No abstract

1. Some criticisms of the classical approach to constructing a selection index are advanced. This approach (the economic model) is contrasted with an alternative one (the biological model) which is derived from the concept of the biological efficiency of production of lean tissues. The selection objective which is proposed from the biological model is the improvement of lean tissue feed conversion. The most important single means of achieving this is increasing the lean tissue growth rate. 2. The approaches were compared in three studies: (a) the effects of errors in economic and genetic parameters of the economic index; (b) phenotypic relationships between biological objectives and other measures of performance; (c) a theoretical model based on the utilization of metabolizable energy. 3. In a fixed situation the results were similar for both models, but the main advantage of the biological model is that it gives good indications of what happens, and what selection policies should be, in different situations. In addition, lean tissue feed conversion and lean tissue growth rate are closely related to concepts widely applied in biology, and their use as selection objectives should be helpful in enabling workers in many disciplines to make use of each other's knowledge.

1. From a review of the literature it has been shown that there are two opposing views regarding the best method of interpreting growth data, which arise from conflicting opinions as to the role of fat deposition in the growth of the animal.2. Data of McMeekan and Palsson and Verges have been re-analysed and their own results are compared with results obtained when the effects of variation in fat content are eliminated.3. No evidence has been found of any effect of plane of nutrition on the total weights of bone and muscle relative to the weight of bone plus muscle together.4. The weight of bone plus muscle in the head and neck was increased relative to the total weight of bone plus muscle during periods of restricted nutrition. Apart from this there was no clear evidence of a relationship between the order of maturity of the joints and their relative retardation of development.5. Huxley's allometry equation was found appropriate for standardising the measurements, and the exponent was taken as a numerical expression of the relative maturity of each tissue or part.

A rapid method for the determination of lipid is described in which extraction of the lipid from the sample is carried out in a sealed tube by using a chloroformwatermethanol solvent system. The weight of lipid in a volumetric sample of the chloroformlipid solution is determined after evaporation of the chloroform.Estimates of precision give coefficients of variation of 0.26 per cent. for recoveries of pure lipid, 0.56 per cent. for muscle samples and 0.89 per cent. for fat tissues. Comparison of fatty-acid recoveries in the extracts from chloroformwatermethanol extraction and from Soxhlet extraction with diethyl ether shows that the chloroformwatermethanol system is, in all the instances examined, equal or superior to the Soxhlet method in efficiency.The time required for one determination is about 1 hour, but an output of sixty determinations per operator per working day can be achieved without difficulty. The method is recommended as a routine laboratory procedure, particularly for materials that contain a high proportion of lipid in association with protein.

Two groups of weaned sheep weighing about 30 kg, one born in March and the other in September, were each divided into two and given ad libitum one of two pelleted diets, ruminant diet A or ruminant diet AA6. The March-born sheep commenced experiment in July and those born in September in January. The experiment continued for 4£ years. At intervals sheep were killed and the fat protein and ash contents of their digesta-free bodies determined.The voluntary intake of feed showed a seasonal periodicity with minima in the winter and maxima in summer. The amplitude was 30 % in the 1st year and in subsequent years averaged 13%. Those given the higher quality diet (AA6) consumed slightly less than those given the poorer one.Mean daily feed intake averaged over 6-month periods from January to July and July to January was invariant with age during 4 years of observation. In this time the sheep increased in weight from about 30 kg to about 130 kg. There were, however, considerable differences between individual sheep in the amount of feed they habitually consumed.The body weight of the sheep increased and eventually plateaued. The asymptotic weight defined as A in the equation W = A -Be~k t , where W is weight at time t and B and k are constants, was related to the mean daily feed intake averaged over 6 months; mean daily feed intake measured over 6 months was proportional to a fractional power of body weight indistinguishable from 0-75, the interspecies power to which metabolism is proportional. Growth of wool during successive 6-month periods did not vary with age of animal but differed significantly between animals.Fasting metabolism of the sheep was 316 kJ/kg W 0 ' 76 for wethers and 336 kJ for rams.Analysis of the bodily composition data showed that over a range of digesta-free body weight from 46 to 130 kg it was not possible, on statistical grounds, to distinguish between linear relationships between body weight and its fat, protein, ash and water content, and allometric ones. The linear relationships had marginally smaller residual standard deviations and the regression coefficients show that the gain of the empty body in these sheep consisted of 68 % lipid, 8 % protein, 1 % ash and 24 % water. The lipid in the carcass accounted for 88 % of the total lipid gain and half the accretion of protein and ash was in the carcass. These results confirm those of Searle, Graham & O'Callaghan (1972) based on tritium dilution which showed that post-puberal growth in sheep is of constant composition.The results of the metabolic studies are shown to be consistent with the growth studies.Growth to maturity, as affected by different hypotheses related to the determinants of maintenance energy expenditure and the regulation of appetite, was examined algebraically. It is shown that mature weight is the rate of feed intake divided by the rate of maintenance metabolism per unit weight, and for defined feeding systems the rate constant for the approach to mature weight is the rate of maintenance metabolism divided by the feed equivalent ...