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SUMMARYIn an egg-transfer experiment, seven breeds of sheep differing widely in mean mature ewe weight were used: Oxford Down, 96 kg; Borderm Leicester, 77; Finnish Landrace, 55; Southdown, 52; Tasmanian Merino, 42; Welsh Mountain, 34; and Soay, 22. Body weight and gestation length of recipient ewes are briefly examined. Of 60 recipient ewes, 95% became pregnant. Embryo survival averaged 69% and appeared to be independent of breed of embryo. Litter size of Welsh Mountain, Border Leicester and Finnish Landrace ewes each given four eggs averaged 2·6, 2·9 and 2·9 respectively so that most of the normally observed breed differences are mainly due to differences in ovulation rate. Observed birth weights were in general agreement with those predicted from litter size and donor and recipient ewe weights. The maternal capacity of Finnish ewes was close to average. Litter size affected survival mainly through its effect on birth weight. For Oxford lambs in litters of 3 to 5 from Finnish recipients, birth weight was 42% that of normal single-born Oxfords and only 42% survived. For Soays the corresponding values were 59% for birth weight and 85% for survival. Finnish lambs were exceptionally small at birth but had the highest viability of all breeds. Lamb mortality was higher for males (18%) than females (9%) especially in large litters (26 v. 11%) but about the same in both sexes for Finnish lambs.

SummaryA short-term selection experiment was undertaken to estimate the realized heritability for birth weight in mice.A cross of two strains was used as a base population, from which heaviest and lightest birth weight individuals were selected to initiate an up (JU) and a down (JD) selected line. Selection was applied on a within-litter basis for six generations. By the end of the experiment, the two lines differed by 0·20 ± 0·01 g. The heritability, estimated from the regression of cumulative response on cumulative selection differentials and adjusted to that of individual values, was 0·14 ± 0·02. Six-week body weight, averaged over both sexes, was higher in line JU than in JD by 1·0 ± 0·3 g at generation 6. Litter size was higher in line JU in every generation, the regression of the divergence between JU and JD on generation number being 0·13 ± 0·03.

SUMMARYA theoretical equation is proposed which describes the dynamics of foetal growth in mice. The equation is basically of exponential form and takes into account the effects of stage of gestation (T) and number of foetuses (N) on the instantaneous relative growth rate. The model was tested against experimental data by using a stepwise regression technique. Both stage of gestation and number of foetuses had significant effects (P< 0·01) on the instantaneous growth rate. When the model was tested against another set of data 44 out of a total of 52 comparisons had a prediction error less than 15 %. As a result, the following equation is proposed to describe the foetal growth of miceW = e-14·34Ne(1·302–0·0276T–0·00059)TAn equation of this form could be useful in interpreting prenatal growth data for other species. Other factors which may influence the dynamics of foetal growth, such as maternal age or weight, could also be incorporated into the equation.

Milk production records from 5740 first lactation Holstein-Friesian daughter-dam pairs were studied to determine if heritability was related to herd production level. Heritability from daughter-dam regression was essentially constant at 10 environmental levels based on either the dam's or daughter's herdmate deviation from season average yield. The heritability estimate over all levels of yield was 0·43 (with standard error 0·03) which is higher than most estimates. Estimates of heritability from paternal half-sib correlations for four environmental levels and based on data from 2568 daughters of A.I. sires generally increased as the herd production level increased. The highest half-sib estimate was 0·42 and the lowest 0·17. Daughter-dam estimates from the same data were essentially constant over the four herd levels.Total variation increased as herd level increased, in agreement with previous reports.Heritability was found not to differ between seasons of calving.

SUMMARYEwes of seven breeds (Oxford Down, Border Leicester, Finnish Landrace, Southdown, Welsh Mountain, Tasmanian Merino and Soay) were treated with progestagen-treated sponges to synchronize oestrus for an egg-transfer experiment. Ewes of five of the breeds were treated with superovulating doses of pregnant mare serum gonadotropin (PMSG). Mean body weight ranged from 89 kg for Oxford Downs to 20 kg for Soays. Interval from sponge withdrawal to oestrus differed significantly between breeds, ranging from 1·83 days in Finnish Landrace to 3·00 days for Soays, but was not consistently related to ewe weight. Treatment with PMSG prior to sponge removal reduced the interval to oestrus; fertilization rate at the first synchronized oestrus was low. There were marked differences among breeds in proportion of ewes showing oestrus following PMSG treatment, total follicular response, and proportion of follicles ovulated. Ewes of the Finnish Landrace breed, with the highest natural fertility, were equal or superior to the other breeds in every component of response, and yielded more than double the number of fertilized eggs per ewe of the next best breed. Finnish Landrace females 6 to 9 months old made very satisfactory donors. Natural ovulation rates of adult recipient ewes were as follows: Welsh Mountain (23 ewes)—1·43; Border Leicester (15 ewes)—2·00; Finnish Landrace (26 ewes)—3·31.

Effects of castration on growth, wool production, carcass characteristics, and meat quality in sheep were investigated.The experiment included two flocks (similar in their effect on pre-weaning performance, but representing high and low planes of nutrition after weaning), two years, and two slaughter ages. Rams weighed approximately 5 % more thanwethers at weaning, and 15 % and 23 % more as yearlings on the low- and high planes of nutrition respectively. Differences in favour of rams in terms of skeletal and muscle size and wool production were also greater on the higher plane of nutrition. Carcass weight was approximately 4%, 8% and 19% higher for rams among lambs, low plane and high plane yearlings, respectively. Proportion of carcass weight in the forequarters was 0·5 %, 1·1 % and 2·3 % higher for rams in these three groups. Rams had less kidney fat and the same percentage of the carcass weight in loin and trimmed leg as wethers in all groups.Carcass grade and eating quality as evaluated by a taste panel did not differ between ram and wether lambs. Among the lighter yearling group, meat from the loin of ram carcasses was less tender. Quality was appreciably lower for rams in the heavier yearling group. Meat from ram carcasses had 1 to 2% higher cooking loss than that from wethers.