest correlation between estimated and actual genetic worth of the individuals. According to Gregory (1965), it is also necessary to evaluate differences in an environment where genetic differences are expressed to the maximum and when differences in random or chance environmental variation is minimum. There are several difficulties concerned with the set up of performance testing procedures. One is identifying the breeding value or genetic worth of an animal from

25 phenotypic measurements. Also, since only a limited number of animals can be tested, unless a representative sample is obtained, little relevant information about herd differences may be collected. Lastly, even under extremely standardized conditions, a large fraction of the differences in records is not due to heredity. Often the results from bulls evaluated at central test stations may be over-publicized. All data from a beef bull central test are collected on young growing bulls maintained on drylot. The bulls tested are nominated by and remain the property of contributing breeders and consequently constitute a selected population from the surrounding geographic area. The central test procedures only provide a scheme for selecting for those traits which can be measured on a growing bull'and also provide a response only under high concentrate feeding. Performance testing of beef bulls is usually conducted from a conventional weaning age of between six to eight months and terminates after a fixed period of time on test. There is variation in age and weight of the bulls at the start and end of the test. The performance records should therefore be adjusted to reduce or discount known environmental differences between anaimals so that genetic differences will tend to be a larger portion of

26 10 the total differences actually measured or observed. Traits Measured Postweaning performance is the most important phase of an animal's life to measure. The individual is on its own and is not environmentally influenced by the dam. It is for this reason the postweaning performance gives a more accurate indication of the individuals real growth potential (Ohio, Bulletin 503, 1978). According to Gregory (1965), in the development and evaluation of procedures for measuring a trait it is necessary to consider the relationship of the measures to the actual economic trait of primary concern, the heritability of the measures and the genetic correlations of the measures with other traits affecting economic returns. Growth rate and efficiency of gain for beef cattle are of primary economic importance to the beef cattle industry. Growth rate has a direct effect on net return and is positively correlated with efficiency of gain, weight and value of retail product (USDA, Beef Improvement, 1981). Growth rate has been recommended as an important selection criterion because of the ease with which it can be measured and the accumulation of evidence that it is closely related to feed efficiency (Barlow, 1978). As a result, central testing of bulls is primarily concerned with the evaluation of bulls on the basis of

27 11 their growth rate and conformation. Measuring growth rate in bulls on a relatviely high concentrate ration from weaning to twelve to fifteen months of age is a close approximation of the period of the life cycle in which the industry is most interested. According to Baker (1975), for the benefit of a beef industry dependent on producers, feeders, processors and consumers, three key points in beef improvement programs cover the major traits. These are weaning weight for the benefit of the producers, yearling, weight and feedlot gain for the benefit of feeders and feedlot operators and fat and lean composition for the benefit of processors and consumers. DuBose et al. (1968) found highly significant (P<.01) correlations among weaning weight, test gain, final weight and weight per day of age, ranging from 0.40 to Birth Weight Birth weight measures prenatal growth. The correlations between birth weight and other traits are of interest because they give an indication of the usefulness of this early weight as an indicator of subsequent development of the animal. Brown and Honea (1969) reported relatively low values of the phenotypic correlations between birth weight and weight per day at 240 days, initial test weight, daily

28 gain during test, final test weight and weight per day at end of test, 0.18, 0.03, 0.16, 0.13, -0.25, respectively. Data were from 299 Hereford and 319 Angus bulls completing a postweaning feed test between 1955 and These results indicate that birth weight is a poor indicator of subsequent performance of the bull calf. These authors found that no more than 8% of the variance of a performance trait was associated with the variance in birth weight. In contrast to these findings, Koch et a l. (19 73 and 19 74) and Brinks et al. (1964) reported a high phenotypic correlation between birth weight and postweaning average daily gain, yearling weight and mature weight. The genetic correlations of birth weight with traits which are emphasized in selection programs for gaining ability are also of interest. Since birth weight must not exceed some maximum threshold level associated with calving difficulties, the extent to which birth weight may be altered by selection for heavier weight and faster growth rates is of importance to the breeder (Brown and Honea, 1969). Genetic correlations between birth weight and weight' per day at 120 days, weight per day at 240 days, initial test weight, daily gain during the test, final test weight and weight per day at the end of the test were high, ranging from 0.50 to 0.86/ in the study by

29 Brown and Honea (1969). The genetic correlations of birth weight with 200 day weight, postweaning daily gain and 452 day weight reported by Koch et al. (1973) for Hereford bulls were 0.41, 0.42 and 0.53, respectively. Brinks et al. (1962) found the genetic correlation of birth weight with 196 day gain and final weight of Hereford bulls to be 0.71 and 0.75, respectively. The high genetic correlation of birth weight with later growth performance traits suggests that the genes responsible for heavier weight at birth also contribute to heavier weights and faster gains later in the animal's life. Weaning Weight Weaning weight has been shown to be largely a function of birth weight, preweaning nutrition, weaning weight of the dam and sex of the calf (Christian et a l., 1965). The influence of weaning weight on economic traits expressed at later ages could be due to a carry over effect of maternal or other environmental factors or even the influence of the same genes which acted prior to weaning. The genetic and phenotypic correlations of weaning weight with yearling weight are in the range of 0.70 to 0.84 (Koch et al., 1973, 1974; Kennedy and Henderson, 1975a, b ). The range of genetic.correlations between weaning weight and 18 month weight reported by Preston

30 14 and Willis (1970) were of similar size. These authors also reported high positive genetic correlations of weaning weight with final test weight, ranging from 0.33 to Estimates of the phenotypic correlation between these two traits were also found to be similar, 0.16 to The range of genetic and phenotypic correlations of weaning weight with postweaning test gain were from 0.22 to 0.83 and from 0.03 to 0.32, respectively (Preston and Willis, 1974). Koch et al. (1973) reported a phenotypic correlation of 0.20 for weaning weight and postweaning average daily gain of bulls. Kennedy and Henderson (1975a, b) found phenotypic correlations of 0.31 and 0.66 for Hereford and Angus cattle, respectively, for the same two traits. Weaning weight and postweaning average daily gain were found by these authors to have a higher genetic correlation for creep-fed Hereford and Angus cattle, 0.87 and 0.92, respectively. Average daily gain to weaning has a high genetic correlation with postweaning daily gain, yearling weight and mature weight (Koch et a l., 1973 and 1974; Kennedy and Henderson, 1975a, b; Brinks et al., 1964). Schalles and Marlowe (1967) also found that as pretest average daily gains of bulls increased, 365 day weights and lifetime average daily gains increased. This is to be expected since these components are part-whole relationships.

31 15 The genetic correlation of average daily gain to weaning and postweaning gain was found to be 0.30 and 0.62 for Hereford and Angus cattle, respectively (Kennedy and Henderson, 1975a, b ). For creep-fed Hereford and Angus the genetic correlation between preweaning and postweaning average daily gain was found to be much higher (0.82 and 0.92). Koch et al. (1973) found a much lower genetic correlation between average daily gain to weaning and test average daily gain for bulls, Koch et al. (1973) found a positive environmental correlation between preweaning and postweaning daily gain in bulls, suggesting that in the feedlot, bulls that gained more before weaning, for environmental reasons, would have a competative advantage for feed consumption over their genetically comparable mates. The phenotypic correlation of test gain andpreweaning gain has been found to range from 0.14 to 0.57, while the genetic correlation estimates among these two traits are slightly lower, 0.06 to Several authors have found negative or essentially zero relationships between preweaning and. postweaning gains under conventional late weaning systems (Dahmen and B o g a r t, 1952; Pierce et al., 1954; Bogart and Frischnecht, 1967). Willis and Preston (1970) studied the interrelationships among bulls weaned at 90 days of age and tested up to

32 kg liveweight. They found that preweaning gain was not significantly related to gain on test (r=0.11) nor was 90 day weight to gain on test (r=0.26). Their findings agree with those reported for conventional late weaning systems. Kennedy and Henderson (1975b) reported little phenotypic relationship between preweaning and postweaning gain. However, a strong positive genetic correlation was found, indicating that many of the same genes are responsible for growth during the two periods. Despite the lack of a strong phenotypic relationship between preweaning and postweaning gain, selection in one stage of growth should result in genetic improvement in the other. The low phenotypic correlation between preweaning and postweaning growth can be attributed to the presence of a strong negative environmental correlation between the two traits. This negative environmental correlation can be attributed to compensatory growth. Calves that are environmentally or nutritionally deprived during the preweaning period can often compensate during the postweaning period with gains greater than they might have achieved otherwise. Christian et al. (1965) reported a greater influence of weaning weight on daily gain after 365 days of age than on gain from 240 to 365 days of age. This agrees with the

33 17 results of Swiger (1961) who found that a good preweaning environment handicapped early postweaning gains but enhanced later gains. Influences of pretest environment can be minimized in evaluation of results if the final report includes both pretest and test gains. If test gain alone is used, cattle on a suboptimum pretest feeding level which did not permit full expression of inherent ability to grow are likely to compensate with inflated test gains. Compensatory growth is defined as the physiological response achieved during ad libitum feeding following a period of undernutrition (Dalton and Morris, 1978). Using both pretest and test gains should avoid labeling an excessively high test gain as the animal's real gaining ability. This can be done either by averaging pretest and test gains or if the test starts immediately after weaning, by computing a final weight as a standard weaning weight (205 days) plus test gain (USDA, Bulletin 286, 1981). Selection for improved weaning weight will result in a slight increase in feed utilization efficiency over the postweaning period when feeding is over a weight constant or time constant period, but a slight decrease when feeding is from actual weaning weight (fixed age) to a fixed slaughter age (Barlow, 1978).

34 18 Age of Dam Age of dam influences the early performance of individuals due mainly to the amount of milk available for the calf's nutrition and growth. As a result weaning weights of calves are routinely adjusted for age of dam differences. Cundiff et aju (1966a,b) suggested that additive correction factors were the most suitable for adjusting calf weaning weights since standard deviations for weaning weights of calf from different age dams were fairly constant. O'Mary and Ament (1961) also reported that weaning weight adjusted on an additive basis was the most effective in reducing variation due to age of dam. They found the correlation between calf weaning weight and age of dam was 0.93 when unadjusted, 0.61 when adjusted on a f multiplicative basis, and when adjusted on an additive basis. The additive factors for age of dam adjust weaning weights more correctly than do multiplicative factors since, unlike multiplicative factors, the additive factors neither favor heavy birth weights nor inflate the effects of extra weight gain due to creep feeding on weaning weight. Bailey and Gilbert (1962) reported that age of dam had a significant effect on initial weight at the start of the test and on final weight. In a study of central

35 tested bulls conducted by Schalles and Marlowe (1967), age of dam was reported to have a significant positive effect on 365 day weight, lifetime average daily gain and test average daily gain. It was found that bulls from younger cows gained less than those from older cows. Bulls from cows 12 years and older had the fastest lifetime average daily gain, test gain and heaviest 365 day weight. This does not agree with most reports but, according to the authors, these results probably can be attributed to breeders selecting bulls to be tested from cows which had produced good performing calves previously. Brinks et al. (1964) found that the effect of age of dam was significant for birth weight, 180 day preweaning gain, 180 day weaning weight, weaning score and final weight off test. However, no significant effect of age of dam on 196 day postweaning gain was found. Swiger (1961) studied the effect of age of dam on postweaning gains of Hereford calves and found that the age of dam effect depended on the length of the feeding period and did not necessarily diminish with time on test. There was a tendency for calves from young and old cows to gain more in the first 28 day postweaning period and gain less toward the end of the test than did calves from cows five to ten years of age. Swiger et a l. (1963) also found that age of dam had no appreciable effect on postweaning gains

36 20 or weights after adjustment for this factor at weaning. Initial Age and Weight There is usually variation among cattle entering a central performance test with respect to age and weight. It is logical to suspect that these variables might affect subsequent test performance. For example, bulls going on test in a thin underfed condition, especially if they are a little older than weaning age, will tend to make rapid compensatory gains on test. To avoid this problem most central test stations require a minimum weight per day of age for bulls entering the test. Initial age was found to be an important source of variation in gain during a 140 day test in an analysis of 1,840 Hereford bulls by Thomas and Cartwright (1962). They recommended that the entry age of bulls be not more than seven months of age. Moore et al. (1961). found a significant partial regression of for the effect of initial age on rate of gain, indicating that a bull 60 days older than average would be expected to gain nearly 0.20 pounds per day less than average if initial weight was constant. Brown (1965) reported that 60% of the variance in average daily gain of 122 Hereford and Angus bulls tested for 154 days was accounted for by initial weight and feed

37 21 consumption. Moore et al. (1961) found a significant partial regression coefficient of for rate of gain on initial weight which indicates that if an animal was 100 pounds heavier than another, the heavier one would be expected to gain almost 0.30 pounds more per day on test. Moore et al. (1961) explained that if two animals were placed on test at the same initial weight the younger one would be expected to gain more or if they were the same age the heavier one would be expected to have faster gains. The combination of these two factors indicates that animals having the largest pretest weight for age would be expected to make larger gains during the test period. Willis and Preston (1967) found that initial age and weight significantly affected the rate that animals reached 400 kg. Brinks et al. (1962) studied the effects of initial age of calf on final test weight and postweaning gain. They found that initial age effects were significant only for final weight. Schalles and Marlowe (1967) studied the effects of initial age and pretest gains on 365 day weight and lifetime average daily gain. They concluded that initial age had a significant negative influence on 365 day weight; younger bulls had a heavier 365 day weight. Initial age was found to have no effect on lifetime

38 22 average daily gain. There was a small tendency for the older bulls to gain faster on test. As pretest average daily gain increased the authors found that 365 day weight and lifetime average daily gain also increased. Pretest gains were positively related to gains on test (P<.10). In contrast, Swiger (1961) studied the effect of initial age (at weaning) on five 28 day period post weaning gains and reported the sum of the regressions of postweaning gains on age over the total 140 day test period to be 0.26 pounds. He concluded that a 56 day difference in weaning age would result in a difference of only 0.10 pounds in average daily gain. However, the regression of gain on initial age was larger for the earlier periods, indicating that initial age would have a larger effect on gains made during a shorter test period. Swiger et al. (1963) confirmed these results. They analyzed the effect of age of calf on the gains of several postweaning growth periods and found that for bulls the effect of age decreased from to pounds per day of age through the postweaning periods. These results suggest that bulls begin to plateau in growth rate after about one year of age. The authors concluded that the effect of initial age on gain from 200 to 500 days of age was small enough that neglecting to adjust for this effect

39 23 would not seriously decrease the accuracy of computing adjusted 550 day weights when a 90 day breeding season is used. Dalton and Morris (1978) also found that initial age and weight did not affect test gain per day of age. O'Mary et al. (1959) reported that age at the start of the test was not an important factor in the subsequent gaining ability of animals. Warwick and Cartwright (1955) found that initial age and weight of beef cattle at gain evaluation centers had no important influence on subsequent gain on test. They found that differences in initial weight accounted for only approximately 1% of tlie variation in subsequent gain, and age had apparently no effect when comparing initial weight to final gain ratio. Results of a study by Wilton et a l. (1973) indicated that age and initial condition had little effect on rate of gain for station tested bulls given a 28 day adjustment period before the start of the test. Other authors concluded it was unnecessary to adjust daily gains for initial age or weight even though the initial age ranged from 241 to 338 days in these studies (Swiger and Hazel, 1961; Brown, 1963).

40 24 Traits Measured at the End of the Test Final weight at 12 to 18 months, standardized for age differences, is a more highly heritable measure of differences in growth rate than any individual component of final weight such as birth weight, preweaning gains or postweaning gains (USDA, Bulletin 286, 1981; Gregory 1961). The yearling or 365 day weight combines the weaning weight and postweaning weight into one meaningful value. According to Peters (1975), many cattle producers consider this particular weight the most important single measure of a bull's potential. Adjusted 365 day weight, computed by adding postweaning gain during a standard time constant postweaning test period to weaning weight, adjusted for age of dam and calf, is highly heritable and is a good measure of genetic differences in growth rate, according to Cundiff and Gregory (1968). However, among bulls from different herds in a central test, care must be used when comparing yearling weight measurements because the weaning weight portion was not made under comparable conditions. Cundiff and Gregory (1968) state that estimates of genetic correlations among components of final weight are positive but the environmental correlations are negative. As a result, according to the authors, this should cause final weight to have a higher heritability than any of its components because the positive genetic covariances among

41 25 the components tend to increase the percent of variation in final weight which is accounted for by differences in breeding value among individuals and the negative environmental covariances among the components act to reduce the environmental variation when the components are combined into one trait, final weight. Estimates of heritability for postweaning growth rate and yearling weight, measured in time constant periods have been high (Warwick, 1958; Gregory, 1961; Clark et ajl., 1963). Genetic correlations between feed conversion (feed consumed per unit of weight gain) and test gain are negative and large, ranging from to (Preston and Willis, 1974). This indicates that selection for increased test gains may also improve feed conversions of animals on test. Results of Koch et al. (1973) indicate that approximately 0.80 as much change in feed efficiency may be expected by selection for postweaning growth rate as selecting for feed efficiency itself. Knapp and Clark (1947) reported higher estimates of heritability for growth rate toward the end of a 252 day postweaning feeding period than at the begining. This is consistent with the opinion that some of the variation in gain, caused by differences in previous environmental conditions, tends to be compensated for with time (Gregory, 1965).

42 26 Measures of liveweight gain are not a measure of the actual or real economic trait of pounds of edible meat (Gregory, 1965). Swiger e t al. (1964) reported that differences in final weight accounted for approximately 88% of the variation in pounds of boneless retail trimmed beef on an age constant basis which leaves only a small amount of variation to.be accounted for by differneces in composition and dressing percentage. Therefore, according to Gregory (1965), liveweight gains meet the requirement of being highly related to the actual or real economic product. Length of the Test Period and Relationships of Traits Measured Knapp and Clark (1947) calculated heritabilty estimates for gain during each of three 84 day periods of a 252 day feeding trial of beef steers. The estimates of heritability for each of the three periods were 0.10, 0.54 and 0.84, respectively. The authors also calculated the phenotypic, genetic and environmental correlations between gains of these consecutive 84 day feeding periods. The phenotypic correlations between periods one and two, one and three, and two and three were 0.26, 0.18 and 0.39, respectively. The genetic and environmental correlations between these same periods were 0.82, 0.45, 0.70 and 0.11,

43 , -0.32, respectively. These heritabilities and correlations indicate that gains of animals during the first period are controlled largely by environmental effects while third period gains are affected more by heredity. Knapp and Woodward (1951) reported similar findings in their study of heritability estimates of live weights at 28 day intervals during a performance test. The heritability estimate for the first period was Estimates of heritability for each successive period increased sharply up to the sixth 28 day period and then plateaued, The ninth 28 day period had a heritability estimate of 0.80 to These authors concluded that a feeding period of 112 days would be sufficient to indicate growth potential of performance tested animals. Dinkel (1958) computed heritabilities for rate of gain for 140, 16 8 and 196 days of feeding of 0.45, 0.52 and 0.65, respectively. He concluded that selection on the basis of 140 day gain would be 94% and 79% as effective as it would be in the 168 and 196 day periods, respectively. Dearborn and Dinkel (1959) analyzed the effectiveness of using final weight as a criterion in the selection of bulls performance tested for the same periods of time. The final weights studied were those obtained after 140,