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 amylase (Am I) genotypes have been determined in sera from 834 cows comprising 562 Friesians, 77 Ayrshires, 106 Guernseys, 62 Jerseys and 27 crossbred cows. All of these cows had completed their third lactation and many were much older. There was a highly significant excess of heterozygotes at the Am I locus. This excess of heterozygotes was highest in high-yielding Friesian herds and there was a significant correlation between percentage heterozygotes and herd yield in the Friesians.

1. In two experiments, each using three groups of six littermate castrated male pigs per group, the effects on cereal composition, apparent digestibility and nitrogen retention of micronizing barley and maize were studied.

2. The process increased the dry matter (DM) content and in vitro starch availability and decreased the nitrogen content of both cereals. When maize was flaked (rolled) subsequent to the process the ether-extract content was markedly decreased.

3. Nitrogen retention was not affected by the process but apparent digestibility of the DM, nitrogen and gross energy was improved. These effects were most marked when the process was followed by flaking, the improvements in such cases in digestible energy content over the unprocessed grain being 4·3% (P>0·05) for barley and 6·6% (P<0·05) for maize.

4. Soaking the unprocessed and micronized and flaked cereals before feeding did not significantly affect the results.

1. In each of two 2 × 3 factorially designed experiments 72 pigs were individually- and scale-fed diets containing 86·5% of maize, wheat or barley in ground or micronized, flaked and ground (micronized) forms.

2. In both experiments, the first between 17 and 20 kg and the second between 22 and 90 kg live weight, pigs offered the micronized maize and barley diets grew significantly faster and converted the dry matter of their diets significantly more efficiently than did pigs offered the ground maize and barley diets. There were no significant differences between the micronized and ground wheat diets in the first experiment but in the second experiment the micronized diet gave significantly inferior results.

3. In pigs slaughtered at 90 kg live weight those fed all the micronizedbased diets had significantly higher killing-out percentages than those fed the diets based on the ground cereals. Other within-cereal significant differences were those of increased carcass length in pigs given micronized maize and increased fat deposition in pigs given micronized wheat.

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.

1. Four British Friesian steers were reared to a live weight of 340 kg as pre-ruminant animals, by the use of milk substitutes based on liquid skim milk, spray-dried whey powder and an ultra-high-fat (68%) milk powder.

2. Except for one case of hypomagnesaemia, the steers remained healthy. Live-weight gain/day, age at slaughter and feed conversion ratio were 1·23 kg, 253 days and 2·31 kg dry matter/kg weight gain respectively.

3. No marked development of the rumen had occurred; gut fill in relation to fasted live weight was 3·4% and killing-out percentage was 61·2.

1. The experiment was designed to investigate the effect of growth at a high ambient temperature, 33–35°C, on the protein and energy metabolism of pigs, compared with 25°C. The high temperature caused an increase in rectal temperature of 1·4° to 1·7°C. The animals were given two low levels of feeding while growing between 25 and 70 kg live weight; energy and nitrogen retention were measured periodically.

2. The apparent digestibilities of the dietary dry matter and energy were decreased and urinary nitrogen losses were increased at the high temperature. Heat production was increased at the high temperature, by between 2% and 10%, the effect becoming more pronounced during the course of the experiment. Retention of energy and nitrogen was reduced at the high temperature.

3. The net efficiency of utilization of metabolized energy above maintenance was between 63% and 67%, and was not significantly affected by live weight or temperature. The results suggested that maintenance requirement was increased at the high temperature.

4. Analyses of sample joints indicated that the high temperature caused an increase in fat percentage only at the higher level of feeding; this result agreed with fat and protein retention calculated from the balances.

1. Three groups of six castrated male cattle aged 6,18 and 36 months and three corresponding groups of sheep received in turn three diets consisting of high-quality dried grass (A), low-quality grass (B) or 60% of B with 40% barley (C). For one-half of each 6-week period the grass was eaten in the long form (L) and for the other half, it was ground and pelleted (P).

2. Pelleting increased intake by 45% in sheep, from 56·8 to 82·4 g dry matter per kg W-75 per day, but only by 11% in cattle, from 81·8 to 90·7. The increase was greater for diet B (44%) than for A (19%) or C (15%), and greater for the youngest animals (38%) than for the middle-aged (17%) or oldest (20%).

3. Dry-matter digestibility was reduced by pelleting from 67·2% to 58·6% in sheep and from 69·9% to 56·9% in cattle. The reduction was greater for diets A (71·2% to 56·1%) and B (65·7% to 54·1%) than for diet C (68·7% to 63·2%).

4. A hypothesis based on the importance of particle size of digesta leaving the rumen provides a possible explanation ofthese interactions between form of roughage on the one hand, and species and age of animal or diet composition on the other.

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.

Fibre number and fibre size were measured in the flexor digiti V brevis muscle and other muscles in pigs of different breeds. A large variation in fibre number (200%) was found among the breeds. However, the fibre number in the flexor digiti V brevis was closely correlated with fibre number of other muscles in the same animal. Fibre number and size measurements were also carried out on animals at different ages taken from a relatively pure breed of pig. Fibre size was found to increase to differing extents in different muscles, although the increase in the flexor digiti V brevis was almost the same as in the longissimus dorsi. Fibre number did not change during growth. On the basis of these findings it is suggested that the flexor digiti V brevis is a suitable indicator muscle for genetic experiments.

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.

An experiment designed to study the response in milk production to dried grass given as an addition to a basal roughage diet and to assess its value as a replacement for conventional dairy concentrates is described. A randomized block design with 34 lactating cows was used. The cows received supplements of either dried grass (dry-matter digestibility, 70%, 0·28, 0·39, 0·50 or 0·61 kg/kg milk) or a cereal-based dairy concentrate (0·39 kg/kg milk) in addition to silage offered in amounts to meet the maintenance energy requirements. Milk yield, milk energy output, live-weight gain and the fat, solids-not-fat and protein contents of the milk increased with level of addition of dried grass. Interrelationships between the level of feeding of dried grass and milk output and body-weight change are given. The derived relationships between dried grass input and animal output were used to predict the replacement value of dried grass for conventional dairy concentrates. These indicated that dried grass offered at 042 kg/kg milk would produce equivalent milk yield and body-weight changes to concentrate given at 0·39 kg/kg milk. Ration digestibility and nitrogen balance results are also given.

Multiple birth percentage and wool production were studied on 338 ewes (907 lambing and 873 shearing records) representing two purebreds, Oxford (O) and Suffolk (S), and four crossbreds, O × S, S × O, Cheviot (C) × O and C × S mated to purebred and crossbred rams during 9 years. Oxford ewes had 24–32% (P<0·01) fewer multiple births than ewes of the other groups, while little difference between S and the crossbreds was observed. The heterosis of the OS and SO groups was 14·6 ± 4·1%. Multiple births tended to increase 0·8% for each kg increase in body weight of the dam (r = 0·13). The repeatability estimate for multiple births was 0·24. Fleece weight was significantly affected by age and by breed group, with OS and SO crosses producing 17% more wool than their parental breeds. The repeatability of fleece weight was estimated at 0·52 ± 0·15. Fleece weight was significantly correlated with multiple birth percentage (0·09) and body weight (0·32).

Suffolk ewes averaged 9·5 kg heavier than O ewes throughout their reproductive life. The two reciprocal crosses were heavier than both parental breeds. Suffolk ewes reached their maximum body weight at 4 years of age while Oxfords continued to gain weight up to 6 years.

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.

The effectiveness of silastic rubber implants impregnated with melengestrol acetate (MGA) to suppress heat in heifers being finished for beef was tested. Nine uniparous females received MGA implants, nine received 0·5 mg MGA orally and eight similar non-treated animals served as controls for a 110-day experimental period. Based on occurrence ofheat and ovarian examination during treatment and at slaughter, the silastic implants were effective in suppressing heat throughout the experimental period. The animals with implants also had a significantly higher rate of daily gain than the controls (P<0·1). In a second experiment the implants were effective in suppressing heat but the increase in daily gain did not reach significance. Silastic implants of MGA are effective for the long-term suppression of oestrus in finishing heifers and their effective life span is sufficiently long for use under commercial farming conditions.

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%.

Pushing the testes of lambs 1 to 2 weeks old into the body cavity and applying a rubber ring to cause atrophy of the empty scrotum, did not prevent most testes growing in a subcutaneous position. A modified technique used in a second experiment, which caused ablation of the tunica vaginalis as well as the scrotum, impeded more effectively the descent and growth of the testicles.

In the two experiments growth rates of the induced cryptorchids were, respectively, 15·3% and 6·0% (non-significant) greater than those of castrated males, dressing percentages were about 2 units lower, and fat depth over the eye muscle of 13th rib chops was 2·6 compared with 2·9 mm (non-significant) and 1·9 compared with 4·1 mm. Chops from the cryptorchids had a visual appeal similar to those from castrated male and from female lambs; differences in flavour detected by taste panel had no significant effect on overall flavour acceptability.

Tenderness did not differ significantly between sex groups, but age and Warner-Bratzler shear values were significantly and positively correlated as were ultimate pH of the meat and age at slaughter. Cryptorchidism induced by the modified technique probably permits the production of heavier carcasses with lower fat cover than on castrated males and avoids practical disadvantages that may be encountered in raising entire male lambs.

Apparent mortality among 280 215 embryos from the 4th to 16th week of gestation was studied from returns to service records following first inseminations with fresh semen from 100 bulls of five dairy and three beef breeds of cattle over a period of 8 years.

The average incidence of apparent mortality was 20·3% for embryos conceived by fresh semen first inseminations. Differences in rates of apparent embryonic mortality between breeds of sire and also between sires within the Guernsey, Ayrshire and Dairy Shorthorn were statistically significant although confounded with the breed of dam. The highest incidence was from matings by bulls of the Channel Island breeds. These results suggest that hereditary factors influenced embryonic mortality in cattle.

The rate of apparent mortality was higher among embryos conceived by frozen semen than by fresh semen. Ageing of fresh semen extra-corporeally also increased the rate of apparent embryonic mortality but to a lesser extent than frozen semen.

The rate of apparent embryonic mortality increased with the greater number of inseminations required for conception possibly because of a deteriorating maternal environment.

Concentrations of copper in blood plasma were determined in January and in February 1971 and in February and March 1972 in a grassland flock of sheep comprising the Finnish Landrace and (Tasmanian) Merino breeds and their reciprocal crosses. Numbers of animals ranged from 30 to 100. The unadjusted mean copper levels of the flock varied from 63 to 81 μg/100 ml. The Finnish Landrace had a markedly lower plasma copper concentration than did the Merino, the mean difference after adjustment for other factors varied from 16 to 54 μg/100 ml. First crosses had levels halfway between those of the parental breeds. Copper concentration fell in the 4-week period between February and March 1972 when most of the ewes were pregnant. This fall was greatest for the Finnish Landrace which started with the lowest level and least for the Merino.

Neither the breed differences in the change in level nor the actual concentration appeared to be related to the number of lambs (0 to 4) carried by the ewes. Variation in copper level associated with the age of ewe, significant for the two bleedings in 1972, was largely attributable to the higher level among the youngest age class, approximately 10 months old when bled. The stage of pregnancy at the time of blood sampling, as reflected by date of lambing, did not significantly affect plasma copper concentration in this flock.

Sixty Scottish Blackface ewes were used in a 2 × 2 factorial experiment in which two levels of metabolizable energy (ME) were given during the final 6 weeks of pregnancy. In two treatments 1600 and 2000 kcal M E were given daily over the total period while in the remaining treatments daily ME intakes were 1200 and 1600 kcal ME during the first 3 weeks of the feeding period and 2000 and 2400 kcal ME during the last 3 weeks. Digestible crude protein (DCP) intakes were constant at approximately 30 g per head daily in the constant energy treatments and 15 and 45 g per head daily in the first and second periods respectively for the low-high energy treatments.

Energy intake had no statistically significant effect on lamb birth weight nor on ewe net body-weight change (change from the start of the experimental feeding period to immediately post partum). Ewes on low-high energy intakes had a significantly lower net body-weight loss than did ewes on constant energy intakes. Pattern of feeding had no significant effect on lamb birth weights. Negative nitrogen balances were found during the first feeding period where the daily DCP intake was approximately 15 g per head.