Developments, both recent and potential, in procedures for manipulating embryos are described. The procedures considered include: embryo transfer, multiple ovulation and embryo recovery, recovery of oocytes, in vitro maturation (IVM) and fertilization (IVF) of oocytes, in vitro culture of zygotes, embryo splitting and nuclear transfer, embryo storage, embryo sexing, gene transfer and embryo stem cells. The impact of these procedures on breeding strategies such as multiple ovulation and embryo transfer (MOET) nucleus breeding schemes and progeny testing are discussed for both dairy and beef cattle.

For MOET nucleus schemes all these procedures have potential applications in producing maximal rates of genetic progress for a fixed rate of inbreeding. With the current effectiveness of the procedures, embryo sexing and nuclear transfer would have the most impact. The potential for increasing genetic progress through progeny testing is enhanced using multiple ovulation, embryo recovery and transfer in cows to breed bulls, but no other procedures appeared to offer major benefits. The efficiency of beef production from the dairy herd could be increased either by using IVM and IVF to produce more beef-type calves or, potentially, by cloning and embryo transfer, to produce pure beef calves. Procedures leading to the production of clone families would make an impact on the evaluation of genotypes and environments. Gene transfer may be used to modify the composition of milk including the production of pharmaceutical proteins, and to increase milk yield or the efficiency of lean meat production.

It is concluded that, although much further research is required, the procedures discussed will have major implications for the structure and organization of dairy and beef cattle herds over the next decade.

The value of cloning in MOET nucleus breeding schemes has to be considered within the constraint of fixed resources. Under this constraint cloning was found to add to genetic progress only when (i) the heritability is low and (ii) it is used at the expense of a reduction in the number of bull families. This course would exacerbate inbreeding and other potential problems with MOET. All other options for using clones lead to a reduction in genetic progress due to a loss of selection intensity that is not made up for by gains in selection accuracy.

Two pedigree indexes were calculated for 754 Friesian/Holstein bulls born between 1974 and 1980 and tested by the Milk Marketing Board. Correlations between the indexes and the average progeny performance were used to compare the indexes calculated using information from different ancestors. The first index, or estimated predicted difference (EPD) was calculated as one-half the sire progeny test, known as an Improved Contemporary Comparison (ICC), plus one-quarter the maternal grandsire ICC. The second index or estimated ICC (EICC) was calculated from one-half the sire ICC plus one-half the dam cow genetic index (CGI). The correlations between the deregressed bull evaluations (DICC) and EPD, calculated from 310 bulls, were 0·40 for milk yield, 0·40 for fat yield and 0·39 for protein yield. The corresponding correlations between the DICC and EICC, calculated from 314 bulls, were 0·43, 0·40 and 0·41. The regression coefficients for predicting average progeny performance from the EPD index were 1·00 (s.e. 0·13), 0·97 (s.e. 0·12), 0·96 (s.e. 0·13), 1·09 (s.e. 0·12) and 1·08 (s.e. 0·10) for milk, fat and protein yield, fat and protein percentage respectively. Those for predicting progeny performance from the EICC index were 0·95 (s.e. 0·11), 0·81 (s.e. 0·10) and 0·84 (s.e. 0·10) for milk, fat and protein yield respectively. Although the correlations were lower than the expected values of 0·50 (EPD) and 0·55 (EICC) the regressions were near the expected value of 1. The results indicate that the indexes are useful as a preliminary means of selecting bulls prior to progeny testing.

Fifty-four Charolais-cross weaned suckled calves were used in an experiment to investigate the effects of feeding level during the post-weaning winter on their subsequent performance, when grazing different vegetation types in summer. During winter they were given grass silage and barley at one of three levels (low, medium and high). The winter live-weight gains were 0·50, 0·75 and 0·96 (s.e. 0·019) kg/day (P < 0·001) for the low, medium and high treatments respectively. During summer they grazed either a sown ryegrass pasture (S), a hill reseed (R) or part of an unimproved hill (H).

Live-weight gain during summer was inversely related to winter feeding level on all grazing treatments, the mean live-weight gains being 1·01, 0·79 and 0·65 (s.e. 0·027) kg/day (P < 0·001) for the low, medium and high winter feeding levels respectively. Summer treatment significantly affected performance, the live-weight gains being 0·88, 0·94 and 0·61 (s.e. 0·027) kg/day for the S, R and H treatments respectively. There was no interaction between winter and summer feeding treatment on live-weight gain. The organic matter intake was highest on the H treatment but the digestibility of herbage consumed was the lowest, resulting in the lowest digestible organic matter intake.

Similar and high levels of performance were obtained on sown ryegrass pastures and reseeded hill land, while unimproved hill vegetation supported only moderate levels of live-weight gain. Compensatory growth occurred when a wide range of vegetation types were grazed in summer.

Six groups of eight 2-day-old calves were placed successively in a controlled environment chamber. Three groups were exposed to air temperatures of 10° and 25°C. A liquid diet of skimmed-milk substitute and 4 I/day of drinking (free) water was offered to each calf. Within each group, calves were allocated to either a low (<0·2 m/s) or a high (>3 m/s) air velocity and to either a low (30 g dry matter (DM) per kg M0·75 per day) or a high (40 g DM per kg M0·75 per day) feeding level. At 8 days of age the apparent digestibilities of DM at air temperatures of 10° and 25°C were 0·77 (s.e. 0·126) and 0·82 (s.e. 0·126) respectively (P > 0·05). The apparent digestibilities of DM were greater at the low feeding level with low air velocity than for either this feeding level with high air velocity or the high feeding level at both air velocities (P < 0·05) between which there was no significant difference (P > 0·05). At 8 days of age there were significant air temperature × air velocity (P < 001) and air velocity × feeding level interactions in the intake of free water (P < 005). There was a significant air temperature × feeding level interaction for total water intake (P < 0·05). Urinary water loss relative to total water intake was significantly greater at the low air velocity than at the high air velocity (P < 0·05).

In a further two groups of eight calves given 30 g DM per kg M 75 per day at 8 days of age, the apparent digestibilities of DM at air temperatures of 10° and 25°C were 0·71 (s.e. 0·020) and 0·90 (s.e. 0·013) respectively (P < 0·01). In the same calves given 40 g DM per kg M0·75 per day at 20 days of age, the apparent digestibilities of DM at air temperatures of 10° and 25°C were 0·89 (s.e. 0·009) and 0·93 (s.e. 0·011) respectively (P < 0·05). The free and total water intakes, the ratios of (total water intake-faecal water loss): total water intake and the urinary losses of water were significantly greater at the air temperature of 25°C than at 10°C (P < 0·05). Live-weight gains were lower at 10°C than at 25°C (P < 0·01).

The results suggested that air temperature, air velocity and feeding level can affect the health and growth of calves less than 4 weeks of age.

Twelve Friesian calves were each fitted with an abomasal cannula. The calves were placed in a controlled environment chamber and given a milk substitute diet in buckets at 12-h intervals. The calves were exposed to air temperatures of 25°C from 4 to 10 days of age, 5°C from 11 to 14 days of age, and 25°C at 15 days of age. At 9, 10, 11, 14 and 15 days of age, the abomasal contents of the calves were sampled at 2-h intervals and the temperature and pH of the abomasal contents were recorded at 40-min intervals for a period of 12 h. The rectal and abomasal temperatures of the calves were lower at 11 days of age than at 10 days of age, and were greater at 15 days of age than at 14 days of age (P < 0·05).

In a similar experiment four Friesian calves were each fitted with an ileal re-entrant cannula. The calves were exposed to air temperatures of 25°C from 5 to 19 days of age, 5°C from 20 to 26 days of age, and 25CC at 27 days of age. At 18, 19, 20, 26 and 27 days of age the ileal contents of the calves were collected and sampled for a period of 12 h.

No significant effects of the air temperatures of 25° and 5°C on the abomasal digestion (acidity and proteolytic activity of the abomasal contents) and ileal apparent digestibility of the diet were found.

To study, after weaning at 6 weeks of age, the effect of age on energy utilization of growing calves and to estimate the requirements of metabolizable energy (ME) for calves weighing less than 100 kg, 243 energy balance trials were conducted using 68 Holstein-Friesian castrated male calves at 7, 9 and 13 weeks of age. Regression analyses of energy retention (ER, kj/kg M0·75) on intakes of ME (kJ/kg M0·75) gave the following equations at the respective age: ER = 0·479 (s.e. 0·070) ME — 197, for 7 weeks, ER = 0·521 (s.e. 0·052) ME - 216 for 9 weeks and ER = 0·523 (s.e. 0·043) ME - 236, for 13 weeks. Amounts of ME for maintenance (MEm) calculatedas ER = 0 were 413, 415 and 451 kJ/kg M0·75 for calves at 7, 9 and 13 weeks of age, respectively. The efficiency of utilization of ME for growth was not significantly different among the ages and was 0·500 (s.e. 0023). ME required for growth was estimated to be 510 kJ/kg M0·75 for each kg gain in weight, irrespective of age. The requirements of ME of calves weighing less than 100 kg were lower than those estimated by extrapolation of British and American standards.

Two experiments were carried out to examine the relative value of soya-bean, sunflower and fish meals as protein supplements for growing beef cattle offered grass silage ad libitum. In each experiment, well preserved grass silage (116 and 119 g crude protein (CP) per kg dry matter (DM) and 69 and 76 g ammonia-N per kg total N in experiments 1 and 2 respectively) was supplemented with (1) barley (107 g CP per kg DM), (2) barley/soya-bean meal (224 g CP per kg DM), (3) barley/sunflower meal (223 g CP per kg DM) and (4) barley/fish meal (218 g CP per kg DM). Each of the supplements was offered at a rate of 1·3 kg per head daily to 32 British Friesian castrated male cattle (mean initial live weight 334 kg) n i each experiment. The treatments were imposed for 118 and 134 days in experiments 1 and 2 respectively and the animals were slaughtered 8 months after the end of the treatment periods. On average over the two experiments for supplements 1 to 4 respectively, silage DM intakes were 6·2, 6·3, 6·3 and 6·2 (s.e. 0·053) kg/day; live-weight gains 0·69, 0·78, 0·71 and 0·80 (s.e. 0·039) kg/day; carcass weights 375, 377, 377 and 374 (s.e. 8·1) kg, carcass fat classification (1 = leanest, 5 = fattest); 3·5, 3·5, 3·4 and 3·6 (s.e. 0·14) and areas of m. longissimus dorsi 19·8, 19·9, 19·7 and 19·3 (s.e. 0·66) mnv/kg carcass weight. It is concluded that when part of a barley supplement given in addition to well preserved grass silage was replaced with soya-bean, sunflower or fish meals, silage intake was not affected. The inclusion of fish meal in the diet increased live-weight gain by HOg/day and the response to soya-bean meal was 0·78 of the response to fish meal per unit of crude protein. Sunflower meal did not affect performance. The treatments did not affect carcass weight or fatness 8 months after the end of the treatment periods.

Four silages were prepared from a sward of perennial ryegrass (Lolium perenne cv. Melle). Two were ensiled directly either without (UWN) or with formic acid at 31 1/t fresh material (UWA). The remaining herbage was left undisturbed in the swath for 43 h (WL) or was spread and harvested after 19 h (WS). The dry matter (DM) concentrations at harvest for the unwilted herbage were 229 (UWN) and 238 g DM per kg (UWA) and wilting increased DM concentration to a similar extent for both treatments (WS, 339 v. WL, 346 g DM per kg).

Forty British Friesian steers initially 3·5 months of age and 105 kg live weight were used for the trial. Eight steers were allocated to an initial slaughter group and the remainder were offered the four silages at a restricted level to give 940 kJ digestible energy per kg M0·75. These animals were slaughtered after 112 days to estimate the incremental increases in the components of the body.

Loss of DM in the field by respiration increased from 22·2 to 40·6 g/kg DM cut with increased length of wilting. In silo losses were greater with unwilted (UWN, 222, UWA 183 g/kg DM ensiled) than with wilted silage (WS, 133 WL, 122 g/kg DM ensiled). All silages were well preserved. Wilting was more effective than formic acid in reducing total fermentation acids but a lower proportion of these acids was in the form of lactic acid.

Live-weight gain was lower for the steers given the control (UWN) silage (398 g/day) than for those given silages UWA (463 g/day), WS (476 g/day) and WL (470 g/day). However, a lower proportion of the gain by steers given UWN was in the form of gut contents so that gains in fat, protein and energy were not significantly affected by treatment. In conclusion, when steers were given similar amounts of digestible energy and unwilted silage was well preserved, neither the use of formic acid nor wilting influenced gains in protein and energy. Further, the duration of wilting, in the absence of any differences in DM content, had no effect on animal performance.

Three experiments of 2 × 2 × 2 factorial design were made with sheep or cattle given a basal diet of barley straw. Fish meal (FM) and unmolassed sugar-beet pulp (SBP) were given either separately or in combination as supplements to untreated (US) or ammonia-treated barley straw (AS) to form eight diets. The diets were given for 8 weeks in experiment 1 (24 sheep), 10 weeks in experiment 2 (24 steers) and 20 weeks in experiment 3 (32 steers and bulls). Urea was added to the shredded straw US and AS, to give a final concentration of 17 g nitrogen per kg dry matter (DM). n I experiments 1 and 3, both FM and SBP, given alone or in combination, significantly increased the DM and organic matter digestibility of urea-supplemented US, but not of AS. Ammonia treatment of straw given alone increased DM intake from 414 to 729 g/day in sheep and from 4·75 to 6·09 kg/day in cattle. All animals given US as the sole food lost body weight, but cattle gained weight when US was combined with either 50 g/kg FM or 15 g/kg SBP. It was necessary to combine the two supplements to achieve a significant gain in body weight of sheep. Although the diets based on AS promoted growth in all the animals, the body-weight gains achieved with US combined with 50 g/kg FM or 15 g/kg SBP were greater than those when AS was given as the sole food.

In two experiments, six silages were offered either alone or with constant daily levels of a barley-based concentrate (450 or 900 g fresh weight per ewe) to Finn Dorset ewes carrying between one and four foetuses in the last 6 weeks of pregnancy. The silages (SI to S3, experiment 1 and S4 to S6, experiment 2) had dry matter (DM) concentrations of 256, 190, 278 and 294, 216, 201 g/kg fresh weight and crude protein (CP) concentrations of 106, 164, 212 and 119, 148, 194 g/kg DM. Mean metabolizable energy (ME) concentrations of the silages (digestible energy × 0·81), in weeks 16 and 20, were 9·2, 10·3, 12·0 and 8·8, 10·9, 11·0 MJ/kg DM. Total ME intakes were 12·6, 13·5, 18·6 and 14·5, 15·1, 16·8 MJ/day in week 16 of gestation and were 11·6, 11·2, 17·2 and 12·2, 11·8, 13·3 MJ/day in week 20. Total ME intakes were increased by offering higher levels of concentrates and for levels of 0, 450 or 900 g/day these were 12·3, 14·8, 17·9 MJ/day in week 16 and 9·6, 12·8, 15·5 MJ/day in week 20. Intakes were similar for ewes carrying twins or multiples in experiment 1, but in experiment 2, intakes by ewes with twins or multiples were proportionately 0·97 and 0·85 of those for ewes with singles. Replacement rates of forage by concentrates were —0·09, —0·08, —0·60 and —0·06, —0·25, —0·38 g silage organic matter (OM) per g concentrates OM for silages SI to S3 and S4 to S6. Only on silage S6 did the replacement rate differ significantly from zero.

A total of 52 ram lambs of different morphological types slaughtered at a comparable stage of body development (carcass weights = 19·5 (s.d. 3·10) kg) were used to study the variation in the relationship between muscle and bone development in the hind limb using multivariate techniques. Development of individual muscles was expressed relative to the weight of the major bone to which they are closest, considered here as their carrier bone.

Variability of muscle: bone ratios of individual muscles varied considerably from one muscle to another. The coefficient of variation (CV) ranging from 0·111 to 0·462 (average CV = 0·199). Relative to the same bone, the variability of muscle: bone ratios for each of the three anatomical regions of the hind limb was still important. This suggests that variability may be due to other factors besides muscle weight, such as bone shape.

The correlation between the various muscle: bone ratios was low in some cases. However, a significant association existed between muscle: bone ratios of individual muscles and the total muscle: bone ratio in the hind limb which indicated that some muscle: bone ratios can be used as indicators of muscling in the selection of meat animals such as m. semimembranosus/femm, m. gluteobiceps/femur, m. adductores/'femur and m. semitendinosus/femur.

The most discriminant variables of the population in multivariate analysis were m. soleus/tibia, m. extensor digitorium lateralis/tibia, m. quadmtus femoris/coxal, m. gemelli/coxal and m. vastus medialis/femur. It was also possible to distinguish the different morphological types of the different breeds by the values of the ratios between these variables.

The effect of undegradable protein supply was studied in overfat lambs given low-energy diets to reduce carcass fatness. Two trials involving sequential batches of 48 overfat Suffolk × lambs investigated the effects of supplementing an ad libitum straw diet with either 0 or 100 g fish-meal pellets daily for 14-, 28- and 42-day periods. Lambs supplemented with fish meal in trials 1 and 2 lost significantly less weight. Weight losses (g/day) were - 2 v. -129 (s.e.d. 26) and -5 8 v. -188 (s.e.d. 29) respectively. There were highly significant and favourable effects of fish-meal supplementation on carcass weight and composition (recorded in the second trial only). Carcass weight (kg) after 14, 28 and 42 days was 21·8, 20·4 and 18·3 for unsupplemented lambs and 22·4, 21·9 and 21·6 for supplemented lambs; saleable lean (kg) as determined by a commercial boning-out process was 12·1, 11·3 and 10·0 v. 12·7, 12·6 and 12·3 respectively. There were corresponding changes in conformation but fat trimmed off the carcass was not significantly reduced by the dietary treatments. There was, however, a reduction by one-fifth in excess carcass fat (about 0·5 kg) over the period 14 to 42 days on trial, and over the trial as a whole, fat trim fell from an estimated proportion of carcass weight of 0·17 to 012, equivalent to a fall in Meat and Livestock Commission fatness score from 4H to 3L. Results indicated very favourable financial rewards for supplementing diets of overfat lambs with fish meal.

Path analysis was applied to correlations among lamb carcass measurements. Correlations were obtained from lamb carcasses averaging 15·7 kg and 270 g fat per kg carcass weight. Correlations were adjusted to represent within breed, sex and management group relationships. The purpose of the analysis was to study the relationships of carcass weight with carcass measurements and to identify carcass measurements that are independent other than through their relationships with carcass chemical fat concentration and carcass weight. The direct effect of live weight on all carcass measurements was positive. Thus, heavier carcasses with the same fat concentration had larger fat and tissue depths, muscle measurements, leg measurements and kidney fat weights. The results of correlations among the residuals suggest that fat depths C and J, specific gravity, muscle width A and kidney fat weight are nearly independent estimates of carcass composition in the sense that they are only correlated through their relationships with carcass weight and carcass fat composition. Robust predictors of carcass composition were developed by first adjusting carcass measurements for the direct effects of carcass weight and then computing regressions on the adjusted measurements. These predictors appeared to be better predictors of environmental and genetic differences than least-squares multiple regression yet reduced the accuracy of within group prediction only slightly.

The effect on the reproductive performance of Greyface (Border Leicester × Scottish Blackface) ewes of a low level food intake and associated loss of live weight from either 14 days before mating, or from the time of mating, until 11 to 26 days after mating, was investigated. Ewes (252) were allocated to one of three treatments with ewes within each treatment divided into two flocks (flock A: 16 ewes per treatment; flock B: 68 ewes per treatment). Ewes of treatment LL were given a ration providing proportionately 0·5 estimated metabolizable energy (ME) requirements for maintenance from 2 weeks before mating. Those of treatment HL were given a daily ration providing 1·5 estimated ME requirements for maintenance until mating and the restricted ration thereafter. Ewes of treatment HH were given the higher ration throughout the experimental period. Flock A ewes were slaughtered at 11 days post mating and flock B ewes at between 18 and 26 days post mating. Treatment differences in the ovulation rates of flock A ewes were not statistically significant but in flock B, ewes of treatment LL had a lower mean ovulation rate (1·81) than those of treatments HL (2·23) and HH (2·09) (P < 0·001). The lower ovulation rate of LL ewes relative to HL ewes in flock B was reflected in a lower mean potential lambing rate per ewe pregnant than in the HL treatment (1·58 v. 1·79; P < 0·01) and per ewe put to the ram (1·37 v. 1·65; P < 0·01). HL ewes had a slightly lower mean potential lambing rate per ewe pregnant (1·79 v. 1·97; P < 01) and per ewe put to the ram (1·65 v. 1·82; P < 0·05) than HH ewes. Ova wastage rates of LL + HL and HH ewes were 0·26 and 014 (P < 001) respectively at 24 days post mating. Values for LL and HL ewes (0·27 and 0·25 respectively) were not significantly different.

Estimated mean conceptus lengths were 370, 500 and 1400 μin for LL, HL and HH ewes respectively (P < 0·05).

It is concluded that low food intake before mating reduced the mean ovulation rate and low intakes after mating compromised embryo growth rate and induced a higher rate of ova wastage; this increase in the incidence of ova wastage was not significantly exacerbated by the low levels of intake prior to mating.

In order to determine the relative digestibilities of nitrogen and amino acids in foodstuffs for pigs and poultry, and the effects of manufacturing methods, equal quantities of soya-bean meal, sunflower meals [hulled (sunflower meal 1) and dehulled (sunflower meal 2)], meat meals [made with (meat meal 1), and without (meat meal 2), blood added at 250 g/kg meat tissue (wet weights)] and rapeseed meals [seeds heated at 80°C (rapeseed meal 1) or 100°C (rapeseed meal 2)] were mixed with protein-free ingredients. The diets were given to five growing pigs with ileo-rectal anastomoses, and, by crop-intubation, to 12 caecectomized and 12 intact cocks. Excreta were collected over 48-h periods. Endogenous excretion was estimated by giving protein-free diets.

In the order, soya-bean meal, sunflower meals 1, and 2, meat meals 1, and 2, rapeseed meals 1, and 2, true digestibilities were: with pigs, of nitrogen, 0·81, 0·80, 0·79, 0·64, 0·79, 0·73, 0·70 (s.e.d. 0·030), of lysine, 0·84, 0·83, 0·84, 0·65, 0·84, 0·76, 0·72 (s.e.d. 0·032); with caecectomized birds, of nitrogen, 0·92, 0·91, 0·91, 0·66, 0·78, 0·74, 0·75 (s.e.d. 0·018), of lysine 0·92, 0·91, 0·93, 0·62, 0·79, 0·70, 0·70 (s.e.d. 0·020); with intact birds, values were similar to those with caecectomized birds for soya-bean, and the sunflower meals, but lesser for meat meals 1 and 2; the solubilities of nitrogen in pepsin were 0·96, 0·92, 0·93, 0·80, 0·89, 0·87, 0·87.

Two hundred and eighty pigs (initial live weights 10 kg) were used to compare growth response to free lysine with that to lysine in soya-bean meal and sunflower meal 2. Lysine availabilities, assessed by analyses of regressions of live-weight gain against lysine intake were 0·82 (s.e. 0·12) for soya-bean meal, and 0·82 (s.e. 0·18) for sunflower meal 2.

Amino acid digestibilities of the sunflower meals were similar to those of soya-bean meal, and were not influenced by dehulling; values for the rapeseed meals were lower, and unaffected by differences in heating severity; values for the meat meal were reduced by blood addition. Values differed between pigs and poultry, but there was consistency in the extent to which each species discriminated between some foodstuffs. The pepsin test was insensitive. The large standard errors associated with availability values prevented meaningful comparisons with digestibility values.

One hundred and two Large White × Landrace Fl hybrid sows were allocated over four parities in a factorial design to two levels of target P2 backfat thickness at parturition (20 mm, F v. 12 mm, T), two levels of lactation feeding {ad libitum, H v. 3 kg/day, L), and two sizes of sucking litter (six v. 10). Sows attained 13 mm P2 when first mated at 126 kg live weight. Fatness (P2, mm) at weaning was significantly influenced by target fatness at parturition (14·2 v. 9·3), lactation feeding level (13·7 v. 10·0), and litter size (12·7 v. 11·0). Changes in backfat (P2, mm) during 28-day lactation were significantly influenced by target fatness at parturition (—5·0 v. —2·5), lactation feeding (-2 0 v. —5·4), and litter size (—2·9 v. -4·6). Sow live weight (kg) at weaning was significantly influenced by target fatness at parturition (211 v. 192), lactation feeding (218 v. 186), and litter size (208 v. 196). Changes in live weight (kg) during 28-day lactation were significantly influenced by target fatness at parturition (—26 v. — 12), lactation feeding (—5 v. —31), and litter size (—12 v. —25). With multiparous sows only, total food intake during 28-day lactation was negatively related to total food intake in pregnancy. Change in backfat (P2, mm) during 28-day lactation = -0·28 - 0·27 P2 at parturition + 0·04 lactation food intake — 0·50 litter size. Change in live weight (kg) during 28-day lactation = -3·8 — 0·15 live weight post partum + 0·36 lactation food intake — 3·3 litter size. Sows with target fat levels of 20 mm P2 at parturition had better food conversion efficiencies than sows with target fat levels of 12 mm. Target fatness at parturition, and especially lactation food intake, but not litter size, significantly influenced the interval (days) from weaning to oestrus in parity 1 (9·1 v. 14·2 and 7·8 v. 15·3, but 11·6 v. 11·5), while n i subsequent parities only litter size influenced the interval (days) from weaning to oestrus (6·0 v. 8·0). Birth weight (kg) of piglets was influenced only marginally by target fatness at parturition (1·4 v. 1·2) in parity 1, and not by the other factors, or in subsequent parities. Piglet growth rate was affected by both target fatness at parturition and litter size, but by lactation feeding level only in the last week of lactation. The relationship between fatness at weaning (mm) and the weaning to oestrus interval (days) for primiparous sows can be expressed as 26·6 — 1·28 P2. High level feeding in lactation imparted production and efficiency benefit in both primiparous and multiparous sows, while pregnancy feeding to a target of 20 mm rather than 12 mm at parturition was of benefit for primiparous sows.

The physical and chemical composition of sows was determined at first mating (no. = 6), weaning the first litter (12) and 14 days after weaning the fourth litter (24). The sows were from 108 Large White/Landrace Fl hybrid gilts allocated in a factorial arrangement according to two levels of subcutaneous fatness at parturition (12 v. 22 mm P2), two levels of lactation feeding (3 v. 7 kg) and two sizes of sucking litter (six v. 10). Treatments significantly influenced the composition of dissected carcass fat and chemical lipid, but not composition of dissected lean and chemical protein. The final body protein mass of well fed sows at the termination of parity 4 was 41 kg, and the total content of gross energy (GE) in excess of 3000 MJ, with an average of 12·4 MJ GE per kg live weight; equivalent values for the less well fed sows were 33 kg and 9·4 MJ GE per kg live weight respectively. The weights of chemical lipid and protein could be predicted from the equations: lipid (kg) = -20·4 (s.e. 4·5) + 0·21 (s.e. 0·02) live weight + 1·5 (s.e. 0·2) P2; protein (kg) = -2·3 (s.e. 1·6) + 0·19 (s.e. 0·01) live weight - 0·22 (s.e. 0·07) P2. On average, sows lost 9 kg lipid and 3 kg protein in the course of the 28-day lactation; these being proportionately about 0·16 and 0·37 of the live-weight losses respectively. Maternal energy requirement for maintenance was estimated as 0·50 MJ digestible energy (DE) per kg M0·75, while the efficiency of use of DE for energy retention was 0·28.

Operant conditioning was used to measure the effect on the feeding motivation (hunger) of pigs of diluting nutrient restricted food allowances with straw. Twelve Large White × Landrace boars were maintained in individual pens where they received their daily food allowance. Prior to the experiment, the boars were conditioned in separate rooms to press a panel so as to receive small rewards of food. The effect of diluting meal with straw on panel pressing was investigated in two trials. In trial 1, the boars were allocated to the following treatments: high (H) (3·5 of maintenance (A/)), low (L) (1·5 M) and low/straw (LS) (1·5 M + 0·15 times the dry weight of meal as chopped straw). This dilution increased the food volume by proportionately 0·29. Operant response rates were measured in six 1-h tests in each of two periods at the start (days 3 to 10, period 1) and at the end (days 20 to 28, period 2) of the 28-day experiment. In trial 2, nine of the above boars were allocated to the following treatments: high (H) (3·0 A/), low (L) (1·5 M) and low/straw (LS) (1·5 M + 0·30 the dry weight of meal as ground straw). This dilution increased the food volume by proportionately 0·75. Operant response rates were measured as above between days 10 to 17, following a 9-day adjustment period to the new regimes.

In both trials, operant response rate was strongly affected by treatment. Overall in trial 1, animals on the H treatment made few responses (mean 60 responses per session), animals on the L treatment made significantly more responses (mean 825, P < 0·001) and animals on the LS teatment significantly more responses yet again (1263, P < 0·05). This last difference was associated with the difficulty of the LS animals consuming all of the diluted diet during period 1. In trial 2, response rates were again higher on the L than on the H treatments. There was, however, no difference between the L and LS treatments (means: 60 v. 1035 v. 1200). Proportionately only 0·04 of the diet was refused during the period of testing.

The present results show that short-term satiety arising from gut distension does not reduce the feeding motivation of animals on chronic nutrient restriction. This suggests that hunger will remain high i n pigs on restricted food allowances, even when they have access to high fibre foods. This hunger will be a potential source of stress, particularly in housing systems involving physical restraint such as tether stalls.