Four groups of six cows were fed from 12 weeks before calving to achieve condition scores at calving of 3·15 (F) and 2·15 (T). For the first 20 weeks of lactation all cows were given 8 kg concentrate (13·93 MJ metabolizable energy (ME) per kg dry matter (DM), 200 g crude protein (CP) per kg DM) which contained either a high or a low dietary concentration of undegradable dietary protein (UDP) (70 g/kg DM, H, and 49 g/kg DM, L) and 3 kg sugar-beet pulp. Hay was also offered ad libitum.

There was no significant effect of treatment on milk yields over the first 20 weeks of lactation (overall mean yield 27·3 (s.e. 3·65) kg/day) but cows in group FL tended to produce less (P > 005). Milk composition was similar for all groups (48·8 (s.e. 5·12) g fat per kg, 27·2 (s.e. 1·48) g protein per kg and 49·0 (s.e. 2·52) g lactose per kg). DM intakes for groups FH, FL, TH and TL were 18·1, 16·9, 17·5 and 18·2 (s.e.d. 0·58) kg/day, respectively (TL v. FL, P < 0·05; FH v. FL, P < 0·05). Mean condition scores for groups FH, FL, TH and TL respectively, were 3·T7, 3·13, 2·21 and 2·08 (s.e.d. 0·18) at calving; 2·25, 2·50, 2·21 and 1·83 (s.e.d. 0·27) in week 10; and 2·46, 2·83, 2·46 and 1·96 (s.e.d. 0·35) in week 20, of lactation. Groups FH, FL, TH and TL lost proportionately 0·09, 0·02, 0·01 and 0·04 (s.e.d. 0·035) of their calving live weights, respectively, over 20 weeks of lactation.

It is concluded that increasing the supply of UDP to cows which are fat at calving results in a greater negative energy balance and a small though non-significant increase in mobilization of body fat reserves. At the dietary ME and protein concentrations used, the thin cows could not respond to increased UDP supply and their intake was limited by physical restriction.

Responses to metabolic challenges were measured in 8-month-old Friesian bull calves with a view to identifying possible predictors of genetic merit for milk fat production. Seven ‘high’ breeding index bulls (mean breeding index for milk fat = 128·9 (s.d. 4·8) percentage units) were compared with six ‘low’ breeding index bulls (mean 107·3 (s.d. 2·4) percentage units). Bulls were from two selection lines and breeding indices calculated as the average of parental breeding index values.

Challenges involved intravenous injection of the following (dose rates per kg body weight): adrenalin (1 μg); glucose (0·17 g); glucagon (0·175 μg); insulin (0·01 mg); and arginine (40 mg). Blood samples were withdrawn prior to and after each challenge (two challenges daily). Bulls were then fasted for 3 days before being refed.

Relative to the low breeding index bulls, those in the high breeding index group exhibited: (a) greater pancreatic sensitivity to circulating glucose as indicated by increased insulin concentrations following the glucose challenge; (b) more rapid clearance of glucose from plasma following the insulin challenge; (c) reduced sensitivity to the glycogenolytic/gluconeogenic effects of glucagon as indicated by lower blood glucose concentrations following intravenous injection of this hormone; (d) elevated plasma insulin and growth hormone concentrations during the fasting period; (e) elevated plasma concentrations of insulin, glucose and urea during refeeding; (f) no difference in responses to intravenous arginine or adrenalin, or in circulating concentrations of Insulin-like Growth Factor-1 or alkaline phosphatase. Results are consistent with previous observations that metabolic differences between cattle in the Massey University high and low breeding index lines are most evident in the metabolism of glucose and insulin. Metabolic challenges offer a potentially useful means of predicting genetic merit for milk fat production but the conditions under which repeatable differences between the lines can best be demonstrated are yet to be determined.

Somatomedin C and other hormones, as well as blood metabolites, were measured during the dry period and during lactation in dairy cows, given different amounts of energy and protein, to study metabolic and endocrine adaptations. Somatomedin C, specifically measured by radioimmunoassay after separation from its binding protein, did not exhibit typical diurnal variations, in contrast to somatotropin and insulin, which increased particularly after concentrate intake. Somatomedin C markedly decreased at parturition and reached lowest values around the peak of lactation, while levels of somatotropin, nonesterified fatty acids and ketone bodies were high and those of glucose, insulin, thyroxine and triiodothyronine were low. Thereafter somatomedin C values slowly increased up to the 12th week of lactation and remained elevated. Low energy and protein balances were characterized by particularly low somatomedin C concentrations. An additional protein deficit at peak lactation, when cows were already provided with low amounts of energy, did not further decrease somatomedin C levels. However, when high amounts of energy were given in the form of starch or crystalline fat, somatomedin C increased. Overall, there was a positive correlation of somatomedin C primarily with energy, but also with protein balances and a negative correlation with milk yield. Conversely, somatotropin increased markedly after parturition and was positively correlated with milk production and negatively with protein and energy balances. Thus, somatomedin C levels were paradoxically low in the presence of high circulating somatotropin. Insulin most closely paralleled somatomedin C levels. Therefore the anabolic state of metabolism at the end of pregnancy was characterized by high somatomedin C and insulin and relatively low somatotropin, whereas the catabolic state of early lactation was characterized by high somatotropin, low somatomedin C, insulin and thyroid hormones.

An experiment with two groups of 24 cows during weeks 3 to 22 of lactation examined the influence of 7 (L) or 11 (H) kg concentrates per day on the prevalence of lameness. Grass silage was offered ad libitum. The cows were scored on a 1 to 5 scale for their locomotion on a weekly basis (1 = normal locomotion; 5 = severely lame). Cows scoring 3 or more were considered to be clinically lame. This allowed the prevalence of lameness, its severity and duration to be measured and statistically analysed. There were significantly more observations of lameness in the H cows than the L cows (0·077 and 0·021 observations per cow week) and the severity and duration of lameness incidents were significantly greater. The major causes of lameness were solar problems in the hind feet. The prevalence declined in both treatments as lactation progressed. The predisposing causes of the higher incidence in treatment H could have been the higher concentrate: forage ratio, the greater daily metabolizable energy intake or the greater daily crude protein intake. For treatments L and H, mean milk yields were 20·5, 23·7 kg/day; milk fat 41·7, 39·0 g/kg; milk protein 31·4, 32·0 g/kg; live-weight change -0·12, +0·11 kg/day; and condition score 2·04, 2·18.

Records of beef cows on a ranch in south-east Mexico gave 1377 ages at first calving (AFC) and 3932 calving intervals (CI) for the comparison by least-squares analysis of the genetic groups: commercial zebu (CZ), Brahman, other zebu breeds and their zebu crosses (zebu), F, crosses born to zebu dams by Bos taunts or Santa Gertrudis sires, and 3/4 bred zebu and 3/4 Bos taurus. Subsamples of 1090 ages at first calving and 3154 intervals were analysed considering the breed groups: CZ, Brahman, Guzerat, Gyr and Indo-brazil, and the F1 crosses resulting from matings between these zebu breeds, Santa Gertrudis and Charolais as sire breeds and the CZ as the dam breed. Three populations were analysed for each model, the original ranch population and two populations reduced by retrospective culling. The original population had a mean AFC and CI of 1226 and 489 days respectively reduced by the first and second culling by 24 and 37 days and by 62 and 83 days respectively. Repeatability of CI increased from 0·06 to 0·11 and 0·14 respectively.

Genetic group significantly influenced AFC. In the original population F1 heifers had a significantly younger AFC than Brahman and zebu heifers but not significantly younger than the other genetic groups. Heifers with 0·25, 0·75 and 1·00 zebu genes did not have significantly different AFCs. After the retrospective culling only the 3/4 bred zebus were significantly superior to the worst group, the zebus. The effect of breed group was only significant in the original population but generally the same rank order was maintained after the retrospective culling. F, Charolais × CZ heifers had the youngest AFC, a superiority of 67 days over CZ and 92 days over Brahman. Purebred zebus and their F, crosses with CZ did not calve earlier than CZ dams. Genetic group had a highly significant effect on CI. Females with 0·50 and 0·75 Bos taurus genes had shorter CIs than females with 0·75 and 1·00 zebu genes. The superiority was maintained after culling. Breed group had a significant effect in each population. F1 Charolais × CZ dams had the shortest CI. CZ had equivalent or shorter CI than all zebu breeds and their F1 crosses with CZ. Heterosis effects for AFC and CI in the zebu crosses were generally advantageous but small, while the heritabilities for the zebu population were negative for AFC and 0·05, 0·22 and 0·06 for CI in the original and two culled populations respectively.

The four sex phenotypes were used to study the relationship between carcass composition and serum profiles of hormones thought to affect growth. Forty calves (10 bulls, 10 steers, 10 heifers and 10 ovariectomized heifers) from Hereford dams crossed with Simmental-Hereford bulls (¾ Simmental-¼ Hereford) were given a high energy diet from 10 to 16 months of age. Sequential blood samples (20-min intervals) representing 6-h windows were obtained from each animal at the outset of the experiment and at 28-day intervals thereafter. Samples were assayed for growth hormone (GH), insulin, triiodothyronine, cortisol, testosterone, oestradiol-17β, urea and albumin. The 9-10-llth rib was dissected into fat, lean and bone, and chemical analysis was performed on the dissected soft tissue. Bulls accumulated more lean tissue and less fat than any other group. Heifers and ovariectomized heifers accumulated the least lean and most fat while steers were intermediate between bulls and the two heifer groups. Bulls may have accumulated more lean and less fat than the heifers due to higher GH and testosterone concentrations, and lower circulating concentrations of cortisol and insulin. The known function of these four hormones and their relative concentrations between bulls and the two female groups were consistent with their suggested role in the relative carcass compositions. The intermediate composition of the steer carcass may have resulted from an interplay between the proteogenic influence of higher GH concentrations, the proteolytic influence of elevated cortisol, the absence of testosterone and the lipogenic influence of higher insulin.

In three experiments, 16, 24 and 22, May-calving Aberdeen Angus × British Friesian cows were subjected either to single suckling of their natural calves or to single suckling in addition to ‘foster’ suckling for the first 8 to 9 weeks of lactation. Foster suckling was achieved by the removal of cows from pasture twice daily and suckling of male Friesian calves while cows were confined in a creep. Average milk yield per cow removed by foster suckling and milk secretion rates of individual cows were measured at intervals and cows were grazed at equal grazing pressure. Single plus foster suckling increased estimated total yield over single suckling proportionately by 0·55, 0·18 and 0·34 in the three experiments respectively. After foster calves were weaned, milk secretion rate of foster-suckling cows was always higher than that of naturally suckling cows. In addition to a mean yield of about 300 kg milk per cow to foster calves, single plus foster suckling increased calf gain in the natural calves in each experiment although the increases were not significant. The apparent conversion of extra milk consumed by natural calves in the single plus foster suckling treatment was generally low in relation to figures quoted in the literature.

An experiment has been carried out to examine the effects of implantation with hormonal growth promoters on the response in the performance of castrated male cattle (steers) to protein supplementation of a silage-based diet. The treatments consisted of grass silage offered ad libitum and supplemented with 2·7 kg per head daily of either a low protein (LP, 114 g crude protein per kg dry matter (DM)) or a high protein (HP, 206 g crude protein per kg DM) concentrate. Half of the animals, given each of the two concentrates were implanted with 20 mg oestradiol plus 200 mg progesterone and 300 mg trenbolone acetate at the beginning of the experiment and after 77 days, while the remainder of the animals received no implant, giving four treatments in a 2 × 2 factorial design. Sixty-four British Friesian and Simmental cross steers which were initially 426 (s.e. 3·5) kg live weight were used. The treatments were imposed for 157 days. The silage used was well preserved and of high digestibility. For LP and HP concentrates given to non-implanted animals and LP and HP given to implanted animals respectively silage DM intakes were 5·7, 5·8, 6·3 and 6·6 (s.e. 0·18) kg/day; live-weight gains 0·96, 0·92, 1·22 and 1·33 (s.e. 0·045) kg/day; carcass gains 0·58, 0·53, 0·78 and 0·81 (s.e. 0·024) kg/day; carcass subcutaneous fat depth 6·0, 6·5, 5·8 and 5·5 (s.e. 0·35) mm; areas of m. longissimus dorsi 69·1, 58·5, 69·6 and 74·7 (s.e. 1·91) cm2 and marbling scores 3·0, 3·6, 3·3 and 3·1 (s.e. 0·25). It is concluded that protein supplementation of well preserved grass silage produced a marginal increase in the performance of implanted finishing steers but did not affect carcass fatness. With non-implanted steers protein supplementation produced a marginal depression in performance and increased carcass fatness.

In an experiment during weeks 3 to 26 of lactation, two groups of 12 cows were offered a mixed concentrate/grass silage diet containing 161 g crude protein per kg dry matter, and two similar groups a diet containing 198 g crude protein per kg dry matter. The concentrate: forage ratios and the daily metabolizable energy intakes were the same for all four groups. The cows in one of the two groups at each crude protein level had their hooves trimmed prior to parturition, the other group remaining untrimmed. The high protein diet significantly increased locomotion score (higher scores indicate poorer locomotion), the number and duration of clinical cases of lameness and outer toe length. Trimming of hooves reduced locomotion scores and the number and duration of clinical cases. Hoof growth was significantly increased by trimming. Mid-sole hardness was negatively correlated with locomotion score. At the two extremes, the low protein trimmed group had five cows lame for an average of 1·0 weeks, whereas the high protein untrimmed group had nine cows lame for an average of 4-2 weeks. Condition score and live-weight change were negatively correlated with locomotion score. There were no significant differences between treatments in milk solids yield.

Monthly herd recording data from Friesian cows in south-east Queensland are used to evaluate 20 models of the lactation curve, 10 being drawn from the literature and 10 being original. The curve of Wood (1967) with equation y =a nb exp(–cn),and its modificationy =a nfc/cosh(en), generally gave the lowest values for the error mean square across all seasons of calving. However, the model which predicts a constant initial yield, followed by a linear decline,y =a forn =Sc, y =a - b(n –c) forn > c, is as good as the previous two models for winter calving cows. Hence it is a useful representation of the lactation curve in south-eastern Queensland, where about 40% of cows are calved in winter. The inverse polynomial modely =nl(a +bn + en2) is a useful model for lactations commencing in the summer months.

Two experiments have been carried out to compare silage-based and dried forage-based diets for finishing beef cattle, and to examine the effect of supplementing the silage-based diet with additional protein. The three diets used in experiment 1 consisted of (1) grass silage supplemented with a low-protein, cereal-based concentrate (98 g crude protein (CP) per kg dry matter (DM)) (2) grass silage supplemented with a high-protein, cereal/soya-bean meal concentrate (208 g CP per kg DM) and (3) grass hay supplemented with a cereal-based concentrate (130 g CP per kg DM). The two diets used in experiment 2 consisted of (1) grass silage supplemented with barley and (2) a mixture of grass hay and artificially dried grass supplemented with barley. All the diets contained approximately 700 g forage and 300 g concentrates per kg DM except diet 3 in experiment 1 which contained 240 g hay and 760 g concentrates per kg. The silages were of high digestibility (digestible organic matter in dry matter 0·72) and were well preserved (ammonia N 36 g/kg total N). The diets were offered to castrated male cattle which were initially 384 kg in experiment 1 and 515 kg in experiment 2. For diets 1 to 3 in experiment 1 and diets 1 and 2 in experiment 2 respectively metabolizable energy intakes were, 92, 94, 94, 124 and 120 MJ/day; live-weight gains were 1·21, 1·16 and 1·21 (s.e. 0·044) and 1·25 and 1·22 (s.e. 0·060) kg/day; carcass gains were 0·76, 0·76 and 0·75 (s.e. 0·034) and 0·80 and 0·74 (s.e. 0·036) kg/day; carcass fat classifications (five-point scale; 1 = leanest, 5 = fattest) were 2·4, 2·9 and 2·2 (s.e. 0·09) and 3·6 and 3·6 (s.e. 0·18); mean subcutaneous fat depths were 6·8, 8·1 and 6·6 (s.e. 0·74) and 7·8 and 7·4 (s.e. 0·48) mm; areas of m. longissimus dorsi at the 10th rib were 66·4, 69·4 and 71·3 (s.e. 3·16) and 77·6 and 72·3 (s.e. 1·52) cm2 and marbling scores (eight-point scale; 1 = leanest, 8 = fattest) for m. longissimus were 1·7, 2·5 and 2·2 (s.e. 0·24) and 3·2 and 2·8 (s.e. 0·16). It is concluded that performance, efficiency of energy utilization and carcass fatness were similar for silage-based and dried forage-based diets, and that protein supplementation of a silage-based diet did not affect performance but tended to increase carcass fatness.

Purebred cows were evaluated for reproductive performance and live weight, and for pre-weaning growth and viability of crossbred progeny sired by unrelated breeds. The cows comprised three indigenous breeds, Mashona, Nkone and Tuli and four exotic breeds Africander, Brahman, Sussex and Charolais. Over 3 years, the mean calving rates % (s.e.) were 55·5 (3·0), 76·0 (3·8), 63·1 (4·5), 69·6 (3·5), 70·0 (3·8), 60·0 (4·3) and 67·0 (5·0) for Africander, Mashona, Nkone, Tuli, Brahman, Sussex and Charolais cows, respectively. Bos taurus and Brahman cows were heavier at parturition and at calf weaning than indigenous cows. Mashona cows were lightest at both stages. Calves of Bos taurus cows were heaviest at birth and those of Mashona lightest. Calves of Brahman cows had the most rapid growth rate and at weaning (240 days) weighed in kg (s.e.) 207 (3·1), compared with 184 (2·1), 172 (2·2), 187 (2·6), 184 (2·1), 178 (2·7) and 186 (2·9) for progeny of Africander, Mashona, Nkone, Tuli, Sussex and Charolais, respectively. Pre-weaning survival rates of the breeds were not significantly different but viability was higher in progeny of indigenous than exotic breeds. The productivity estimates combining calving rate, cow weight, calf weaning weight and pre-weaning viability demonstrated the superiority of indigenous breeds and the Brahman over Africander and Bos taurus breeds.

Crossbred cows were evaluated for reproductive performance and live weight, and for pre-weaning growth and viability of progeny sired by seven unrelated breeds. They were also evaluated on overall productivity expressed as weight of weaned calf per cow mated per year, per 100 kg of cow per year and per 100 kg metabolic weight of cow per year. The cows comprised reciprocals of Africander with Mashona, Nkone, Brahman and Sussex, one-way crosses sired by Sussex out of Mashona, Nkone and Brahman dams, and by Charolais out of Africander, Mashona, Nkone, Brahman and Sussex dams. These represented sanga × sanga, sanga × zebu, Bos taurus × sanga, Bos taurus × zebu and Bos taurus × Bos taurus crossbred types. Bos taurus × zebu crosses were superior to other breed types for all the three productivity indices and for virtually all the individual components of these. There was little difference between the other four breed types in overall productivity of components of these. Differences between reciprocals were observed in sanga × sanga and sanga × zebu, cows with Africander dams being more productive than reciprocals with Mashona, Nkone and Brahman dams. The Bos taurus × Bos taurus cross was inferior in overall productivity primarily due to large cow weight and relatively low growth rate of progeny. Progeny sired by Friesian, Simmental and Brahman had higher weaning weights than those sired by Hereford, Aberdeen Angus, Africander and Tuli. Overall, heterosis was positive for all traits and averaged 0·15 for the three measures of cow productivity. Heterosis was three times higher in the Bos taurus × Bos indicus cross than in the Bos indicus crosses.

The response of Hereford × Friesian heifers to large intakes of two types of high glucosinolate rapeseed meal were studied during two 5-month periods in consecutive years. In year 1, six heifers were given a compound concentrate food containing 250 g/kg extracted rapeseed meal and six control animals were given concentrate containing 210 g/kg soya-bean meal. In year 2, a further six heifers were given concentrate containing 320 g/kg expeller rapeseed meal and six control animals were given the same soya-bean meal compound concentrate. The crude protein (CP): metabolizable energy (ME) ratio of all diets was 14·3 g CP per MJ ME. Barley straw was the only roughage offered.

There were no differences between groups in ovarian activity assessed from plasma progesterone concentrations, nor in behavioural activity around oestrus. The heifers were slaughtered 6 weeks after artificial insemination. Pregnancy rate was proportionally 0·58 (7/12) in the rape-fed and 0·67 (8/12) in the control heifers (P > 0·05). Foetuses from heifers given rapeseed meal were slightly smaller than those from control heifers; the difference was significant in year 2 (P < 0·05).

Thyroid glands of heifers given rapeseed meal showed histological evidence of goitrogenicity but weights of the glands did not reflect this. Plasma thyroxine values were depressed in heifers given rapeseed meal (year 1, P < 0·001; year 2, P < 0·05). Plasma thiocyanate was elevated in rapeseed meal heifers in both years (P < 0·001) compared with values for control animals.

A comparison of anabolic treatment regimes was made in 761 autumn-born Friesian steers on 14 farms. Prior to turn-out for grazing at 6 months of age and approximately 180 kg live weight, steers were weighed and divided into three equal-weight groups. Group 1 received silastic implants containing 45 mg oestradiol-17β. Group 2 received 36 mg zeranol and group 3 were untreated controls. All steers were weighed at intervals of approximately 3 months (i.e. mid summer, yarding at autumn, mid winter and pre-slaughter) and group 2 steers received a further 36 mg zeranol at the second, third and fourth weighing. At the third weighing half the cattle in each of the treatment groups 1 and 2 received 300 mg trenbolone acetate. Also at the third weighing, group 3 (controls) were subdivided into three equal-weight groups, the first of which received 45 mg oestradiol-17β and 300 mg trenbolone acetate and the second 36 mg zeranol and 300 mg trenbolone acetate. The third subgroup remained as untreated controls.

From 10 farms a 25% sample close to group mean weight at mid winter were slaughtered on the same day and subjectively assessed for subcutaneous fat score and conformation on 15-point scales. The left thin flank was separated into tissues. Samples of lean fat, liver and kidney were analysed for zeranol residues. All implant treatments resulted in higher live-weight gain, heavier slaughter weights and earlier slaughter. Trenbolone increased daily gain only during the first 3 months after treatment. Implanted carcasses were heavier both in the fore- and hindquarter but trenbolone also produced heavier forequarters. Carcasses from treated steers had more subcutaneous fat, less perinephric and retroperitoneal fat and less intermuscular fat in the thin-flank joint. Zeranol residues in implanted steers were not significantly higher than controls except in the kidney, but even these were significantly below accepted tolerance levels.

The effects of average carcass fat concentration of a group of lambs on correlations among lamb carcass measurements within the group and of slaughter age on variances of carcass measurements were studied. Three-hundred and sixty-eight crossbred lambs from one experiment were slaughtered on 10 days over a 2-year period. Average carcass fat concentration of the 10 groups ranged from 179 to 326 g/kg carcass weight. Variances of carcass fat, water and protein concentrations and of muscle measurements did not increase at later slaughter ages but variances of kidney fat weight, fat depths and total tissue depths increased. Several correlations among carcass measurements had significant regressions on average carcass fat composition of the groups slaughtered. When measurements were adjusted for age but not weight, correlations of carcass weight, kidney fat weight, specific gravity, tibia and tarsus length and m. longissimus depth with carcass fat, water and protein concentrations weakened as carcass fat concentration of the group increased. Correlations of kidney fat, specific gravity and m. longissimus width with fat, water and protein concentrations adjusted for carcass weight weakened as average carcass fat concentration of the group increased. Multiple regression equations did not reveal any advantage in using combinations of muscle or leg dimensions to predict fat concentration.

In 22 cows given complete diets of 500 g straw and 500 g concentrates per kg fresh weight the outflow rate of small fibrous particles was determined on two occasions to investigate the consistency of this measurement and its association with apparent digestibility. The correlation between the outflow rates on the two occasions were r = +0·90 and the correlation with apparent digestibility was r = −0·80.

In a second experiment, the outflow rates of long and small particles were determined together with rumen retention time. There was no difference between outflow rate of long and small particles but rumen retention times were greater with the long particles. The correlation between the two estimates of outflow rate was r = +0·84. The outflow rate of fish meal was found to be 0·070 while that of short fibrous particles was 0·031 per h. The average liquid outflow rate was 0·146 per h.

The three cows with the highest outflow rate and the three with the lowest outflow rate from experiment 2 were selected for experiment 3. While voluntary food intakes of the two groups were similar, the differences in outflow rates between the cows persisted both with ad libitum and restricted intakes of both a high and a low roughage diet. The differences were reflected in significant differences between the two groups in apparent digestibility of the diets. The practical implications for breeding and selection are discussed.

The use of oestradiol-17β was evaluated in cereal-based beef production on 14 farms with 450 bulls. Half of the bulls were implanted subcutaneously in the ear with a silastic rubber implant containing 45 mg oestradiol-17β Representative bulls close to mean weight for each group were selected for slaughter on the same day at the end of the trial for carcass measurements. The remaining bulls were slaughtered at the farmers' discretion. Where possible, slaughter live weight (208 bulls), carcass weight (412) and European Economic Community carcass classification (353) were recorded. Implanted bulls had higher gains only in the first 3 months (+76 (s.e. 17) g/day; P < 0·001) and had heavier carcasses with the extra weight in the forequarter (+2·3 (s.e. 0·61) kg; P < 0·001), better carcass shape (+0·9 (s.e. 0·3) points; P < 0·01) and more subcutaneous fat in the thin flank (+1·3 (s.e. 0·6) g/kg; P < 0·05). Implanted bulls selected for slaughter by farmers were not slaughtered earlier, but killed out better (+0·7 (s.e. 0·3) g/kg; P < 0·05) and had heavier carcasses (+4·6 (s.e. 0·5) kg; P < 0·01) of better shape (+0·8 (s.e. 0·3) points; P < 0·05).

The use of zeranol was evaluated in cereal beef production on 15 farms with 385 bulls. Half received 36 mg zeranol which was repeated after 6 months. Forty-four bulls were selected for detailed carcass assessment as above. Implanted bulls had daily gains 0·06 (s.e. 0·01) kg greater (P < 0·001) than controls. Farmers chose to slaughter implanted bulls 8·3 (s.e. 3·8) days earlier (P < 0·05) at only marginally heavier weights (+4·8 (s.e. 3·1) kg; P = 0·08). More implanted bull carcasses were distributed in better shape and higher fat classes (P < 0·01). There were no significant differences in fore- and hindquarter and thin-flank tissue proportions. Oestradiol treatment marginally decreased testis weight in slaughtered bulls whereas zeranol treatment had no significant effect although epididymal weight was decreased in zeranol treated bulls. There were no clear effects of either treatment on testicular structure, but both oestradiol and zeranol caused a degree of disruption to the epididymal epithelium. Zeranol concentrations in treated animal tissues were not significantly different from those in control animals.

Fat depths of Southdown and Suffolk rams were ultrasonically measured several times prior to 18 months of age. Ultrasonic measurements were regressed on live weight and deviations from this regression were standardized and summed to select rams with high and low fat depth to produce crossbred progeny for slaughter. An average of 3·24 standard deviations separated high and low sires. Progeny were slaughtered at two ages to determine differences in carcass fatness and its development. A significant difference of 0·5 mm fat depth over the loin eye muscle was found between progeny of low and high sires. Progeny from Suffolk high sires had greater tissue depth over the rib and higher kidney fat weight than those from Suffolk low sires but differences were small or reversed in Southdown progeny. Low progeny had significantly less estimated carcass chemical fat than high progeny. Fat depth C increased less in low progeny between early and late slaughter ages. However, no difference in partition of carcass weight gain to fat weight gain was observed between high and low progeny. Differences in fat weight between high and low progeny, as well as between Southdown and Suffolk progeny, appeared to have occurred prior to the earlier slaughter age.

An experiment was conducted using medium-sized crossbred beef heifers to study how manipulation of weight gain in late gestation affected production. Forty heifers were fed individually to gain according to one of three regimens for an approximate 90 day period prior to parturition: (1) 1·2 kg gain per head per day for 45 days followed by no gains for 45 days; (2) 0·6 kg gain per head per day for the entire period; (3) no gain for the first 45 days followed by 1·2 kg gain per head per day for the last 45 days. Milk production was measured by weighing of calves before and after suckling at 30, 60, and 150 days post partum. Heifers were weighed, measured for heart girth and hip weight, and visually scored for condition at the start, middle and end of the feeding period to quantify growth and condition changes. Heifers that had a high rate of gain for a short time produced more milk in early lactation than heifers that gained at a slower rate for a longer time pre-partum (P < 0·05). Calf weaning weights and dam reproductive performance were unaffected (P < 0·05) by pre-partum weight gain. It is concluded from these results, that as long as the recommended weight gain is achieved during late gestation, the timing of the weight gain is not critical to sustain productivity.