Known genetic loci that affect metric traits may be useful in livestock improvement. Their value depends on the proportion (R) of the total additive genetic variation due to the known loci relative to the heritability of the trait concerned and on the form of selection practised. When normal selection is effective, further information on known loci can add only a little to the rate of improvement. But if normal selection is not very effective, as for characters of low heritability, or if indirect selection on relatives must be used (as for sex-limited or carcass traits) then known loci may add significantly to the rate of improvement possible.

Sampling errors in the estimated effects and in the proportion (R) may cause selection effort to be misdirected and may even lead to losses rather than gains in improvement. Such errors are most likely to occur when the heritability of the character is low.

Reports on several loci with large effects in the various farm species have been summarised, but the evidence is often inconsistent and contradictory. At present, there appear to be no loci that could be used with confidence in the improvement of economic traits in farm animals.

From the published data summarised in Table 1, it was calculated that, at an average degree of feed restriction of 15·8 %, daily weight gain decreased by 12·50% (or 0·78% for each 1% restriction), feed conversion efficiency improved by 3·96% (or 0·31% for each 1% restriction) and backfat thickness diminished by 7·63% (or 0·53% for each 1% restriction).

Equations were calculated to show the degree to which daily weight gain, feed conversion efficiency and backfat thickness change with increasing feed restriction (Figures 1, 2 and 3).

As the degree of feed restriction increases, the decrease in daily weight gain becomes relatively greater and the decrease in backfat thickness relatively smaller. Efficiency of feed utilisation, however, improves at first, but eventually decreases as restriction becomes more severe.

It is thought that the equations found can be used, within certain limits, to calculate the average effects of feed restriction for different breeds of pigs, which when fed ad libitum consume 2·7 kg. meal ration (of normal composition) per day and have a growth rate of about 750 g. daily from 30 to 90 kg., and backfat thickness of about 37 mm. when kept under average environmental conditions.

Female sheep of five breeds (Scottish Blackface (B), South Country Cheviot (C), Welsh Mountain (W), Lincoln Longwool (L), and Southdown (S)) were purchased as lambs of 5–6 months old and subsequently kept as a single flock on a grassland farm until they were 4½ years old (Southdowns to 3½ years). The sheep were used in a crossbreeding experiment with up to six breeds of ram.

The sheep were weighed and 8 body parts measured at frequent and regular intervals. The Blackfaces and Southdowns grew, relative to their mature size, a little faster initially than the Cheviots. Mature weights (lb.) were approximately B: 146, C: 145, W: 101, L: 197 and S: 129. The breeds differed in conformation but the order of maturity of the body parts was the same for each breed. Variances for body measurements decreased slightly with increasing age for early-maturing parts and increased a little for later parts. The coefficients of variation decreased slightly with increasing age. Variance in weight increased markedly with age (but not the C.V.) and showed the only large breed differences in variance.

Fleece weight, after the first shearing, was strongly affected by the amount of wool shed prior to shearing. Shedding differed significantly between the breeds, Cheviots shedding most. Shedding was also related to the number of lambs born per ewe. The total weights (lb.) of wool produced from four shearings were B: 23·2, C: 20·0, W: 15·6, L: 55·5 and S (estimated from 3 shearings): 20·8.

The numbers of lambs born per ewe totalled over three lamb crops were B: 5·81, C: 5·21, W: 4·53, L: 4·85 and S (estimated from two crops): 4·55. In survival to weaning, single-born lambs were no better than twins and crossbred lambs no better than purebred. Lambs born to Welsh mothers had the best survival; the other breeds did not differ significantly from each other. The breeds of ewe differed in the proportions of their twin lambs which had to be assisted at, or soon after, birth.

When each breed of ewe was mated to the same breeds of ram, the weights of lambs born to Blackface mothers did not differ significantly at birth from those of lambs born to Cheviots but were heavier at weaning. Lambs from Welsh mothers were the lightest at birth and weaning.

In relation to (live-weight)0·73 the maternal performance of the Welsh females was at least as good as that of the Blackfaces but the wool production was slightly poorer.

1. Two experiments are described in which the effects of ammonium acetate on the yield and composition of the milk from heifers given (1) a fat-depressing diet consisting mainly of cereals or (2) a normal diet of silage and sugar beet pulp for maintenance and rationed concentrates according to yield.

2. In the first experiment acetate increased the content of butterfat in the milk from 2·76 to 3·12. This difference was almost significant (P < 0·1). There was no significant effect of acetate on the total solids content of the milk.

3. In the second experiment acetate very slightly depressed the total solids and the butterfat content of the milk.

4. In Experiment 1 there was no significant difference between treatments i n the pH of rumen liquor, or in its concentration of total steam volatile fatty acids, but the proportions of acetic and isobutyric acids were higher on the acetate treatment.

5. In Experiment 2 there were no differences in the relative proportions of the fatty acids although the total steam volatile fatty acid concentration was higher on the acetate treatment.

Data on 2,055 Welsh Mountain lambs showing wide variation in birthcoat type were used to study the association of birthcoat type with lamb survival and subsequent performance. Each lamb was assigned to one of six grades on the basis of its distribution of halo hair.

Lamb mortality up to weaning was associated with birthcoat type though the effect was significant only in the deaths of live-born lambs before 14 days of age. During this period fine birthcoated lambs showed the highest losses with a mortality rate of 13% compared with rates up to 5% for intermediate and hairy birthcoat lambs. The lowest overall mortality rates occurred among lambs with intermediate birthcoats.

Growth of 669 surviving ewe lambs did not show any significant correlation with birthcoat grade at any stage up to 18 months of age. However, the carcass weights and dressing-out percentages of 570 wether lambs were significantly higher among fine birthcoat lambs.

Fine birthcoats were also associated with better quality fleece grades, 86% giving rise to kemp-free fleeces with short fine fibres, while 92% of hairy birthcoats gave rise to kempy fleeces with coarser fibres.

Twelve inbred (coefficient of inbreeding 25%) and 12 outbred, singlereared, Scottish Blackface castrated male lambs were taken at six months of age from amongst the progeny of three rams. They were housed and fed individually on a pelleted diet in three separate phases. In the first two phases restricted amounts were offered. These were calculated to produce, firstly, a small rate of live-weight gain and, secondly, a small rate of live-weight loss. In the third phase feed was offered to maximum voluntary intake during 2 two-hour periods daily.

On restricted intakes there were no significant differences between the groups. In the third phase the outbred sheep consumed 17% more feed than the inbred group and the daily rates of live-weight gain and wool growth were 33% and 27% higher, respectively. Since the inbred lambs were smaller, the voluntary intake per unit live-weight did not differ significantly between groups.

The estimated requirements of feed for maintenance (20·1 ± 0·4 and 21·9±0·6 g./kg. live-weight, respectively, for the outbred and inbred groups) differed significantly. There were no apparent differences in the estimated efficiency of conversion of available surplus feed to body tissue or wool growth.

1. A growth trial and nitrogen balance studies were carried out on 32 early-weaned calves fed on a basal diet containing fish meal or diets in which the fish meal nitrogen was partially replaced by urea or ammonium acetate or totally replaced by urea.

2. Growth rate and feed conversion efficiency were highest on the fish meal diet and fell as urea replaced the fish meal.

3. Rumen ammonia levels and urinary nitrogen excretion were highest, and nitrogen retention lowest, in the calves receiving the diet in which fish meal nitrogen was completely replaced by urea nitrogen.

Experiments were carried out to measure the effects of wearing jute coats on the heat production and external insulation of Blackface and Cheviot sheep during exposure to different types of cold environment. The results showed that wearing coats did not appreciably increase external insulation in cold conditions at wind speeds up to 10 mph when the fleece was dry. In cold, wet and windy conditions, wearing coats prevented moisture from penetrating the fleece of the sheep so that external insulation was 25–27 units when the sheep were wearing coats, but only 15–18 units when they were not.

It has been shown that few environments likely to occur in Britain constitute a cold stress for grazing sheep if their external insulation is above 25 units. Wind and rain reduced the external insulation of sheep with full winter fleeces below this value only when they were not wearing coats. It is concluded, therefore, that jute coats have a considerable value in ameliorating the effects of cold stresses of the type likely to be encountered by hill sheep wintered out of doors in the British Isles.

1. An analysis has been made of the association between transferrin genotype and progeny test for both milk yield and fat content in a sample of 879 bulls of five dairy cattle breeds, the majority used in artificial insemination in Great Britain. Although significant effects were found in only one of the ten analyses when the breeds were considered separately, a significant effect on milk yield was found when the results from all breeds were combined. A significant effect was not found for fat content, although the effects of the allele substitutions were in the opposite direction to that on milk yield in five cases out of six.

2. Although our results were in the same direction as those of Ashton, the effects were smaller. It was estimated that the locus accounted for 1·1% of the genetic variation in yield and 0·4% of the genetic variation in fat content.

3. Some information based on the production records of 178 experimental cows with known genotypes was also analysed. For milk yield, the pattern of effects was similar to that in the sire data though the differences were much larger. The variance removed by fitting constants was not statistically significant for milk yield or fat content. It was estimated that the sire data contained some fifty times as much information as did the cow data.

Three groups of eight yearling Friesian steers were fed on pelleted diets containing ground barley straw, maize and soya bean meal, the percentages of straw in the diets being 10, 30 and 50 (treatments 1, 2 and 3). The crude protein level was adjusted to 12% in all diets on an air-dried basis.

Daily live-weight gain for treatment 2 was 7% greater than for treatment 1, and 12% greater than for treatment 3. Voluntary feed intake was also greater in treatment 2, by 16% when compared with treatment 1 and 4% when compared with treatment 3. The efficiency of feed conversion decreased by 10% for each 20% increase in dietary straw level.

Weight loss during lairage was greater on treatment 3, as were total gut fill and omasum weight. The bone content of carcasses was similar for all treatments; carcass lean was increased in treatment 2. Carcass and channel fat was severely depressed in treatment 3.

The results are discussed in relation to dietary effects on carcass composition.

Non-genetic sources of variation in pre-weaning death losses due to enteritis or pneumonia have been investigated using records of 15,329 rabbits born during the years 1946–61 in the random bred colony at the U.S. Rabbit Experiment Station at Fontana, California. Under consideration were periods of years, season within years, litter size, parity of doe, and fostering.

For enteritis there was a statistically significant effect of season due to a negative regression of death rate on ambient temperature. For pneumonia, periods of years were statistically significant due to an increase in later years in deaths during the first 14 days of life. The other sources of variation studied were of negligible importance.

Six dry cows, six in-calf heifers and six yearling calves grazed in groups of six, arranged either in single age groups or mixed age groups; the arrangements alternated, and each was repeated three times in a continuous grazing sequence with six-day periods. All the animals grazed together for a preliminary period of seven days. Under each arrangement the cattle grazed on a mature sward for two periods, and on an aftermath for one period. Access to fresh and grazed herbage was restricted by electric fences which were moved daily, but the amount of fresh herbage offered was well in excess of appetite at all times.

Records of live-weight, herbage intake, grazing behaviour and social dominance ranking were obtained for all individuals. There was a close relationship between live-weight (LW) and herbage organic matter intake (OMI) in each grazing period, and the regression equations for the six periods were very similar. The best-fit exponent of LW for the pooled results was 0·61±0·038; this was significantly different from 0·73 and 1·0. The exponent increased to 0·71±0·045 when the cow data were excluded. The inclusion of weight change as a third variable was only of value in the mature sward.

The calves ate significantly more, and the cows tended to eat more, when grazing in mixed age groups. These differences could not be explained by differences in the time spent grazing.

There were high simple correlations between LW and OMI, between social dominance rank and OMI, and between LW and rank, particularly in the mixed age groups. When variations in LW were excluded, dominance rank had little influence on OMI in the mixed age groups, but the independent effects of LW and social rank on OMI were of approximately equal importance in the single age groups.

1. Investigations into possible ways of rearing artificially the lambs surplus to the ewe's capacity are described and the results discussed.

2. A ewe-milk replacer with a high fat content was used from 2 days of age until weaning at 11.4 kg live weight. Two levels of milk feeding, restricted and ad libitum, were given in the second year. Significant differences were observed in live weight gain per day, milk consumption per kg live weight gain and solid food intake prior to weaning. Live weight gain and solid food intake in the period immediately after weaning also differed markedly. Sex and birth weight had little effect on performance.

3. Food costs were lower on the restricted level of milk feeding than on the ad libitum level.

1. Homologous antibodies to Salmonella pullorum were produced by immunising pigs between 100 and 200 lb. live-weight.

2. Standardised doses of antibody, extracted from whole serum after these pigs were slaughtered, were administered to 245 piglets by stomach tube during the first 24 hr. after birth.

3. The marked variation in uptake of the marker antibody, determined by direct agglutination tests on the blood serum of the dosed pigs, was chiefly due to variation between litters. This variation was attributed to maternal effects, associated with agents secreted in varying quantities into colostrum. Concentration of antibody in the dose administered to piglets, time elapsing between birth and dosage and differences between sire families, were subsidiary causes of variation. Possible mechanisms influencing efficiency of antibody absorption were examined and discussed.

Heritability estimates of death from enteritis or pneumonia, the two major causes of pre-weaning death at the U.S. Rabbit Experiment Station, Fontana, California, have been estimated. The data consisted of 15,329 observations made at the station from 1946 to 1961.

From birth to 14 days of age the death rate over the years increased from 0·2 to 6·1% During the period from 15 days to 56 days of age (weaning) the enteritis death rate was quite constant over the years (7·5%) while that of enteritis or pneumonia increased from 8·3% to 13·2%.

Heritability of death from enteritis or pneumonia for the period 15 to 56 days of age was estimated on the basis of paternal half sib analysis to be 0·12 ± 0·02. Based on an analysis of differences between dams within sires the corresponding estimate was 0·58 ± 0·05. The difference between the two estimates was attributed to maternal and non-additive genetic effects.

The genetic correlation between the two diseases under discussion was estimated to be 0·66 and 0middot;57 on the basis of differences among sires and among dams within sires respectively.

Breeding plans are suggested on the basis of the parameters estimated.

A trial designed to investigate the effects of two environmental temperatures, three feeding regimes and the interactions between them is described. A temperature of 72° F. was better than one of 54° F. for bacon pigs between 40 lb. and 200 lb. weight. The higher temperature resulted in faster growth, more efficient feed conversion and increased length of carcass. Other carcass characteristics were not significantly altered. Ad libitum feeding resulted in faster growth and fatter carcasses than restricted feeding, but did not have a significant effect on efficiency of feed conversion. When feed intake was restricted, feeding pigs once daily or twice daily resulted in similar performance and carcass composition.

There was a significant interaction between environmental temperature and feeding method for average daily gain in that pigs fed ad libitum grew faster at the low temperature and pigs fed restricted amounts of feed grew faster at the high temperature. No other interaction reached significant levels.

1. Sixteen foods, made up with differing proportions of roughage and concentrates and ranging in crude protein content from approximately 2·6% to 10·0%, were each offered ad libitum to Blackhead Persian wether sheep. The nitrogen balance of the sheep and the distribution of nitrogen in the urine were measured.

2. Urinary nitrogen excretion decreased with the digestible nitrogen intake. There was a highly significant (P < 0·01) positive correlation between either excretion of urea or of allantoin and digestible nitrogen intake.

3. The amount of creatinine excreted varied little with diet but was directly related to the body weight of the sheep.

4. For 8 diets, hippurate excretion by the sheep was directly related (P<0·01) to intake of digestible nitrogen, while a significant (P<0·05) positive correlation between the same measures was found for the other 8 diets.

5. It is suggested that with certain types of diets the excretion of either nitrogen, urea or allantoin in the urine may be useful indicators of digestible nitrogen intake of low-protein diets by sheep.

Comparisons of performance for growth rate were made on 15 Suffolk rams and their 362 Suffolk × Welsh progeny from 2 to 16 weeks of age or to 100 lb. live-weight, whichever was earliest.

The Suffolk rams were reared artificially from 7 days of age.

A high negative correlation was found between growth rate and efficiency of food conversion.

The correlation between the sires' growth rate and the growth rate of their progeny was 0·25. This was increased to 0·46 by limiting the performance test of progeny to the period between 8 to 16 weeks of age.

Heritability estimates increased as animals grew older. An estimate of 0·52 ±0·012 was obtained for the period 8–16 weeks of age.

1. Sixteen sows were immunised in late pregnancy with Salmonella pullorum antigen. Sows showed marked differences in serum antibody titre at parturition.

2. Twelve hours after birth, serum antibody titres in the 173 piglets born to the 16 sows were measured. They were positively related to the serum titres of their mother.

3. Marked variation existed in the antibody titres of colostra from different teats and from different sows. No relationship was found between colostral titres and the titres in sow or piglet sera.

4. Sow and piglet serum titres were negatively related to piglet and litter weight gain from birth to 7 days of age.

5. Those piglets with high serum antibody titres at 12 hr. after birth displayed better growth rates and enjoyed lower mortality than piglets with low antibody titres.

1. Two experiments concerned with the voluntary intake of food by artificially reared lambs are described and the results discussed.

2. When the diets were pelleted the addition of finely ground oat husks to form as much as 40% of the diet had practically no effect on live weight gain since the lambs controlled their consumption to achieve the same energy intake on the various diets. When the diets were given as a meal, the addition of 20% oat husks caused food intake to increase, digestible dry-matter intake to remain the same and growth rate to be somewhat poorer; but the addition of 40% oat husks caused no further increase in intake so that digestible drymatter intake fell markedly and growth rate was much reduced. It is argued that the relationship between food intake and digestibility is dependent on the physical form of the diet.

3. Addition of beef tallow to the meal diets had very little effect on the lambs' performance.

4. Lambs which had received a restricted quantity of milk replacer from birth to 11·4 kg live weight had a slower growth rate and poorer food conversion efficiency during the subsequent feeding period (13·6–34·1 kg) than lambs which had received milk ad libitum. This effect was greater with meal diets containing 40% oat husks than with more concentrated diets.

5. On a pelleted diet containing 85% barley a conversion ratio of 3·21 was achieved over the whole fattening period, indicating the commercial feasibility of intensive lamb production on cereal diets.