When comparing progeny-testing schemes with individual performance selection, one should use equivalent selection intensities and population structures in the two systems. Adapting formulae from open-nucleus theory, the relative efficiency of progeny testing has been tested for a range of heritabilities, fertility levels, upward gene-transfer rates, mating ratios and numbers of sires selected. In sheep and beef cattle breeding, the heritability of the trait under consideration has usually to be very low to make progeny testing worth-while. High fertility increases genetic gains but does not change the relative efficiency of progeny testing. Opening the nucleus to females from the base increases genetic gains and reduces relative efficiency of progeny testing, both effects being of small magnitude. For fixed mating ratios, only a small fraction of females should be in the nucleus, but for fixed sire numbers approximately one-third of the population should be mated to proven sires. In these analyses, variances were adjusted for the effects of repeated selection and for the effects due to mixing groups with different means. This adjustment did reduce absolute genetic gains, but did not change conclusions on progeny testing efficiency and optimum design.

The concepts of metabolic time and size are used, respectively, to compare maturing time and energy exchanges during growth in homeotherms. Maturation rate, expressed per unit of mature size (A1/4) is relatively constant for mammals, irrespective of attempts by man to select for rapid growth. Precocial birds mature more rapidly, however, and altricial birds more rapidly still. These differences are explained in terms of different patterns of energy exchange. It is proposed that maturation time is ultimately determined by size-related constraints on rate of metabolism during growth.

A multivariate restricted maximum-likelihood procedure was used to estimate variance and covariance components between and within sires. This method, which considered all lactations simultaneously, accounted for the bias in later lactation records due to selection on dairy performance. Analysis was carried out for a mixed model with herd-year-seasons as fixed and sires as random effects, and fitting lactation length, calving age and month within season of calving as covariables. The data included 26 176 first, 19 978 second and 14 868 third lactation records for 679 test sires, and were analysed in 13 subsets. Additional records for proven sires, treated as fixed effects, were included to improve the data structure.

Estimates for all components were higher in later than in earlier data sets, probably, to a large extent, because of a scale effect. Pooled estimates of heritabilities for lactations 1 to 3 were 0·28, 0·19 and 0·24 for milk yield, and 0·27, 0·21 and 0·25 for fat yield. Genetic correlations of 0·91, 0·91 and 0·95 for milk yield, and 0·91, 0·91 and 0·99 for fat yield, were found between lactations 1 and 2, 1 and 3, and 2 and 3, respectively. Results suggest that dairy performance in all lactations is almost identical genetically.

A multibreed cattle experiment involving 25 British breeds was set up to study genetic variation between breeds and genetic inter-breed relationships for a wide spectrum of traits in order to examine the problems of between-breed testing and selection. The experimental design is described and results on between-breed variation are presented for four traits.

All animals were housed indoors and from 12 weeks of age were given a single complete pelleted diet ad libitum through a system of Calan-Broadbent electronic gates. Females were mated to produce one purebred and three crossbred calves, which were reared to slaughter in order to measure the efficiency of the cow-calf unit of production.

Results based on a total of 292 animals, with an average of 12 per breed, are presented for body weight, cumulated voluntary food intake, daily weight gain and daily food intake over the age range from 12 to 72 weeks. The 25 breed-mean curves for body weight and cumulated food intake displayed a remarkably uniform pattern of rankings at all ages and the rankings were very similar for both traits.

The multibreed design used was effective in estimating between-breed variation as a proportion of total variation for the four traits examined. After approximately 1 year of age, the proportion of variation between breeds was approximately 0·70 for body weight and 0·60 for cumulated voluntary food intake. Changes in these traits could therefore be brought about more effectively by selection between breeds rather than within breeds. For average daily weight gain measured over 12-week intervals, between-breed selection was estimated to be most effective in the period of maximum growth rate between 6 and 9 months of age, when between-breed variation was 0·52 of the total. For average daily food intake, measured over 12-week intervals, between-breed selection was likely to be effective beyond 6 months of age, when the proportion of between-breed variation plateaued at 0·48.

At all ages, the coefficient of genetic variation between breeds was approximately 0·14 for body weight and daily gain, and remarkably constant at approximately 0·12 for both daily and cumulated food intake. It is suggested that, for growth and intake traits, the genetic variances within and between breeds remain proportional to each other at all ages.

Gestation length and birth weight of 939 calves born at Santa Monica Experimental Station, Valenc, a, State of Rio de Janeiro, were studied. The calves were of six red and white Holstein-Friesian (HF) × Guzera (G) grades: 1/4, 1/2, 5/8, 3/4, 7/8 and ≥ 31/32.

As the grade × sex interaction for birth weight was significant, data for each sex were analysed separately. A model, including the effects of grade, year-season of birth, grade × year season interaction and age of dam as a covariate, resulted in the following least-square means for the six grades in the above order (± s.e.): gestation length for cows carrying male calves, 290·0 (± 0·9), 281·2 (± 0·9), 285·3 (± 0·8), 278·8 (± 0·9), 280·5 (± 0·9) and 279·3 (± 0·9) days; gestation length for female calves, 287·5 (± 0·9), 280·1 (± 1·0), 285·3 (± 0·9), 274·5 (± 1·1), 279·2 (± 0·9) and 276·4 (± 1·0) days; birth weight for males, 34·6 (± 0·7), 28·6 (± 0·8), 34·2 (± 0·7), 32·4 (± 0·7), 35·0 (± 0·7) and 34·7 (± 0·7) kg; birth weight for females, 29·9 (± 0·6), 29·5 (± 0·7), 33·4 (± 0·6), 31·9 (± 0·7), 33·6 (± 0·6) and 32·6 (± 0·6) kg.

Direct (g1) and maternal (gM) breed additive effects (HF - G), and direct (h1) and maternal heterosis (hM) were estimated. The estimates of g7, gM, h1 and hM were respectively, for gestation length for male calves: -21·5 (± 1·9), 10·6 (± 2·1), 1·5 (± 1·8) and -1·6 (± 11) days; for gestation length, for female calves: -21·9 (± 2·1), 14·6 (± 2·5), 5·7 (± 2·1) and -1·5 (± 1·2) days; for birth weight of male calves: -3·5 (± 1·5), 7·1 (± 1·8), -0·6 (± 1·5) and 1·0 (± 0·9) kg; and for birth weight of female calves: 4·7 (± 1·4), 4·1 (± 1·6), 2·8 (± 1·3) and 2·0 (± 0·8) kg. The difference in birth weight between males and females was 4·7 (± 0·9) for G-sired calves and 0·8 (± 0·4) for HF-sired calves.

Over a 2-year period, 80 dairy cows were used to investigate the relationship between nutrition and fertility. The plane of nutrition of half the cows was raised for a 9-week period around the expected time of artificial insemination in both breeding seasons. The net effect was that the high-plane cows received 15% extra metabolizable energy during the 9-week period. This was achieved by giving these cows a high-energy concentrate (calculated value 13·1 MJ metabolizable energy per kg dry matter), whereas the control cows received a concentrate with a calculated energy density of 12·5 MJ metabolizable energy per kg dry matter. All cows received a basal ration that supplied an average of 75 and 80 MJ metabolizable energy per cow per day in the 2 years, respectively. Control cows received their calculated amount of dry matter using the control concentrate, whereas the high-plane cows received their calculated dry matter using the high-energy concentrate. The high-plane cows also received an additional 0·5 kg dry matter per head per day of their concentrate. They also received additional concentrates (2 kg per head per day) for 8 weeks, commencing 12 weeks before the second calving.

There was no effect of dietary treatment on the reproductive performance of the cows, although some measures of milk yield, live weight and body condition were affected significantly.

Analysis of data from all cows, regardless of treatment group, showed that the probability of pregnancy to an insemination was most strongly associated with milk yield on day 21 of lactation. Only the cumulative yield to day 21 was related to the probability of pregnancy after accounting for the trend with yield on day 21. The chance of pregnancy was lower in cows whose milk yields on day 21 were high but, among cows with similar yields on day 21, those with the higher cumulative yields at that time tended to become pregnant more readily. Thus both yield per se and rate of increase in yield in early lactation were associated with subsequent reproductive performance.

Age and weight at puberty, age at first conception and number of services per conception of 90 females at Santa Monica Experimental Station, Valencia, State of Rio de Janeiro, were studied. The females were of six red and white Holstein-Friesian (HF) × Guzera (G) grades: 1/4, 1/2, 5/8, 3/4, 7/8 and ≥ 31/32.

A model (1) including the effects of grade and season of birth resulted in the following least-squares means for the six grades in the above order (± s.e.): age at puberty, 770 (± 17), 725 (± 18), 799 (± 26), 788 (± 23), 777 (± 17) and 803 (± 24) days; weight at puberty, 309 (± 8), 334 (± 9), 316 (± 13), 311 (± 11), 303 ((± 9) and 298 (± 12) kg; age at conception, 865 (± 19), 773 (± 19), 867 (± 31), 808 (± 25), 852 (± 22) and 837 (± 25) days; number of services per conception, 1·9 (± 0·2), 1·7 (± 0·2), 1·5 (± 0·3), 11 (± 0·2), 1·6 (+ 0·2) and 1·3 (± 0·2).

A second model (2) was fitted, including the effects of season of birth, direct (g1) breed additive effects (HF - G) and heterosis effects (hl). g1 and h1 were estimated by the partial regression coefficient of the traits studied on, respectively, the expected HF gene frequency and the expected heterozygosity of the females. The estimates of g7 were not significantly different from zero except for age at first conception (g7 = -102 (s.e. 46) days) (P < 0·05). Heterosis effects were significant for age at puberty (h1 = -86 (s.e. 34) days), weight at puberty (h1 = 44 (s.e. 17) kg) and age at first conception (h1 = —119 (s.e. 37) days). These three estimates of h1 amounted, respectively to —0106, 0151 and —0·132 of the estimated mean of the parental breeds.

F-tests on the extra variation due to fitting model 1 after model 2 were not significant (P < 0·05) indicating that epistasis or other genetic effects, not included in model 2, were of little importance for the traits studied.

A mathematical model based on the energy requirements of the dairy cow and the lactation model of Wood was constructed to determine the effect on net returns over concentrates of different calving intervals for different weeks of the year under different feeding management conditions. The model was computerized, and this enabled an investigation of the effect on net returns of varying silage metabolizable energy levels and different nutritional management systems. The importance of the modelling approach in examining a range of interacting factors simultaneously is highlighted. To illustrate this approach, results are presented here with 1981 prices for milk, concentrates and calves kept constant over a period of 4 years and also, to validate this approach, using actual prices paid over the period 1976 to 1981.

A field study involving 1087 cows and 191 heifers on nine carefully selected English dairy farms was conducted to evaluate the effect of nematocidal treatment on subsequent milk production. As many of the cattle as possible were allocated to pairs according to specific criteria and dosed with either levamisole and oxyclozanide, or oxyclozanide alone, soon after calving. The remaining unpaired animals were assigned alternately to the two treatment groups according to calving date. The milk production and health status of all the cattle in the study were closely monitored.

Analysis of pooled milk production data over a 2-year period failed to show a statistically significant response to nematocidal treatment. When individual herd data were analysed, a significant response to treatment was recorded on one farm. It was noted that the apparent value of the response varied according to method of analysis.

Factors considered likely to account for differences in response between farms and also between trials are considered.

Two randomized-block experiments have been carried out to examine the effects of increasing the digestibility of grass silage offered to beef cattle by harvesting three crops of grass after shorter growth intervals rather than two crops after longer growth intervals. In experiment 1 early-cut silages were harvested on 1 June, 16 July and 24 August, and late-cut silages on 20 June and 22 August 1979. In experiment 2 the early-cut silages were harvested on 19 May, 2 July and 20 August, and the late-cut silages on 5 June and 12 August 1980. Early- and late-cut silages were made from each of two cultivars of perennial ryegrass (cultivars Cropper and Talbot). The silages were unwilted and had formic acid applied at 2.5 1/t fresh crop.

In experiment 1 the silages were offered ad libitum, both unsupplemented and supplemented with 2·2 kg concentrates per head daily, to 88 cattle of mixed beef breeds and mean initial live weight 337 kg, in a 2 × 2 × 2 factorial design. In experiment 2 the silages were offered ad libitum and supplemented with 2·4 kg of concentrates per head daily to 56 Charolais × (Aberdeen Angus × Friesian) cattle of mean initial live weight 351 kg, in a 2 × 2 factorial design. Grass cultivar did not significantly affect silage intake or animal performance but intake, live-weight gain and carcass gain were significantly higher for the early-cut than for the late-cut silages in both experiments. In experiment 1 silage dry-matter intakes were 6·33, 5·12, 5·80 and 4·67 (s.e. 0·124) kg/day; live-weight gains were 0·72, 0·89, 0·47 and 0·76 (s.e. 0·029) kg/day; and carcass gains were 0·46, 0·60, 0·27 and 0·48 (s.e. 0·016) kg/day, for the unsupplemented and supplemented early-cut silages, and the unsupplemented and supplemented late-cut silages, respectively. In experiment 2 silage dry-matter intakes were 5·49 and 4·95 (s.e. 0·056) kg/day; live-weight gains Were 0·95 and 0·76 (s.e. 0·033) kg/day; and carcass gains were 0·67 and 0·50 (s.e. 0·17) kg/day, for the early- and late-cut silages, respectively.

1. A deterministic model and computer program of the energy system of lactating and pregnant cows have been developed.

2. Input data for cow, nutritional, management and environmental factors are required.

3. The output of milk production, live weight and heat production is a time series, with a weekly time-step.

4. Suckler and dairy cow examples are given to illustrate the use of the model to evaluate the effects of quality, quantity and pattern of food intake, and shelter.

Body condition score, assessed subjectively on the live animal, was related to the directly determined body composition of 73 mature, non-pregnant, non-lactating cows of Hereford × Friesian, Blue-Grey, Galloway, Luing and British Friesian genotypes. Relationships between condition score and chemically determined body fat were all very highly significant, and considered to be of value for predictive purposes. Differences between genotypes in the proportion of fat stored in the main depots of the body resulted in differences in the relationship between condition score and body fat. British Friesian cows had a higher proportion of their fat in the intra-abdominal depots and the lowest proportion of subcutaneous fat, resulting in their being fatter at any given condition score. Hereford × Friesian cows had the highest proportion of subcutaneous fat and were thus the least fat at any condition score. One unit change in condition score was associated with a change of 2242 (s.e. 103) MJ of body tissue energy in Hereford × Friesian, Blue-Grey, Galloway and Luing cows and 3478 (s.e. 392) MJ in British Friesian cows. These figures may be used to bring a greater degree of precision to the nutritional management of beef and dairy cows.

A number of possible indices of body composition (live weight, skeletal size, total body water as estimated by deuterium oxide dilution, blood and red cell volumes as estimated by Evans Blue dilution, ultrasonic measurements of subcutaneous fat depth and eye-muscle area, and body condition scoring) was examined using 73 non-pregnant, non-lactating, mature cows of Hereford × Friesian, Blue-Grey, British Friesian, Galloway and Luing genotypes, ranging in body condition score from 0·75 to 4·5. Direct measurements of body composition in terms of water, fat, protein and ash were made following slaughter.

Live weight, deuterium oxide dilution, ultrasonic measurements of subcutaneous fat depth and eyemuscle area, and body condition score were all considered to be potentially useful predictors of body composition. Combinations of techniques offered better predictions than did any single index. Using a combination of measurements it was possible to predict body fat and protein with a residual s.d. of 13·1 kg and 3·15 kg respectively. Breed differences in the partition of fat among the main adipose tissue depots necessitated the development of specific prediction equations for body fat based on condition score and subcutaneous fat depth for different breeds. Equations remain to be developed for predicting body composition in cows in different physiological states.

1. The model of the energy system of lactating, and pregnant, cows proposed by Bruce, Broadbent and Topps (1984) has been evaluated using data relating to 166 lactations for dairy cows and 48 lactations for beef cows. The cows in both sets of data were subjected to various nutritional and managerial regimes.

2. The discrepancies between observed and predicted values of milk (energy) and live weight were small, both as a proportion of the measured values and total energy intake. For Danish Black and White dairy cows the discrepancies for milk were virtually zero on both bases; those for live weight represented 0·020 of the total energy intake, or an over-prediction by 0·054 in late lactation. For Hereford × British Friesian beef cows, suckled by one or two calves, the discrepancies as a proportion of the total energy intake ranged from 0·005 to 0·051 for milk and 0·001 to 0·063 for live-weight change across the nutritional and managerial regimes.

3. It was concluded that the model of Bruce et al. (1984), which considers genetic potential for milk production and growth, nutritional (energy), reproductive and environmental status, may be used to predict milk yield and live-weight change simultaneously from food (energy) input.

Automated feeding systems for dairy cows offer the benefits of ‘little and often’ feeding. Published data were collected and analysed to establish the extent of such benefits. Twenty-three publications reporting the results of 35 experiments were examined. Several statistically significant positive responses indicated that milk fat concentration, and to a lesser extent yield of milk, could sometimes be increased by increasing the feeding frequency. However, for ail experiments the average proportional increases (± s.e.) in milk fat concentration and milk yield were fairly small at 7·3 (± 3·3) % and 2·7 (± 1·3) %, respectively. The responses of milk fat concentration and milk yield were apparently correlated (r = 0·43, s.e. = 0·20), and the mean proportional response of milk fat yield was 8·3 (± 3·1) %. There was no evidence that milk protein concentration, lactose concentration or changes in body weight were affected by changes in feeding frequency. Increases in food intake were sufficient to explain some, but not all, instances of increased milk fat production. All statistically significant responses to increased feeding frequency occurred when the milk fat concentration was originally depressed, milk fat depression generally being due to feeding pelleted or highly concentrated diets. Increases in milk fat concentration through increased feeding frequency were generally insufficient to bring the milk fat concentration up to a commercially acceptable level. All statistically significant responses were observed on moving from one or two to three or more meals per day, but the possibility of further responses beyond four meals per day could not be ruled out. It was concluded that cows producing milk of commercially acceptable milk fat concentrations were unlikely to benefit from increased feeding frequency.

1. Two experiments were conducted with four sheep and four lactating Friesian cows to investigate the effect of food intake on the outflow of protein supplements from the rumen. Protein supplements treated with sodium dichromate were used.

2. Dichromate treatment had little or no effect on particle size distribution or density of the protein supplements. The dichromate-treated proteins were rendered completely undegradable in the rumen and the rate of outflow of chromium-treated protein from the rumen was estimated from the concentrations of chromium recovered in the faeces.

3. There were highly significant linear effects of feeding level on fractional outflow rate per h of chromium-treated protein supplements from the rumen of sheep and cows. Fractional outflow rates of chromium-treated fish meal and soya-bean meal, respectively, from the rumen of sheep were 0·0080 and 0·0076; 0·0182 and 0·0259; 0·0319 and 0·0335; and 0·0383 and 0·0385 per h at 0·2, 1·0, 1·5 and 2·0 times the energy requirements for maintenance, respectively. There were no significant differences between the two tested chromium-treated protein supplements in fractional outflow rates per h when the particle size was similar, nor any difference in particle size or density. Soya-bean meal was ground to achieve a particle size distribution similar to that of fish meal. For the dairy cows, the fractional outflow rates of fish meal from the rumen were 0·065, 0·072, 0·091 and 0·088 per h, at 1·5, 2·0, 2·5 and 3·0 times the energy requirements for maintenance.

4. There were significant differences between sheep in fractional outflow rates per h of protein supplements from the rumen due to the considerably higher values obtained for one animal.

East Friesland × Blackface (EFX) ewes with sustained lactation (S) and pure Scottish Blackface (BF) ewes with normal lactation (N) were mated to Suffolk rams. Twenty pairs of twin lambs born to the EFX ewes were used in the experiment. One lamb of each pair was re-allocated as a single lamb to either an EFX or a BF ewe (ER) and the second lamb was artificially reared (AR). At pasture the AR lambs were fed four times daily at levels corresponding to the milk intake of the ER lambs, which was measured weekly. Herbage intake was measured only in wether lambs.

The intake of milk and herbage between weeks 3 and 12 was lower in the AR groups than in the corresponding ER groups but the differences were not significant.

Total metabolizable energy intake as milk in the ER-S group from weeks 3 to 12 was significantly higher than that in the ER-N group (668 v. 450 MJ). Corresponding intakes in the AR-S and AR-N groups were 613 and 416 MJ. Herbage intake over the same period was 187 and 238 MJ for ER-S and ER-N, and 137 and 219 MJ for AR-S and AR-N. Total intake from milk plus herbage was significantly higher in the S group (855 and 750 MJ, respectively, for ER and AR lambs) than in the N group (688 and 634 MJ).

There were negative correlations of herbage intake with milk intake (–0·533) and with live-weight gain (–0·094), and positive relationships between live-weight gain and milk intake (r = +0·832), and between live-weight gain and total intake (r = +0·900). Differences in milk intake resulted in significant differences in live weight at 12 weeks (respectively, 34·6 and 29·7 kg for ER-S and ER-N; 32·3 and 29·1 kg for AR-S and AR-N).

It is concluded that lambs reared artificially at pasture may be used to investigate the relationship between milk and herbage intake. It was suggested that if milk does not meet the requirements of voluntary intake or growth potential, herbage intake may be increased to fill the deficit but this does not compensate fully for a lower milk supply.

Physiological differences between cows in hormone and metabolite concentrations were sought that may account for genetic differences in milk production.

Two groups of New Zealand Friesian cows, differing in genetic merit for fat production (high breeding index (HBI) and low breeding index (LBI)) by approximately 23 breeding index units (20 kg fat on the United Kingdom improved contemporary comparison scale), were each divided into two further groups, one of which was given freshly cut pasture to appetite and in the other intake was restricted to 70% of appetite. Eighteen blood samples were collected over a 24-h period from each of 16 cows fitted with indwelling jugular catheters. Samples were analysed for growth hormone, insulin, free fatty acids and β-hydroxybutyrate. Mean concentration of insulin was significantly higher in the HBI underfed group than in any other group (P < 0·01). Glucose concentration was significantly lower in the LBI group (P < 0·05), whose intake was restricted. The concentration of (3-hydroxybutyrate was significantly higher in the cows whose intake was restricted (P < 0·01) and the concentration fell markedly after feeding. Growth hormone concentrations were higher in the cows on restricted intake (P < 0·05) and in HBI cows but the differences were not statistically significant.

It was concluded that different patterns of blood metabolites and hormones are associated with cows that differ in genetic merit. Aspects of insulin and glucose metabolism, particularly at low planes of feeding, appear to offer most promise as indicators of high milk-solids production.

Yearling steers, 39 Friesians on an 18-month beef production system (group 1) and 38 Hereford × Friesians on a 20-month beef production system (group 2), were weighed, blood sampled and then half of them were implanted subcutaneously with 300 mg trenbolone acetate and 30 mg hexoestrol. All steers were then weighed and blood sampled at 1-month intervals for a further 3 (group 1) or 4 (group 2) months. Serum was assayed for prolactin, insulin, growth hormone, glucose and blood urea nitrogen concentrations. Steroid treatment increased daily live-weight gain by 28 to 37% over the experimental period and the proportional response appeared to be correlated with the plane of nutrition. Neither prolactin nor glucose concentrations were affected by the treatment. However, in both groups 1 and 2, growth hormone concentrations were significantly higher in implanted steers, whilst urea-nitrogen concentrations were significantly lower. Insulin concentrations were consistently, although not significantly, lower in implanted than in control steers.

1. Energy costs of eating were determined from the increased rates of oxygen uptake by five steers aged 18 to 20 months and weighing 298 to 407 kg.

2. Five diets were tested: pelleted concentrate (500 g barley grain per kg, 400 g lucerne meal per kg, 90 g soya bean meal per kg, and 10 g NaCl, trace mineral and vitamin supplement per kg); pelleted lucerne; lucerne hay; chopped-grass hay (700 g brome per kg, 300 g fescue per kg); and chopped fresh turnips. The dry-matter concentration of the pellets and hays was approximately 900 g per kg while the turnips contained only 140 g per kg.

3. The rates of ingestion differed markedly between diets during the (15 to 50 min) twice-daily (morning and evening) feeding periods. On a dry-matter basis, the pellets were consumed most rapidly at a rate of 130 to 138 g per min, while the hays were consumed at approximately 38 g per min and the turnips at 30 g per min.

4. The energy costs per min spent eating were similar for all rations (27·6 to 35·6 J/ kg live weight). However, because of different rates of ingestion, the energy costs per kg DM ingested were different: 222 to 238 J/kg live weight for the pelleted foods, 1029 J/kg live weight for the hays and 1427 J/kg live weight for the turnips.

5. The energy cost of eating is more a function of time spent eating than a function of the amount of food ingested. Thus, rate of ingestion and duration of the meal are key factors in determining the energy cost of eating in cattle.