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Five nonlinear models were used to fit weight-age data for female cattle of diverse breeding and management. Comparisons were made among these models for goodness of fit, biological interpretability of parameters and computational ease. Three models, von Bertalanffy, Gompertz and logistic, consistently overestimated weights at early ages and the logistic underestimated mature weight. A four-parameter model, Richards, more accurately fit the data but was computationally more difficult than the three-parameter models. The Richards model which has a variable point of inflection is most appropriate for fitting weight-age data when goodness of fit, especially prior to 10 months of age, is critical. The Brody model is computationally easier and fit the weight-age data for ages past 6 months almost as well as the Richards model. Parameter estimates, or functions thereof, from all models can be interpreted as measures of size and rates of gain and maturing. However, parameter estimates from different models, although having the same biological interpretation, were not perfectly correlated. Genetic variations in rates of gain and maturing and mature size were observed.

The weight-age curves of 288 Hereford (H) and 296 Angus (A) females were used to obtain weights and gains at specific ages during the interval from 4 months to maturity. The effects of calving year, season of birth, sire and age of dam were used in a linear model to contrast the influence of genetic and environmental sources of variation on weights, gains and rate of maturing. Additive genetic variation (Va) was larger for immature H weights than A weights, but the A showed much larger Vavalues for monthly gains than the H. The mean estimates of h2for body weight were not very different for the two breed groups but the A showed a larger range in h2and had much smaller estimates of h2at 4 and 36 months than the H.Large gains at young ages were associated with early maturing females while genotypes required for sustained growth rate into the advanced ages were associated with late maturing females.

PAST genetic studies in beef cattle have emphasized point evaluation of growth over relevant economic time periods. In general, the measurements considered have been confined to constant ages, constant time periods or, occasionally, over a constant weight period (Swiger et al., 1962; Brown and Gifford, 1962; Thrift et al., 1970; Nelms and Bogart, 1955). Many aspects of development patterns and interperiodic relationships have been neglected as a consequence of using a few select measurements to evaIuate growth. Certain characteristics of growth such as rate of maturing and mature size cannot be measured on immature animals, yet these or related measures are necessary to give perspective to the single measurements made on animals. This concept was considered in some detail by Brody and his coworkers, yet it has received only limited consideration since 1950 (Brody, 1945; Taylor, 1965; Fitzhugh and Taylor, 1971; Timon and Eisen, 1969).One objective of this study was to obtain estimates of mature weights and rate of maturing for Hereford and Angus females.

TEN body measures, including weight, were taken on 267 Hereford (H) and 283 Angus (A) bulls at 12 months of age. The genetic and phenotypic correlations (rGand rP) between the individual measures and four feedlot traits including test gain, feed consumed (FCONS), feed conversion (FCONV) and final test weight (FWT) were discussed. The 10 body measures were also used in a principal components analysis for each breed group. The principal components explained the covariance structure among these 10 body measures and the various components provided a means of evaluating bulls according to size and shape. The rGand rPbetween several principal components from each group and the four feedlot traits were considered as measures of the relationship of size and shape to test performance.The rGbetween individual measures at 12 months and measures of feedlot performance of H and A bulls in the feedlot were generally large and positive, with several of the A correlations being smaller than those of H bulls. The rGbetween body measures and FCONV were different for the groups. Genes with a positive effect on the body measures improved FCONV of H bulls but had an adverse effect on FCONV of A bulls. The rPwere moderate, positive and very similar for all test traits except FCONV which were negative but sufficiently small to be of little use.

The correlations of overall size, shape and ten body measurements at 4 and 8 months to feedlot performance were obtained for 550 Hereford (H) and Angus (A) bulls. Size and shape description were obtained from principal components analyses of the ten body measures for each breed at each age. The correlations indicated that, generally, an increase in any one body dimension was associated to some degree with increased gain, weight and feed consumption in both breed groups. Increases in body dimensions had different effects on feed conversion of the two breed groups. Increasing the dimensions of H bulls improved feed conversion but increasing any one dimension of A bulls had an adverse effect on feed conversion. The first principal component which combined all body measures including weight into a single linear index contrasted large framed bulls with small framed bulls. The conclusions from the correlations between overall size and feedlot performance were similar to those drawn from single measurements.Three concepts emerged from the correlation analyses of shape and performance. Immature body shapes were associated with subsequent performance on test and more than one shape showed positive relationships to efficiency and rate of gain on test. Bulls which months subsequently grew well on test. Tall were larger in all measurements at 4 and 8 and narrow bulls of both breed groups at 4 and 8 months were observed to eat more feed, weigh more at the end of test and gain more but less efficiently than short, wide bulls. The correlations between shape and performance indicate that correlations between single measures and performance traits should be qualified. Logically, the effect of one body dimension on performance of a bull depends upon its relationship to other body dimensions. The data support the hypothesis that shape may be the result of developmental homeostatic mechanisms attempting to compensate for extremes in any one dimension. Several immature shapes were identified which had acceptable performance on test. With a variety of body shapes showing acceptable feedlot performance, the problem concerning the breeder becomes one of identifying those body types which are consistent with efficiency in other phases of beef production.

Nine skeletal measures and body weight were recorded on 267 Hereford (H) and Angus (A) bulls at 4, 8 and 12 months of age. A total of six principal component analyses, one for each breed and age, were obtained. In addition to a better understanding of the dependence structure among skeletal dimensions at a given age, the principal components estimated in these analyses measured differences in size and shape. The first principal component (PC1) at each of the three ages accounted for 56 to 68% of the variation in the 10 measurements and provided a linear function of size with nearly equal emphasis on all 10 standardized traits. The second principal component (PC2) for each of the three ages contrasted tall, narrow bulls with short, wide bodied bulls and this contrast in shape accounted for more than 10% of the variation in the dependence structure of the system of ten variables. Therefore, nearly 75% of the variation in the dependence structure of nine immature skeletal dimensions and body weight was explained either on the basis of overall size or by the shape contrast of short, wide vs.tall, narrow bulls. The consistency and importance of the shape contrast provided by the second principal component at 4, 8 and 12 months suggests that tall, narrow bodied and short statured, wide bodied bulls represent two basic shapes in cattle at preyearling ages. The correlations between PC2 at 4, 8 and 12 months indicated that bulls of these two body shapes remained relatively unchanged over the ages examined. The failure of body length to consistently receive a weighting equal to height or width in the second component contrasting shape suggests there was more phenotypic latitude for length in cattle of different heights and widths than there was in width for cattle of different heights.The genetic and phenotypic correlations between principal components at different ages indicated that the complex of genes influencing size or shape tended to have a similar effect at the ages of 4, 8 and 12 months. However, the correlations also indicated that, when equal weighting was given to each of the 10 measures of size as in the first principal components, bulls selected at one age on this single composite value could be quite different in shape at later ages. This result pointed out the likelihood that unrestricted selection based on a composite character such as weight, size or gain would produce animals of variable dimensions or shape.