<p>Since the last century, many biologists have studied the effects of size differences between species on the rate of their metabolic processes. in 1980, Taylor published the genetic size-scaling theory which incorporated the existing knowledge on size effects, and introduced two formal scaling rules and the concept of genetic size. Genetic size is expressed at all developmental stages of growth, but is most clearly visualized by the size of the mature animal; usually genetic size is quantified by the weight of the mature animal. In the scaling theory, genetic size is defined as the major genetic factor that controls the growth rate of an organism from its early embryonic stages to full maturity, and continues to determine rate and duration of life processes during the remaining part of life.<p>The genetic size-scaling theory states that variation between animals can partly be explained by differences in scale, expressed by the genetic size factor. By applying scaling rules, the differences in scale can be eliminated. What remain are size- independent differences. The size scaling theory may be very useful for animal breeding, as it provides a biological basis for explaining genetic variation. In practice, breeding is mainly based on selecting fast growing animals for meat production and high yielding animals for milk and egg production. These traits include variation due to differences in genetic size. Genetic size, however, is not related to the individual biological efficiency. If it would be possible to define size-independent traits, selection on biological efficiency would be more effective. Especially in tropical animal production systems, application of the genetic size concept can be very fruitful for a proper breed evaluation of differently sized indigenous breeds, imported exotic breeds and their crossbreeds.<p>As outlined in the general introduction of this thesis, concepts of genetic size are not adopted in animal production mainly due to the lack of a more precise and practical measure than adult equilibrium weight. This study was undertaken to solve some of the implementation problems. The objectives of this thesis were:<p>1. To improve the practical applicability of the genetic size scaling theory at the individual genotype level and at the breed level. The study particularly focused on exploration of alternative measures for genetic size.<br/>2. To elaborate methods for evaluation of growth characteristics among individual genotypes and breeds which incorporate the genetic size concept, and which enable assessment of the relative importance of genetic size in breed differences.<p>Chapter 1 of this thesis starts with a brief overview of the main developments in the study of metabolism and scaling, from the initial surface law, the mouse-to-elephant curve, to the genetic size-scaling theory. The outline served to identify areas where additional basic research is needed for successful application of the genetic size-scaling theory in practice. Two basic problems were identified: rules for intra-specific scaling are not firmly established, and a practical accurate method for size determination is lacking. Conditions for an ideal practical measure of genetic size were formulated, and current methods for size determination in ruminant species were discussed. It was concluded that a promising approach for determination of genetic size is to look for characteristics in (relative) growth patterns of body components that are linked to functional stages of development.<br/> <p>In order to gain more insight into the effects of genetic size and possible measures, a long term experiment with two goat breeds was carried out. Data on growth, ad <em>libitum</em> feed intake, body composition and body frame development of castrated males were recorded during 3 years (1987-1990) from birth to a weight level near the asymptotic weight. The two goat breeds used in the trial differed widely in adult size: the Saanen breed, a large sized European dairy breed, and the West African Dwarf goats (WAD goats), one of the smaller goat breeds. After weaning, the Saanen breed (group S) and half of the WAD animals (group DH) were fed pellets of feed H (10.7 MJ ME/kg DM). The other half of the WAD animals (group DL) received pellets of feed L (7.5 MJ ME/kg DM). A serial slaughtering scheme was designed to examine the changes in body composition. Breed effects were analyzed by comparing group S with DH, and feed effects by comparing group DH with DL.<p>In Chapter 2, animal management, feed composition, digestibility experiments and other experimental methods related to the recording of feed intake and growth were described in detail. An integrated feed intake and growth model was presented, based on the relationships between respectively weight (W) and cumulative feed intake (F), and W and age (t). The models were fitted to longitudinal data sets of 10 animals of group S, 8 of DH and 8 of DL and breed and feed quality effects on animal variation were examined for each model parameter. Feed quality did not affect the asymptotic weight (A) of the WAD goats (DH, 48.1 kg; DL, 52.1 kg). The feed intake curve against u (=W/A) of DH, was more convex in approaching adult intake than the curve of DL. The feed efficiency parameter (AB) was lower for the L feed (DH, 0.262; DL, 0.172). The maturation rate (du/dt) of DL was lower than the rate of DH. There was a strong breed effect on A (S, 109.6 kg versus DH, 48.1 kg). The shape of the size-scaled feed intake curve was similar for S and DH, but its magnitude was at comparable u higher for S. At u = 1 the estimated scaled intake of S was 1.18 times the intake of DH. Group S converted their feed more efficiently than DH (AB for S: 0.291). There was no breed effect on (du/dt). Size-scaling of the time component dt by A <sup>0.27</SUP>in maturation rate (du/dt) resulted in a higher scaled maturation rate for S. It was concluded that in this specific case, growth traits of both breeds could be better compared on basis of normal age than scaled age.<p>In Chapter 3, breed and feed quality effects on body composition were studied. Cross- sectional slaughter data, that covered postnatal growth over a period of nearly 3 years, of 27 animals of group S, 31 of DH, and 31 of DL were used in the analysis. Water, protein and ash accretion were described against fatfree weight (FFW) by the simple allometric model. The allometric coefficients (b) showed a very consistent pattern among the groups, with minor effects of breed and feed. For all groups, FFW showed a decreasing water proportion toward maturity accompanied by higher protein and ash proportions. Fat deposition was described against FM by a diphasic allometric model. The extra phase improved the fit for S and the combined DH and DL group. Feed quality did not affect b in both phases, nor the FFW level at which the second phase started. S had a higher b than DH in the first phase, and the same in the second. S entered the second phase at a higher weight than DH, but this could be mainly attributed to their larger genetic size. It was concluded that the consistency in parameters of the diphasic allometric model among breed and nutrition groups supported the concept of distinct phases of fat growth.<p>In Chapter 4, effects on growth of lungs, heart, liver, kidneys, spleen, forestomachs, abomasum, duodenum, jejunum/ileum and the humerus bone were examined by both monophasic and multiphasic analysis of their growth relative to FFW. Cross-sectional data of the animals of Chapter 3 were used. Analyzed organs were consistently single- phased or diphasic in both breeds. The same consistency, except for heart, was observed among both feed quality groups. Only heart growth (for S and DH only), and abomasum and humerus were single-phased. The allometric coefficients of the diphasic organs indicated in their first phase a proportional increase of organ and thereafter a proportional decrease. The goats in DL developed a larger digestive tract, heart, kidneys and liver than goats of equal weight in DH. The breeds had very similar allometric coefficients, which differed only for the second phase of the lungs and jej./ileum, and for both phases of the liver. The main part of the breed variation was expressed by genetic size-dependent parameters. The consistently higher weight levels at transition for group S reflected the larger genetic size of the Saanen breed.<p>In Chapter 5, breed and feed quality effects on chest girth (CG), length of trunk (TL) and ulna (UL) were examined by monophasic and multiphasic analysis of their growth relative to W. Longitudinal data of 15 animals of group S, 12 of DH and 9 of DL were used. Allometric coefficients from the monophasic allometric model indicated proportional enlarging of CG, and decreasing of TL and UL. The diphasic model described CG better than a monophasic model. Allometric coefficients increased from the first (0.31) to the second phase (0.45), and were not affected by breed or feed. Breed estimates of transition zones between phases in CG could serve as indicators of genetic size. TL and UL were not fitted adequately by a diphasic model. However, separate analyses of preweaning and postweaning growth improved the overall monophasic fits in most cases. Monophasic residuals of UL showed systematic changes in postweaning growth patterns, and indicated the absence of epiphyseal closure in these castrated animals.<p>In Chapter 6, the results and conclusions of the foregoing chapters were integrated to evaluate genetic size measures that are accurate and practical in use. Different types of genetic size measures were assessed on basis of criteria outlined in Chapter 1. It was concluded that asymptotic weight (A) is not a suitable measure to describe genetic size of animals in normal animal production environments. Among body component-age relations, in particular bone lengths offer perspectives; mature bone lengths or characteristics from multiphasic analysis could be applied. However, size determination on basis of bone length can be complicated in castrated animals. Both simple allometry and multiphasic allometry provide information on genetic size. It is explained how the phenomenon of allometric transposition is related to genetic size differences. Transition zones in multiphasic allometry can be used as genetic size indicators. Multiphasic allometry in chest girth offers a possibility to determine genetic size from an external measure. It was concluded that, although limited in number, some measures offer perspectives for improvements that enable a better use of the genetic size-scaling theory. Prospects for use of genetic size measures at production level and experimental level were briefly discussed.<p>The following general conclusions were drawn:<br/>1. Genetic size measures are available that offer perspectives for a more accurate and practical determination of genetic size than a measure based on adult weight.<br/>2. Allometric growth patterns are very useful in early determination of genetic size. Allometric transposition, and transition in multiphasic allometry yield information on genetic size.<br/>3. Analysis of (multiphasic) allometry in body components should be based on fatfree weight as the explanatory variable.<br/>4. Size determination on basis of body dimensions is more complicated in castrated animals than intact animals.<br/>5. Equal degree of maturity in terms of proportions of genetic size is a more stable basis for comparison of genotypes than metabolic age.<br/>6. At equivalent developmental stages, allometric growth patterns were very similar for the Saanen breed and the WAD breed, the size-scaled feed intake of the Saanen goats was higher than intake of the WAD goats, and the efficiency in the conversion of feed into gain was higher in the observed developmental range.<br/>7. Use of the genetic size-scaling theory provides a powerful method for elimination of animal variation due to different stages of development and genetic size, and contributes to a better understanding of genotypical differences.
|Qualification||Doctor of Philosophy|
|Award date||19 Oct 1993|
|Place of Publication||S.l.|
|Publication status||Published - 1993|
- genetic variation