The effect of changes in the mating system on the rates of genetic progress and rates of inbreeding are considered for MOET nucleus breeding schemes. Methods are derived to calculate best linear unbiased predictors of breeding value in MOET schemes and the rate of inbreeding under selection. These are applied to different mating systems in which the numbers of sires and dams, and the number of offspring per sire and offspring per dam, remain constant.

Results showed that compared with nested mating systems, factorial mating systems in which the maternal half-sibs are produced instead of full-sibs, could increase genetic progress by 1·12-fold with no additional inbreeding. The increased progress arose through an increase in the selection intensity applied. The rates of inbreeding derived were found to be approximately double those estimated by the formula of Wright (1931) in the absence of selection.

In practice, even if a complete factorial system were to increase the generation interval and consequently reduce progress below that predicted, changes in the mating system avoiding this problem could be implemented that would be of immediate benefit.

During weeks 3 to 26 of lactation, two groups of 24 cows were offered either a 60:40 (high) or a 40:60 (low) ratio of concentrate dry matter:silage dry matter. The 40:60 diet was restricted to give the same daily metabolizable energy and crude protein intake. The hooves of half the cows on each diet were trimmed prior to the experiment, the other half remaining untrimmed. The high concentrate ratio significantly increased locomotion score (high scores indicating poorer locomotion), and the number and duration of clinical cases of lameness. Trimming of hooves significantly reduced the number and diiration of clinical cases. Hoof growth was significantly increased by trimming. Hoof hardness was significantly reduced by the high concentrate ratio. The low concentrate ratio had 8/24 cows lame for an average of 3·3 weeks compared with 11 cows for 3·9 weeks on the high ratio. There were 7/24 trimmed cows lame for 3·1 weeks compared with 12 untrimmed cows lame for 4·2 weeks. The main hoof lesions were solar ulcers and heel erosion. There was no significant effect on milk yield, but milk protein was signficantly increased by the high concentrate ratio. Condition score was significantly negatively correlated with locomotion score.

Data were collected on 19 high-producing Holstein dairy cows from day 20 post partum until they were either determined pregnant or reached 150 days post partum. Blood metabolites (progesterone (P4), glucose, beta-hydroxybutyrate (BHBA) and free fatty acids (FFA)) were related to the interval from parturition to first luteal phase, first visual oestrus and conception. Prior to data analysis, cows were divided into two groups based on their 120-day milk production and designated as below herd average (BHA; no. = 9) or above herd average (AHA; no. = 10). Average 120-day milk productions for BHA, AHA and herdmates (no. = 56) were 3932 (s.e. 157), 4841 (s.e. 60) and 4353 (s.e. 96) kg, respectively. Days from parturition to first luteal phase were not different between the BHA and AHA groups (31·7 (s.e. 1·7) and 31·5 (s.e. 6·9), respectively). In contrast, days from parturition to first visual oestrus and to conception oestrus were less for the BHA group than for the AHA group (46·8 (s.e. 4·6) and 76·9 (s.e. 7·5) v. 67·7 (s.e. 5·2) and 102·5 (s.e. 9·2) days, respectively (P < 0·05)). Further, average glucose concentration from day 20 to first luteal phase was higher (P < 0·05) for the BHA group than for the AHA group (64·1 (s.e. 1·6) v. 58·4 (s.e. 1·6) mg/dl). From day 20 to first visual oestrus, plasma glucose concentration increased while BHBA and FFA declined; all three metabolites then remained relatively constant until conception. These data suggest that as the amount of milk a dairy cow produces increases, the interval from parturition to first visual oestrus and conception increases. These data are consistent with a suppression of oestrus behaviour rather than with a delayed interval to ovarian cyclicity.

Dry-matter (DM) intake and water consumption were recorded in Borana cattle (Bos indicus) which were subjected to infrequent watering regimes, prolonged walking and night-enclosure. DM intake and water intake were depressed in cows watered every 2nd day (0·04 and 0·16 respectively) and in cows watered every 3rd day (0·1 and 0·3 respectively) as proportions of the food and water intake of daily watered cows. DM intake was further reduced by proportionately 0·05 as a result of a 40-km walk every 3rd day throughout the dry season and by 0·05 as a result of night-enclosure. Digestibility of food was unaffected by treatments. Water intake increased in some months as a result of walking but not as a result of night-enclosure. It is thought that DM intake was depressed with decreasing frequency of watering owing to the physiological effects of dehydration resulting in inappetence. Water consumption fell owing to the constraint imposed by rumen volume at drinking. Walking and night-enclosure independently reduced DM intake as a result of the greatly decreased time available for grazing.

A sequential procedure for the development of breeding objectives for beef cattle improvement is presented. The procedure is general, and could be used for other types of domestic livestock. The development proceeds in four phases: (1) specification of the breeding, production and marketing system; (2) identification of sources of income and expenses; (3) determination of biological traits influencing income and expense; (4) derivation of the economic value of each trait, based on discounted gene flow methods. As an example, a breeding objective is developed for a pure breeding herd from which male calves are marketed at 9 months of age and surplus heifers are sold at 15 months of age. Situations examined included: (1) the effect of discounting on the breeding objective (income and expense per year v. discounting at 0, 5 and 10%); (2) the effect of ignoring the economic value for food intake in the objective; (3) the effect of inclusion of yearling weight or the deletion of calving date from the selection index. In general, correlations between objectives for discounting and income and expense per year were greater than 0·95, whereas correlations between indices were greater than 0·85. Setting food costs equal to zero shifted emphasis from reproduction to growth traits, with a consequent increase in food intake. The addition of yearling weight to the index caused a small increase in the accuracy of the index, whereas the deletion of calving date had a large effect, halving the accuracy of the index. The practical implications of the results are discussed.

Cloning of cattle from embryos has been achieved by nuclear transfer. Repeated cloning to produce large clones of genetically identical individuals for commercial use may soon become possible. The main advantage in breeding will come from identifying and using superior clones, selected to fit particular husbandry and marketing niches. Two types of cattle clones will be required, terminal clones chosen for economic merit in production traits, such as growth and carcass traits, and maternal clones chosen for reproduction and maternal traits, such as short calving interval, ease of calving, milking and mothering ability and small mature size. Clonal selection and use will provide several steps of improvement. An initial gain will come by recruiting clones from elite stocks and further responses by selecting the best clones for commercial use. This will reverse the normal improvement lag between breeding and commercial stocks. Heterosis will also be exploited by using crossline clones. For continuous genetic improvement, genetic variation will be maintained by breeding the next generation from a number of selected male and female clones and testing and selecting their progeny, the new set of clones, as before. For maximum improvement rates, both male and female clones will need to be tested. In the short term (5 to 10 years), benefits will come from testing a large number of clones by which gains in economic merit of proportionately 0·15 to 0·30 of the mean can be achieved. Further improvements will come by rebreeding the best clones and testing and selecting the new set of clones from which annual genetic response rates of proportionately 0·02 to 0·03 per year are possible. Cloning thus offers considerable advantages in the improvement of beef cattle.

The study aimed to determine the effect of the growth promotant avoparcin on the digestion and absorption of nitrogen and amino acids from pre-ruminant milk-fed calves. Eighteen male Friesian calves, 45 kg live weight, were allocated to three diets: a commercial high-quality milk replacer compound (control); control with 100 mg zinc bacitracin per kg (positive control) and control with 40 mg avoparcin per kg. The diets were given once-daily at a set rate of intake during a 14-day metabolic study. On the 15th day the calves were fed at 2-h intervals and ileal digesta were sampled (8 h after the start of feeding) from the euthanazed animal. Estimates of apparent ileal digestibility were determined by reference to an indigestible marker, with the comparable faecal estimates determined following the total collection of faeces.

There was no significant effect of avoparcin or zinc bacitracin addition on faecal apparent digestibility of dry matter, nitrogen, gross energy and amino acids or on ileal apparent digestibility of nitrogen and amino acids. Neither was there a significant effect of either additive on nitrogen retention, daily urinary urea and total nitrogen excretions or portal and peripheral plasma urea concentrations. Ileal apparent digestibility coefficients determined on the control diet ranged from 0-82 for glycine to 0-96 for phenylalanine, tyrosine and methionine. The ileal measures of apparent digestibility were more variable than the comparable values determined over the entire digestive tract. Measurements on the faeces compared with measurements on digesta at the terminal ileum gave higher values (P < 0·05) of apparent digestibility for nitrogen and 10 of the 16 amino acids.

Selection on the basis of estimated breeding value for cow fertility was successful in establishing two distinct lines of high (H) and low (L) fertility in a herd of Droughtmaster (Brahman × Shorthorn) cattle. The average difference in pregnancy rates over 3 years between the H and L line was 12% (P < 0·01) in the selected cows. In lactating cows this difference was 17% indicating that lactational anoestrus was an important component of the line difference in fertility. The total calf losses between confirmed pregnancy and weaning were 17%, but there were no line differences. Cow mortalities were 5% higher (P < 0·01) in pregnant cows than in empty cows but were similar in the H and L 'lines.

H cows were proportionately 0·06 lighter in body weight (P < 0·01) at mating and 0·075 lighter (P < 0·01) at weaning than L cows. Much of this difference was due to the greater number of calves carried and weaned by H cows during their lifetime. Smaller lactation effects on weight gains and weight losses in H cows indicated that these cows were less sensitive in terms of weight gain to seasonal fluctuations in nutrition. H cows conceived earlier than L cows in 2 out of the 3 years and reared calves which were proportionately 0·04 lighter at weaning (P < 0·01). It is suggested that lower milk production in H cows leading to shorter periods of lactational anoestrous, combined with lower sensitivity to nutritional stress, was responsible for their higher fertility.

Milk consumption was estimated for crossbred suckled calves from birth to weaning during 2 years. Breed groups utilized were Aberdeen Angus (A) and Charolais (C) sires on Fl Angus-Hereford (and their reciprocals) dams in the 1st year and in the 2nd year, sires A and Nelore (Bos indicus) (N) were used on the same type of dam. The weigh-suckle-weigh technique was employed twice a day to estimate milk consumption on a weekly basis. Estimations were adjusted by least-squares for the effects of genetic group, week of estimation, and their interaction, with sex of calf and calf and cow weights in the week of estimation as covariables. Then a non-lineal regression equation of the form: y = b0Wb1e -b2b2W was fitted for each individual, where y = milk consumption and W = week of estimation. Curves for A and C were very close in their shape, but with a greater consumption (P < 0·01) for C. For the 2nd year, there were differences among A and N both in the shape of the curve and in consumption at the peak of lactation (P < 0·01). Differences in time to peak lactation were also found: for A calves, it was week 9, for C calves week 10 and for N calves week 3·8. The N group showed a sharper decline in persistency than the other groups, suggesting different patterns in milk consumption.

Post-partum interval to oestrus and conception was studied in relation to previous and current lactation (milk yield and lactation length), age of ewe and treatment with intravaginal sponges in a total of 186 Awassi ewes lambing in December/January. In the first trial, 62% of ewes, tested for oestrus twice a day with vasectomized rams until the end of May, showed oestrus between 1 and 170 days after lambing with a mean interval of 83 (s.d. 31·9) days. Neither current nor previous lactation had important effects on the rebreeding interval (P > 0·05). In another trial where ewes were exposed continuously to fertile rams until the end of March, 50% showed oestrus with a mean interval of 56 (s.d. 23·7) days (range 33 to 113 days). Of those showing oestrus, 86% conceived with a mean post-partum interval of 70 (s.d. 33·0) days (range 33 to 133 days). Lactation yield had no significant effect on the lambing-to-oestrus interval (P > 0·05) but ewes with longer lactations showed longer lambing-to-conception intervals (P > 0·01). Treatment with progesterone-impregnated vaginal sponges at 1, 2 and 3 months post partum showed an increase in the percentage of ewes showing oestrus from 35% to 75% and in those conceiving from 25% to 70% as the timing of treatment increased from 1 to 3 months post partum. It is concluded that lambing three times in 2 years is biologically feasible and that hormone treatment can facilitate its operation in practice.

The response to long-term continuous infusion of gonadotrophin releasing hormone(GnRH)was monitored in progesterone-treated seasonally anoestrous ewes. Using osmotic pumps, groups of five ewes eachreceived 0(controls),125, 250, 500 or 1000 ng GnRH per h subcutaneously for a period of 21 days. Bloodsamples were collecte dat 30-minintervals from 6 h before until 24 h after the start of treatment and then for 8-h periods on days 2, 8, 15 and 21. After 21 days of treatment all the ewes were slaughtered to determine pituitary GnRH receptor numbers. Continuous infusion of GnRH resulted in a short-lived (2day)increase in plasma LH and oestradiol concentrations after which they were not different from the pre-treatment values. Over the later period of treatment when the pituitary gland was not responding to the exogenous GnRH(days 8, 15 and 21), LH episodes(presumably due to endogenous GnRH secretion fromthe hypothalamus) were observed. Continuous infusion of GnRH was also associated with a suppression in plasma FSH concentrations, the duration of which was dose-dependent. Only at the highest GnRH dose level(1000 ng/h)was there a significant reductionin pituitary GnRH receptor content.

The effects of the routine stressful stimuli of handling, transport and slaughter on the plasma concentrations of cortisol and β-endorphin have been studied in lambs. Blood samples were obtained from group 1 lambs after rounding up, after transport and at slaughter. Group 2 lambs were treated similarly except that blood was collected only at slaughter. Group 3 lambs served as controls and were blood sampled twice daily for 5 days to accustom them to handling before being slaughtered. Plasma cortisol and β-endorphin concentrations were increased above control levels by rounding up and transport, and were further increased at slaughter. Group 3 lambs, however, had very much lower β-endorphin levels at slaughter than the other two groups, although their cortisol levels were similar, β-endorphin concentrations declined during the 5-day blood sampling period in group 3 animals but cortisol levels were unchanged. The results suggest that although levels of both hormones are increased by stress, they are not necessarily released concomitantly.

A total of 96 gilts were used in two experiments to determine the effect of live weight and body composition at about 170 days of age on age of attainment of puberty, ovulation rate and subsequent reproductive efficiency.

In the first experiment 48 crossbred (Large White ♂ × Landrace ♀) gilts were randomly allocated at 21 days of age to each of six treatment groups which gave six different intakes of a protein-adequate diet. The food intakes were designed to achieve six live weights ranging from 59·6 to 118·0 kg at boar introduction at 170 days of age. Age at puberty declined linearly from 202 to 173 days with increasing live weight at boar introduction (P < 0·01). Ovulation rate at puberty was also positively affected by live weight at 170 days (P < 0·05).

In the second experiment, four littermates from each of 12 litters were randomly allocated at 28 days of age to four treatment groups. The groups were offered diets of varying protein adequacy at different feeding levels up to boar introduction at 165 days of age. The treatment groups comprised a factorial design with the factors being live weight (light, L or heavy, H) and fat level (lean, 1 or fat, f relative fat content). At boar introduction, mean live weights were 70·6, 71·5, 101·3 and 100·0 kg, and mean backfat depths were 11·3, 20·9, 21·7 and 27·0 mm for the LI, Lf, HI and Hf groups, respectively. Heavier gilts at boar introduction were younger (183 v. 199 days, P < 0·01) at puberty. Ovulation rate at puberty was also greater (11·0 v. 9·5, P < 0·05) in the heavier gilts. Gilts which were leaner at boar introduction also tended to attain puberty earlier (186 v. 195 days, P < 0·10) than fatter gilts.

In both experiments neither the subsequent reproductive efficiency of gilts after parturition nor the performance of their piglets was significantly affected by body weight or body composition at boar introduction.

The inheritance of the time taken to react to halothane anaesthesia was examined as a possible quantitative measure of penetrance, which might allow the advantage of the halothane gene in lean content to be separated from its disadvantage of stress susceptibility. Restricted maximum-likelihood analyses were conducted on a total of 2207 pigs from 118 sire families in British Landrace and Pietrain-Hampshire lines selected for a high frequency of positive halothane reaction over 6 years. Reaction time averaged 94 s with a CV of 0·53, and was significantly influenced by line, season and live weight, but not sex. Estimated heritabilities averaged 0·07 (s.e. 0·06) and 0·16 (s.e. 0·12) in the two lines respectively, giving a pooled estimate of 0·09 (s.e. 0·05). Due to the high CV, family selection with a heritability of around 0-1 could be expected to give a rapid genetic change in reaction time. It is therefore concluded that selection for slower reaction to halothane could provide a means for reducing the incidence of stress susceptibility while retaining the advantage of the gene in lean content. However, further evidence is required on the correlation of reaction time with stress susceptibility and lean content.

An experiment was conducted to evaluate the effects of diet on fur colour and quality of bromocriptineand melatonin-induced winter pelage in mink. At weaning, 90 standard dark female kit mink were assigned randomly to one of two dietary regimens. Diet 1 (no. = 30) was a basal ranch mink diet, and diet 2 (no. = 60) was the basal diet, supplemented with liver and eggs. On 26 June 1986, mink given each diet were assigned to treatment (no. = 15) and control groups (no. = 15). Animals receiving diet 1 were treated with 120 mg melatonin implants (silastic) while those given diet 2 were treated with 120 mg melatonin implants, 60 mg bromocriptine pellets, or 60 mg bromocriptine pellets plus 18·6 mg melatonin implants (silastic). Control groups on each diet received no implants. Subsequently, fur growth was measured at biweekly intervals. Mink treated with 120 mg melatonin, 60 mg bromocriptine, and 60 mg bromocriptine plus 18·6 mg melatonin all moulted 1 month earlier than controls and exhibited significantly greater fur growth during the months of August and September (P < 0·05). Mink implanted with 120 mg melatonin, and bromocriptine plus melatonin were considered to be in fully prime pelage by mid October and were pelted by October 23. Mink receiving bromocriptine alone were not considered to be in prime pelage in October and were pelted in December with controls. Neither diet supplementation with liver and eggs, nor treatment to induce early fur growth had an effect on fur colour or quality. It was also concluded that treatment of mink with bromocriptine may not completely mimic the effect of exogenous melatonin on winter fur growth.

The effect of routine induction of parturition on some reproductive and productive parameters during a year-long trial in 50 California doe rabbits allocated in two groups of 25 animals (treated and control), was studied. Intramuscular injections of 50 ug of the synthetic analogue of prostaglandin F2ct (Etiproston), were given at 10.00 h on day 29 of pregnancy. Control animals were untreated. When compared with the controls, treated does showed a significant reduction in the individual weight of pups at birth (49·7 (s.e. 0·41) and 52·2 (s.e. 0·53) g), number born dead per litter (0·42 (s.e. 0·11) and 0·81 (s.e. 0·16)), and parturition interval (42·9 (s.e. 1·5) and 53·5 (s.e. 1·7) days), and an increase in both sexual receptivity on days 6 to 9 post partum (70·4% and 43·0%), and in the conception rate to remating on days 6 to 7 post partum (82·2% and 66·6%), respectively. Litter size at birth, overall conception rate, mortality from birth to weaning, litter weight at 21 days, litter size at weaning, milk production, and mortality of does were not affected by the prostaglandin treatment.

Methods of inducing ovulation 3 weeks prior to the onset of the breeding season were evaluated in lactating adult red deer. Following 11 days of intravaginal progesterone pre treatment, hinds were either untreated (control), or given 300 i.u. PMSG i.m. or 500 mglh GnRH s.c. by osmotic pump. All hinds were laparoscoped 7 days after progesterone withdrawal to record the presence or absence of a corpus luteum on the ovaries, Laparoscopy showed 0113 control, 11113 PMSG and 8/13 GnRH-treated hinds ovulated indicating that during lactation, both methods of inducing ovulation are similarly effective, However, although these treatments induced ovulation, fertility as assessed from calving records, was poor.

Litters of piglets were given ad libitum access to a sow-milk replacer (SMR) during lactation (200 g SMR per I) and the 1st week after weaning (100 g SMR per I). Weaning was at 3 and 4 weeks of age in the first and second studies respectively. SMR was readily consumed but was associated with a high incidence of scouring, reduced intakes of creep food, and (in one study) increased mortality. There was a trend towards increased litter weight in the 4-week weaning study only.

Live-weight growth was studied for 64 female, 64 castrated male and 64 intact male indigenous Nigerian pigs at 16 ages from birth to 672 days. The weights obtained were fitted into four different growth functions to derive prediction equations for the models used. The quadratic (W = a + bt + ct2) and the Schmalhausen functions (W = at) described live-weight growth better than the simple linear function fW = a + bt). Estimating live-weight growth using Brody's function (Y = aebt) gave the least exact predictive ability. Linear growth in indigenous Nigerian pigs occurred between 112 and 336 days. Castrated male pigs exhibited higher live-weight growth rates than either the female or intact male pigs. Differences in the prediction of live weights using the four growth functions and the actual live weights are presented and discussed.

Fourteen adult ewes from each of three breeds, Karagouniko (K), Serres (S) and Chios (C), were maintained, barren, as a single flock at Diavata (latitude 40·5°N) for 2 years. Oestrus was checked twice daily by vasectomized rams for 17 months from April 1979. For K, S and C ewes, respectively, the mean dates of onset were 7 August (s.e. 6·5), 31 July (s.e. 8·2) and 28 July (s.e. 7·3) (P < 0-05) and the spread of first oestrus extended to 100, 94 and 95 days, respectively. Over all breeds, the dates of first oestrus ranged from 23 May to 19 September. Mean dates of last oestrus were 16 April (s.e. 8·5), 5 April (s.e. 9·5) and 20 May (s.e. 7·0) (P > 0·01) and the mean durations of the breeding period were 253 (s.e. 11·0), 247 (s.e. 12·2) and 301 (s.e. 10·2) days, respectively (P > 0·01). In the second season, the mean dates of first oestrus were 4 September (s.e. 2·4), 28 August (s.e. 5·1) and 25 July (s.e. 6·2), respectively (P < 0·01).