Twenty-four lactating British Friesian cows were used in a four-period partially balanced change-over design experiment to evaluate eight treatments consisting of two silage types (wilted and unwilted), each offered in addition to supplements containing both two crude protein (CP) levels (160 and 210 g/kg fresh weight) and two energy levels (10·8 and 12·9 MJ metabolizable energy (ME) per kg fresh weight) obtained by including 100 g spray-dried tallow per kg, in a 2 × 2 × 2 factorial arrangement. All supplements were offered at ttie same rate of 6·8 kg/day and total diet digestibility and food utilization studies were carried out on all animals at the end of the final period of the experiment. A separate two-period change-over design expeximent, using two lactating cows fitted with rumen cannulae, was undertaken to determine the influence of the two silages on volatile fatty acid (VFA) proportions in the rumen and to measure the rates of disappearance of dry matter (DM) and nitrogen in the silages and supplements from the rumen.

There were no significant interactions between silage and supplement type in terms of animal performance. Animals offered unwilted silage consumed proportionately 003 less DM but produced 0·03 more milk than those offered wilted silage. Although total diet digestibility was not influenced by silage type because of the higher gross energy concentration of the unwilted silage the ME intake with this diet was higher than with the diet based on wilted silage and the ratios of milk energy output to ME available for production (0·54) were similar with both silages. Silage type had no effect on the rumen VFA proportions but influenced the jates of disappearance of DM and nitrogen from the rumen.

Increasing the CP concentratioryof the supplement had no effect on the intake of silage DM but resulted in a significant incrase in milk yield (0·55 kg/day) but due to a reduction in milk fat concentration there was np etfect on yield of either fat-corrected milk or milk energy. The use of the higher energy supplement depressed the intake of silage DM by 0·46 kg/day but resulted in an increase in milk yield of 0·74 kg/day. Neither CP nor energy concentration of the supplement had any effect on DM or energy digestibility of the total diet but there were considerable differences between supplements in the patterns of nitrogen and DM disappearance from the rumen.

Thirty-four British Friesian bull calves were used in experiments to identify diurnal patterns of respiratory quotient (RQ), as an indicator of substrate utilization and to measure energy balance when the same daily amount of milk replacer was given on either 1, 2, 4 or 6 occasions. Each calf spent two 4-day periods in an open-circuit respiration chamber followed immediately, in selected calves, by an 8-day period in a metabolism crate, period 1 starting when calves were 12 days of age and period 2 at 28 days of age. The amount of milk replacer given daily was 32 and 48 g/kg M0·75 during periods 1 and 2 respectively.

Neither rate of live-weight gain nor the energy balance of the calves was affected by frequency of feeding. However, raising the frequency of feeding from one to four times daily significantly affected the pattern of RQ. Reduced feeding frequency tended to raise the mean maximum and lower the mean minimum values of RQ; reducing the frequency of feeding significantly increased the range in RQ (F < 0·01).

The apparent dry-matter digestibility of the milk replacer was higher in 36-day-old than in 20-day-old calves (0·93 v. 0·88; s.e.d. 0·011, P < 0·01). The effect was mainly due to an increase in the digestibility of fat (0·82 v. 0·73; s.e.d. 0·019). In 20-day-old calves, there was a linear increase in fat digestibility with increased frequency of feeding (P < 0·01) rising from 0·67 in calves given milk once daily to 0·85 when milk was given in six meals.

The results suggest that calves given milk replacer once daily (at a level of intake of 32 rising to 48 g milk powder per kg M0·75) do not pass through a diurnal period of severe nutrient deprivation and that raising frequency of feeding would do little to improve efficiency of energy utilization.

Two feeding trails were made and in both all the diets given contained about 11 MJ metabolizable energy (ME) per kg dry matter (DM).

In trial 1, diets containing three concentrations of crude protein (CP) were given to a total of 84 Friesian bulls in each of three stages of growth. They were, in stage 1 (150 to 250 kg live weight) 130, 145 and 160 g CP per kg dietary DM; in stage 2 (250 to 350 kg live weight) 110, 130 and 145 g CP per kg dietary DM; in stage 3 (350 kg to slaughter) 95, 110 and 130 g CP per kg dietary DM. In stages 2 and 3, the source of supplementary nitrogen at each CP concentration was either soya-bean meal or non-protein nitrogen (NPN) giving six combinations of concentration and source of dietary CP. Live-weight gains in stage 1 were 1·52, 1·60 and 1·58 kg/day respectively (P > 0·05). In the stages involving both concentrations and source of nitrogen (soya-bean meal in parentheses), the mean daily live-weight gains were (1·40), 1·38, (1·32), 1·34 (1·34) and 1·31 kg for stage 2 (F > 0·05) and (1·14), 1·14, (1.24), 1·16, (1·06) and 1·08 kg for stage 3 (P < 0·05), for the diets of 130, 145 and 160 g CP per kg diet respectively.

Trial 2 consisted of six stages of growth, the first five of 45 days duration and the sixth of varying length up to a predicted slaughter weight, and seven dietary treatments. Treatment 1 was a negative control (NC) with a dietary CP concentration of 90 g/kg DM. For the other six treatments two concentrations of dietary CP were given; 120 g/kg and 140 g/kg in stages 1 and 2 and 100 g/kg or 120 g/kg in stages 3 to 6. Dietary CP concentration was increased by the addition of NPN, fish meal or soya-bean meal. A total of 98 Friesian bulls were used with an initial average live weight of 185 kg. Daily gain of the NC group in the six stages of the trial was 0·86, 0·97, 101, 1·13, 1·07 and 1·00 kg for stages 1 to 6 respectively. From the results it was concluded that feeding NPN is not efficient at live weights lower than 250 kg and that there is no advantage to feeding fish meal over soya-bean meal in diets of medium energy concentration at live weights heavier than 250 kg.

Sixty-four Friesian/Holstein cows culled from dairy herds were selected for uniformity and general health. They were allocated to five groups of 12 on the basis of body condition and live weight excluding the two cows at each extreme. These four cows indicated the range of condition and live weight in the sample and were slaughtered at the beginning of the trial with one group of 12 cows. Of the remaining four groups, two were implanted with 300 mg trenbolone acetate (Finaplix, Hoechst) and two used as unimplanted controls; one implanted group and one control group were slaughtered after 60 days and the remaining cows after 100 days. All cows were individually fed to appetite on a pelleted diet of 500 g dried lucerne and 500 g barley per kg. After slaughter the left side of each carcass was cut into primal joints and then separated into fat, lean and bone.

Implantation resulted in greater live-weight gains to 60 days (1·35 v. 112 kg/day, P > 0·05) and to 100 days (1·31 v. 0·92 kg/day, P < 0·01), greater lean meat production and an increase in the lean proportion of carcasses.

Feeding cows for 100 days rather than for 60 days led to higher slaughter weights but daily live-weight gain decreased and food intake increased progressively between 60 and 100 days. Carcasses from cows slaughtered at 100 days had significantly higher lean, fat and bone weights (P < 0·01) but as a proportion of side weight only the fat proportion of the carcass was significantly increased (P < 0·05).

It was concluded that the supplementary feeding of cull cows produced heavier carcasses but at these high rates of gain increased yield was at the expense of proportionately more fat in the carca?.s. This problem was partially countered by implantation with trenbelone acetate.

Forty-eight Hereford × Friesian and Blue Grey cows ranging in body condition score from 1·75 to 4·0, 12 weeks before calving, were fed to achieve three levels of body condition score at calving. Half the cows were fed on an increasing plane of nutrition as pregnancy advanced and half were fed on a flat-rate feeding system. The condition scores achieved by the three groups at calving were 2·28, 2·47 and 2·70 (s.e.d. 0·071; P < 0·001). Differences in cow body condition at calving were reflected in cow live weight and condition 6 weeks later, but there was no effect on calf performance. Hereford × Friesian cows lost more weight in early lactation, tended to produce more milk and their calves were heavier. Pattern of feeding had no effect on cow condition score or weight at calving or on subsequent performance.

It was calculated that each unit of body condition-score loss in late pregnancy contributes the equivalent of 3200 MJ dietary metabolizable energy while 6600 MJ dietary metabolizable energy are required for 1 unit of condition-score gain.

Hypothermia with starvation is a major cause of neonatal mortality among lambs born outdoors. The results of selection for resistance to hypothermia (cold resistance) in newborn Scottish Blackface lambs are described. Cold resistance was defined as the time required to induce a decline in body temperature of about 4·5°C by means of tests involving part-immersion in a cooled water bath. From about 200 tested lambs (half males) four ram lambs with the lowest cold resistance and four with the highest resistance were selected and each mated randomly at 7 months of age to about 30 ewes to establish upward and downward selection lines. The progeny were subsequently tested for cold resistance and the selection process was repeated for four male generations. Tested females were allocated at 18 months of age to high or low lines according to their performance as lambs.

Preliminary half-sib analysis and sire-offspring regression gave estimates of 0·3 for the heritability of cold resistance. Response to selection was rapid but asymmetrical with a realized heritability for cold resistance of 0·27 (s.e. 0·13) for upward selection, 0·01 (s.e. 0·16) for downward selection and 0·17 (s.e. 0·09) for the line divergence.

Body weight at test was phenotypically, but not genetically, correlated with cold resistance. Rate of recovery from induced hypothermia was not significantly correlated with cold resistance. The results are discussed in relation to lamb mortality in the field.

Reproductive performance, body growth, wool production and ewe survival were examined in Scottish Blackface sheep. The data set was from a large randomly selected population maintained under hill conditions between 1954 and 1974. Paternal half-sib analyses of variance were used to estimate heritabilities and genetic correlations, while maternal half-sib analyses of variance indicated the relative importance of maternal effects (both genetic and environmental).

Among reproductive components, the heritability of fertility (ewes lambing per ewe mated) and lamb survival (lambs weaned per lamb born) were about zero. Litter size (lambs born per ewe lambing) had an estimated heritability of 0·12 (s.e. 0·04) and a repeatability of 0·22 (s.e. 0·02), indicating some scope for improvement. Additionally, the strong genetic correlation between adult body weight and litter size (0·66, s.e. 0·18) suggested greater response from a combination of indirect selection (on body weight) and direct selection (on dam's litter size). Prior to weaning, direct genetic effects on body weight were very small (heritability less than 0·1) with a large influence of maternal environment. After weaning, these maternal influences gradually diminished and the heritability of weight increased. Genetic correlations between body weights showed a similar increasing trend with age. Genetic improvement of early growth rate would be more effective by selecting on a post-weaning rather than a pre-weaning body weight. Fleece weight had a high heritability (about 0·5) but was genetically uncorrelated with other measures of performance. Ewe survival had a small genetic component although estimates of heritability were very imprecise.

A selection experiment with Scottish Blackface sheep was used to compare predicted and realized responses to selection. Three closed lines, of approximate annual size of 270 ewes and 10 rams, were maintained between 1956 and 1974, in which selection was at random, or for high and low values of an index of cannon-bone length at 8 weeks of age adjusted for body weight at the same age. An unselected base flock (1954-55) and the randomly selected line were used to estimate base population parameters, while the selected lines were used to estimate realized responses to selection.

Heritabilities and genetic correlations were obtained in the unselected lines from a variety of collateral and ancestral relationships. The important components of phenotypic variance were estimated and likely responses to selection predicted for the index and its two component traits. Realized responses to selection were estimated from the regression of response on selection differential. The expected variance-covariance matrix of observed responses was included in generalized least-squares estimates of these regressions.

The realized heritability of the index under selection, estimated from the divergence of selected lines, was 0·52 (s.e. 0·02). After allowing for the expected reduction in heritability arising from linkage disequilibrium, this was very similar to the base population estimate of 0·56 (s.e. 0·04). The responses in the component traits of the index were also very close to those expected from base population parameters

A selection experiment with Scottish Blackface sheep was used to compare predicted and realized correlated responses to selection. Three lines were maintained between 1956 and 1974 in which selection was either at random or for high and low values of an index of cannon-bone length at 8 weeks of age adjusted for body weight at the same age.

There was no evidence of asymmetrical responses in any trait. Selection for increased cannon-bone length, adjusted for body weight, resulted in (i) increased body weights at all ages between birth and maturity, (ii) increased reproduction rate, principally from increased litter size but also from small responses in ewe fertility and lamb survival and (iii) decreased survival of adult ewes. The realized genetic correlations were relatively small (up to 0·3) but mostly significantly different from zero, whereas base population genetic correlation estimates were very imprecise. The realized responses in reproduction rate were probably a consequence of the genetic response in body weight.

A study was made of the levels of oestrous activity of two indigenous breeds of sheep (Somali and Nandi) and three exotic breeds of sheep (Merino, Karakul and New Zealand Romney Marsh) over a period of 3 years, in an equatorial environment. Breed was the only significant source of variation for the length of the oestrous cycle (P < 0·01). The mean lengths of the oestrous cycle were 17·2 (s.d. 3·21), 17·5 (s.d. 2·24), 17·9 (s.d. 2·99), 17·5 (s.d. 2·57) and 16·5 (s.d. 3·41) days for the Somali, Nandi, Merino, Karakul and Romney Marsh breeds, respectively.

The mean percentage of ewes of the different breeds showing oestrus in 20-day periods were 69·8 (s.d. 22·57), 49·9 (s.d. 18·67), 63·4 (s.d. 25·70), 79·2 (s.d. 20·30) and 33·2 (s.d. 23·50) % for the Somali, Nandi, Merino, Karakul and Romney Marsh breeds, respectively. Time-series analysis did not detect any evidence of seasonal variation in oestrous activity, although there was an indication that the Merino and Romney Marsh breeds showed a marked increase in oestrous activity following, the introduction of rams. It was concluded that the variation in level of oestrous activity was short term and random.

Two experiments were designed to study the effect of rearing method, female company, and repeated libido and semen tests on the pattern of sexual development of ram lambs. Four artificial rearing treatments (AR) were compared with natural rearing (NR) in one experiment and four artificial compared with three natural rearing treatments in a second experiment. Lambs on AR treatments in both experiments, as well as naturally reared lambs in the second experiment, were reared with or without females either before and/or after weaning. Half the lambs in the second experiment were libido and semen tested, and their mean plasma testosterone level was also measured, while the rest were only tested at the end of the experiment. The effect of method of rearing and female company on body growth, testes diameter, libido and semen quality were also investigated.

A close relationship was found between body weight and sexual characteristics (for example, testis diameter, semen traits, libido, testosterone level and age at the onset of sexual maturity). NR resulted in faster growth than AR but was not appreciably different from early weaned-naturally reared treatments. It was also observed that mating performance improved steadily with advancement in age and increase in body weight. The onset of sexual maturity was attained at a minimum of 23 kg body weight and a minimum of 198 days of age. However, the heavier the lamb when sexually mature, the better its mating performance.

Previous libido tests of ram lambs improved their subsequent mating performance, but had no effect on semen quality. However, libido tests did not improve the performance of those ram lambs which did not exhibit initial libido.

It is suggested that early weaning at 2 months of age could be a suitable system for ram lambs weaned off their mothers for breeding purposes. These lambs should be reared as an all-male group but libido tested at monthly intervals commencing at 3 months of age. The criteria for selecting these lambs at an earlier age should be body weight and testis diameter.

Ultrasonic measurements of fat thickness and m. longissimus depth 25 and 50 mm lateral to the mid line at the last rib and 100 mm caudally to the last rib were made, before slaughter, on 273 ram lambs using two probes: the Scanogram and Krautkramer. The lambs were from three strains: a meat-type sire strain and two fecund dam strains. They weighed on average 37·3 kg at scanning, (range 29·0 to 51·3 kg), well within the weight range over which Canada Al-grade lambs are marketed in Canada. Their carcasses contained on average 437 g trimmed boneless meat per kg (s.d. pooled within strain = 24·1 g/kg).

The precision with which the ultrasonic measurements combined with live weight at scanning predicted trimmed boneless meat (weight and proportion) was examined. Fat thickness measurements had no predictive value (P > 0·05). The residual standard deviations for the prediction of trimmed boneless meat in the half carcass (weight and proportion) from weight at scanning were 0·29 kg and 21·5 kg, respectively; the addition of the m. longissimus depth measurement reduced the residual standard deviations by 0·02 kg and 1·1 g/kg, respectively. The weight of trimmed boneless meat was predicted with more precision than the proportion of trimmed boneless meat. Strain, location of measurement site or type of instrument did not change the precision of prediction.

It is concluded that prediction of trimmed boneless meat in young ram lambs based on live animal ultrasonic measurements made with the Scanogram or Krautkramer lacks the level of precision necessary for practical application.

One hundred and ninety-two individually fed pigs, half of them castrated males and half females, were used to examine the effects of reducing the concentration of dietary lysine below the value of 0·84 g/MJ digestible energy suggested by the Agricultural Research Council (1981). Lysine was varied by altering the inclusion of either free lysine hydrochloride or soya-bean meal. Over the whole experiment from 18 to 65 kg live weight, there were significant reductions in live-weight gain and increases in food: gain ratio in response to reducing lysine concentration; these changes were greater with soya-bean meal than with free lysine. During later growth (45 to 65 kg) alterations in lysine concentration had no significant effect.

For pigs given soya-bean supplements, both the live weights after fasting and the carcass weights were significantly lower than for those given free lysine hydrochloride, suggesting that at least part of the difference in growth between pigs given free lysine hydrochloride and soya-bean was attributable to this difference. The food required per kg carcass weight gain, or per kg lean tissue gain, was not affected by the source of lysine.

In a subsidiary experiment using nitrogen balance, supplements of five amino acids in addition to lysine did not increase nitrogen retention, providing further evidence that lysine was the only amino acid responsible for the response seen with soya-bean meal. It was concluded that for growing pigs with daily food intakes of 1·5 to 1·8 kg, a lysine concentration of at least 0·81 g/MJ digestible energy is required to maximize performance.

Halothane positive and negative selection lines were established to estimate the effects of the halothane gene on performance in British Landrace pigs. Incidences of positive reaction diverged from 0·12 in the foundation generation to 0·93 in the positive and to 0·02 in the negative line in generation 4. Differences in litter productivity as a trait of the dam were estimated from a total of 399 positive × positive and negative × negative parity 1 and 2 matings in generations 1 to 4.

Compared with negative contemporaries, positive females showed non-significant reductions in litter size at birth (−0·10, s.e. 0·26) and at 42 days (−0·28, s.e. 0·26), accompanied by significant reductions in average piglet weights at birth (−0·11, s.e. 0·02 kg) and 42 days (−0·4, s.e. 0·2 kg). There were no differences in conception rate or adult live weights. A subsample of 69 parity 2 and 3 sows slaughtered 30 days after mating showed no significant difference in ovulation rate or embryo survival, but for positive dams the length of embryos was significantly reduced (−3·1, s.e. 1·1 mm).

The study suggests that the principal effect of the halothane gene was in reducing foetal growth rather than litter size. However, the phenotypic difference is expected to under-estimate the genetic difference between homozygotes, and it was not possible to distinguish the effects of dam and offspring genotypes. The present economic loss of roughly £4·70 per positive litter would be unlikely to justify elimination of the gene from a purebred maternal Landrace herd.

The effects of halothane phenotype on growth were estimated from generations 1 to 3 of British Landrace positive and negative selection lines. A total of 241 full-sib pairs (castrated male and gilt) from 133 second litters were performance tested from 25 to 85 kg live weight on either ad libitum or scale (0·69 g/g ad libitum daily food intake) feeding.

Compared with negative reactors over feeding regimes, positive pigs showed significant advantages in food conversion ratio (−0·07, s.e. 0·02), eye-muscle area (0·8, s.e. 0·3 cm2), proportion of lean in sample joints (16, s.e. 7 g/kg), and visual conformation scores, accompanied by disadvantages in meat colour (5·9, s.e. 2·1 EEL units), incidence of pale, soft, exudative (PSE) meat (0·36 v. 0·05, P < 0·001), post-weaning mortality (0·09 v. 0·01, P < 0·001) and ad libitum daily food consumption (−0·08, s.e. 0·02 kg). In addition, there were non-significant indications of an advantage in carcass lean proportion (11, s.e. 10 g/kg), with disadvantages in carcass length (−14, s.e. 7 mm) and proportion of lean in high-priced cuts (−6, s.e. 2 g/kg). Only daily food consumption showed significant selection line × feeding level interaction.

Averaged over feeding regimes, an estimated extra return of £1·70 (s.e. 0·50) from lean growth would be offset by increased PSE and mortality to give a net loss of roughly £3·90 (s.e. 1·00) per positive reactor. Taking these results as the minimum difference between homozygotes, and if the gene is additive for lean proportion but recessive for stress susceptibility, the heterozygote would have an advantage of £0·90 over the normal homozygote. The effects of the gene on growth and carcass traits in British Landrace appear similar to those reported in other breeds, although the advantage of positive reactors in lean proportion may be slightly smaller.

The effects of an internal recirculating air filter on air hygiene and pig performance were studied using two groups each of 100 early-weaned pigs from a commercial herd. The two groups were housed in separate rooms under identical conditions except that one room contained a recirculating air filter. Five trials were carried out, but the same records were not kept in all trials. The air throughput of the filter was similar to the installed ventilation capacity (0·9 m3/s). The filter material nominally removed particles > 5 μm. Distribution ducts discharged the filtered air downwards over the pigs' lying area at a velocity of 0·3 m/s. At locations close to the pigs, the most representative proportional reductions of total particles, dust mass, and bacterial colony-forming particles were 0·58 (P < 0·01), 0·73 (P < 0·01), and 0·51 (P < 0·01), respectively. No differences in growth rate or health of pigs after 3 to 4 weeks were observed between the rooms but the pigs originating from the room containing the air filter reached a marketable state 6 to 8 days earlier than the control group pigs on the three occasions that this aspect was monitored (P < 0·0·01).

A trial was carried out to examine some of the factors involved in the possible deterioration in pig meat quality associated with the trend towards leaner carcasses. Five batches of pigs, each one as far as possible from the same producer, were identified at each of 10 abattoirs. Three gilts and three entire males were selected from each batch to show a range of fatness. Overall mean fat thickness measurements over the m, longissimus at the last rib (P2, mm) for lean, average and fat carcasses were 8·8, 11·6, and 16·9 (gilts) and 8·4, 11·7 and 15·5 (entire males). The overall mean carcass weight was 58 kg. Loin and leg joints were assessed for cutting and presentational characteristics by a panel of 45 butchers. Loin chops and shoulder and leg joints were assessed for eating characteristics by consumer panels involving a total of 500 families. Butchers judged the fat of lean carcasses to be softer and the meat to be floppier and with more tissue separation. Entire males also had slight disadvantages in these respects. Consumers found the chops of lean carcasses to be less juicy on average (0·16 lean carcasses were judged to have dry chops compared with 0·09 for fat carcasses) with a tendency towards toughness and less flavour. There were no differences in overall acceptability. Meat from the two sexes did not differ in eating quality or overall acceptability to consumers. The butchers were more critical of overlean carcasses than were consumers. The trial indicates that the trend towards leaner carcasses is likely to create butchery problems but not consumer dissatisfaction.

Evaluations of meat quality were made of 300 carcasses, of average weight 58 kg, falling into three fatness groups, 8, 12 and 16 mm for P2 fat thickness. There were equal numbers of entire males and gilts. An increase in fat thickness was associated with an increase in the firmness of loin and shoulder backfat, a reduced incidence of fat separation and changes in the composition of both muscle and backfat: the concentration of lipid increased and that of water decreased, more markedly in backfat. In addition, the concentration of collagen in backfat decreased. The loin joint was visually most attractive when P2 was 11 mm, attractiveness decreasing below and particularly above this value. Eating quality of grilled chops was assessed in a representative subsample of 96 pigs falling into the 8-mm and 16-mm P2 categories. The only statistically significant difference observed between these extremes of fat thickness was in juiciness which was lower in the leaner pigs; the correlation between juiciness score and m. longissimus lipid concentration (‘marbling fat’) was 0·31. Juiciness did not influence the assessment of overall eating quality which showed no association with fatness. Entire males of similar backfat thickness to gilts (12 mm P2) had a higher concentration of water and lower concentration of lipid in backfat and slightly softer backfat. There were no differences in eating quality, or in the presence of abnormal odours or flavours, between entire males and gilts. These results confirm those obtained in butcher and consumer panel tests in the same carcasses.

The use of profit equations for deriving economic weights (the value per unit improvement in a trait) in the genetic improvement of livestock has led to anomalies both in theory and in practice. These anomalies can be removed by imposing two conditions. One is that any extra profit from genetic change that can be matched by rescaling the size of the production enterprise should not be counted since it can be achieved without any genetic change. Only savings in cost per unit of product value should be included. The second condition is that changes that correct previous inefficiency in the production enterprise should not be counted. Thus, it is assumed that resources are efficiently used, and changes in output will require proportional changes in input. This means that fixed costs, like variable costs, should be expressed per unit of output, rather than as a fixed total enterprise cost. Improvement effort spent on traits that redress current inefficiencies in a production enterprise is of local and temporary value and is at the opportunity cost of improvement of other traits that save costs per unit of product value and are of permanent and general value. Application of these two conditions is shown to give economic weights that are identical on different bases; scaled output value, input or profit; fixed output value, input or profit; and zero profit. They were also equivalent to economic weights derived as the cost per unit product value from an economic efficiency index. The dilemma of different economic weights for different perspectives in production (unit of product, animal, producer, investor, and consumer) is also resolved.

One of the major limitations to the widespread use of artificial insemination (AI) in the United Kingdom (UK) sheep industry is that with frozen-thawed semen current insemination techniques result in lowered fertility. Consequently, only fresh or liquid-chilled ram semen can be used if commercially acceptable conception rates are to be achieved.