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Twenty-two unmated female Ayrshire twin cattle, that had initially been maintained for prolonged periods on one of six constant feeding levels until an equilibrium weight was attained, were subsequently moved up to higher constant feeding levels including ad libitum. In all, results were obtained for 44 equilibrium periods mostly of 96 weeks duration.For controlled feeding levels, the log-log,. regression of equilibrium body weight on food intake, within animals, was 0·999 (s.e. 0·045). For all results, including mature equilibria, the within-animal regression was 1–014. There was thus no systematic change in an individual's equilibrium maintenance requirement per kg body weight in the range from 25%, to 100%, mature. Efficiency of food utilization for equilibrium maintenance was found to be independent of age also, except for a small increase at advanced ages beyond 8 to 9 years.There were significant differences between animals in equilibrium maintenance efficiency, the genetic coefficient of variation being 6·4%. The most striking result, however, was a within-animal repeatability of 0·7, which meant that almost the same efficiency was re-attained when, after a prolonged period of many years on one constant feeding level, the same animal was allowed to re-establish a new equilibrium on a higher level.Following prolonged periods of food restriction, the animals showed remarkable capacity to recover in body weight even at very advanced ages, but nevertheless substantial stunting of mature weight did occur.

The relationship of efficiency to weight and feeding rate has been studied in 38 twin calves from 2 to 18 weeks of age. Following birth or weaning heavier animals were more efficient, but tended to be slightly less efficient when fully established on a diet. On balance, they possessed an advantage.Heavier twins and heavier pairs were also on average more efficient. The increase in mean efficiency for contemporary animals was about 1% for each 1 lb. superiority in weight over the 4-month period. These results contrast with the well-known decline in efficiency as animals increase in weight. Different pairs declined in efficiency with increasing live weight at distinctly different rates judged by the resemblance of twin and co-twin, but evidence for genetic control was slight.More efficient animals were associated with a relatively more rapid decline in efficiency, leading to a general convergence in efficiency as animals grew heavier.The relationship of efficiency to feeding rate was very similar to the relationship with weight.

Efficiency of food utilization from 2 to 18 weeks of age has been examined in 12 MZ and 7 DZ pairs of twin calves fed standard rations. Mean efficiency reached a maximum at about 6 weeks of age; weaning brought about a rapid decline followed by a recovery. Animals became more alike in efficiency as their ages increased.Those animals least efficient shortly after birth were most adversely affected by weaning, although they tended to be relatively more efficient when fully established on either milk or solid food.Relative to the fairly closely replicated age-efficiency trends of identical twins, different pairs followed different trends. These trends, which primarily reflected the variable success with which the calves adapted to new diets, were subject to some genetic control.

Since the year 1900 the number of Border Leicesters has been fairly stable, between 3,000 and 5,000 females being registered each year. The number of flocks, however, has shown a fairly steady increase from 200 to over 600. There has been a decline in the number of ewes registered per flock from 16 to about 6 to 8. The important flocks would seem to number 15 to 20; only a few flocks persist for any appreciable length of time. Generation intervals are short, being only about 2 years on the sire side and 3½ years on the female side.As with other breeds, relatively few rams have an importance out of proportion to their numbers—14 rams and 2 ewes with direct coefficients of relationship greater than 5% were found. Inbreeding per generation was only 0·32%, and there was no evidence of strain effects.

Twenty-two Ayrshire twin female cattle were kept for up to seven years of age on six equally spaced levels of constant food intake, ranging from 35 to 140 lb per week of a complete pelleted diet. Analysis was confined to the period between 5 and 7 years when growth had practically ceased and equilibrium maintenance requirements could be determined.Equilibrium weight maintained was directly proportional to the level of constant food intake.This relationship obtained by controlling food intake was significantly different from the 0·73 power relationship that applies across species. In establishing the present relationship, systematic genetic differences were excluded, and this is probably the main condition required for weight eventually maintained to be proportional to food intake. In contrast, the well-established proportionality of food intake to metabolic weight is essentially based on a species comparison and is thus due almost entirely to genetic differences.A mathematical model is given which combines (1) the genetic relationship of proportionality of voluntary food intake to mature metabolic weight and (2) the nutritional relationship of proportionality of equilibrium weight maintained to level of constant food intake. The model is then used to partition variation into systematic and random components.The experiment provides some indication of the extent of individual variation in efficiency of maintenance within breeds, the amount which is genetic, and the repeatability of this efficiency from age to age. Some information is also provided on variation in voluntary food intake, mature weight maintained and efficiency of maintenance on ad libitum feeding.Methods of estimating efficiency of maintenance using ‘feed and weigh’ techniques, selection for efficiency of maintenance, the correlated response in mature weight, and selection for relative food capacity are briefly discussed.

Individual variation in the triple relationship of food intake, live weight and gain was examined in 40 twin calves. Appetite largely determined food intake.Regression analyses of fortnightly gain on protein and starch equivalent and mean weight are presented. Gain per lb. of P.E. or S.E. showed real but irregular changes from fortnight to fortnight. The food differential between heavier and lighter animals decreased during adaptation to new surroundings and diets.

1. It is possible for a breed of cattle to persist and to expand slowly, despite the existence of a genetic defect which prevents 50% of the calves born being used for breeding. Moreover, it is possible to exert a certain amount of selection in such a breed. The Dexter breed provides an example.2. Before 1924, about 64% of entrants to the breed were from inspected animals. Between 1924 and 1939 the breed contracted. From 1940 to 1947 the breed expanded.3. During the period of decline almost every Dexter-type female born was registered and raised. The wastage rate and age distribution of the cows are similar in Dexters to those of other breeds. There would appear to have been comparatively little breeding from Kerry types. The average registered reproductive rate of Dexter-type females was about 0·8.4. While it was not possible to examine the breed structure in the period of expansion, this was probably accomplished by lengthening the average productive life of the female, and possibly also by breeding from Kerry types.5. The possibility of exercising selection in Dexters is shown to exist. Only about 13% of the females registered are used for producing registered bulls. Of the total bull crop, only about 7% are used to produce registered females, but the bulls selected have different numbers of offspring. The use of bulls selected after progeny testing is hardly feasible in Dexters. The increased numbers of females necessary in order to produce an adequate number of heifers, renders this technique almost impossible.6. There has been little inbreeding in Dexters. This is possibly due to deliberate policy, as some of the breeders associate inbreeding with the production of monstrous calves.7. It would, therefore, appear that the damaging effect of a lethal in a breed could be overemphasized.

The effects of birth weight, maternal age and parity on survival of single lambs have been studied in a Blackface and a Welsh Mountain flock.In both flocks the maximum survival rate was found to occur among lambs with birth weights just above the mean. Mortality increased as lambs' birth weight increased or decreased from the optimum, but was especially heavy at the lower extreme of the range of birth weights.Mortality decreased with age of dam to 14·3% for lambs from 4- to 6-year-old Blackface ewes and to 9·4% for lambs of 3- to 4-year-old Welsh ewes. Ewes having their first lambs gave mortality rates twice as great as for the mature ewes in the same flocks. The lower chances of survival of first born lambs irrespective of dam's age accounts for the apparent trend of mortality with maternal age in the Blackface.Changes in mortality rates associated with variation in lamb's birth weight and with the age structure of the ewe flock were estimated. Possible means of improvement of survival rate are discussed.

A study has been made of the reactions of three breeds of sheep (Blackface, Cheviot and Wiltshire) to four different environments. Evidence was obtained that these breeds react differently to these different environments. No changes in rank occurred but the Blackfaces grew relatively faster on a good plane of nutrition than the other two breeds. Their wool production on a sample area was also relatively greater, due primarily to an increase in diameter of the non-medullated fibres. These differences indicate fundamental physiological differences in reaction to environment.

A genetico-nutritional experiment is described in which 22 Ayrshire twin cattle, both one-egg and two-egg, were reared and maintained for up to seven years on a standard all-pelleted ration. Six equally spaced constant levels of food intake ranging from 35 1b. to 140 1b. per week were used. Each twin was reared on nearly ad libitum feeding until it had grown sufficiently to be able to consume its pre-assigned level of constant food intake. Thereafter its weekly food intake remained unchanged until the end of the experiment. The number of animals on each of the six feeding levels varied from two to six.On each constant level of food intake, animals continued to grow fairly rapidly for two or three years, but thereafter their growth tended to slow down and usually an equilibrium weight was eventually attained. The resultant mean weight curves constituted an apparently simple, regular and systematic family with each curve eventually separated from the next by a relatively constant weight difference.Coefficients of variation were about 3% for unrelated animals strictly conforming to the design, but increased at later ages to around 6% when animals with different pre-treatment were included. Coefficients of variation, however, did not change over the ascending series of feeding levels.The advantages of the design for exploring the triple relationship of gain, weight and food intake are outlined. The potential use of such results as an empirical feeding scale and the possibility of obtaining paths of maximum efficiency from such a controlled experiment are described.

The effects of maternal age and birth weight on survival of twin lambs has been studied in a Blackface and a Welsh Mountain flock. The time of death was also investigated for both singles and twins.Twin mortality was higher than mortality of singles, mainly due to the lower average birth weight of twins. Twin mortality declined with increasing birth weight although it showed a tendency to increase at the top of the scale. Weight for weight, mortality was similar for singles and twins. Twin mortality declined with increasing age of ewe in a very similar pattern to single mortality.Lamb mortality was heavy at birth and during the first 14 days of life with only 30% of the mortality occurring after this age. Approximately 12% of the dead lambs were stillborn. Among singles a similar proportion died as a result of difficult births, but this cause of mortality was rare in twins.Stillbirth mortality rates were very high for very small birth weights and declined with increasing weight of lamb. Difficult births, on the other hand, increased with increasing birth weight. The association of birth weight with mortality was particularly important in the first 14 days of life but thereafter its significance was less.Although mortality in lambs of younger ewes was heavier than that of older ewes, similar causes of death operated. Younger ewes, however, having lower birth weights tended to have more stillbirths but few losses due to difficult lambings.

In an experiment to investigate the interrelationships of genetics and nutrition in the growth of cattle up to 2 years of age, five monozygotic and six dizygotic Ayrshire twin heifers were reared under close nutritional control on an all cubed diet.Two control systems were used. Some animals were fed similar amounts of food, and differences in growth rate and efficiency examined while others were made to grow alike and differences in food consumption and efficiency examined. Overall, similarly fed animals had similar growth rates and efficiencies and similarly grown animals similar food intakes and efficiencies. Variation in growth, food consumption and efficiency was much less than in an ad lib. feeding system.

An experiment to investigate the differential response of lambs of various breeds and crosses to different planes of nutrition is described.Two groups of twin lambs were used comprising Blackfaces and their crosses ( x Lincoln, Wiltshire and Border Leicester rams) and Welsh and their osses ( x Suffolk and Wiltshire rams).From October to January, each cross was divided between high and low planes of nutrition. Approximately half of the twin pairs were split and the remaining pairs distributed between the environments. Significant differences were found between the crosses and twin pairs of the same cross for body weight, body measurements and wool production but not for various blood characters. For almost all characters studied, plane differences were found. There was, however, only one significant (P < 1%) cross-plane interaction—for non-protein nitrogen concentration in the blood.At the end of January, each plane was subdivided to produce high-high, high-low, low-high and low-low groups. The lambs remained in these groups until May. As before, differences between crosses were found for body weights and measurements, and wool characters but not for most blood characters. The majority of characters were affected by plane of nutrition. Cross-plane interactions were not found except in antibody response toErisipelothrix rhusiopathiavaccine (P < 5%).At the end of May the high-high group were slaughtered and the remaining three groups fattened on grass until each reached the same average weight as the high-high group. Carcass measurements showed that all the crosses shared a striking ability to recover from previous poor nutritional treatments. One significant (P < 5%) interaction of cross and plane was found in weight of cannon bone.The most characteristic feature of the results has been the similarity of response of the different genotypes to the different nutritional environments. The few interactions found to be statistically significant by conventional methods are difficult to interpret because of the many tests of significance carried out in the analyses.

The effects of inbreeding of the sow and plane of nutrition after mating on litter size at 28 days were investigated.Outbred Large White sows were mated at first heat after weaning to inbred boars, and inbred sows to outbred boars, so that both groups of sows carried the same type of embryo. The sows were then placed at random on either high or low planes of nutrition until slaughter 28 days later, when their corpora lutea and number of embryos were counted.Litter size was significantly higher in outbred than in inbred sows by 2·52 ± 0·93 pigs. This resulted from 2·89 ± 0·94 more ova shed and 0·37 ± 0·93 more embryos dying. The general conclusion is that inbreeding of the sow reduces early litter size mainly by depressing ovulation rate.The different planes of nutrition produced negligible differences in within-litter mortality and, therefore, on litter size, but resulted in a marked difference in conception rate. All animals on the high plane were pregnant, whereas on the low plane 25% were empty, this difference being highly significant.Age of the sow, but not her weight, was found to be significantly correlated with number of corpora lutea (r = 0·39). The latter had a correlation with litter size of 0·55 and with mortality of 0·45. The correlation between litter size and mortality was −0·50.Parity increased and inbreeding reduced the length of the uterus which was not, however, significantly correlated with litter size.

Natural scrapie can develop in sheep at any age later than about 18 months old. In order to formulate a realistic policy for experimental investigation of the disease as it occurs naturally, it is necessary to understand what factors might affect the incidence in sheep of different ages. Any attempt to use data from commercial sheep flocks to draw conclusions about the age distribution of scrapie cases must make allowance for the fact that the population at risk is being culled on commercial criteria and incurs losses by death from various causes.