The published theoretical estimates of equilibrium genetic change possible by progeny testing in dairy cattle are too high. Much of the overestimation is because no account has been taken of the temporary reduction in genetic variation due to selection of parents and ancestors. A particular efficient progeny testing scheme was modelled theoretically and simulated on computer using a continuous infinitesimal additive model. To make the simulation feasible an elite breeding herd of females was used, and progeny testing was done outside the herd. The proportional reduction in genetic variance due to previous selection was 0·25, 0·27 and 0·29 for original heritabilities of 0·05, 0·25 and 0·50, with the theoretical and simulated results being very similar. The theoretical and simulated proportional losses in equilibrium selection responses were 0·22 and 0·26, 0·21 and 0·22 and 0·21 and 0·25, respectively, for the three levels of heritability. Thus, most of the loss in response in the simulation results came from the reduction in genetic variance due to selection.

Theoretical rates of genetic response are calculated for adult multiple ovulation and embryo transfer (MOET) nucleus breeding schemes and conventional breeding systems in dairy cattle. For a single trait breeding objective, the relative responses with MOET schemes decline as the heritability falls and the number of progeny in the progeny test scheme increases. Selection for an aggregate breeding objective of milk yield and another trait, for different values of heritability, correlation and relative economic importance, is also examined. Generally, the overall response of efficient MOET schemes exceeds the progress of an efficient conventional breeding scheme with 50 to 100 daughters per bull. Only if the relative economic weight for the lowly heritable trait exceeds 0·60 and the progeny test is on 200 offspring are conventional systems superior. Testing additional paternal half-sib groups from a MOET scheme gives a considerable extra response, especially when lowly heritable traits have a dominating economic importance.

Six steers, each fitted with a ruminal cannula and T-type duodenal and ileal cannulae, were used to investigate protein and fibre utilization from lucerne, birdsfoot trefoil and sainfoin preserved both as hay and silage. Steers were fed at 2-h intervals during six 12-day periods in an experiment with a 6 × 6 Latin-square design. Animals were fed all-forage diets of six treatments (lucerne hay and silage, birdsfoot trefoil hay and silage, sainfoin hay and silage). Samples were collected from all alimentary sites twice daily during the last 3 days of each period. Acid insoluble ash was used as a solid marker and Cr-ethylene diamine tetra-acetic acid as a liquid marker. Organic matter (OM), hemicellulose (HC) and cellulose (CL) total tract digestibilities of lucerne, birdsfoot trefoil and sainfoin were similar (OM: 599, 608, 580 g/kg; HC: 499, 497, 480 g/kg; CL: 590, 618, 608 g/kg). However, crude protein (CP) digestibilities were lower (P < 0·001) for sainfoin (582 g/kg) than for lucerne (732 g/kg) or birdsfoot trefoil (693 g/kg). Nitrogen (N) flows at the duodenum or ileum were similar among forages despite lower N intake for sainfoin. Presence of tannins in sainfoin may have been responsible for low protein degradation in the rumen, and reduced N digestion in the small intestine. Forage preserved as hay or silage had similar CL, HC and OM digestibilities, while CP digestibility was higher (P < 0·05) for silages.

The carcass composition of Hereford × Friesian (HE), Friesian (FR) and Charolais × Friesian (CH) steers finished on diets of high (H) and medium (M) dietary metabolizable energy (ME) concentrations and slaughtered at low (L), normal (N) and heavy (W) carcass weights was examined in a 3 × 2 × 3 (no. = 9 per individual treatment) factorial experiment. A pre-finishing slaughter group of nine animals of each breed type was also included, giving a total 189 animals in 21 experimental groups. ME concentrations of the H and M diets which were offered ad libitum were 12·6 and 10·4 MJ/kg dry matter. Target carcass weights for L, N and W were 260, 300 and 340 kg for HE and FR and 260, 320 and 380 kg for CH.

Carcass side weights (before tissue separation) of the HE, FR and CH pre-finishing slaughter groups were 90·8, 970 and 101·0 (s.e.d. 3·9) kg. Corresponding tissue proportions were 188, 199 and 200 (s.e.d. 4·3) g/kg bone, 663, 686 and 690 (s.e.d. 5·3) g/kg muscle and 135, 99 and 96 (s.e.d. 5·7) g/kg fat. Main effect side weights of the finished groups were 152·3, 151·4 and 162·2 (s.e.d. 1·4) kg for HE, FR and CH. 158·5 and 152·2 (s.e.d. 1·1) kg for H and M and 131·2, 155·2 and 179·6 (s.e.d. 1·4) kg for L, N and W, respectively. Tissue proportions in the same order were 146, 160 and 157 (s.e.d. 2·0), 149 and 159 (s.e.d. 1·6) and 163, 154 and 146 (s.e.d. 2·0) g/kg bone, 579, 601 and 635 (s.e.d. 5·5), 600 and 610 (s.e.d. 4·5) and 637, 599 and 574 (s.e.d. 5·5) g/kg muscle and 264, 228 and 195 (s.e.d. 6·4), 240 and 219 (s.e.d. 5·2) and 188, 235 and 264 (s.e.d. 6·4) g/kg fat. CH had more (P < 0·001) of their muscle in the pelvic limb and less (P < 0001) in the thorax than HE and FR. HE had more (P < 0·001) of their carcass fat in the subcutaneous depot and less (P < 0·001) in the intermuscular depot than FR and CH. The allometric regression coefficients for the main joint and tissue weights on side weight were <1·0 for both limbs, loin, bone and muscle. Coefficients were >1·0 for the thorax, ribs, flank and fat. The regression coefficients for the main muscles of the pelvic limb and loin and total thoracic limb muscle on total side muscle were <l·0, while the coefficients for the flank, ribs and thorax muscles were >1·0. Similarly the regression coefficients for the bones of the two limbs on total side bone were <10, while the coefficients for the loin, ribs and thorax bones were >10. It was calculated (for the H diet) that at a carcass weight of 300 kg, HE, FR and CH would have carcass tissue proportions of 576, 600 and 642 g/kg muscle and 261, 227 and 180 g/kg fat. The three breed types would have similar carcass fat contents at carcass weights of 264, 300 and 376 kg for HE, FR and CH, respectively.

For the Mpwapwa cattle breed in Tanzania, the efficiency of various breeding schemes including an open nucleus was investigated by model calculations. Artificial insemination and intensive recording of production are assumed to be applied in a nucleus which is the main breeding unit. As a pre-nucleus, associated herds with less intense data recording serve as the basis to screen superior cows for nucleus replacements, provide the capacity for progeny testing, and operate as bull multipliers for commercial herds.

The criteria of efficiency were genetic gain and profit from selection for a dual purpose breeding objective (milk and beef) in a total population of 10 000 cows. Introducing a two breeding tier scheme through separating all recorded cows into a nucleus and pre-nucleus leads to an increase of the genetic gain rather than the profit. Further improvement is obtained by introduction of artificial insemination in pre-nucleus herds. The nucleus size should not exceed about 5% of the cow population and an optimum size of the pre-nucleus is about 15%. Opening the nucleus to replacement cows coming from the pre-nucleus affects the aggregate genetic gain very little although it can be recommended if milk yield is to be mainly improved or if the total profit is taken into account. Furthermore, the nucleus should be opened if there is little difference between the heritabilities in the nucleus and pre-nucleus and also in order to avoid detrimental effects of inbreeding and genotype × environment interaction.

To separate ingestive factors involving palatability from post-ingestive factors responsible for forage intake being depressed by ensiling, silages were evaluated using sham-fed animals. Protocol for all experiments was to formulate low dry matter (DM) silages from a high DM lucerne silage reconstituted with either water, solutions containing constituents commonly found in silage, or extracts of low DM silage. Solutions of silage constituents were adjusted before reconstitution to a pH typical of silage. Tests were performed by offering silages one at a time once daily to oesophageal fistulated sheep, maintained on lucerne hay, but starved for 5 h prior to the test. Intake was measured over 30 min. Sheep were adapted to intermediate levels of each test ingredient(s) before experiments were done to evaluate effects of four to five levels of each ingredient using Latin-square designs. Animals were sham fed during both periods of adaptation and of testing ingredient levels. Mean of 30-min intakes for control silages was 573 g DM. Lactic and acetic acids added together, to levels of 53·2 and 35·4 g/kg, respectively, enhanced intake with a linear response due to level (P < 0·05). Acetic acid added by itself to 88·0 g/kg depressed intake with a linear response due to level (P < 0·05) in which 10 g/kg added acetic acid depressed 30-min intake by 13·5 g. Acetic and butyric acids added together to levels of 36·2 and 54·4 g/kg respectively, were without effect as was ammonia added to 3·8 g/kg as ammonia nitrogen (N). A mixture of free amino acids added to a level of 9·2 g amino-N per kg was without effect, but at a level of 13·8 g/kg, intake was depressed to proportionately 0·75 of the control. A mixture of two amines and gamma amino butyric acid added to a level of 4·6 g N per kg caused a quadratic response (P < 005) in which intermediate levels enhanced intake but the highest level was without effect. An extract from good-quality silage had no effect on intake and that from a poor-quality silage enhanced intake in a linear manner (P < 0·05). These data indicate that elevation of acetic acid in silage without increases in amounts of other constituents decreased intake of silage through an effect on palatability. This conclusion modifies interpretation on the significance of palatability, determined from studies of ingestive behaviour of ruminants, as a factor to explain reduced intake of sheep given silage.

Nine male and nine female lambs each from the Karagouniko (K), Chios (C) and East Friesland (F) breeds were weaned at a mean age of 50 days and were killed over a commercial range of live weight from 16 to 44 kg (carcass weights 6 to 22 kg).

Carcass weight was linearly related to fasted live weight prior to slaughter (r = 0·994). There were significant but small differences between breeds in mean weight of lungs, pelt and internal fat. The carcass components of bone, muscle and fat tissues were related to carcass weight by linear or quadratic regression. Adjusted to mean carcass weight, the mean weight of bone did not differ between breeds (2·82 kg, s.e. = 0·056 at 12·8 kg carcass). The K and C lambs had significantly less muscle than the F lambs (6·77, 6·63 and 8·20 kg, respectively, s.e.d. = 0·094) and more fat (2·87, 2·95 and 1·29 kg, respectively, s.e.d. = 0·017) when compared at mean carcass weight. The breed differences were significant over the whole range whether compared at equal weight or at an equal proportion of estimated mature weight. At the same proportion of carcass fat, corresponding to the United Kingdom market average (250 g/kg) the calculated carcass weights were 16, 17 and 30 kg, respectively, for K, C and F lambs.

It is concluded that the Friesland breed differs from many others in the proportions of muscle or fat at corresponding carcass weights but this difference is not a general characteristic of dairy sheep. Comparison of breeds at defined proportions of carcass components, as determined by market standards, can be effectively achieved by serial slaughter over a suitable weight range.

The study examined early measures of growth and their relationships with mature body size. Data on 792 purebred females of the Chios breed, born and reared on two experimental farms between 1978 and 1986, were analysed. Pedigree information and records on live weight at birth, at weaning, 105 days of age and at first and subsequent matings was recorded. There were flock-year differences for all traits studied (P < 0·01) and birth type was important for all traits except post-weaning growth rate. Dam lactation number had no significant effect on birth weight and pre-weaning growth rate, but it significantly affected all subsequent weights and post-weaning growth rate. Estimated heritabilities increased with age and ranged from 0·13 (s.e. 0·14) (birth weight) to 0·30 (s.e. 0·15) (mature weight). Genetic correlations were generally positive between weights and growth traits. Phenotypic correlations were positive and followed the pattern of the genetic correlations. The results suggest that selection for 105-day weight will result in increased mature weight.

Water balance, electrolyte and acid-base status of a group of five South African Mutton Merino wethers given chopped lucerne hay ad libitum was determined before and after ileorectal anastomosis. In general, the sheep recovered rapidly from surgery, resuming, within 10 days, a level of dry matter intake only slightly less than pre-operative levels (1323 (s.e. 147) and 1419 (s.e. 196) g/day respectively, P > 0·05). A new water balance was established at about the same time, when sheep with an ileorectal anastomosis drank about 2500 ml/day more water, while losing about 2400 ml/day more water in the faeces and about 365 ml/day less water in the urine than did the control sheep. Most of the electrolyte and acid-base parameters tested were not significantly affected by the anastomosis but the concentrations of oxygen and potassium in the blood of sheep with an anastomosis were significantly lower (P < 0·001), while those of chloride (P < 0·001) and bicarbonate (P < 0·05) were significantly higher, than before surgery. These observations, together with normal blood pH values, suggest that bypassing the large intestine caused a compensated metabolic alkalosis.

Energy balance was determined on the five sheep with an ileorectal anastomosis as well as five control animals in an indirect, open-circuit calorimeter. Despite a significantly lower (P < 0·05) loss of nitrogen and energy via the urine (due to a lower output of urine) in sheep with an ileorectal anastomosis, total heat loss, whether absolute or relative to energy intake, did not differ from control values and therefore indicated that basal metabolic rate was not significantly affected.

From post-mortem examinations, anastomoses appear to have induced a marked atrophy of the bypassed caecum and colon, whilst the mass of the large intestine was significantly less than in control animals, despite the significant enlargement of the rectum. On the other hand, the small intestine appeared unaffected, either macroscopically or histologically.

The most significant physiological changes that occurred on bypassing the large intestine were related to loss of water reabsorbtion. However, good adaptation to the increased turn-over of water was evident, providing free access to water and a salt lick was allowed.

Carcass and meat composition and eating quality of m. longissimus lumborum (LI) and leg joints were compared in entire male (ram), castrated male (wether) and female (ewe) Dorset Down-cross and Suffolk-cross lambs at about 20 weeks old. Rams grew faster than wethers and ewes particularly in the earlier maturing Suffolk crosses. Carcass weights ranged from 13 to 24 kg and at the mean carcass weight of 17·7 kg ram carcasses yielded larger shoulder joints. Ram carcasses were assessed visually leaner than those from the other sexes with similar conformation scores. Dorset Down carcasses tended to be fatter than Suffolk carcasses and the leanest carcasses were from Suffolk rams. Intramuscular fatness in LI was similar in all sexes. Intramuscular collagen contents were higher in ram LI than in other sexes. There were no differences in protein content, pH or colour of raw or roast LI. In assessments of eating quality of LI by triangle tests, male and female assessors differentiated twin rams and wethers equally. Category scaling showed LI from ewes to be slightly tougher than those from rams and wethers. Consumer evaluation of leg joints did not detect any differences in odour due to sex and ram meat was assessed better than that from wethers or ewes.

Groups of wether lambs were fed on four concentrate diets, from a live weight (LW) of about 25 kg until they had grown to about 50 kg, when they were killed, minced and analysed. A fifth group was killed at the start of the trial to provide data on their initial composition. The dietary treatments were identical except in their concentrations of calcium (Ca) and phosphorus (P) and supplied, with some approximations: (A) 0·75 times the requirements for Ca and P estimated according to the recommendations of the AFRC Technical Committee on Responses to Nutrients (TCORN, 1990); (B) a close match to the estimated requirements for both elements; (C) 1·5 times the estimated requirements for both elements; (D) 1·5 times the Ca requirement and 0·75 times the P requirement.

The retentions of Ca and P in the lambs were closely similar with diets B and C (6·8 and 7·3 g Ca per kg LW and 4·1 and 4·5 g P per kg LW) and significantly lower with diet A (4·8 g Ca and 3·0 g P per kg LW) and diet D (5·0 g Ca and 2·7 g P per kg LW). Parallel differences in response to diet were evident in the composition of the metacarpi. Plasma Ca and P concentrations showed no significant differences between the dietary treatments A, B and C, but plasma P concentrations were markedly depressed with lambs on diet D and plasma Ca concentrations were elevated in this group. Rumen P concentrations were also markedly lower in lambs on diet D and their food intakes and growth rates were lower than in lambs in other groups. The results indicate that Ca and P retentions were not increased by feeding these elements in excess of their estimated requirements and were reduced when Ca and P or P alone was reduced proportionately to about 0·75 times requirement. The data are in accord with the TCORN recommendations and suggests that they provide a satisfactory basis for defining Ca and P requirements for growing lambs.

The composition and degradability of four varieties of rice straw were examined. The varieties were tall or short and early or late maturing. The straws and their botanical parts (leaf blade plus leaf sheath, internode, node and chaff) varied in acid-detergent fibre, ash and silica contents, but not in nitrogen and neutral-detergent fibre. There were some trends but no significant difference in proportions of parts between varieties. Milled samples of whole straw and its parts were either untreated, treated with a cellulase preparation plus propionic acid, treated with ammonia, or treated with cellulase then ammonia. Rumen degradability was measured by incubating the samples in polyester bags suspended in the rumen of sheep. The soluble fraction and potential degradability were greater in untreated short and early than in tall and late varieties, but rates of degradation did not vary significantly. Internode was more degradable than leaf. Cellulase and/or ammonia treatments increased the soluble fraction, especially in tall and late varieties, and increased potential degradabiiity except in internode. Degradability correlated positively with ash and silica contents.

Different procedures for measuring cold resistance and metabolic rate of newborn lambs were evaluated by varying the extent of induced hypothermia, the rate of cooling and the method of rewarming. Relatively fast cooling followed by a simple self-rewarming procedure proved harmless and satisfactory.

The effect of age, from birth up to 2 weeks, on thermoregulation was studied. There was no difference in cold resistance between 0·5 h and 30 h after birth, and between 1 day and 2 weeks after birth, despite a large increase in insulation, body weight and coat depth over this period. Weight-specific resting metabolic rate and cold-induced peak metabolic rate similarly did not change significantly in the first 30 h, although resting metabolic rate tended to be lower at birth than at 30 h of age. Peak metabolic rate decreased significantly between 1 day and 2 weeks of age.

The effect of fasting, for 3 to 4 h after birth, on thermoregulation was also studied. Cold resistance and peak metabolic rate were not significantly affected by fasting. Recovery from hypothermia was slightly slower in fasted lambs.

These results may reflect the newborn lamb's initial reliance on heat production derived from brown fat and non-shivering thermogenesis. Older lambs, which benefit from better insulation, rely more upon shivering. Fasted lambs showed a tendency to rely more on insulation and slightly less on heat production than suckled lambs.

Effects of food intake (FI) and ad libitum food intake capacity (FIC) were investigated in 438 pigs fed in seven batches. Batches 1 to 6 consisted of barrows and batch 7 of gilts. Restrictedly fed animals had one to three ad libitum fed litter mates. In batches 6 and 7, animals were fed ad libitum until 48 kg live weight. Thereafter, two-thirds of the animals were fed according to a fixed weight scale. FIC is considered as a trait of the animal, independent of its actual FI. With ad libitum feeding FIC can be measured directly, with restricted feeding FIC has to be estimated.

Two methods of estimating average FIC from about 27 to 107 kg live weight were applied. The first method consisted of assigning to each restrictedly fed animal the average FI of one to three ad libitum fed litter mates. The second method was applied in batches 6 and 7 only and consisted of estimating FIC with multiple regression based on individual 'ad libitum performance' in the first part of the growing period.

Effects of FI and FIC on daily gain, food conversion ratio, backfat thickness, lean tissue proportion, fatty tissue proportion, lean tissue growth rate, fatty tissue growth rate and lean tissue food conversion were investigated. FI had significant effects on each trait, except on lean tissue food conversion. FIC had significant effects on body composition traits, but not on daily gain and food conversion ratio.

Irrespective of the method used to estimate FIC, results showed that animals with a higher FIC produced more fat and less lean from the same amount of food than animals with a lower FIC. It was suggested that the partition of food energy between energy for maintenance, protein deposition and fat deposition is associated with FIC. The findings confirmed that selection for leaner and more efficient pigs may result in animals with lower FIC, irrespective of the feeding regimen during performance testing.

From each of 159 litters, between one and three pigs were fed ad libitum and between one and four pigs were fed at several restricted feeding levels from about 27 to 107 kg live weight. Effects of litter by feeding regimen (ad libitumv. restricted feeding) interactions (L × F) and sex by feeding regimen interactions (S × F) on production traits were investigated. In experimental groups where restrictedly fed animals received an amount of food based on their estimated voluntary food intake capacity (FIC), interactions were non-significant. In groups where restricted feeding was independent of FIC, significant L × F or S × F interactions were found for daily food intake (FI), daily gain, lean tissue growth rate and fatty tissue growth rate, but not for body composition and food conversion traits.

Because of between-animal variation in FIC, restricted feeding according to a fixed scale restricted intake of some animals to a higher degree than others and degree of food intake restriction (DFR) was calculated from actual FI and FIC for animals fed at a restricted level. After correction of FI and production traits of restrictedly fed animals for differences in DFR, L × F and S × F interactions were not significant.

It was concluded that the poor relationships often found between test-station results of boars and results of their progeny in practical environments may be caused, to an important extent, by differences in DFR in each environment. To overcome these genotype by environment interactions it is recommended that animals be fed ad libitum in both test and commercial environments.

The non-linear model y = ae(−hW−c/W) was fitted to weekly calculated daily gain (DG), daily food intake (FI) and food efficiency (FE) of 653 barrows and gilts fed ad libitum or restrictedly during a growing period from 27 to 108 kg live weight. Where y was DG, FI or FE, W was live weight and a, b and c were parameters. The model fitted well to the expected course of the traits, with an accuracy similar to that of quadratic polynomials. Parameters for one trait could simply be derived from the parameters for the other two traits. For each trait, four basic patterns were distinguished, depending on the signs of b and c. Curves with a maximum (b > 0 and c > 0) occurred most frequently. In cases of curves with a maximum, the model could be reparameterized to a model with parameters having a simple biological meaning.

Coefficients of determination in barrows averaged 0·29 for DG, 0·88 for FI and 0·45 for FE, whereas these values were somewhat lower in gilts. With ad libitum feeding, a DG curve with a maximum was fitted in proportionately 0·83 of the barrows and 0·61 of the gilts. The maximum DG was on average at live weights of 64 kg for barrows and 77 kg for gilts. A maximum in the FI curve was predicted in proportionately 0·60 of the barrows and 0·39 of the gilts. Curves for DG and FI in gilts were less curvilinear than in barrows. FE curves, with ad libitum feeding, had a maximum in proportionately 0·59 of the barrows and 0·52 of the gilts. This predicted maximum FE was, on average, before the start of the growing period. Gilts had a higher FE than barrows from 35 kg body weight onwards, and the difference increased with increasing live weight. Differences in FE between ad libitum and restricted feeding were small, with a tendency for animals fed at a restricted feeding level to be more efficient at the end of the growing period.

Average FE curves and individual FI or DG curves were used for indirect prediction of individual DG or FI curves, respectively. The correlation between directly and indirectly predicted values of DG and FI at live weights of 30, 65 or 100 kg was about 0·7 in ad libitum fed barrows and gilts, and greater than 0·8 in pigs fed at a restricted level. This indicates that the model is suitable to predict and control the course of individual daily gain by influencing the course of food intake.

The effect of monosodium L glutamate (MSG) on diet palatability and piglet performance during the suckling and weaning periods was studied in the course of five successive experiments involving a total of 230 litters during the suckling period and 120 pens of seven piglets during the post-weaning period. During the suckling period, the addition of 5 g MSG per kg diet led to a significant 0·36 proportional increase of creep food intake from the 18th day post farrowing. However, no increase in weaning weight was observed subsequent to the intake of the diet with added MSG. Moreover, in an experiment where MSG was added with an associated commercial flavour, no flavour enhancing effect of MSG was detectable. Few health problems were encountered at weaning and the number of scouring piglets was not related to one particular diet. During the first 13 days after weaning the addition of MSG to the diet increased daily food intake by proportionately 0·1 (P < 0·01) as well as growth rate by proportionately 0·07 (P < 0·01). Moreover, the addition of MSG to the diet was more effective with piglets of low weaning weight and thus contradicted the classical relationship between post-weaning performance and weaning weight. Finally, in an experiment where weaning occurred at lower ambient temperature, no improvement of post-weaning performance was observed following the addition of MSG.

In comparative digestibility trials with growing pigs Latin squares are widely used as experimental designs. In this paper conditions for the application of Latin squares are explicitly stated and checked with real data. Data were collected at the Institute for Livestock Feeding and Nutrition Research in the Netherlands and consisted of digestibility trials run on pigs of 40 to 100 kg live weight. There is strong evidence for a certain form of period × animal interaction in the data. This may seriously affect estimates of treatment effects and results of significance tests. It is concluded that Latin squares are unsuitable for comparative digestibility trials with pigs growing from 40 to 100 kg. When there is no interest in statistical comparison of the diets under study, Latin squares can still be used. Some suggestions for alternative designs are given.

Priming with a progestagen-impregnated vaginal pessary followed by an injection of 750 i.u. pregnant mare serum gonadotropin at pessary withdrawal was used to induce oestrus and ovulation during anoestrus (13 June, latitude 57°N) in 58 Border Leicester × Scottish Blackface ewes that had lambed previously in late March/early April. At 54 to 58 h after pessary withdrawal the uterus of each ewe was viewed by laparoscopy and 50 × 106 viable sperm deposited into each uterine horn. Mean ovulation rate was 2·9 (s.d. 0·12; range 1 to 5). Plasma progesterone concentrations remained >1·5 μg/l in 54 (93%) of the ewes until at least day 21 after insemination implying a high fertilization rate. After allowing for three ewes that were observed to abort between 39 and 55 days after insemination and another three, carrying nine normally developed lambs, that died between 102 and 136 days of gestation, 40 of the remaining 48 ewes (i.e. 69% of the original 58) produced 76 viable lambs in early November. These conception and lambing rates obtained following the intrauterine insemination of 100× 106 viable sperm per ewe are more than double those obtained following natural mating or conventional cervical insemination with four times the number of viable sperm and imply that intrauterine insemination may enhance lamb production from ewes induced to breed during seasonal anoestrus.