1. Seventy-two Blackface ewes with 96 Greyface (Border Leicester ♂ × Blackface ♀) lambs were allocated to three groups. Twenty-four ewes with 32 suckling lambs were set-stocked on 2·4 hectares (ha) of grass (A); 32 lambs were weaned and set-stocked on 2·4 ha of grass (B) and 32 lambs were weaned, housed and given concentrates (C). Beef cows with calves were used to control the grass in the two grazing groups.

2. The average initial live weight of the lambs was 18 kg. Treatments had a significant effect on performance, the live-weight gains per lamb over the initial 35 days being 11·2, 9·1 and 5·9 kg for groups A, B and C respectively. The total gains over 70 days, when the suckled lambs (A) were weaned, were 17·6, 15·5 and 14·3 kg.

3. The total live-weight gain per ha from lambs and calves over 88 days was higher on treatment B than on treatment A. The lambs weaned on to grass took 9 days longer than the suckled lambs to reach a slaughter weight of 42 kg and had significantly poorer killing-out percentages.

4.The lambs given concentrates had an overall food conversion efficiency of 4·89 kg concentrates per kg live-weight gain.

Four Finnish Landrace and four Scottish Blackface rams were exposed at 21-day intervals over a period of 19 months to two ovariectomized ewes, which had been induced to show oestrus. The behaviour of each ram was observed for 10 minutes with each ewe. Over the period of the experiment the Finnish Landrace rams consistently mounted the ewes more often than did the Blackfaces and in this respect in particular the reproductive behaviour of the males of these breeds was found to be positively correlated with that of the females of their breed.

Initially, however, the Finnish Landrace rams mated more ewes than did the Blackfaces, but from the time that all rams became sexually active it was impossible to differentiate between the ability of the two groups of rams to mate the ewes presented to them. Semen collections from each ram were attempted by the use of an artificial vagina, again at three-weekly intervals. The motility, density and proportion of live sperm did not differ between the two groups of rams.

The average number of mounts accredited to each ram during the test increased from around 5 in the summer to around 35 in the autumn.

A total of 156 pigs of the Large White breed were used in a series of four experiments to test the suitability of barley which had been stored with a high moisture content as an ingredient in the diets of growing pigs. Barleys with mean moisture contents of 26·3%, 28·1% and 21·2% and a range from 19·6% to 30·5%, from three successive harvests were used.

In all the trials there was evidence that the use of moist barley in diets for growing pigs had an adverse effect on live-weight gain and feed conversion ratio, the effect being independent of whether feed intakes were equated on the basis of the dry-matter content of the diets. In one of the experiments performance was depressed to the extent of causing changes in carcass characteristics. The performance of pigs given rolled barley was similar to that of others given ground barley.

A numerical model of AI selection procedures has been constructed in an attempt to assess the efficiency of current selection methods and to examine possibilities for their improvement. The results indicate that (1) present selection methods are relatively efficient in their utilization of test resources, but much less efficient in their usage of tested bulls. Additional investments in bull testing are likely to profit the dairy industry. (2) The main effects of improved bull utilization are (a) to permit increases in selection pressure with no increase in cost over current selection procedures and (b) to extend the limits to profitable selection. (3) The adoption of alternative stud management systems, in which bulls are slaughtered early in life, are likely to require excessively large investments in semen processing and storage.

The potential benefits to be obtained from the adoption of performance testing for meat production in a dual-purpose breed have been examined and contrasted with those obtainable from improvements in dairy progeny test procedures.

It was concluded that capital investments in performance testing are likely to prove profitable, but less profitable than similar investments in the improvement of the dairy progeny test. Where performance testing is practised, there appears to be little advantage to be gained from the application of selection rates in excess of 1 in 3.

Two progeny tests for beef, one using 452 calves brought centrally to a test centre, the other using 768 calves commercially fattened on 81 farms, were compared to isolate environmental sources of variation. It was concluded that appreciable reduction of error variance ensued from the use of a growth curve, the maintenance of pen identity, and the removal of variation due to pens and months of birth. The fixed effects model underlying the experimental design on the centre yielded relatively robust estimates of variance components.

Herbage from the same timothy/meadow fescue/white clover sward was ensiled at four different dry matter contents. The resulting silages had dry matter contents of 19·0, 27·3, 32·3 and 43·2%, the dry matter content increasing with the length of the wilting period. An experiment was carried out to determine the voluntary intake of the silages. Each silage was given to 7 animals individually, the mean live weight of these being 334 kg.

Although the silages made from wilted herbage were lower in digestibility than that made from unwilted herbage, wilting increased dry matter intake and metabolizable energy (ME) intake. The mean daily intakes of digestible organic matter were 53·0, 58·1, 59·6 and 59·6 g/kg W0·73, for silages of increasing dry matter content. The corresponding ME intakes, expressed as a multiple of the ME requirement for maintenance, were 1·17, 1·29, 1·30 and 1·28. The percentage of acetic acid in the silage dry matter was significantly (r = −0·56) and linearly related to voluntary intake. The relationship between lactic acid concentration and voluntary intake was significantly curvilinear (r = 0·48).

A total of 64 Border Leicester × Greyface ewes were offered eight diets comprising four levels of digestible crude protein and two levels of metabolizable energy (ME) intake during gestation. After parturition the ewes on each gestation treatment were offered a high or low level of ME intake at a constant level of crude protein intake. Nitrogen balances were carried out on all ewes during weeks three and four of lactation.

Digestibility coefficients and output of nitrogen in the milk measured during the lactation balance periods were not significantly affected by the gestation treatments. Body nitrogen balance during lactation was significantly affected by the level of protein offered during gestation.

Empirical equations relating digestible crude protein intake and ME intake to milk nitrogen production and body nitrogen balance indicated that a digestible crude protein intake of 7·9 g/kg W0·75 per day and a ME intake of 220 kcals/kg W0·75 per day resulted in the production of 0·70 g milk nitrogen per kg W0·75 or approximately 100 g of milk per kg W0·75. The corresponding level of body nitrogen retention was 0·162 g/kg W0·75 per day and showed no consistent trend due to level of protein intake during gestation.

1. Experiments carried out in two herds to examine the effects of a treatment designed to induce oestrus and ovulation during the lactational anoestrus of the sow and to allow the establishment of pregnancy during lactation are described. Two aspects are examined: (a) the effects on the reproductive performance of the sow of repetition of the treatment in successive lactations and (b) the effects of the treatment on the suckled young.

2. The treatment involved the separation of each sow and litter for 12 hr on days 21, 22 and 23 of lactation followed by an injection of 1500 IU pregnant mare serum gonadotrophin (PMSG) at the end of the third period of separation.

3. It is concluded that oestrus and ovulation can be induced with some consistency in the fourth week of lactation, that lactation is not incompatible with the first two to three weeks of pregnancy, and that the establishment of pregnancy during the fourth week of lactation is not detrimental to the growth of the suckled young when supplementary food is provided.

The experimental animals consisted of 71 heifers or cows of the Black and White Danish breed (SDM), 61 heifers or cows of the Red Danish breed (RDM), and 8 heifers or cows of the Jersey breed. Highly significant differences between breeds were found in yield of colostrum at first milking after calving and in the concentration of immunoglobulin (Ig) in colostrum. The RDM animals had the highest colostrum yield but the lowest Ig %. The total yield of Ig was not significantly different between RDM and SDM. The individual variation in colostrum yield, Ig %, and Ig yield was very marked.

Heifers had a lower colostrum yield and Ig yield than cows in second and later lactations. For SDM the Ig % was not dependent on the lactation number. In contrast the older RDM cows had a significantly higher Ig % in colostrum than the younger cows. No effect of season of the year was found on colostrum yield, Ig %, and Ig yield. An increase of the interval between calving and the first milking caused a very significant drop in colostrum Ig %. Loss of colostrum from the udder before milking increased the probability of getting colostrum with low Ig %. The repeatability in Ig % at successive calvings was found to be high.

One hundred and forty-one calves, belonging to three different breeds, were each given one single feeding of a variable amount of colostrum from their dams at the age of 2, 6, 10, 14 or 20 hr, respectively.

The increase in serum Ig concentration during the first 24 hr after colostrum feeding (Δ Ig % 24) was a function of the mass of Ig fed to the calf, the age at colostrum feeding, and the birth weight of the calf. Among these three factors the mass of Ig and the age of the calf were the two predominant factors. The absorption coefficient, expressing the absorbed fraction of a given amount of Ig, was primarily determined by the age of the calf at first feeding. Thus the absorption coefficient was reduced linearly to about half by delaying the feeding from 2 to 20 hr. Except for a negative effect of increasing amounts of colostrum in one of the breeds, no other factors were detected as responsible for variation in the efficiency of Ig absorption. Thus, the absorption coefficient appeared to be unaffected by the Ig % in the colostrum and also by the quantity of Ig given to the calf.

Previous studies have shown that reciprocal recurrent selection (RRS) would be ineffective in the early cycles of selection if over-dominant loci were at equilibrium gene frequencies in both selected populations. It is shown in the present study that response to RRS may be reduced by this unstable equilibrium even when gene frequencies are as much as 30 % removed from the theoretical equilibrium frequency. Reducing one population to a bottleneck of two individuals for one generation or more before initiating RRS (RRSB) was very effective in overcoming the unstable equilibrium. RRS with recurrent inbreeding, then outcrossing in both populations each cycle of selection (RRSC) was not effective in overcoming the unstable equilibrium, but yielded greater response per cycle after selection response began. The effectiveness of RRSC was inversely proportional to the heritability of the trait. Use of these modifications to increase the effectiveness of RRS in poultry breeding is discussed.

Selection based on average carcass densities of sibs was effective in breeding for leanness and for fatness in Large White pigs. Four herds were established for the five-year experiment, in which 615 pigs were slaughtered at bacon weights. Responses were obtained with both feeding to appetite and restricted feeding, but it was not possible to decide which was the preferable method.

Computer simulations using parameters based on results from experiments showed that by chance a certain frequency of hypo-gammaglobulinaemia would always occur. This was due to variations in birth weight, concentration of immunoglobulin in the ingested colostrum, amount of colostrum offered, and age at first feeding. It was not necessarily associated with an absorption block as sometimes postulated. The probability of an individual getting hypogammaglobulinaemia can be minimized by early feeding of a large quantity of colostrum at the first feeding after birth.

A temporary tremor condition in adolescent pigs is described which shows similarities with a tremor condition known to occur in young piglets. Growth and carcass traits were on average superior in affected pigs compared with unaffected litter-mates, paternal half-sibs and genetically unrelated pigs. Females were over-represented among the affected pigs which were born in large litters.

The relation between the weight of the alimentary tract contents and the weights of body fat and the uterus was examined in 23 Blackface ewes. Six ewes were not pregnant, five had single foetuses and 12 had twins. The pregnant ewes were slaughtered three or four days before their expected lambing date. There was a negative relation between uterine weight or body fat weight and the weight of alimentary tract contents. The effect of uterine weight was small and not significant when only the pregnant animals were considered. It was unlikely that physical restriction of the alimentary tract by the uterus played more than a minor role in limiting voluntary food intake under the conditions of this experiment.

The litter size and lambing rate of 1531 ewes of British hill breeds which had been mated to Finnish Landrace or Border Leicester rams was found to be independent of breed of ram. It was concluded that Finnish Landrace rams do not have a particular effect on the reproductive performance of ewes to which they are mated.

Identical group-feeding experiments (each of 90 days duration) were conducted in the summer and in the winter of 1968. The purpose was to study the feed-lot performance of the Grey Shirazi breed on high- and low-concentrate diets with or without diethylstilboestrol (DES) in two different seasons of the year. Lambs on the high-concentrate diet gained more than those on the low-concentrate diet both in summer and in winter. On both diets lambs performed better under cold than under warm conditions. There was no difference when DES was given orally with the above two rations.