Sheep from the Soay, Finnish Landrace, Southdown and Oxford Down breeds were serially slaughtered at about 40, 50, 60 and 70% of their estimated mature body weight. Breed and sex comparisons were made at these degrees of maturity. Large differences were found in the weight of muscle, fat and bone, total muscle ranging from a mean of 3·3 kg in the Soay to 13·0 kg in the Oxford Down; the corresponding range for dissected fat was 1·2 to 9·2 kg.

Most of the breed and sex differences observed at the same degree of maturity disappeared when expressed as a percentage of body or carcass weight. Thus both sexes had almost the same body and carcass composition in terms of fat, muscle and bone percentage. There were breed differences in fat percentage, with the Soay significantly less fat than the other breeds. Total muscle as a percentage of body weight was remarkably close to 28·5% in all breeds in both sexes, and at all four stages of maturity.

Records from a selection flock of Galway ewes collected over a 6-yr period (1966 to 1971) were used to obtain estimates of the phenotypic and genetic parameters of ewe productivity. The number of lambs born (NLB) and weaned (NLW), the weight of lamb weaned (WLW), ewe body weight (EBW) and greasy fleece weight (GFL) were used as measures of productivity, mean values being 1·25, 0·88, 25·6 kg, 52·8 kg and 2·8 kg, respectively. Each ewe's annual performance was adjusted to a 4-yr-old ewe equivalent and expressed as a deviation from the appropriate flock × year subgroup mean; these deviations were combined into a single index value for each ewe for NLB, NLW and WLW. Records for 975 individual ewes representing 84 sires which had three or more progeny were used in a half sib analysis of variance.

Estimates of heritability were 0·18, 0·24 and 0·25 for NLB, NLW and WLW based on index record and 0·07, 0·16 and 0·30 respectively for 2-yr-old ewes. Heritability estimates of EBW and GFL were 0·73 and 0·52 respectively. The genetic correlations between EBW and NLB2, NLW2, WLW2 and GFL were 0·85, 0·42, 0·62 and 0·53 respectively; those between EBW and index records were slightly negative. Phenotypic correlations were generally lower than the corresponding genetic ones.

Ewes born as multiples were more productive than singles. They were 1·8 kg lighter (P < 0·01) at 18 mo and weaned 2·5 kg more lamb (P < 0·05) at 2 yr of age. The repeatability of NLB ranged from 0 to 0·2.

The susceptibility of whole and rolled cereal and legume seeds to microbial attack within the rumen of cows was examined by incubating seeds in nylon bags in the rumen for 24,48 and 72 hr. With whole, intact seeds the husked cereal grains were less susceptible than naked grains and legume seeds. Inter-varietal differences were observed with maize, oats and barley. Breaking the cereal grains before incubation considerably increased the losses of dry matter. Digestibility trials with non-lactating cows confirmed that barley, sorghum, wheat and oats were poorly digested when given whole, and considerable variation was recorded between individual cows in their ability to digest whole grains. Rolling the grains before feeding made the greatest improvement in digestibility with sorghum, wheat and barley and the least with oats.

The nutritional value of a pelleted fattening concentrate mixture, in which heat-sterilized poultry litter (PL) was included at rates of 0, 15, 25 and 35%, was studied. A digestion and nitrogen balance trial was carried out with rams, and a feeding trial was carried out with intact male cattle from the age of 8 mo to slaughter at 14 mo.

Digestibility of crude protein was related inversely to PL content of the diet. This decrease in digestibility was not accompanied by a decrease in the quality of the protein digested.

Gross and digestible energy contents of the diets were related inversely to their PL content. There were no significant differences in blood urea concentration. A concentrate mixture containing 1% urea but otherwise similar to the control, was included in the feeding trial.

There were no significant differences in rate of live-weight or carcass gain between treatments. The differences in degree of fatness of the carcasses tended to be related inversely to PL% in the diet, but they were small and reached significance only with the extreme treatments. Feed intake on PL-containing diets was higher than on the control diet, but feed conversion efficiency was impaired when the proportion of PL exceeded 25%. No significant correlations were found between total volatile fatty acids in rumen liquor and daily live-weight gain.

1. Hereford × British Friesian castrate male cattle, growing from about 80 to 500 kg, were given equal amounts of metabolizable energy (ME) from dried grass pellets or from a pelleted diet of barley and a protein supplement. Two animals on each diet were fed close to appetite (High) and two at a level mid-way between maintenance and appetite (Medium). The rations for all animals were reduced to approximately maintenance for 4 weeks on three occasions equally spaced throughout the experiment. Measurements of energy and nitrogen balance were made for each animal at intervals of 6 to 8 weeks. Total body-water content was estimated from D20 dilution at intervals of 110 kg live weight.

2. Values for the metabolizability of the gross energy of the dried grass and barley/protein diets were 0·55 and 0·67.

3. Metabolic heat production during growth was closely related to body weight to the power 0·75. Predicted basal metabolism was 395 kJ/kg 0·75 per 24 hr.

4. Estimates of the net efficiency of utilization of ME for growth were unaffected by small variations in the exponent of W used to describe the effect of body size on heat production. Both diets were utilized above maintenance with a net efficiency close to 0·62.

5. Nitrogen balance trials seriously overestimated nitrogen retention. Gains in fat-free body mass estimated from D2O dilution were combined with calorimetric measurements of energy retention to predict the relationship between energy retention and weight gain. The agreement between observed and predicted live-weight gain was good at the lower body weights and on the lower ration. For fatter animals eating the high ration at higher body weights, live-weight gains were less than predicted.

1. Three pure breeds, Aberdeen-Angus (AA), Ayrshire (AYR) and British Friesian (FR), and three crossbreds, Aberdeen-Angus × Ayrshire (AA × AYR), Aberdeen-Angus × British Friesian (AA × FR) and British Friesian × Ayrshire (FR × AYR) were studied. Each breed type was represented by 12 male castrates which were the progeny of at least seven sires in each case.

2. The purebred cattle of dairy type (FR and AYR) were faster growing (P < 0·001), heavier at slaughter (P < 0·001) and produced heavier carcasses (P < 0·001) than the pure beef breed. There were no significant differences between the breeds in the lean contents of their carcasses and there was no indication of any systematic breed effect on the distribution of lean within the carcass. The AA had carcasses with a higher fat content (P < 0·05), a lower bone content (P < 0·001) and a higher lean:bone ratio (P < 0·001) than the FR and AYR.

3. In general the values for performance and carcass data of the crosses between the three purebreeds fall mid-way between their parent values or do not differ significantly from this value. The hypothesis that crossbred cattle produce carcasses with higher muscle: bone ratios than purebred cattle is refuted by these data.

A technique for evaluating beef carcasses in terms of their retail value, has been applied to 132 half-carcasses from seven groups of cattle (Jersey, Hereford, Polled Hereford, Lincoln Red, Devon, South Devon and Charolais × Jersey) to test the general applicability of the method, in particular the regression equations for predicting retail value. Significant differences in retail value were found between the groups of animals, South Devon and Charolais × Jersey having a high retail value per kg side weight, and Jerseys a low one. There was a very high correlation between retail value per kg side weight and the proportion of muscle in the side. Relationships between retail value and proportion of fat in the side were also investigated. It is not possible to decide, from these data, whether the differences found in retail value are a true breed effect, or whether they are due solely to differences in composition.

Effects on cow reproductive performance, of cow age, parous state, body weight and body-weight change over the breeding season were examined using 6490 annual records from indigenous cows in Botswana.

Calving percentage rose from 69% at 2·5 yr of age to a maximum of 82% at 6 to 7 yr and declined in cows over 8 yr of age. Heifers and previously non-parous cows had a calving percentage of 85%; 8, 18 and 30% higher respectively than parous cows that had calved at the beginning, middle and end of the previous calving season. Cows weighing less than 300 kg at the beginning of the mating season achieved a 50% calving. This rose steadily to 85% as body weight increased to 430 kg.

Cows that lost weight over the mating period achieved a 67% calving, while gains of 20 kg raised this to 76%. Increased weight gains thereafter had little effect on calving percentage. In all cases, as percentages increased, days to conception decreased in a similar manner.

Under these range conditions in Botswana, animals least likely to achieve high calving percentages were young cows; cows which calved previously at the end or middle of the calving season; cows less than 310 kg in weight; and cows gaining less than 20 kg body weight during the mating season.

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.

A Piétrain boar was placed on each of six farms where the terminal sire was a Large White and the breeding females were either Large Whites or Large White × Landrace. Cooperators were requested to obtain 10 litters by each boar and record their performance to slaughter at 64 kg live weight. Farrowing rate to first service was relatively higher for sows mated with Large White boars but not significantly so. Pietrain-cross piglets were heavier at weaning (7·5%) but breed of boar did not influence other litter traits or age of progeny at slaughter. Pietrain crosses had higher carcass yields (2·4%), shorter carcasses (2·5%) and larger eye muscles in cross-section (10·6%) and in commercial evaluations relatively more were placed in the highest grade.

Carcasses of Piétrain-cross pigs slaughtered at 64 kg live weight and purchased by retail butchers were accompanied by questionnaires aimed at obtaining an appraisal of them relative to the butchers' normal purchase. Consumer reaction to loin chops from Pietrain crosses and contemporaries sired by the Large White was determined by panel—, household- and shop-testing respectively. Retail butchers showed a strong preference for Pietrain-cross carcasses on lean content but not shape and few considered that the muscle tissue had defects. In panel— and household tests participants showed a strong preference for Pietrain-cross chops on their appearance but broad equality for the two types of chops on organoleptic properties. In shop tests a preference was shown for Piétrain-cross chops over Large Whitecross chops and those normally stocked by the shop.

The optimum design of genetic controls is considered with respect to several factors, such as a fixed total expenditure, differing costs for males and females, different plans for retaining parents and choosing replacements, the value of keeping spares and the effects of mortality and infertility among individuals in the control.

The effects of mortality and infertility are important for these will tend to reduce the annual effective size and lower the optimum age for replacement of parents. Keeping individuals to a fixed age, without spares, will usually be preferable in practice to the lifetime retention of parents with spares, since spares will be at the expense of other breeding animals. It is shown that with the same total expenditure, the annual effective size of a control may be substantially increased by the appropriate choices among the various factors studied and methods are given for finding the optimum design.

Expressions are given for the variance of estimates of breed differences for direct and indirect comparisons among foreign breeds, using either purebreds or crossbreds. These expressions allow for planned genetic relationships among the groups tested.

An experiment was conducted to study the post-weaning response of young pigs to a flavouring substance (Firanor No. 3) designed to be incorporated into the sow's milk if added to her diet during lactation. During the first 3 weeks after weaning the average feed intake and growth rate of all pigs were 0·755 kg/day and 0·385 kg/day respectively. The average feed intake and growth rate of pigs weaned from sows given a flavoured diet during lactation and subsequently offered a flavoured weaner diet were 0·818 kg/day and 0·450 kg/day respectively, each significantly greater than the corresponding results for pigs which had no previous experience of the flavouring or which had access to flavoured feed in only one period.

Eighty pregnant ewes were housed in December in each of 2 yr approximately 14 weeks before lambing and offered grass silage. Four nutritional regimes were imposed during the final 7 weeks of pregnancy. They were (a) no concentrates given, and three other treatments each aiming to provide 25 kg concentrates on (b) a weekly ascending scale, (c) a level scale, or (d) a weekly descending scale. Concentrate feeding caused the anticipated improvement in performance particularly with respect to ewe live-weight loss and the live weight of multiple litters, but pattern of concentrate feeding appeared to have no significant effect on any performance parameter. It is concluded that a level pattern of concentrate feeding would be simpler and provide a more even distribution of feed among housed pregnant ewes, which vary in expected date of lambing, than the currently recommended ascending pattern of concentrate feeding.

The concentrations of maternal immunoglobulin in 2-day-old lambs born to Scottish Blackface ewes which had been mated when 6 to 7 mo old were significantly lower than in lambs born to these ewes a year later. But the mortality rates were almost identical. The concentrations in lambs born to the same ewes in successive years were significantly more highly correlated than the concentrations in lambs born to older females in successive years, examined in a previous study.