Estimates were obtained of the variability among herds of dairy cattle of the standard deviation (s.d.) and coefficient of variation (CV) of milk, fat and protein yield. For fat yield, for example, the mean s.d. within herds for heifers was about 30 kg, the s.d. among herds was 5·2 kg and the mean and s.d. of CV were 0·15 and 0·026, respectively. Correlations of both s.d. and CV were high between both heifers and cows in the same herd and year and between heifers or cows in the same herd in successive years, showing that variability was consistent over time and over age groups.

Ways of correcting for the heterogeneity of variance for use in cow index calculations are suggested; because CVs are heterogeneous, simple log transformation is not sufficient.

Progesterone concentrations were measured by radioimmunoassay in fat-free milk samples obtained three times weekly from calving until day 60 of pregnancy from over 700 cows in commercial dairy herds. Profiles of progesterone concentration during the ovulatory cycle and early pregnancy were also examined. In 94% of the animals cycling had commenced by 60 days post partum and a further 3% had shown some ovarian activity. In 34% of cows, the resumption of regular ovulatory activity was preceded by short periods of progesterone secretion which did not meet the criteria for a 'normal cycle’. Peaks of progesterone cross-reactivity (>0·75 ng/1) were detected at the time of ovulation in proportionately 0·06 of all cycles. There was no relationship between the detection of oestrus and the length of the following progesterone cycle or the concentrations during that cycle. However, there was a tendency for the first progesterone cycle post partum to be more variable in length than later cycles (0·56 within the range 18 to 24 days compared with 0·70 of later cycles; P < 0·001). Extension of the length of the luteal phase occurred in 58 second and later cycles (proportionately 0·141) following inseminations which did not result in calving. However, since the comparable figure for 1053 cycles where no insemination was given was 0·048, the overall figure for embryo mortality later than 24 days after insemination was estimated as 9·3%. Concentrations of progesterone in pregnant animals, compared to non-pregnant animals, and the distributions of luteal lengths between 8 and 21 days long following inseminations not resulting in calving could both be explained by early maternal recognition of pregnancy.

Genetic lines for high and low liquid milk production were established within the British Friesian and British Jersey breeds by random matings of experimental dams to the ‘best’ and ‘worst’ nationally available progeny-tested sires. Some Friesian dams could also be classified as either high or low for genetic potential for milk yield on the basis of previous, but less rigorously controlled, matings to high-or low-production sires. The dams and their high and low potential-production female progeny were reared indoors, and given a single complete pelleted diet ad libitum from weaning until leaving the experiment after their third calving. Height at withers and width at hooks were recorded monthly, cumulate food intake and body weight fortnightly and milk yield, fat and protein concentration weekly, throughout life in the experiment. Measures of lactation production, food intake and efficiency of conversion of food to milk product during the whole calving-to-calving interval were obtained. Yields were about 0-8 times national average yields. Differences between high and low genetic lines appeared similar for the two breeds. High potential-production progeny produced more liquid milk, fat and protein but at a lower fat and protein concentration than did low-potential progeny. High-potential progeny consumed more food from calving to calving and had higher food conversion efficiencies to liquid milk, fat and protein. The response in efficiency accompanying a given change in production was close to that predicted by phenotypic regression of efficiency on yield with a 0-75% increase in efficiency for every 1% increase in yield. The likelihood of smaller returns in efficiency at higher yields is discussed. High potential-production progeny lost more body weight than did low during lactation, suggesting a greater withdrawal of energy from body reserves. High-potential progeny were neither heavier nor taller but were marginally narrower at the hooks than were low-potential progeny, suggesting the possibility of increased calving difficulties as selection for increased yield continues.

Friesians produced 50% more liquid milk, 13% more milk fat and 29% more milk protein and consumed 22% more food from calving to calving than did Jerseys. As a consequence Friesians were markedly more efficient at producing liquid milk (+23%) but essentially no more efficient at producing milk energy (+2%) than were Jerseys. In terms of both biological and commercial application of the results, any biases in the experiment probably operated against the Jersey relative to the Friesian.

Predictions of food intake using accepted feeding standards underestimated observed total food intake during lactation by a factor of 0-8. Linear regression indicated underestimation of maintenance requirement as a likely explanation.

In each of 2 years, 20 Friesian cows were given either a low-energy or a high-energy complete diet to appetite during weeks 3 to 20 of lactation. The diets for trial 1 contained concentrates, lucerne silage and chopped grass hay in the proportions 33:47:20 or 58:29:13. In trial 2, concentrates and lucerne silage were mixed in the proportions 38:62 or 75:25. These mixtures had energy concentrations of 10·2 or 11·2 and 10·3 or 11·9 MJ metabolizable energy per kg dry matter (DM) and crude protein concentrations of 144 or 160 and 140 or 157 g/kg DM in the low- and high-energy diets of trials 1 and 2 respectively.

Cows offered the low-energy diet consumed less DM daily (14·5 v. 17·4 (P < 0·001) and 15·2 v. 18·2 (P < 0·001) kg DM per day in trials 1 and 2). Daily milk yields were lower with low-energy diets (19·2 v. 22·3 (P < 0·001) and 18·7 v. 24·2 (P < 0·001) kg/day in trials 1 and 2), and there were significant differences between treatments in milk composition and live-weight changes.

The correlation between food intake and milk yield during weeks 3 to 20 was moderately strong (r2 = 0·58). Variation in food utilization efficiency between diets and parity groups was small in comparison to variation within parity groups.

During weeks 21 to 44 of lactation the cows were at grass. In trial 2, but not in trial 1, cows previously given the low-energy diet gave less milk (1999 v. 2644 (P < 0·001) kg) but gained more live weight. There were also differences in the composition of milk produced in this period.

Voluntary food intake and body weight were examined over 4-week intervals between 14 and 70 weeks of age in 306 females from 25 British breeds of cattle. At each age, the relationship of the natural logarithm of voluntary food intake to that of body weight was examined by linear regression both within and between breeds.

Of the total variation in voluntary food intake, the proportion accounted for by body weight was extremely high between breeds (phenotypically, 0·80 or more; genetically 0·88 or more, at most ages) but phenotypically low within breeds (0·33 or less). The mean voluntary intake of a breed at any age could be predicted from its mean body weight at the same age with a coefficient of variation (CV) among breeds that declined with age from 0·08 to 0·04. Within breeds, the corresponding CV for individual intake was between 0·12 and 0·15 beyond 9 months of age, and even higher at early ages.

Within breeds, the regression coefficient of log intake on log body weight was close to the value of 0·7 at all ages. Between breeds, it was over 0·8 at early ages, declining to about 0·7 beyond 1 year of age. Thus, genetically larger breeds voluntarily consumed relatively more food at early ages compared with later ages. Breed size should therefore be taken into account when recommending food intake requirements. Breed deviations for high and low appetite are discussed.

Selection indices to improve the efficiency of lean meat production in cattle were derived with an aggregate breeding value which comprised growth rate, food conversion efficiency, killing-out proportion and carcass lean proportion. Index measurements were growth rate, food conversion efficiency and ultrasonic fat area. Relative economic values of traits in the aggregate breeding value were calculated for an 18/20-month beef system, assuming a fixed national output of lean meat. Literature estimates of phenotypic and genetic parameters were used. Two indices were derived, one with a complete restriction on genetic changes in birth weight, and the other without restriction. Correlations between the index and the aggregate breeding value were 0·53 for the restricted index, and 0·57 for the unrestricted index. The maximum proportional reduction in expected economic response, due to complete restriction of birth weight was about 0·08. Selection on either index would actually lead to a slight decrease in carcass lean proportion, but this was less than the decrease expected from selection solely on growth rate. Correlations between the indices and the aggregate breeding value (measuring the accuracy of selection) fell by only about 0·01 when ultrasonic measurements were omitted from the index, but fell by about 0·09 when food conversion efficiency was omitted. Sensitivity of the indices to changes in parameters was also examined. With proportional changes of ±0·5 in individual economic weights, or absolute changes of ±0·2 in genetic correlations or −0·2 in heritabilities, the efficiency of selection ranged from 0·93 to 1·00.

Four diets containing 9·6 MJ metabolizable energy (ME) per kg dry matter (DM) (LMEC), and four diets containing 11·3 MJ/kg DM (HMEC) were formulated. One diet of each ME concentration contained 90 g crude protein (CP) per kg DM and served as a negative control (NC). In the three other diets of each ME concentration, the level of CP was raised to 140 g/kg DM by inclusion of poultry litter (PL), non-protein nitrogen (NPN) or fish meal (FM). These diets were examined in a digestion and N balance trial and in a feeding trial.

For the NC, PL, NPN and FM subtreatments of the LMEC diets, DM intakes were 107·1, 112·4, 100·6 and 130·2 g/kg M075 respectively; digestibility coefficients of organic matter (OM) were 0·64, 0·70, 0·67 and 0·71 respectively; and N retention 0·40, 0·76, 0·53 and 1·21 g/kg M075 respectively. For the respective subtreatments of the HMEC diets, the values were: 100·4, 119·4, 109·3, 107·5 for DM intake; 0·64, 0·73, 0·73, 0·73 for digestibility coefficients of OM and 0·40, 0·87, 1·06 and 1·03 g/kg M075 for N retention.

Daily gain on the respective diets in the first 130 days of the feeding trial was 0·85, 0·90, 0·91, 1·04 kg for the LMEC and 1·09, 1·21, 119, 1·24 kg for the HMEC diets, and from the 131st day to slaughter was 0·98, 1·02, 100, 1·08 kg for the LMEC and 1·03, 1·02, 1·07, 1·05 kg for the HMEC diets respectively. Daily carcass gain was 0·49, 0·51, 0·51, 0·53 kg for the LMEC and 0·55, 0·59, 0·59, 0·60 kg for the HMEC diets, respectively.

The relationship between ME concentration in the diet and source of CP is discussed.

Thirty-eight barley-fed entire and castrated male Friesian calves were used to examine the effects of different implanted anabolic agents on growth and carcass and tissue composition. Entire males were implanted with trenbolone acetate plus hexoestrol alone or following an earlier implantation of zeranol, or with zeranol alone. Control bulls (no implant) and steers treated with trenbolone, acetate plus hexoestrol were also studied. The anabolic agents tended to increase growth rate in bulls, and caused an increase in fat deposition. Treated steers had more abdominal fat than all bulls, but did not have more carcass fat. However, the carcasses contained less saleable meat and had a lower retail value because of lower lean-to-bone ratio. There were no significant differences between any of the groups in the proportions of lipid and water in the lean tissue. Transporting the bulls resulted in a high pH and a dark colour in m. tongissimus. The results suggest that anabolic agents have small biological effects which directly modify the quantitative and qualitative meat production characteristics of bulls.

Primary growths of perennial ryegrass (Lolium perenne) were cut on 22 May and 12 June and wilted for 24 h prior to ensiling. A 40: 60 mixture of the two silages was offered ad libitum with two pelleted supplements. Supplement Ba was a mixture of (dry matter (DM) basis) 932 g rolled barley and 68 g fish meal per kg. Supplement SBP/FF consisted of 555 g unmolassed beet pulp, 314 g extracted rice bran, 56 g fat prills and 75 g soya-bean meal per kg. The factorial combination of the two supplements (Ba and SBP/FF) given at two levels, 6 kg DM (L) or 12 kg DM (H), provided the four treatments imposed over weeks 3 to 10 of lactation on 40 British Friesian cows. During weeks 12 to 20 of lactation (post-experiment period) the cows were given an equal mixture of the two concentrates at 9 kg DM per day.

Supplement Ba contained less ash (31 v. 94 g/kg), acid-detergent fibre (68 v. 218 g/kg) and fat (29 v. 77 g/kg) but more starch (558 v. 89 g/kg) and digestible organic matter in the dry matter in vitro (DOMD) (781 v. 627 g/kg) than SBP/FF. The concentration of total nitrogen (N) was similar at 25·6 g/kg. Silage had a DOMD of 643 g/kg, a pH of 3·8 and proportionately 0·82 of fermentation acids as lactic acid.

Apparent digestibility of gross energy was higher for Ba diets (0·748) than for SBP/FF (0·704). Cows given SBP/FF ate 0·9 kg more silage DM than those given Ba (P < 0·01) but there was no difference in digestible energy intake or in substitution rate (−0·37 kg silage DM per kg additional supplement DM). Increasing the amount of supplement increased milk yield by 3·9 kg/day (P < 0·001). Cows given SBP/FF yielded on average 1·6 kg more milk than those given Ba (P < 0·05). However, this increased output consisted almost entirely of lactose and water as a result of a high concentration of fat in the milk of cows given Ba at the low level (46·3 g/kg). Further, the concentration of protein was less with SBP/FF (28·0 g/kg) than with Ba (29·2 g/kg).

The effects of SBP/FF in early lactation were translated into a positive residual effect in mid lactation equivalent to the immediate effect. In contrast, raising yield by increasing the amount of supplement did not result in increased output subsequently. The results show that a supplement of fibre and fat despite having a lower digestibility than barley can produce more milk but a similar yield of fat and protein provided silage is offered ad libitum.

The effects of level of supplemental carbohydrate from turnips and level of rumen degradable nitrogen on bacterial cellulolysis, food intake, live-weight gain and diet digestibility were examined with 36 300-kg Friesian steers allocated, in a randomized-block design, to one of six dietary treatments: (i) sodium hydroxide treated straw plus a supplement of urea (30 g urea per kg straw dry matter (DM)) (uO); (ii) uO plus turnips (23·5 g DM per kg M0·75); (Hi) uO plus a high level of turnips (47 g DM per kg M0·75); (iv) (v) and (vi), the same three diets, but with additional urea (60 g urea per kg straw DM). In two separate experiments each lasting for 56 days, the turnip allowance was given in either two feeds, or one feed per day and the straw was given to appetite. Rumen fluid was obtained by stomach tube from all steers before feeding (07·30 h) and after feeding (11.00 h) on days 10 and 24 of each experiment.

Weight gains for all groups given straw plus turnips were high (> 0·90 kg/day). Increasing the quantity of turnips gave significantly increased DM intakes and weight gains but then turnips substituted for straw, and the proportion of straw consumed decreased from 0·8 to 0·6 of the dietary DM (P < 0·01) There were no significant interactions between frequency of feeding of turnips and level of urea on the variables food intake and weight gain. Fibre digestibility was not depressed by the addition of turnips to the diet. Rumen fluid pH decreased after the steers ate turnips but tended to remain above 6·7. There were no large changes in the total number of viable cellulolytic bacteria as a result of either addition of urea or turnips to the diet; counts remained in the order of 107 per ml. Steers consumed large quantities of sodium hydroxide treated straw; turnips were therefore only a small proportion of the overall DM intake with relatively little effect on bacterial cellulolysis.

Twenty-one pairs of male cattle twins, 10 of which were monozygotic, were used in a study of the effects of a combined implant of trenbolone acetate (140 mg) and oestradiol-17B (20 mg) on growth and carcass composition. There were three comparisons within pairs: (1) between bulls and steers (no implants); (2) between bulls and implanted steers; and (3) between bulls and implanted bulls.

Implants were given twice, at 44 to 73 days and at 300 days of age. Animals were slaughtered at 400 days following recorded consumption of a complete pelleted diet given ad libitum. Carcasses were separated into tissues and lean and fat were chemically analysed. The effects of castration and/or implantation on growth and carcass composition were assessed by comparison with the untreated (control) bull values within each group and expressed as a ratio (treatedxontrol). These ratios were then compared between groups.

Bulls were significantly heavier than steers at slaughter (29·6 kg, P < 0·05), but were not heavier than implanted steers. Implantation of steers tended to increase appetite; it proportionately increased lean and protein production by about 0·1, and partly reversed the effects of castration on the length of limb bones and the weights of reproductive organs and hide. The proportions of carcass tissues were very similar in the untreated and implanted steers. In bulls, implantation increased the weight of fatty tissue in the subcutaneous and intermuscular depots, but it had small effects on other carcass characteristics.

Three randomized block experiments, involving a total of 80 castrated male cattle, were carried out to examine the effects of plane of nutrition and type of diet offered during a winter store period from about 10 to 15 months of age on the subsequent performance of late-maturing, Friesian steers slaughtered at 22 to 23 months of age. A control diet of medium-quality grass silage (mean digestible organic matter in the dry matter (DOMD) 0·617) offered ad libitum and supplemented with 0·3 kg concentrates (17·1 to 25·6 g nitrogen per kg dry matter) per head daily, which was designed to sustain a live-weight gain of 0·4 kg/day, was compared with three diets which were designed to sustain a live-weight gain of 0·7 kg/day. These were medium-quality silage supplemented with 1·6 kg concentrates, medium-quality silage supplemented with 0·6 kg concentrates and 0·25 kg fish meal and high-quality silage (mean DOMD 0·704) supplemented with 0·3 kg concentrates. The animals were at pasture for 5 months after the treatment period and were then given silage and concentrates for 3 months until slaughter. The low and high planes of nutrition sustained mean daily live-weight gains of 0·43 and 0·73 kg respectively, giving a difference in live weight at the end of the treatment period of 45 kg. Compensatory growth at pasture reduced this difference to 30 kg. This resulted in a difference of 17 kg carcass weight between the two planes of nutrition. The type of diet offered at the high plane of nutrition did not affect subsequent performance. It is concluded that, when the live-weight gain of late-maturing, Friesian, yearling cattle was reduced below about 0·7 kg/day, there was relatively little compensatory growth during the subsequent period at pasture and consequently the optimum live-weight gain during the winter for this type of animal may be higher than that previously recommended.

Barley straw constitutes up to half of the dry-matter intake of Awassi sheep in areas of Syria where the rainfall is 200 to 350 mm. The genotype of barley planted could therefore, through the nutritive value of its straw, significantly affect flock production levels. Selection of suitable barley genotypes may have greater economic potential as a means of improving roughage feeding quality in these areas than chemical or physical processing.

The voluntary intake and digestibility of the unsupplemented straw of three genotypes of barley was evaluated with Awassi castrated male sheep. The voluntary organic-matter (OM) intakes of handharvested Beecher, Arabic Abied and ER/Apam straw were 22·2, 34·7 and 27·0 g/kg M0·75 per day and OM digestibility coefficients were 0·39, 0·44 and 0·42. The factors affecting voluntary intake and digestibility appeared to be the proportions of leaf and stem. Beecher straw had less leaf and more stem than the other two genotypes. The chemical composition and in vitro cellulase digestibility of the leaf indicated that it had a higher potential feeding value than stem.

ER/Apam appears to be an example of a genotype which under dry environmental conditions combines higher barley grain yields with more acceptable straw feeding value than several other genotypes evaluated.

Twenty-one pairs of male cattle twins, 10 of which were monozygotic, were used in a study of the effects of a combined implant of trenbolone acetate (140 mg) and oestradiol-17β (20 mg) on muscle distribution, fat partition and carcass value. There were three comparisons within pairs: (1) between bulls and steers (no implants); (2) between bulls and implanted steers; and (3) between bulls and implanted bulls.

Implants were given twice, at 44 to 73 and 300 days of age, and animals were slaughtered at 400 days following ad libitum consumption of a complete pelleted diet. The effects of castration and/or implantation on muscle distribution, fat partition and carcass value were assessed by comparison with the untreated (control) bull values within each group and expressed as a ratio (treatedxontrol). These ratios were then compared between groups.

Implantation of steers increased slightly the weight of muscles in the neck which in a preliminary study had been shown to be particularly heavy in mature bulls compared with steers. Implantation therefore produced a more bull-like muscle distribution in steers and the effect in bulls was to increase the weight of these muscles even further. In steers and bulls the partition of body fat was not affected by implantation. Both implanted steers and bulls were fatter than their respective control bulls and their fat partition was appropriate to this (higher subcutaneous to intermuscular fat ratio).

Differences in carcass value between the groups, as evidenced by a retail cutting method applied to the right side of the carcass, reflected these differences in muscle distribution and fat partition. Thus, implantation of steers was judged to be economically advantageous despite the marginal reduction in value brought about by enlarged forequarter musculature. Implanted bull carcasses were less valuable than their respective controls mainly because they were fatter and required more trimming.

An experiment with a 2 × 2 factorial design was used to assess the effects of offering either 6 (L6) or 12 (H12) kg concentrates daily from day 13 of lactation until turnout to grazing with grass silage (600 to 620 g digestible organic matter per kg dry matter (DM)) ad libitum and either ad libitum (G) or restricted (g) quantities of grass from turnout to the end of August. In each of 3 years, 60 to 70 January- to March-calving British Friesian cows were used. From calving until turnout to grazing the mean daily DM intakes of silage were 7·2 and 8·9 kg, and of concentrates 8·5 and 4·9 kg for the H12 and L6 groups respectively, giving a mean substitution rate of 0·47 kg silage DM per kg concentrate DM. Mean daily milk yields were 26·7 and 22·9 kg giving a direct response of 0·85 kg/kg additional concentrate DM. Milk from the L6 group had a higher fat concentration and these cows were also lighter at the end of the winter. At pasture, there were no significant interactions for milk variates between winter treatments and grass availability. The stocking rate of cows with restricted grass availability was 0·33 higher from turnout to the end of August. It resulted in a 0·10 proportional reduction in milk yield per cow but per ha there was a 0·21 proportional increase. At pasture, the H12 cows gave more milk at both grass availabilities. The total responses to concentrates were 1·59 and 2·11 kg milk per kg concentrate DM at the ad libitum and restricted grass availabilities respectively. The good response to concentrate coupled with a 0·35 cow per ha increase in annual stocking rate as a result of restricting the grazing area ensured that gross margins and grass utilization were highest for the H12g cows. Conception rates were similar on all treatments and there were no significant differences in live weight at the end of lactation.

In crossbred female lambs reared on a concentrate diet between 8 and 20 weeks of age, a restriction in food intake to 40 g/kg body weight per day decreased live-weight gain (155 g/day) compared with that in control lambs fed ad libitum (284 g/day) and significantly reduced final live weight and the size of the mammary fat pad (P < 0·001). However, restricted lambs at 20 weeks had more total parenchymal deoxyribonucleic acid (DNA; 54·7 v. 47·5 mg) occupying a greater mass of mammary fat pad (17·0 v. 13·2 g) than control lambs (P > 0·10). Restricted feeding had little effect on mean plasma growth hormone (GH) concentrations at 12 and 18 weeks of age, but decreased plasma insulin and prolactin concentrations.

In lambs fed ad libitum, daily subcutaneous (s.c.) injections of 0·1 mg bovine pituitary GH per kg live weight between 8 and 20 weeks of age significantly increased daily live-weight gain (347 g/day; P < 0·001) and also increased total mammary parenchymal DNA (71·2 mg; P < 0·10) and the mass of fat pad occupied by parenchymal tissue (20·6 g; P < 0·05), compared with the control treatment. Daily s.c. injection of bromocriptine (1 mg/day) had no effect on either body or mammary growth, but tended to reduce the effects of bovine GH when given in combination. Secretory activity was observed in the parenchymal tissue of the eight lambs receiving bovine GH alone, and also in five control lambs and three lambs receiving bromocriptine alone. Three lambs that had attained puberty by 20 weeks of age had the poorest mammary development of their respective treatment groups.

Mammary gland development at 20 weeks of age was significantly correlated across treatment means with plasma GH concentrations estimated at 18 weeks of age (r = 0·95; P < 0·05), but no correlations of similar magnitude were found between individuals within treatments at 20 weeks. Strongest correlations were found among 8-week-old lambs, where mammary parenchymal DNA was positively correlated with mean plasma GH (r = 0·62) and the GH:insulin ratio (r = 0·79) and negatively correlated with the prolactin:GH ratio (r = -0·65).

Forty-eight individually-penned lambs (mean live weight 31-4 kg) were offered one of four diets, to investigate response to a supplement of fish meal (0 and 100 g dry matter (DM) per kg M) given with diets of either 60: 40 or 40: 60 forage-to-concentrate ratio. Nitrogen (N) degradability in the rumen and fractional outflow rates of protein supplements were determined. The diets were formulated such that the lambs received about 3 or 9 g undegradable rumen N per kg DM. The diets were given daily to provide sufficient metabolizable energy for maintenance and 150 g gain, and were adjusted according to live weight at weekly intervals.

Fish-meal supplementation improved daily growth over a 49-day period (P < 0·001) and enhanced N retention (P < 0·001). Live-weight gain was also marginally improved on the high-forage diets. (P < 0-05), but there was no protein supplement × forage-to-concentrate ratio interaction. Apparent digestibility of acid detergent fibre was improved by the fish-meal supplement on the high-concentrate diet.

The four diets were also given to rumen-fistulated sheep. The high-concentrate diet was associated with a higher molar proportion of propionate (P < 0·05) and a lower proportion of acetate (P < 0·001). Rumen concentrations of ammonia tended to be maintained at higher levels throughout the day by the fish-meal supplement.

The carcasses of 15 ram and 15 ewe lambs of the Dorset Down breed, weight range 12 to 23 kg, were separated into individual muscles, bones and fat depots.

At the same carcass weight (16-8 kg) the ram lambs contained 41 g more muscle per kg, but there were few important differences between sexes in individual muscle weights; 65 of the 72 muscles differed in weight by less than 10 g.

The distribution of total muscle weight between eight anatomical groupings differed significantly between the sexes; ewes had proportionately more muscle in the proximal pelvic limb (8 g/kg) and less in the neck (7 g/kg) and distal thoracic limb (1 g/kg). Despite these small differences in muscle-weight distribution in higher-valued joints in favour of the ewe lambs, each of their joints contained less muscle than those of ram lambs.

In these lambs, reared under commercial conditions in the United Kingdom, there were no signs of gross morphological differences which could result in their disqualification for the Variable Premium of the European Economic Community Sheepmeat Regime. In particular the m. splenius cervicus was a very small proportion of total muscle in both sexes (1·5 and 2·5 g/kg for ewe and ram respectively).

Seventy-eight lambs, 9 weeks of age, the offspring of sires of the Scottish Blackface, East Friesland, Finnish Landrace, Suffolk and Texel breeds mated to Scottish Blackface females, as a common maternal breed, were given a barley-fish meal diet containing 1-6 g magnesium and 5·9 g phosphorus per kg dry matter for 13 weeks. Urinary excretion of Mg and P was estimated from the ratio of Mg or P to creatinine in spot samples of urine. A relationship between creatinine excretion and live weight was obtained using total urine excretion from a sample of 20 lambs, two of each sex from each sire breed group. Good agreement was found between the actual and predicted urinary excretion of minerals by the 20 lambs on three occasions.

The effect of sire breed on urinary excretion of P was significant (P < 0·05) and the breed types ranked, in descending order of excretion rate, Texel, Blackface, East Friesland, Finnish Landrace and Suffolk, with a four-fold difference in excretion rate between Texel and Suffolk. This difference in urinary excretion of P was taken as evidence of breed differences in efficiency of absorption of dietary P.

Urinary excretion of Mg was correlated with the intake of Mg. The mean estimated efficiency of absorption of dietary Mg varied, over the three occasions, from 0·42 to 0·50 and from 0·39 to 0·45 for total collections and for spot samples of urine respectively. Individual variation in the estimated efficiency of absorption within sires was not significant whereas that between sires was significant (P < 0·05). Heritability was estimated to be 0·62 (s.e. 0·30).

Forty-eight, individually-penned lambs (mean live weight 31·4 kg) were given diets with or without fish meal (0 and 90 g intake daily) in sufficient quantities to provide energy for maintenance and either 100, 150 or 200 g daily gain. Nitrogen (N) degradability in the rumen and fractional outflow rates of protein supplements were determined and the diets formulated such that the lambs received about 2·2 and 6·7 g undegradable rumen N daily on all three feeding levels.

Live-weight gains were generally greater during the first 3 weeks than during the final 4 weeks of the experiment. Lambs given fish meal had greater gains (P < 0·001) throughout the experiment, but there was a feeding level x protein interaction (P < 0·05) during the first 3 weeks, such that the difference between diets with or without fish meal was greater at the highest feeding level.

Retention of N increased with feeding level (P < 0·001) and was greatest when fish meal was given (P < 0·001). There was a feeding level x protein interaction (P < 0·05) for g N retained per 100 g N digested; N retention increased with feeding level only when fish meal was given.