In three trials conducted in 3 successive years, Africander, Brahman, Tswana, Tuli and crossbred cattle were injected intramuscularly with 500 jug of cloprostenol on two occasions 11 days apart to synchronize oestrus. The cattle were inseminated 72 and 96 h after the second injection. Controls of the same breeds were exposed to natural service for a period of 22 days. In all trials the conception rate to natural service was significantly higher (P < 0·01).

Conception rates to both breeding methods were highest in Tuli and crossbred cattle and lowest in the Africander and Brahman. Non-lactating cows showed higher fertility than suckling cows and maiden heifers.

Vasectomized bulls used in one trial were shown to be useful in the selection of breeding cows suitable for oestrus synchronization. Further studies are justified to maximize the benefits of a potentially valuable method of breeding beef cattle in ranching conditions.

Live weights from birth to 3·5 years are reported for beef cattle reared under ranching conditions in Zambia. The 809 cattle were purebred Africanders, Angonis, Barotses and Borans and the reciprocal crossbreds of the latter three breeds born in 2 years. All animals born in the 1st year and half the males born in the 2nd year grazed natural grassland. The remaining males and all females born in the 2nd year received, in addition, dry season supplementary feed from 1·5 years of age.

The interaction of genotype with year-of-birth was important but not the interactions of genotype with management or sex. Purebred progeny of the introduced Africander breed were heavier than the progeny of the indigenous Angoni and Barotse breeds in both year-of-birth groups, but only heavier than progeny of the introduced Boran breed in the first group. On average, the Africander progeny had live-weight advantages of about 16% and 10%, and the Boran progeny advantages of about 12·5% and 5·5% over the purebred Angoni and Barotse progeny respectively. Heterosis estimates tended to increase with age, reaching levels of about 5 to 6% in the Barotse/Boran crosses at and after 1·5 years. Heterosis was not shown by the other crosses. The Barotse and Boran breeds had similar maternal effects which were superior to those of the Angoni breed.

Continuous reciprocal backcrossing results in individuals which are 67% heterozygous, and which, assuming a linear relationship between heterozygosity and heterosis, would be expected to exhibit 67% of the heterosis found in Fi hybrids. An experiment was undertaken to investigate this relationship in the reproductive traits of mice. By crossing and repeated backcrossing in both directions using two unrelated strains of outbred mice, females were produced which were 25, 50, 75 and 100% heterozygous. The crossbred females and the two purebred strains (922 females in all) were mated to a sire line of a genetically distinct strain to standardize foetal heterosis. The reproductive performance of females with different percentage heterozygosity gave estimates of maternal heterosis. A significant positive linear relationship was found between heterozy-gosity and heterosis in litter size, total weight of litter and individual weight of progeny both at birth and at weaning. The relevance of these results for livestock breeding is discussed.

Four animals from a group of eight Hereford × Friesian heifers weighing approximately 365 kg were implanted with 300 mg trenbolone acetate at the beginning of a 60-day trial period. Treated heifers grew more quickly (P<0·05) and converted food to liveweight gain more efficiently (P < 0·01) than untreated cdntrols. They had lower overall concentrations for serum albumin and plasma urea (P<0·01), the differences being maintained throughout the diurnal periods studied. No significant differences were recorded in the mean values for serum total protein, plasma glucose or free fatty acids. Similarly, the absence of significant differences in the mean values for serum growth hormone, insulin and prolactin suggested that these hormones did not mediate the response to trenbolone acetate. Implanted heifers had a significantly greater retention of nitrogen (P<0·01) which was mainly due to a reduction in the excretory products associated with total urinary nitrogen.

The problem of applying a linear constraint on expected selection responses in a two-character independent culling levels scheme is investigated. A general equation is derived, the solutions of which specify the culling levels satisfying the constraint applied. Examples are given to demonstrate how different constraints can be applied by transforming one or more of the characters under selection.

Carcass characters for 365 male castrate cattle of the Africander, Angoni, Barotse and Boran breeds, and the reciprocal crossbreds of the latter three breeds, are reported. In each of the two year-of-birth groups, different slaughter-age/management regimes were used. For all carcass characters, except those related to size, the two sanga breeds, Africander and Barotse, were very similar, as were the two zebu breeds, Angoni and Boran. The introduced breeds, Africander and Boran, which had similar carcass weights, had heavier carcasses (+18 kg, +10%) than the indigenous Barotse and Angoni breeds.

The sanga breed carcasses had less fat cover than those of the zebu breeds. Maternal effects were not important for carcass characters and the Angoni/Barotse and Angoni/Boran crosses showed no heterosis. In the Barotse/Boran crosses, slaughter and carcass weights and eyemuscle area gave between 8% and 9·5% heterosis, and the linear carcass measurements between 2% and 3%.

It was concluded from the management comparisons that there was no economic advantage in delaying slaughter to the later of the two ages compared in the two year-of-birth groups.

1. Gestation diets formulated to contain A, 9 or B, 11 % protein, and lactation diets with C, 13; D, 15; E, 17 and F, 19 % protein provided eight treatment combinations. They were given at 2 kg/day in gestation and approximately 5·7 kg/day in lactation to sows at eight centres, which remained on experiment for 3 to 5 parities and produced in total 529 litters. Mean results for parities 1 to 3 are given in this summary.

2. Sows given more protein during gestation (i.e. diet A v. B) gained more weight in gestation (18·3 v. 22·5 kg net gain) but lost more in lactation (—1·7 v. —5·2 kg). Their offspring were significantly heavier at birth (1·31 v. 1·36 kg) and at 3 weeks old (5·28 v. 5·49 kg), but not at 6 weeks old (weaning).

3. Giving more protein in lactation caused linear increases in piglet weights at 3 and 6 weeks, and a linear reduction in creep feed consumption. From a sample of sows whose milk yields were recorded, extra protein appeared to increase milk yield and its protein content. Extra protein for lactating sows also reduced their live-weight loss in lactation, but this effect was partially counterbalanced by smaller gains in gestation.

4. No treatment had consistent significant effects on litter size or on the interval from weaning to conception. There were no significant interactions between gestation and lactation protein concentrations.

5. The generally large differences between centres were in a few cases associated with centre × treatment interactions, but these were not such as to cast doubt on the general applicability of the main findings of the experiment.

6. From this and an earlier experiment the minimum protein level for diets based on barley was concluded to be 11 % during gestation and 15% during lactation. For lactation diets, however, the economic optimum level may be > 15 %.

Masham ewes were mated at three stages of the breeding season, early (15 September to 20 October), mid (20 October to 24 November) and late (15 December to 19 January), at two different body condition scores, 4 and 2, for 3 successive years. The mean scores achieved for body condition 4 were 3·6, 3·7 and 3·9 for early, mid and late respectively and for body condition 2 were 2·4, 2·3 and 2·2. In the late lambing flock more ewes were marked by vasectomized rams and less ewes were barren than in the early and mid flocks. The ewes in higher body condition at mating had significantly more triplets and fewer singles, and a higher lambing percentage (183 versus 165). This effect of body condition on litter size distribution was significant at each time of mating, with the ewes at the late mating producing significantly fewer triplets and more singles; this difference was caused by a reduction in the performance of the body condition 4 ewes in the late group.

Young ewes in the body condition 4 group produced significantly fewer triplets and more singles than the mature ewes in the same body condition group, but at the lower body condition age had no effect on litter size distribution.

Forty-eight 1-day-old pigs were used to compare lactose and glucose as the carbohydrate source for piglets from 1 to 22 days of age. Average daily gains, gain over feed ratios and the apparent digestion coefficients for crude protein, ether extract, nitrogen-free extract, ash, energy and dry matter were similar for the two groups. Apparent net protein utilization was greater with the glucose diet than with the lactose diet (P<0·05), although their apparent biological values and metabolizable energy values were similar. The serum levels of total glucose, total protein and urea nitrogen were also similar for both groups. Serum levels of total protein and urea nitrogen declined with age while the trend for total glucose was inconsistent. Subsequent nursery performance from 22 to 64 days of age when all pigs were fed the same diet was not influenced by the previous neonatal dietary treatment.

Four experiments were conducted to study the performance of pigs weaned at 10 days on to diets containing different amounts of flaked maize.

Digestibilities of dry matter and energy of flaked maize were found to be 0·87 for pigs between 11 and 16 days of age.

Starter diets containing 0, 250 or 375 g flaked maize/kg, given from 10 to 35 days, resulted in growth rates of 254, 254 and 213 g/day respectively and the reduction with the high level of maize was significant (P< 0·001). Feed conversion ratio was unaffected by treatment and averaged 1·08.

When the starter diets were replaced at about 24 or 27 days by a cereal-based diet containing 200 g dried skim milk/kg there were no significant differences in performance between 10 days and 10 kg live weight due to starter diet, and growth rates were similar t o those obtained when the starter diets were given to 35 days. However, there was a consistent trend of lower food intakes, poorer feed conversion ratio and, consequently, poorer growth during the immediate post-weaning period on the starter diet containing 375 g flaked maize/kg.

Breed and crossbreeding significantly affected variation in 10 fleece, follicle density and fibre diameter traits in 12-week-old lambs. The breeds involved were the Scottish Blackface, South Country Cheviot, Welsh Mountain, Lincoln Longwool, Southdown and Tasmanian Merino, but the last only by rams.

The Southdown came closest to the Merino in affecting density and diameter traits although the Merino crosses remained clearly distinguishable by their high follicle density. The Lincoln and Blackface crosses were notable for their long fibres and the Blackface crosses also had the least dense follicle and coarsest primary fibres. The coarsest secondary fibres were from Lincoln and Cheviot crosses. Heterosis estimates were obtained for crosses involving the Blackface, Cheviot, Welsh and Lincoln breeds. Significant heterosis was present in a wide range of traits but was not apparent in all crosses. Maternal differences were evident for many of the traits but with density these differences were apparently related to maternal effects on live weight of the lamb. The sex of the lamb, its birth type and rearing and its 12-week live weight significantly affected many of the traits.

Among breeds and crosses a strong relationship was found between fibre density and average fibre diameter. The relationship suggested that breed variation in total fibre area per unit area of skin was small compared with the breed variation in its components. Fleece weight per unit area was most closely related to mean fibre length.

Data on 2120 lambs produced over 7 years in the Animal Breeding Research Organization synthetic Dam Line (49% Finn, 22% East Friesian, 16 % Border Leicester and 13 % Dorset Horn) were analysed by least squares. The effects of sex, age of dam, year of birth, birthrearing class, and age of lamb on weights at birth and at 4, 8, 12 and 16 weeks of age were all appreciable. However, pooled within-year correction factors should be adequate in adjusting records for selection. Heritability estimates for weights at the five ages ranged from 0·17 to 0·24 by the sire component method and from 0·18 to 0·28 by regression of offspring on dam. Previous reports of differences in heritability in singles and in twins were not confirmed. Estimates of the genetic correlations among the various lamb weights were high (0·62 to 1·04) indicating that selection for weight at one age should result in increased weights at all ages.

The efficacy of feed additives in enhancing the performance of growing pigs, together with the effect of different protein levels (18 v. 16% from 13 to 18 kg body weight and 16 v. 14% from 18 to 39 kg body-weight), were evaluated in three experiments with 412 pigs. In Experiment 1, from 13 to 39 kg body weight, pigs receiving 55 p.p.m. carbadox (C) produced a greater (P<0·05) average daily live-weight gain than did the non-medicated (NM) pigs and those receiving 275 p.p.m. chlortetracycline-sulphamethazine-penicillin (ASP) with a similar but non-significant (P>0·05) trend n i daily feed intake and gain/feed ratio. The ASP pigs also produced a greater (P<0·05) weight gain than the NM pigs. The higher dietary protein level sequence produced a greater (JP<0·05) weight gain and gain/feed ratio.

In Experiment 2, from 16 to 39 kg body weight, NM control pigs achieved high weight gains, and all the feed additive treatments studied [C+106 p.p.m. of the worming agent pyrantel tartrate (C+P); ASP+8 mg of the worming agent levamisole-HCl per kg body weight fed once on day 7 of the experiment (ASP+L); and 113 p.p.m. tylosin+13 p.p.m. of the worming agent hygromycin B] were similar to the NM controls in daily feed intake, weight gain and gain/feed ratio. However, in Experiment 3, the C+P and ASP+L treatments resulted in a greater (P<0·05) weight gain with a trend toward greater daily feed intake than did the NM control treatment from 17 to 35 kg. The weight gain of the ASP+L pigs was also greater (P<0·05) than that of 165 p.p.m. oxytetracycline and NM pigs from 35 to 57 kg. From 57 to 96 kg all the pigs received the same NM control diet and their weight gains were similar. The gain/feed ratio was similar among the treatments within each of the weight periods in Experiment 3. Pigs slaughtered at the beginning and at various stages during Experiments 2 and 3 were free of internal parasites and migration damage.

Data on 792 Chios lambs born during the 1972/73 and 1973/74 lambing seasons were used to estimate genetic and phenotypic parameters for birth weight, weaning weight, age at weaning, pre-weaning daily gain, body weight at 5, 10, 15 and 20 weeks of age, and postweaning daily gain. Body weight at 15 weeks of age had the highest heritability estimate (0·73 ± 0·17) and that of post-weaning daily gain was also high (0·56 ± 0·15). Selection for either weight at 15 weeks or post-weaning daily gain would be expected to yield a greater response than selection for pre-weaning daily gain or weaning weight. Genetic correlations among weights and/or gains were positive (approximately 0·20). Phenotypic correlations among weights and gains were generally higher than genetic correlations. However, the correlation between pre— and post-weaning daily gain was small (0·08). Likewise, post-weaning daily gain had low correlations with all weights before 10 weeks. Age at weaning had moderate negative associations with all weights but a very low positive correlation with post-weaning daily gain.

1. Many apparently normal nursings occur in the absence of milk ejection, but little is known about their frequency or causes. The present paper reports an observational study in which the incidence of these incomplete nursings was studied in detail.

2. Nine lactating sows were studied throughout a 5-week lactation in each of two environments. One was a normal commercial farrowing house, and the other a cubicle in which the sow and litter were effectively alone. Extended (3 h) observations were made on each litter at various ages.

3. Twenty-seven per cent of all nursings were found to be incomplete with little difference between the grouped and alone sows. Incomplete nursings often occurred within 40 min of a complete nursing and increased the interval between successive complete nursings. They also occurred most frequently during the 2nd and 3rd weeks post partum.

4. Irrespective of the housing condition the sows initiated the same proportion of all nursings and a similar percentage of these were incomplete.

5. Incomplete nursings appear not to be an artefact caused by crowding many lactating sows together, but they may reflect some form of discomfort of the sow. The problem of real and apparent nursing intervals is discussed.

The lactation curves of 21 single- (SR) and 28 twin-rearing (TR)ewes were fitted and milk production was estimated by the model Y = axb e−cx, developed in England for dairy cattle. In addition, persistency of lactation of SR and TR ewes was compared using five procedures reported in the literature. Results indicated that the coefficients that characterize the shape of the lactation curve (b and c values) were of similar magnitude to those reported in dairy cattle, thus denning a curve of similar shape. Lactation curves of SR and TR ewes were essentially parallel throughout lactation, with more milk production from TR ewes. The correlation between total milk production estimated by this model and that based on the sum of partial lactations was 0·98, but variability of milk production as estimated by the latter method was slightly less. Lactations of SR ewes had similar persistency to those of TR ewes.

Observations were made on the distribution of ovulations between the two ovaries, the distribution of foetuses between the two uterine horns, the total numbers of cotyledonary burrs, and the weights and numbers of foetal cotyledons for each foetus in 80 Finnish Landrace × Dorset Horn ewes killed between 50 and 145 days of gestation. While ovulations were randomly distributed between right and left ovaries, viable foetuses were more evenly distributed between right and left uterine horns. Migration of embryos between the horns contributed to this relative balance but early foetal mortality, which increased with ovulation rate, tended to upset it. The mean number of foetal cotyledons per foetus depended on litter size and distribution, and at any fixed litter size was substantially reduced in those ewes with six or more ovulations. Foetal weights were also relatively low in these ewes. The total weight of cotyledons per foetus decreased by about 12% for each increase of one in number of foetuses, as compared with a 20% decrease in number of cotyledons but only a 7 to 11 % decrease in the weight of the individual foetuses. Comparisons between foetuses within the same litter led to similar trends in the ratios of cotyledon numbers, cotyledon weights and foetal weights. It is suggested that the presence of some very small lambs in large litters should not be attributed to maternal under-nutrition in late pregnancy.

Data from 1447 pigs from 438 litters were used to estimate heritability, breeding and mating system differences and heterosis of atrophic rhinitis. Yorkshire, Landrace and Lacombe breeds and their crosses were represented. Rhinitis was measured by necropsic examination of the degree of degeneration of the nasal turbinates. Two traits were considered: rhinitis incidence, based on whether or not pigs showed any nasal turbinate degeneration; and rhinitis score, which ranged from zero to four, based on the percentage of nasal turbinate degeneration exhibited. Heritabilities of rhinitis incidence and score were 0·12±0·09 and 0·03±0·02, respectively. Mating system differences were significant but differences between crosses within mating system were not. Estimates of heterosis of two-breed, backcross and three-breed cross pigs over purebreds were respectively, 1·1, 3·4 and 22·1% for rhinitis incidence and 10·1, 13·2 and 28·5% for rhinitis score.

Whole, dry barley treated with ammonia (A), whole, untreated barley (WU) or ground barley (G) were fed with hay and a little high protein concentrate to 7 to 11-month-old lambs. Most apparent digestibility coefficients were found to be 3 to 5 units higher in the diet with barley A than in diets with barley WU or G, the differences being significant. Calculation of the digestibility of barley by difference showed that the coefficients were 8 to 10 units higher in barley A than in barley WU or G as regards dry matter and organic matter. Digestion of crude fibre was better in both A and WU than in G. These differences were also significant.

Sheep from the Soay, Southdown, Finnish Landrace and Oxford Down breeds were serially slaughtered at 40, 52, 64 and 76% of their estimated mature body weight. Breed and sex comparisons of muscle distribution wee made at these degrees of maturity. Comparisons were based on 12 individual muscles obtained from the prime retailjoints. Together, these 12 muscles represented about 41 % of total weight ofmuscle in the carcass. Highly significant breed and sex differences in the weight of individual muscles were obtained, but these differences were greatly reduced when values were expressed as a percentage of total muscle weight.

On a percentage basis, there were significant changes with stage of maturity in only three of the muscles studied, although most individual muscles showed an initial percentage decrease with increasing maturity (from 40 to 52%) but almost no change thereafter. Significant breed differences in muscle distribution were found. The individual muscles examined, when combined, comprised 40·8, 43·4, 40·5 and 39·8 % of total muscle weight respectively in the Soay, Southdown, Finnish Landrace and Oxford Down. No breed was significantly above average for every individual muscle, but the Southdown had the highest percentage in 6 out of 12 and might therefore be said to have had the most preferred muscle distribution. For most muscles examined, the percentage was slightly higher for females than for males, with totals for all 12 muscles of 41·9 and 40·4% respectively.