1. The heat production of four sows, approximately 17 months old at the start of the experiment, was measured at five ambient temperatures from 23° to 6°C. Food intake was controlled at different levels for different sows; one sow conceived during the experiment.

2. Heat production increased consistently as temperature decreased from 23° to 6°C; the relative effect on heat production of exposure to low temperatures was greater the lower the level of feeding.

3. Estimated values for whole body conductance decreased as ambient temperature decreased to minimum values of 73 to 77 kcal/m2. day. °C for three sows and 87 kcal/m2. day.°C for the fourth sow, which had a considerably thinner backfat than the others.

4. Estimated values for lower critical temperature varied from 10° to 20°C, with the lower values associated generally with the higher levels of feeding; nevertheless heat production was consistently lower at 23°C than at 18°C.

5. Differences in level of feeding were generally associated with differences in live weight, and it was difficult to obtain reliable estimates of metabolizable energy requirements for maintenance and of net efficiency of utilization of metabolizable energy above maintenance; however analysis on the basis of live weight0·75 produced values of 92 kcal/kg0·75 per day at 23°C and 106 kcal/kg0·75 per day at 18°C for maintenance and between 67 and 75% for the efficiency of utilization above maintenance.

The expected annual selection response is a function of selection intensity (i), generation interval (t) and precision (p) of breeding value estimation. These three parameters may, under certain conditions, be reduced to functions of one variable, y, the mean age of breeding animals at culling, which is a simple function of the annual replacement rate. When the number of breeding males and females are different, i and t become functions of two variables ym and yf, the mean ages of males and females at culling. The values of y (or of ym and yt) giving the maximum selection response are established for different domestic species: for individual selection, where the ratio i/t is to be maximized, for selection on a repeatable criterion connected to female reproduction (prolificacy, milk or egg production), where the maximum of ip/t is to be determined, and for male progeny-testing. The results show that, the more prolific a species is, the more rapid the renewal of breeding animals must be and that the renewal should be more rapid for males than for females, particularly in species of low prolificacy. When selecting on female performances which are repeated in time, the optimum renewal rate of females is lowered relative to individual selection on a criterion measured once. Numerical values have been calculated for different species, with given reproduction parameters for each one.

1. According to the Agricultural Research Council (1965) energy retention in cattle is predicted from metabolizable energy (ME) intake, the net availabilities of ME for maintenance (km) and for fattening (kf) and measurements made of fasting metabolism (F). The present experiments were designed to examine the validity of the use of F as a basis from which to predict energy retention.

2. Four British Friesian and four Aberdeen Angus steers were fed, from weaning to slaughter at about 450 kg, a barley-based, pelleted diet at two levels calculated to yield overall efficiencies of retention of ME of 20% and 10% respectively. Successive measurements were made of the energy balance of each animal at intervals of 4 to 8 weeks.

3. The metabolizability of the diet was measured for both sheep and cattle. In both species metabolizability was greater at the higher level of feeding. In sheep kf measured directly or estimated from metabolizability was 0·61.

4. Measured values for F in cattle agreed closely with values given by the Agricultural Research Council.

5. Basal metabolism in the growing animal (F') was predicted by extrapolation to zero intake of measurements made on animals in positive energy balance. Log F' during growth was proportional to log body weight0·73. Expressed in terms of the usual exponent of metabolic body size, F' was about 440 kJ/kg0·75 per 24 hr throughout growth. There were no major differences in F' attributable to breed or to level of food intake.

6. The results indicate that F is not a good basis from which to predict energy retention in steers.

A simplified Net Energy system for ruminants is described. It is based on the Metabolizable Energy system outlined by the Agricultural Research Council (1965) and enables a non-iterative approach to be used in the formulation of rations. The method is suitable for use in linear programming work and is illustrated, with appropriate tables, for growing cattle.

A line of mice, at its limit to selection for high body weight did not decline in performance over 11 generations of random mating, neither did it respond when selection was renewed. The experiment tested a method of improving body weight by a scheme which had earlier increased litter size under similar circumstances. The scheme was to derive partially inbred lines from the plateaued line, to select during inbreeding and, finally, to cross the best inbreds. Body weight was not increased, but the study allowed further examination of the residual genetic variance in the line.

During inbreeding, the inbred lines became clearly differentiated in body weight, proving that loci controlling body weight had not become fixed. There was also a significant response to selection for a lower body weight during inbreeding. The pattern of results suggested the segregation of recessive genes, detrimental to high body weight but which selection had become inefficient at removing. A genetic model compatible with the results accommodated several such recessives, perhaps as many as 10, each with an effect of about two-thirds of a standard deviation (or some equivalent combination of gene number and effect), and at frequencies of around 0·2. Nevertheless, the total improvement in body weight to be gained by their elimination was only half a gram, or less than 2 %. Thus, substantial genetic effects can occur at individual loci despite trivially low heritabilities and negligible potential gains.

Data from a random-bred population of mice were used to examine the sources of variation in litter size and 12-day body weight, and, the genetic relationships between these traits and post-weaning body weights and body-weight gain from 3 to 6 weeks of age (post-weaning gain). The genetic model included maternal genetic effects in addition to the usual direct (individual) genetic effects. Such maternal genetic effects may be important to an understanding of traits like 12-day body weight of mice, where the young depend to a large extent on the mother for early postnatal nutrition.

Analysis of litter size yielded a paternal half-sib heritability estimate of 0·34±0·19, while twice the daughter-dam regression was 0·23 ± 0·08. The repeatability of litter size was 0·51±0·07. The correlation between direct genetic effects on litter size and those on 3-, 6-, and 8- week body weights and post-weaning gain were –0·18,0·36,0-34 and 0·58 respectively. The correlation between direct genetic effects on litter size and 12-day weight was positive (0·41), while the genetic correlation between maternal effects on 12-day weight and direct effects on litter size was negative (–0·37).

Analysis of 12-day weight showed that direct and maternal genetic effects were important sources of variation, but there was a large negative correlation (-0·88) between these effects. The usual genetic correlations between 12-day weight and post-weaning body weights were positive while post-weaning gain yielded a negative estimate. However, there appears to be a general negative correlation between direct genetic and maternal genetic effects in this population, for all such correlations were negative except that between direct effects on 12-day weight and maternal effects on post-weaning gain.

A latin square design with three periods of 3 weeks was used to measure the dry-matter intake, live-weight gain and water intake of 12 British Friesian calves offered an all-concentrate diet ad libitumin wet form. The diets contained 30% (B), 20% (C) or 15% (D) dry matter. A further six calves were given the same diet in dry form (A) throughout the experimental period. Digestibility and nitrogen balance were measured on four calves on each treatment at the end of the experimental period. In addition, two Ayrshire calves, each fitted with a permanent abomasal cannula, were used to study the site of digestion of the feed when given in dry or wet form.

The mean dry-matter intakes of the calves given the dry and wet feeds were 75·5 and 80·8 g/kg W0·75 per day respectively and mean daily live-weight gains were 849 g for the calves on both types of feed. The mean dry-matter intakes of the calves given the wet diets were 80·9 (B), 81·2 (C) and 75·5 (D) g/kg W0·75 per day, and the live-weight gains (g/day) were 857 (B), 879 (C) and 812 (D). None of these differences were significant. The intake of water by the calves was significantly increased by reducing the dry-matter content of the diet. The form or dry-matter content of the diet offered had no effect on the digestibility of dry matter (76·1%), organic matter (77·9%), energy (75·8%) or crude protein (75·8%) or on nitrogen retention (32·5% of N intake).

The rumen was the main site of digestion for all Tour diets and an average of only 7% of dietary starch passed undigested through the abomasum.

Copper concentrations were determined in whole blood, plasma and caeruloplasmin and calculated for red cells and non-caeruloplasmin copper in plasma in 215 female sheep, 9 months old, belonging to the Scottish Blackface, Cheviot and Welsh Mountain breeds of sheep and their crosses. Copper determinations were also made on red cells and plasma, and calculated for whole blood, of a contemporary group of 107 female sheep of the same breeds at the age of 4 years. Further, plasma copper concentrations were determined for these 107 sheep from samples taken on several occasions some years earlier.

Within breed, sheep of type Hb B had significantly higher levels of copper than sheep of type Hb A in whole blood and most of the blood components. The biggest differences were found in plasma and were 19 ng/100 ml for one group and 37 Jig/100 ml for the other. Type Hb AB sheep were intermediate in copper level. The differences in plasma copper level associated with Hb type were present and of similar magnitude at different ages, seasons and average flock levels of copper.

After adjustment for the effects of Hb type, breed differences in copper levels were significant for whole blood and all components but much less pronounced for red cells than for plasma. Copper levels in plasma and red cells were only moderately correlated (r = 0·23–0·40).

Combined evidence suggests the existence of a gene with a marked effect on copper level which is linked to the locus for Hb type.

The reproductive performance of a lowland flock of Clun Forest ewes is reported. During the period from 1955 to 1966, over 1300 ewes entered the flock and more than 4800 lambing records were obtained. The effects of culling and death on the records available were investigated and it was found that the reduction of the material caused by death was random, while that caused by culling was related to previous performance.

Barrenness decreased with age, and for ewes aged 1, 2 and more than 2 years was 44, 7 and 5%. Average litter sizes (ALS) over the same ages were 1·15, 1·55 and 1·75. Accordingly the number of lambs born per ewe mated and present at lambing also increased with age of ewe, and values for the same ages were 0·64, 1·44 and 1·68.

An analysis of over 170 ram mating groups, consisting of ewes older than 1 year, revealed no differences between rams on the ALS of their mates. The results of a similar smaller analysis of barrenness among 1-year-old ewes also showed no evidence of a ram effect.

Phenotypic and genetic parameters for reproductive performance in a flock of Clun Forest ewes have been estimated. Average litter size (ALS) appeared to be more repeatable than the number of lambs born per ewe mated and present at lambing (NLBEM), and the estimates obtained by regression (0·15 and 0·14) were slightly higher than those from the analyses of variance (0·12 and 0·09). The repeatability of barrenness was very low (0·05). It was concluded that little scope existed for the improvement of reproductive performance from a regular policy of culling.

Estimates of heritability by analysis of variance and a method for all-or-none traits were 0·12 and 0·11 for ALS, and 0·15 and 0·10 for NLBEM, whereas barrenness was less heritable (0·07). An estimate of 016 was found for the heritability of ALS performance over two years. The phenotypic correlation between body weight and reproductive performance were positive yet small (ALS:+0·16; NLBEM:+0·10), while at the genetic level ALS was more strongly related to body weight (+0·21) than NLBEM (-0·04). The genetic regressions indicated that an increase of 5 lambs per 100 ewes lambing would accompany genetic gain of 5 kg in ewe body weight, while an increase of 1 lamb per ewe lambing would lead to a correlated response in body weight of 5 kg.

Selection for ALS on the basis of dams' performance, within the flock studied, would be expected to yield a rate of progress of 2% per annum.

1. Observations were conducted to ascertain the social dominance structure in two small groups of castrated male Corriedale sheep. In these and a third group, the order of the sheep past a fixed point was recorded, when they were moving either voluntarily or under force. Records were also made of the way in which the sheep distributed themselves spatially in a confined area, relative to one or two men simulating yard work. In a fourth group divided into two sub-groups, the social dominance structure was determined before and after the subgroups were mixed, to assess the effect of mixing.

2. The small groups showed a distinct social dominance hierarchy of a linear type, similar to that found in cattle and pigs. The dominance status of individual animals was correlated with their body weight, wither height and height at hocks. Dominance status within a subgroup remained stable following the mixing of sub-groups.

3. The sheep adopted consistent positions in forced movement, and in relation to one or two men simulating yard work. There was no consistency in voluntary movement. Dominant animals did not lead the forced movement, were usually some distance from man, and were widely scattered from each other.

4. Dominance status was closely related to positional behaviour.

The concentrations of maternal immunoglobulin in 2-day-old lambs varied widely. They were significantly correlated with the lengths of pregnancy and the serum protein concentrations of the adult female sheep (ewes), but not with the colostrum immunoglobulin concentrations, nor with the weights or ratios of weights of the lambs and ewes. The mean concentrations were as high in Merino as in Blackface lambs.

Although there was some evidence that colostrum production was higher in ewes with twins than in ewes with single lambs, the immunoglobulin concentrations in the lambs fell significantly as litter size increased. They were significantly higher in male than in female twins, and in twins from litters containing both sexes than in twins from litters of one sex, but the difference between single males and females was not significant. Nor was the difference between the first and second born of each pair of twins. Twins and triplets born to ewes aged 7 years or more had particularly low concentrations. No other effects of the age of the ewe were found. The concentrations fell significantly as the date of lambing during the season advanced, and there were significant annual variations which could not be attributed to the weather. The correlation coefficients for the concentrations in lambs born to the same ewe in successive years were 0·309 and 0·298 (P < 0·001), respectively, for Merino and Blackface lambs.

The mean concentration in lambs which died subsequently from infections was significantly lower than in the surviving lambs, but many lambs with very low concentrations survived.

Competition for oat grain between sheep grazing on scant, poor quality pasture was studied in three flocks of sheep. A daily feed of 450 g per head was fed in troughs with the space per sheep varying from 4 to > 50 cm. Records were taken of sheep behaviour during the first 3 min of feeding.

There was no fighting in the competition for feed, but dominance of individuals and of groups of sheep occurred. With abundant trough space, certain sheep led the way to the feed trough and were termed leaders, but ‘leadership’ was a characteristic unrelated to competitiveness during feeding. With restricted trough space most sheep raced to the feed trough at feeding time. The rate of disturbance of the sheep from the feed trough increased with decreased space per sheep, particularly below 12 cm/sheep. As the rate of disturbance increased a progressively greater proportion of sheep stopped competing for feed and became non-feeders. Neither the rate of disturbance of the sheep nor the proportion not feeding was related to body weight, but these were largely individual characteristics of sheep.

In a flock of mixed age Merino wethers, the 1-year-old and 7-yearold sheep were the least competitive, while in a flock of wether sheep of seven different breeds but of similar age, Merinos were least competitive.

From these experiments it is suggested that a minimum trough space of 16 cm be allowed when daily grain supplements are fed to grazing sheep.

The attachment between the ewe and her lamb, as assessed by estimating the distance between ewe and lamb and observing their behaviour, was studied for the 4 weeks after birth. The ewe and lamb kept in close contact throughout this period. They were within 10 metres of each other in 56% of all observations; only in 6% of all observations were they more than 50 metres from each other. With increasing age of lamb, the ewe spent more time grazing while the lamb also spent more time grazing and playing and less time sucking. The lambs showed independent forms of behaviour such as the formation of groups as they got older. The distinct dependent behaviour traits were that if bothewe and lamb were lying or walking, then they were usually close together.

The mean distance between ewes and lambs remained constant for dependent forms of behaviour but gradually increased with age for independent forms of behaviour.

Body weight was studied from birth to 5½ years of age and six linear measures of body size from birth to 4¼ years for sheep of five breeds, Scottish Blackface, Cheviot, Welsh Mountain, Lincoln Longwool and Southdown, and of crosses of these breeds with each other and with the Tasmanian Merino, but not in all possible combinations. The sheep were run as a single flock. Numbers ranged from 753 at birth to 150 at the end of the experimental period.

There were marked differences among the breeds and crosses in all aspects of body size and some differences in rate of maturity. They also differed in conformation independently of body weight. Crosses of the hill breeds with the Lincoln and probably with the Southdown were heavier than the average of the parental breeds and larger in some linear body measurements from about weaning onwards. The weights of crosses of Blackface and Cheviot with Lincoln eventually exceeded those of the heavier parent of the cross. For the crosses among the three hill breeds only the Blackface Cheviot cross showed significant deviations from mid-parent values for weight and size.

Maternal effects were important for all traits studied from birth to weaning and for weight up to a year old. In relation to mature size, Welsh dams produced the biggest and Lincoln the smallest lambs. The data did not permit Southdown maternal effects to be estimated. The effects of birth type and rearing were apparent for late maturing body parts up to a year old and for weight up to 3 years. Males, measured only from birth to weaning, were heavier and generally larger than females.

Residual correlations between the various measures of size varied from 0·1 to 0·7.

Digestibility studies were carried out to establish the inter-relationship between crude fibre and total lipid digestibility in the presence of added calcium or non-ionic detergents. Three levels of calcium, corresponding to daily intakes of 2·3, 4·8 and 22·9 g per sheep, were used in a predominantly ground barley diet (65 to 73%) to which was added 8% beef tallow or maize oil. Two non-ionic detergents were used at 0·1% of the diet: nonyl phenol ethylene oxide and fatty alcohol combined with 3 molecules of ethylene oxide.

With a daily intake of 2·3 or 4·8 g calcium per sheep, supplementation with fat resulted in a reduced fibre digestibility. When the intake was increased to 22·9 g, there was a significant improvement (P<0·05) only for the maize-oil-supplemented diet. Increasing calcium intake, associated with increased calcium digestibility, significantly (P<0·05) depressed total lipid digestibility. Intake of 4·8 g calcium per day was associated with the highest calcium digestibility of 33% digestibility units and the total lipid digestibility of 94%. Maize oil was better digested than beef tallow with the lower level of calcium; the position was reversed with the higher level of calcium supplementation. No advantages were found for crude fibre and total lipid digestibility when detergents were added.

A possible mechanism by which the physical wetting of dietary lipids causes decreased digestibility of the fibre component is discussed.

In three experiments housed Greyface (Border Leicester × Scottish Blackface) ewes of mean live weight 68 kg received 0·91 kg hay plus 0·91 kg molassed sugar beet pulp daily during the second half of pregnancy and in the first 3 weeks of lactation. Comparisons were made with additions of crude protein given as urea or groundnut cake either during the whole period or only during early lactation. In one experiment, increasing the daily intake of digestible crude protein from 88 to about 140 g had no effect on the weight of the lamb at birth or at 3 weeks. In the other two experiments, in which the increases were from 58·70 g to 131·161 g or from 80 to about 125 g digestible crude protein per day there were highly significant increases in the weight of the lamb at birth (+0·43 and +0·31 kg) and 3 weeks (+1·44 and +1·46 kg). Introduction of either urea or groundnut cake to the diet on the day after lambing significantly increased the the 3-week weights of the lambs by amounts which were comparable to those found when urea had also been given during pregnancy. Supplementation with either urea or groundnut cake did not significantly increase ewe live weight.

Concentrations of urea in the blood of the ewes were highly significantly increased by either urea or groundnut supplementation. There were indications that a concentration below 10 mg/100 ml in the blood of the ewes in late pregnancy or early lactation was associated with a reduction in lamb growth rates.

Over a 15-year period nearly 2000 young females were exposed to the ram at 6 to 9 months of age. These Clun Forest females were born between January and April in the Cockle Park experimental flock, and mated between October and December of the same year. On average nearly 42% of those present at lambing did not produce a lamb. Analyses are presented of the reproductive efficiency of these animals and of some of the possible causes of their failure to lamb. Observations in 10 of these years suggested that only 12% had not been mated and might therefore be assumed to be prepuberal. Age at first oestrus for those animals achieving puberty in their first autumn was inversely related to date of birth. The number which lambed increased by 1·1% per kg increase in live weight before mating.

In 3 years simple nutritional and housing treatments were applied to groups of female lambs. Feeding a cereal supplement to animals at pasture before, during and after mating, and housing 1 to 2 weeks after mating, appeared to increase the percentage which lambed. It is suggested that the effect was to reduce the amount of early embryo death caused by nutritional and climatic stresses on the normal grazing animal.

Cobs and pellets which differed in particle size (modulus of fineness, MF) were prepared from dried perennial ryegrass and tall fescue and given to sheep. The MF values of the feeds were 2·45,1·09,0·81 and 0·74 for perennial ryegrass and 2·28,0·84,0·67 and 0·57 for tall fescue. Organic matter digestibility was similar for the two grass species when measured in vivo, but when measured in vitro values for perennial ryegrass were about 3 units higher than those for tall fescue. Voluntary intake and live-weight gain were higher for sheep given tall fescue, the difference in live-weight gain being just significant at P < 0·05. The response to change in particle size was similar for the two grass species. Intake was lowest and digestibility highest for the feeds with the highest MF, but differences between the three finely-ground forms of each species were not significant. There was a tendency for the intake of digestible organic matter to be lower for the feeds with lowest MF than for those with intermediate values for MF. The rate of passage of undigested particles was slower for cobs with the highest MF than for the other feeds, which did not differ significantly one from another.

Twenty-seven North Country Cheviot ewes, each carrying twin foetuses and having a mean body weight at 6 weeks prior to parturition of 81 kg were individually penned and offered a constant daily intake of 16·75 MJ metabolizable energy during the last 6 weeks of gestation. At parturition the ewes were allocated to one of three dried grass/concentrate diets containing (1) 10·3, (2) 13·6 or (3) 16·9 % crude protein (CP) and 10 MJ metabolizable energy (ME) per kg. The mean daily CP intakes for diets 1 to 3 were 273, 340 and 415 g respectively and the mean daily ME intake was 25 MJ. Within each level of dietary CP intake the lambs were weaned at either 25, 35 or 41 days of age.

There were no significant diet × stage of weaning interactions in milk yield or ewe body-weight change during lactation. The mean daily levels of milk production were 2·4, 2·9 and 3·1 kg for diets 1 to 3 respectively. The corresponding values for ewe body-weight loss were 118, 170 and 265 g per day. When the diet contained 13·8% CP the incremental increase in milk production per unit increase in protein intake was 4·2 g/g. This gave an incremental increase in lamb growth rate of 0·79 g/g and was equivalent to the associated incremental increase in ewe body-weight loss.

A reduction in the concentration of CP in the ewes' diet when the lambs are 3 weeks of age may be a useful practical procedure for decreasing milk production and for reducing the body-weight loss of the ewe in a frequent breeding system.