Litters that contained at least 13 mice were reduced at birth to 12, 8, or 4; and any natural litters of these sizes were also kept. The growth of these mice and their subsequent reproductive performance when mated at 6,10 and 14 weeks of age were recorded. All their offspring were weighed at weaning (4 weeks); some were weighed also at 12 days and 6 weeks of age.

The amount of milk supplied was not fully adjusted to the size of the litters. This had lasting effects so that, even at the oldest age for mating, the mice from the small litters were noticeably the heaviest. Litter size effects extended to significant differences in fertility (numbers born) and less obviously to the numbers weaned. The mice that were mated at 14 weeks were the most fertile. There was little to distinguish the mice from natural and reduced litters of the same size.

Small differences in weights of the offspring at 12 days, 4 weeks and 6 weeks of age bore no consistent relation to these factors.

The effects of selection for increased or decreased body weight in mice, at 5 or 10 weeks of age, on the fibre number, fibre diameter and weight of the m. sternomastoideus and m. anterior tibialis muscles were studied. Unselected control mice were also included. Significant effects were detected in the case of each of the traits. However, the magnitude of any particular correlated response to selection varied between muscles and between selection ages in an unsystematic fashion. Most of the changes in muscle weight resulting from selection were attributable to positively correlated changes in fibre number but they were also accompanied by changes in fibre diameter in some cases.

The correlated responses described indicate the difficulty of predicting how selection for body weight changes will influence cellular structure of any given muscle.

Phenotypic correlations between muscle fibre number and diameter were essentially zero. The phenotypic correlations between muscle weight and fibre number and fibre diameter were positive.

Sex had no effect on fibre number but had an effect on fibre diameter which depended on the particular muscle and the age of the animal.

The material for the present investigation included approximately 4000 cows in 50 progeny groups from the dual-purpose breeds Red Danish cattle and Black Pied Danish cattle. The main objective was to investigate production in 305 days of second lactation. The characteristics studied were milk yield, butterfat yield and fat content. Analyses of the influence of the four environmental factors, herd average, age at calving, calving interval and calving season, were carried out using least squares. It was shown that age at calving, calving interval and calving season had a curvilinear effect on the yield traits. Herd average yield had a strong linear effect on the yield traits. The reduction in the phenotypic variance due to regression of yield on environmental factors was about 35% for milk and butterfat, and about 18% for fat content.

Heritabilities for yield traits in second lactation as well as phenotypic and genetic correlations between the yield traits in first and second lactation were presented. The coefficients of repeatability for milk and butterfat yield diminished from 0·5 to 0·3 when the data were corrected for the environmental factors.

The effects of the proportion of hay in the diet (18 or 40%), implantation of diethylstilboestrol and the type of housing on the performance of male calves was examined over the growth period from 250 to 500 kg live weight in a 2×2×2 factorial experiment. The dry-matter consumption per 100 kg of live weight was lower and the live-weight gain and efficiency of conversion of crude metabolizable energy was higher with the diet containing 18% of hay than with that containing 40%. Diethylstilboestrol implantation had only a small effect on performance. Animals that were allowed yard space in addition to pens gained less rapidly, were less efficient in the conversion of energy into live weight, and were somewhat leaner, than those restricted to pens.

Lipid mobilization was investigated in pigs selected for high and low backfat thickness for four generations. Both epinephrine and pituitary lipid mobilizing factors (LMF) induced in vitro lipolysis in pig fat. The release of non-esterified fatty acids (NEFA) from fat pads sampled from the group selected for thin backfat, the control group, and the group selected for thick backfat differed significantly, the means being 1·16 ± 0·18, 0·83 ± 0·06 and 0·40 ± 0·04 μmol/100 mg per 3 hr respectively.

The inhibition by pig serum from the same groups of an LMF- induced adipokinesis in rabbits also differed significantly. The mean NEFA concentrations were, in the same sequence, 5·05 ± 0·15, 3·35 ± 0·27 and 1·29 ± 0·11 milliequiv/litre respectively. A difference in feeding level of about 20% between 25 and 90 kg live weight did not influence lipid mobilization in vitro.

The serum inhibition method may be a useful aid to early selection for leanness, since it appears that the method may be used for pigs 5 to 6 weeks old.

Digestibility of the major chemical components in diets high in plant cell-wall constituents was determined with four pigs. Two were given a commercial pig meal and two were given, in addition, alkali-treated straw (25% of the diet). Dry-matter digestibility of the straw was 32%, and protein excretion in faeces was increased by the addition of straw to the basal diet. Calculations based on microbial protein synthesis per gram of organic matter fermented showed that the additional nitrogen in the faeces may have been almost entirely microbial in origin.

In a second experiment, ground lucerne was progressively introduced into the diet in substitution for commercial pig meal. There was no obvious associative effect on the digestibility of the chemical components examined. Apparent digestibility of dry matter of the all-lucerne diet was 53%.

Forty-eight Suffolk cross lambs were reared artificially from 2 days of age. Sixteen entire males, 16 castrates (castrated by rubber ring at 2 days) and 16 female lambs were allocated at random to a set of 2×2×2 factorial treatments in which feeding was ad libitum or restricted for each of three growth periods—from 2 days old to 15 kg, from 15 to 25 kg and from 25 to 40 kg live weight.

The results showed no apparent interaction of sex and feeding treatment and differences due to sex were apparent only in the period after 25 kg live weight. Entire males grew faster and were more efficient at this stage although there was little difference in voluntary feed intake. Carcasses of the males were lower in fat and energy content and higher in protein content than those of females and castrates.

The feeding treatments in the main had similar effects to those shown in other studies in the series. Restricted milk feeding was compensated for by increased solid food consumption during the milk-feeding stage but in this experiment there was no tendency for restricted lambs to eat more and grow faster when fed ad libitum in the following period. Restricted feeding in the last period, particularly if preceded by restricted feeding, gave higher killing-out percentage, higher fat content and lower protein content in the carcass than feeding ad libitum. This finding was partly explained by the higher weight of the carcasses of the restricted lambs.

Five forages were given to three oesophageally-fistulated sheep held in metabolism cages. Estimates of the N, P and K contents of the forages were derived after correcting the composition of the extrusa for salivary contamination by using tritiated water (TOH) as a marker. Estimated N and P contents of the diet were generally higher, and estimated K content was lower, than in the diet. The estimates were variable and could not be used to estimate dietary N, P or K. It is suggested that the ratio of N, P or K to TOH of the pre-feeding saliva sample may have differed from the corresponding ratio during feeding, or that TOH may have moved independently of salivary N, P or K during feeding and processing. These difficulties might be overcome by marking each dietary component with its own isotope.

Estimates of dietary composition were also derived from analyses of squeezed extrusa without correcting for residual saliva or losses in leaching. The error with which dietary N content was estimated was then sufficiently low for most practical purposes. The equation relating dietary and extrusa N contents yielded similar estimates of dietary N to an equation derived in an earlier report.

Ninety-three Kellakui fat-tailed single lambs were assigned after birth to a 2×2×2 factorial experiment; 46 were uncastrated males and 47 were females. About half the lambs of each sex were docked before they were 1 week old. Half the lambs had unrestricted milk from the ewes until 115 days of age (phase 1) and the other half received all the ewes' milk for the first 30 days and then about one-third of the milk until weaning at 60 days. Creep feed was avail-able to all lambs from about 3 weeks of age, in addition to 200 g of alfalfa dry matter per lamb daily. From 115 days of age, all lambs were fed on a standard fattening ration until slaughtered at weights of 46 to 49 kg for males and 35 to 38 kg for females (phase 2).

Lambs given unrestricted milk were 7·4 kg heavier than restricted lambs at the end of phase 1. Significantly better gains due to docking were observed only for females during phase 2. There was little indication that docking affected feed conversion efficiency. In docked lambs the fat normally deposited in the tail was partially (less than 50%) relocated as subcutaneous plus intermuscular and internal fat. Lean meat percentages of carcasses were similar for docked and control treatments. There is a need to standardize reporting of results from docking experiments to make valid comparisons.

A total of 120, 6–7-month-old fat-tailed uncastrated Kellakui lambs were randomly allotted to three groups, given fattening diets for 93 days and then slaughtered. The diets contained 10 (A), 20 (B) or 30% (C) coarsely chopped wheat straw; the latter was increased at the expense of dried beet pulp. All diets contained 45% ground barley, 10% beet molasses and 1% ground oyster shells; there were minor adjustments in the cottonseed meal percentage to maintain a similar protein level in all diets.

The three diets were also given in sequence to six adult male Kizil (fat-tailed) sheep in a digestibility trial. Further digestion trials were made with chopped wheat straw given alone or supplemented with 400, 800 and 1200 g of a concentrate consisting of 75% dried beet pulp and 25% cottonseed meal; an all-concentrate treatment was also included.

Daily gain (g), feed efficiency (kg feed/kg gain) and dressing percentages for lambs given diets A, B and C were, respectively: 154, 9·78, 54·5; 159, 9·85, 53·5; 127, 11·30, 53·1. A significant decrease in daily gain occurred with the highest straw level and considerably more feed was required per unit of gain. Supplementing straw with 400 g of concentrate increased daily dry-matter intake per sheep from 645 g to 1214 g, with further but smaller increases at higher levels of supplementation. Almost linear increases were observed in dry-matter digestibility as more concentrate was given.

The records of 873 lambs from Ossimi and Barki backcrosses to Merino, and the first and second generations of their interbreeding were analysed. The traits included were: birth, weaning and yearling weights, first fleece weight and lamb mortality.

The interbred groups had about 10–15% lower body and fleece weights than their backcross parents, and almost double the lamb mortality. Differences between the interbred groups and the back-cross parents were predicted from the performance of purebreds, first crosses and backcrosses. There was good agreement between prediction and observation for birth and fleece weights, fair agreement for weaning and yearling weights, but lamb mortality increased more than expected.

Maternal heterosis seems to be more important than individual heterosis for body weight, and the reverse for fleece weight. Both components are large for lamb viability.

Three trials involving 68 Greyface and 31 Dorset Horn ewes were carried out to investigate the effect of intensity of husbandry at lambing time on the viability and subsequent performance of lambs. The results showed that the level of husbandry at lambing could have a large effect on the quantity of immune globulin absorbed by lambs. There was an indication that the quantity of immune globulin absorbed was related to the susceptibility of lambs to neonatal disease. A low level of husbandry at lambing appeared to have increased lamb mortality.

Two experiments involving the sequential slaughter of intensively fattened store lambs were each carried out over a period of 12 weeks. In the first experiment the lambs were offered a concentrate diet ad libitum. In the second experiment they were offered swede turnips ad libitum with a limited amount of concentrates.

In each experiment, similar groups of lambs were slaughtered at equal intervals in order to measure progressive changes in live weight, carcass weight and dry-matter intake. Carcass-weight gain as a proportion of live-weight gain was found to be 74% in the first experiment and 86% in the second experiment.