A theoretical study is reported of the way in which additive genetic improvement, made in a nucleus population, is passed to successive levels in a three-tier multiplication system. The rate at which progress accumulates at each level is approximately equal. The extent to which each tier is genetically behind the previous one has been termed the ‘improvement lag’. These lags can be considerable; their actual size is determined by the annual rate of progress in the nucleus, the age structure in the lower tiers, and by the origins and degree of selection achieved in the animals used as parents in these tiers. Only improvements generated in the top tier are cumulative. The scope for reducing the improvement lag is discussed within the context of livestock production in Britain.

Diets of cooked maize meal and dried skim milk were fed to newly weaned mice for 2 weeks, and to lactating females for 3 weeks. The diets ranged from 16% to 100% maize, with complementary amounts of from 84% to 0% milk and a 5% supplement of yeast.

Live-weight gains from 3 to 5 weeks of age were greatest when an intermediate diet containing 76% maize was fed. Average gains on 16%, 76% and 100% maize were 8·0 g, 17·7 g and 5·5 g respectively. The full sib phenotypic correlation was 0·45 but gains were independent of weaning weight. No carry-over effect was detected from maize-milk diets that had been fed to the dams; the dams' own gains at the same age had little influence on their offspring.

Weight of food consumed per mouse varied little between diets despite marked differences in gain. Food and energy intakes relative to gain showed no trend between 16% and 88% maize but doubled between 88% and 100% maize. Protein was used most economically for gains on 88% maize.

There were health problems. When high milk diets were first used many mice suffered from diarrhoea. A few mice on 95% maize, whose dams had been given the same diet during lactation, died without prior symptoms. Mice on 100% maize were thirsty, but not glycosuric.

Females nursing litters on high milk or high maize diets drew on reserves in the carcass, liver, pancreas, spleen and fat. In addition, high milk diets caused hypertrophy of kidneys and caecum, and diarrhoea.

Effects of constant and of changing diets on responses to selection were studied. Mice were fed maize meal and dried skim milk from 3 to 5 weeks of age and were selected for live-weight gains in that period. Gains in the generation prior to selection were 17·7 g on 76% maize (24% milk), 8·0 g on 16% maize and 5·5 g on 100% maize.

The Fixed optimum line was selected on 76% maize, the Milk line on 16% maize and the Maize line on 100% maize. The diets of three other lines were reviewed at each generation, and two diets were fed on each occasion. The diets of the Shifting optimum line straddled the optimum of the previous generation; those of the Milk-impelled line at first had a little more than the optimum amount of milk, but were made more milky as normal growth resumed; and the Maize-impelled line was similarly directed towards 100% maize. Back selected milk, optimum and maize sublines were taken off the three lines on constant diets. The Control line measured unimproved rates of growth on 16%, 52%, 76% and 100% maize.

Dietary effects were not always repeatable. Controls on the optimum diet, 76% maize, were the most stable across generations and with this diet, gains generally had the lowest phenotypic variance within generations —g2. With 100% maize, this variance was slightly larger—8 g2. The diet with the least stable effect was 16% maize; as Control gains rose from 8 to 15 g, phenotypic variance fell from 21 to 6 g2. On various diets between 88% and 96% maize, the variance of the Maize-impelled line rose from 6 to 34 g2 without commensurate changes in its mean gain.

Responses of generally about 5 g above controls were made in five generations of selection. The Maize line was exceptional in making no forward response. Heritability of growth between forward and back selected sublines was highest on good diets (0·49 on 76% maize) and lower on poor diets (0·20 on 16% maize and 0·28 on 100% maize).

The Fixed and Shifting optimum lines do not as yet differ on comparable diets; nor the Milk and Milk-impelled lines on 16% maize. The Maize-impelled line has not yet been tested on 100% maize and cannot be compared with the Maize line. Family-by-diet interactions were smaller than consistent family differences. It is concluded that genotypic improvements may be effective over a wide range of maize-milk diets.

1. An experiment was conducted with dairy cows over two grazing seasons to investigate the feasibility of fixed rotational grazing. Each year a comparison was made between treatment A, which consisted of four equal paddocks grazed for 1 week and rested for 3, and treatment B, which contained five paddocks grazed for 1 week and rested for 4. The influence of the length of the rest periods on output per hectare and per cow was measured. Within each paddock approximately one-seventh of the area was allocated daily to each treatment group, there were no back fences and the paddocks were not topped or harrowed. Stocking rates were identical on both treatments at all times at 6·2 cows per ha throughout 1964; 7·1 cows per ha for weeks 1·9 inclusive and 6·2 cows per ha for weeks 10·20 inclusive in 1968. The fertilizer nitrogen applied (282 kg/ha) was equivalent to a daily rate of 2·02 kg/ha. The feed intake of each cow was measured on three occasions during 1967 and four in 1968.

2. Differences between treatments in outputs per ha were small and on average about 13 000 kg milk and 890 cow grazing days were produced per ha during the 20-week experimental period.

3. Differences between treatments A and B in herbage intake, milk production, milk composition and live-weight change were not statistically significant.

4. When 3·6 kg of a supplementary concentrate was given to some cows, positive responses in milk production, milk composition and live-weight change were found, but the value of the extra milk produced was less than the cost of the concentrate.

1. Jersey calves were fed on milk substitute at the rate of 10% of live weight per day for 5 weeks and were offered either chopped dried grass or ground, pelleted dried grass to appetite. Calves were slaughtered at birth and after 5, 6, 7, 9 and 12 weeks on experiment, and the weights of the component parts of the alimentary tract and their contents were measured.

2. There were close correlations between the weights of the sections of the alimentary tract or their contents and the solid food intake at slaughter, total solid food intake, or empty body weight of the calves. There were no consistent changes with age in the DM concentration or the density of the digesta in any section of the alimentary tract except the abomasum, or in the digestibility or mean retention time of the diets.

3. Grinding and pelleting the diet increased the DM concentration but did not affect the density of the digesta. The increase in the weight of rumen contents per unit increase in dry-matter intake was much greater for chopped than for pelleted diets. The reverse was true for the contents of the abomasum and small intestine. There were relatively small variations between diets in the predicted weight of rumen contents, total digesta, or the volume of organs in the abdomen, at the within-diet mean levels of dry-matter intake.

1. The primary growth of a sward of S24 perennial ryegrass was harvested between 23 April and 9 May 1968, artificially dried and either chopped or coarsely milled. The two physical forms of grass were wafered either alone or together with 50% by dry weight of rolled, anaerobically stored barley, and given to 7-month-old, Hereford × British Friesian steers in an experiment of 2 × 2 factorial design.

2. A representative group of animals was slaughtered at the beginning of the experiment, and the remainder were individually fed ad libitum for 70 days, after which they also were slaughtered. During the feeding period digestibility was determined by total faecal collection from all the animals individually on two occasions.

3. Dry-matter intake and live-weight gain were similar on all feeds. The weight of gut contents as a percentage of final live weight was significantly lower and the carcass-weight gain was significantly higher for animals given wafers containing barley compared with those for animals given wafers of grass alone.

4. Coarse milling compared with chopping reduced the apparent digestibility of dry matter, organic matter, cellulose and energy; the addition of barley increased the digestibility of organic matter but further reduced that of cellulose.

5. The efficiency of conversion of digested energy to empty-weight or carcass gain was similar for wafers containing the two forms of forage, but the digested energy of wafers containing barley was converted to carcass weight with significantly greater efficiency than that of wafers consisting solely of grass.

6. Physical separation of the tissues of sample joints failed to show any significant differences in the proportion of fat, muscle and bone as a result of either differences in physical form or the inclusion of barley.

7. The results indicate that live-weight gain is an insufficiently sensitive measurement by which to assess the true productive potential of feeds given to ruminants in short-term experiments.

1. Young cattle were fed on four types of wafer containing chopped or coarsely-milled dried grass, with or without 50% of barley.

2. At slaughter, the alimentary tract of each animal was removed and the contents of each part were weighed and sampled to determine the extent and site of digestion. Chromic oxide was used as an indigestible marker.

3. The form in which the grass was processed had no effect on any of the parameters measured.

4. The dry-matter content of the digesta in all parts of the alimentary tract was significantly higher when barley was included in the wafers. The inclusion of barley decreased the proportion of acetic acid and increased the proportion of butyric acid in the rumen liquor. The proportions of propionic and valeric acids were unaffected. Barley also appeared to reduce the proportion of digestible dry matter which disappeared in the forestomachs (31·44%) compared with that when grass was given alone (42·59%).

5. With grass alone about 70% of the apparently digestible cellulose disappeared from the forestomachs, but the technique used did not allow a good estimation of these proportions when mixed diets were given. The retention time of dry matter in the forestomachs was not affected by the diet but the inclusion of barley in the wafers increased the retention time of cellulose in the forestomachs.

6. Of 1·6·1·9 kg of starch ingested in mixed diets, 92-96% was apparently digested in the forestomachs, and only 80-130 g per day escaped rumen fermentation. The intake of starch on diets of grass alone was negligible and almost all the soluble sugars were digested in the forestomachs.

7. More N was recovered at the omasum than was ingested, but the different diets had no effect on the site of digestion of N.

Two experiments were conducted to study the effect of type of cereal and time of weaning on the performance and voluntary intake of lambs.

In the first experiment 20 early-weaned entire male lambs were given diets ad libitum during the fattening period from 15 to 40 kg live weight. The diets were based on either kibbled maize or rolled barley. A high feed efficiency and growth rate were achieved with both diets and differences between them were not statistically significant. The growth rates were 428 and 430 g/day. The cold carcass weights at 40 kg live weight were 19·0 and 19·9 kg and the conversion ratios of dry matter to live-weight gain were 2·33 and 2·22 for the rolled barley and kibbled maize diets respectively.

In the second experiment a comparison was made of the performance of 36 sets of twin lambs, one of each set being weaned at either 6, 13 or 20 weeks of age and the other being left to suck the ewe outdoors. The lambs weaned at 6 weeks had better conversions of dry matter to live-weight gain than lambs weaned at 13 and 20 weeks. The respective conversions of dry matter to live-weight gain were 3·36, 5·64 and 5·52. The lambs weaned at either 13 or 20 weeks had a greater weight at weaning, a greater growth check at weaning and a lower rate of feed intake.

1. Dorset Horn ewes were fed individually to make high (H) and low (L) gains in weight in the last 6 weeks of pregnancy. The gains were 20% (H) and 10% (L) of the live weight in week 14 of pregnancy for ewes with twin foetuses, and 16% (H) and 6% (L) for ewes with a single foetus. Three levels of intake imposed during lactation were denned as low (L) 15·0, medium (M) 22·5, and high (H) 30·0 g of feed dry matter/kg of ewe live weight in week 14 of pregnancy. The lambs were removed 12 to 16 hours after parturition and the ewes were machine-milked twice daily for the first 6 weeks of lactation.

2. Ewes making low gains in pregnancy produced lambs with slightly lower birth weights but the difference was not significant for either twins or singles. Pregnancy treatment had no effect on milk yield or live-weight change in lactation, but fat content was increased in milk from ewes on the H treatment in pregnancy (8·06% v. 7·46%).

3. Level of feeding during lactation did not significantly affect milk yields. The total yields in the first 6 weeks of lactation were (L) 49·4 kg, (M) 60·8 kg and (H) 62·7 kg. However, there were significant effects on milk composition. The treatment means for fat content were (L) 8·34%, (M) 8·05% and (H) 6·84% and for solids-not-fat content were (L) 10·04%, (M) 10·37% and (H) 10·82%.

4. Live-weight losses in lactation were (L) 9·1 kg, (M) 6·3 kg and (H) 3·2 kg.

The distribution of chemical fat throughout the major depots of the fleece-free empty body was studied in 30 mature Scottish Blackface ewes from two sources and covering a wide range of fatness.

Relationships between amounts of chemical fat in individual depots and total body fat are presented. These provide information regarding rates of change in the distribution of fat with changing fatness. Information is also presented which allows estimates of the distribution of chemical fat throughout the body to be made from subjectively assessed condition scores.

The wool growth cycle in three rams and three ewes was studied by taking monthly wool and skin samples for periods up to 3 years. The percentage of inactive follicles and of medullated fibres was recorded, fleece length was measured and change in fibre diameter and pigmentation between summer and winter was investigated.

Casting of the fleece appeared to depend more on the thinning of fibres than on the proportion with brush ends, casting in some instances taking place with only 30% brush ends, while not in others with as much as 70% follicle inactivity.

Follicles began to become inactive in both sexes in September following the autumnal equinox. The secondary follicles had a greater proportion of inactivity (mean of 45% compared with 20% in the primaries) but tended to regrow earlier, and regrowth in the rams tended to begin before that in the ewes. Regrowth as a whole began from January to March. Subsidiary cycles of inactivity of about 2 months' duration were evident.

Maximum growth in length occurred between April and October. A difference in mean diameter of only a few microns could be detected between summer and winter. Maximum medullation occurred in July in the rams and August in the ewes. Decrease in pigmentation in winter was brought about by a lowering of the density of pigment granules within the fibres.

1. Two experiments have been carried out to determine the effect of a high air temperature, 32·33°C, on growth and backfat depth of pigs, by comparison with a thermo-neutral air temperature, 22·24°C. The diet consisted predominantly of whey, since it was expected thatthis would enable appetite to be maintained at the higher temperature.

2. Food intake was reduced by 5% to 7% during the period of exposure to 32°C in Experiments 1 and 2 respectively. Growth rate and FCE were reduced significantly only in Experiment 2.

3. Backfat depths measured above the eye muscle were significantly increased by the hot treatment in both experiments; those measured on the split carcass were consistently greater for the hot treatment.

Three trials, each involving 24 pigs, were conducted to assess the value of Hormonexa as a feed supplement for pigs slaughtered at 90 kg live weight. Rations were compounded from a variety of feeding stuffs to different protein levels and the pigs were fed on various nutritional planes. Hormonexa was added to these rations at the rate of 1·32 kg per 1000 kg meal.

In all three trials growth rate and food conversion efficiency were not affected significantly by the addition of the supplement. There was a tendency for the pigs receiving the supplement to be slightly leaner but in only 1 out of 39 measurements of fat thickness was the difference between control and supplemented pigs statistically significant (P< 0·05).

No benefit from this feed supplement has been demonstrated in terms of animal performance or carcass quality under a range of feeding systems.

A total of 64 nine-week-old pigs were used in two experiments to study the influence on feed intake, growth and carcass quality of dietary restriction at various ages or live weights. Two diets of different energy content were given throughout each experiment, which was divided into four periods on either an age or weight basis. Feed was restricted or offered ad libitum during each of the first three periods, all pigs being fed ad libitum during the last period.

Compensatory intake following a period of feed restriction occurred at most stages of growth but was most marked in the early stages. Compensation when it occurred took several weeks to develop fully and persisted for some time, often after an intervening period of ad libitum feeding. Increased gains resulted from increased intake although some of the early compensatory gains could not be fully explained by increased intake.

There were few appreciable differences between the pigs on the highand those on the low-energy diets. However, in the early stages the latter did not show the same response in intake following restriction as did the former (but in experiment I the latter were considerably lighter).

No effects on carcass quality were demonstrated under the conditions of these experiments.

Forty-eight pigs were used to compare the feeding value of barley which had been stored dry, and then hammer-milled, with that of barley stored moist, either anaerobically or treated with 1·3% propionic acid, and then rolled. The pigs were fed from 30 kg live weight for a period of 11 weeks and during this period all consumed the same allowance of dry matter. The growth rate and feed conversion ratio of the pigs given the rolled moist barley were significantly poorer than those of others given dry milled or acid treated rolled barley. There were no treatment differences in carcass attributes.

Microbiological counts made throughout the experiment showed that there was no significant fungal or bacterial development on the acid treated barley. Similar numbers of bacteria were present on the dry barley as on the moist, but the fungal count for the moist grain was always at least 500 times higher than that of the corresponding dry sample. The dominant organisms were identified.

In a further experiment the apparent digestibility of dry matter and nitrogen of diets containing dry, moist or propionic acid treated barley, processed either by milling or rolling, were determined. The highest values were obtained for dry barley. Acid treated moist barley had lower values than moist barley. The method of processing did not affect the apparent digestibility of the dry matter, but that of the N was significantly lower for the rolled barleys.

Ventilation of hay with oxygen increased oxygen consumption (P< 0·001) and carbon dioxide formation (P<0·01) of two steers, when compared with nitrogen-ventilated hay. Heat production was about 5% higher in the oxygen-hay period than in the nitrogen-hay period (P< 0·001). No difference in digestibility of nutrients was observed between these treatments.

The voluntary intake of water by 24 groups of 3 to 6 Large White pigs (range of mean body weight: 21 to 73 kg) was measured during a total of 48 periods lasting 3 to 12 weeks each in a large calorimeter equipped as a pig pen. The calorimeter temperature was controlled between 7° and 33°C and the pigs were fed at levels ranging from 42 g feed/kg body weight per day to maximal intake. The mean water: feed ratios were between 2·1 and 2·7 at temperatures between 7° and 22°C, and between 2·8 and 5·0 at 30° and 33°C. The range of mean water consumption extended from 0·092 to 0·184 litre/kg body weight per day.

From among a group of ewes in which oestrus had been induced following progesterone and PMS treatment and which had been mated in the spring, those lambing in early October were used for this experiment. They were divided into three treatment groups: (1) reared their lambs for 24 hr, (2) reared lambs for 16 days and (3) reared lambs for 75 days.

For the ewes rearing single lambs only in groups 1, 2 and 3, 100% (8 of 8), 93% (13 of 14) and 65% (11 of 17) respectively exhibited oestrus between lambing time in autumn and early winter (P<0·05). The average interval to post-partum oestrus was 29, 37, 50 days for group 1, 2 and 3 respectively. Group 3 exhibited first oestrus later than group 1 (P<0·01). These data show that lactation even during the peak of sexual activity has some suppressing effect on both the percentage of ewes showing oestrus and on delaying the onset of oestrus exhibition.