Genotype × environment interaction for age at puberty was evaluated using gilts from the Nebraska Gene Pool population derived from lines selected randomly (RS; no. = 48) or for seven generations for decreased age at puberty (AP; no. = 73). Age at puberty was evaluated in two environments: boar exposure for 15 min daily (BE) or no boar exposure (NBE). Pigs were randomly assigned to treatment and mixed into groups of 20 in pasture lots. Oestrous detection was initiated when the oldest gilt in a pen was 125 days of age. Blood samples were taken weekly and assayed for progesterone. Gilts were considered to have expressed oestrus if they exhibited lordosis or had consecutive weekly blood samples with progesterone values above 6·4 × 109 mol/1 (2 μ, g/l). Gilts were removed from the pen upon confirmation of puberty or at 250 days of age. Two gilts failed to reach puberty by 250 days so this value was assigned as their age at puberty. Least-squares analyses of variance were used to analyse the data. A model including line, sire within line, farrowing group, treatment and line × treatment interaction was fitted to the dependent variables age at puberty and percentage cycling by 185 days. Line and treatment each affected age at puberty (P < 0·05) but did not interact. Least-square mean ages at puberty were 154 (s.e. 4·5), 164 (s.e. 4·7), 164 (s.e. 6·1) and 179 (s.e. 5·9) days for AP-BE, AP-NBE, RS-BE and RS-NBE, respectively. Treatment affected percentage of gilts cycling by 185 days (P < 0·05). Least-square percentages were 91 (s.e. 6·9), 76 (s.e. 71), 84 (s.e. 9·2) and 65 (s.e. 8·9) % for AP-BE, AP-NBE, RS-BE and RS-NBE, respectively. These results indicate that expression of response to selection for decreased age at puberty in the gilt is not dependent on stimuli from the boar.

The objective of the experiment was to test whether maintenance energy requirements of non-lactating, non-pregnant beef cows increased with body fatness. Twelve Charolais cows were initially fed to attain either lean or fat body condition. Subsequently, over a period of 66 days food allowances were regularly adjusted for zero live-weight change. For the following 76 days, lean and fat cows received either 400 or 553 kJ metabolizable energy (ME) per kg initial live weight 0·75 per day, respectively. Fat cows maintained their body weight and composition whereas lean cows lost 296 g body weight, 221 g lipid and 65 g protein daily. Calculated maintenance energy requirements averaged 516 and 536kJ ME per kg live weight 0·75 per day for lean and fat cows respectively but were not significantly affected by body fatness. Diet dry matter, organic matter, crude protein and energy apparent digestibilities but not acid-detergent fibre digestibility were significantly higher in fat animals.

1. The growth, feed consumption per unit weight gain, carcass characteristics and some aspects of meat quality were studied in two trials with crosses derived from Landrace (L), Pietrain (P), Hampshire (H) and Large White (LW) breeds.

2. In Trial 1, 96 pigs of both pure Landrace and LW(× (L♀ × P♂)♂ origin were studied up to slaughter weights of 64 and 91 kg live weight. In Trial 2, Landrace, LW♀ × (L♀ × P♂)♂ and LW ♂x (P♀x L♂)♂, (H♀ × P♂ × L♂ and (H♀ × P♂)♀ × LW♂ pigs were compared up to 91 kg live weight using 48 pigs of each type.

3. All crossbred types showed better economy of production than Landrace when assessed by live-weight gain and feed consumption per unit weight gain. Their carcasses contained greater quantities of lean meat and similar quantities of rind, bone and fat than Landrace at comparable weights, but they exhibited a variable tendency to poorer meat quality. LW × (L × P) and LW × (P × L) carcasses at 91 kg live weight were shorter than Landrace; 4% and 14% of pigs in trials 1 and 2 respectively were less than the currently acceptable length of bacon pigs (775 mm). (H × P) × LW and (H×P)×L carcasses at this weight were much shorter; 34% and 38% respectively were less than 775 mm.

This study examined the effects on intake, diurnal rumen pH changes, rumination and digestibility of feeding ruminally cannulated non-lactating cows in a Latin square design (four cows×four periods) with four total mixed rations (TMRs) typical for lactating cows. TMRs were based on: long wheat hay or short wheat hay, wheat silage or wheat silage+1.5% NaHCO3 buffer, as the sole roughage source (30% of TMR dry matter (DM)). The level of physically effective NDF remaining above the 8 mm screen (peNDF) was similar in the long hay and silage-based TMRs (9.45% to 9.64% of DM) and lower in the short hay TMR (7.47% of DM). The four TMRs were offered individually at 95% of ad libitum intake to avoid orts within 24 h. Cows fed long hay consumed less DM than the short hay and silage groups (9.6 v. 10.5 and 10.8 kg/day, respectively) and sorted against large hay particles at 12 h post-feeding. Under the limitations of this study (non-lactating cows fed at restricted intake) short hay TMR prevented sorting within 12 h post-feeding, encouraged rumination per kg peNDF ingested, and had higher average rumen pH (6.24), whereas preventing sub acute ruminal acidosis (SARA, defined as pH<5.8 for at least 5 h/day). In contrast, the long hay and silage-based groups were under SARA. In vitro methane production of rumen fluid was higher in the hay-fed cows than in their silage-fed counterparts, and in all treatments lower at 1 h pre-feeding than at 6 h post-feeding. In vivo DM and NDF digestibility were similar for the short hay and silage TMRs, and higher than those of the long hay TMR. Under the conditions of this study, addition of 1.5% buffer to the wheat silage TMR had no effect on intake, rumen pH, creation of SARA and digestibility.

Despite the importance of the role of Climate Finance to comply with the United Nations Framework Convention on Climate Change 1.5°C objective, there is no consensus on the definition of Climate Finance and the estimated assessment of its aggregated flows and effects remains challenging. Despite being a major emitter and having a significant and cost-effective mitigation potential, the livestock sector has so far only received a marginal share of Climate Finance. As demand for animal protein products continues to increase (68% between 2010 and 2050), there is a compelling case for channeling more Climate Finance investments into the sector to incentivize greenhouse gas emissions reduction at scale. Bottlenecks in linking the livestock sector to Climate Finance include the insufficient capacity to assess the cost-benefit of projects, high upfront cost and risk perception of investors, the informality of the sector, non-existence of Climate Finance instruments dedicated to the livestock sector and lack of cost-efficient Monitoring, Reporting and Verification systems. Nevertheless, recent developments provide avenues to increase the access of the animal protein sector to Climate Finance.

Pure Merinos d'Aries male lambs were reared indoors (IN), with hay and concentrates, or outdoor (OUT) on improved Mediterranean range (Garrigues) and slaughtered when they reached 25 (light) or 32 (heavy) kg live weight. The alimentary tract weight was significantly affected by slaughter weight and by rearing method, which also significantly modified rumen contents. IN lamb body composition was higher in fat and energy and lower in water and protein than OUT lambs but differences induced by the rearing method were reduced when slaughter weight was increased. Allometric equations established for each environment were used to determine body composition and body gain composition at 20 and 25 kg empty body weight. Changes were very limited for IN lambs but protein and fat content of OUT lambs changed greatly as a result of the tremendous increase of grass availability towards the end of the fattening period.

Grazing management has an important impact on dairy ruminants’ performance. References on the intake and milk yield of dairy goats under strip-grazing systems in temperate regions are scarce. In order to study the effect of pasture allowance on pasture intake (PI), milk yield and grazing behaviour, a trial was carried out in spring with 36 Alpine goats in mid-lactation. Three daily pasture allowances (PA=1.7, 2.6 and 3.5 kg dry matter (DM)/day, namely Low, Medium and High, respectively) were compared in a 3 × 3 Latin square design replicated six times during three successive 14-day periods. Goats individually received 268 g DM of concentrate twice daily at each milking and had access 11 h/day to pasture (from 0830 to 1600 h and from 1730 to 2100 h). Pasture intake increased with PA, and more so between Low and Medium than between Medium and High (+216 v. +101 g DM/kg DM of PA). Milk yield was lower on Low than on Medium and High (2.79 v. 3.13 kg/day), as were milk fat and protein yields. Grazing time averaged 476 min/day and was lowest on Low and greatest on Medium. Pasture intake rate was 30 g DM/h lower on Low and Medium than on High. It is concluded that under temperate conditions, when goats are supplemented with 536 g DM of concentrate and have enough access time to pasture (11 h/day), a medium pasture allowance close to 2.6 kg DM/day may be sufficient to maximise milk yield.

Accurate and complete reporting of study methods, results and interpretation are essential components for any scientific process, allowing end-users to evaluate the internal and external validity of a study. When animals are used in research, excellence in reporting is expected as a matter of continued ethical acceptability of animal use in the sciences. Our primary objective was to assess completeness of reporting for a series of studies relevant to mitigation of pain in neonatal piglets undergoing routine management procedures. Our second objective was to illustrate how authors can report the items in the Reporting guidElines For randomized controLled trials for livEstoCk and food safety (REFLECT) statement using examples from the animal welfare science literature. A total of 52 studies from 40 articles were evaluated using a modified REFLECT statement. No single study reported all REFLECT checklist items. Seven studies reported specific objectives with testable hypotheses. Six studies identified primary or secondary outcomes. Randomization and blinding were considered to be partially reported in 21 and 18 studies, respectively. No studies reported the rationale for sample sizes. Several studies failed to report key design features such as units for measurement, means, standard deviations, standard errors for continuous outcomes or comparative characteristics for categorical outcomes expressed as either rates or proportions. In the discipline of animal welfare science, authors, reviewers and editors are encouraged to use available reporting guidelines to ensure that scientific methods and results are adequately described and free of misrepresentations and inaccuracies. Complete and accurate reporting increases the ability to apply the results of studies to the decision-making process and prevent wastage of financial and animal resources.

Selection indices to improve lean meat production of terminal sire breeds of sheep were derived. An aggregate breeding value which comprised growth rate, killing-out proportion and carcass lean proportion was examined. Selection criteria included growth rate and ultrasonic or X-ray computer tomography (CT) estimates of carcass composition. Correlations betwen the index and the aggregate breeding value were about 0·38 for indices using ultrasonic fat measurements, and about 0·40 for those using CT measurements. Dropping ultrasonic or CT measurements from the indices reduced the correlations with the aggregate breeding value by only about 0·01 and 002 respectively, indicating that selection on growth rate alone under current economic conditions was highly efficient. Changes in economic weights, and genetic parameters had little effect on the efficiency of selection. The total national discounted value of returns from a single round of selection on the indices was expected to be between £6 and £30·5 million, depending on penetration rates of improved stock, the breeding scheme used, and with a time horizon of 20 years.

The efficiency of utilization of protein for retention was analysed in pre-ruminant kid goats of the Granadina breed. Sixty male kids were used. Six were slaughtered at birth and the remaining 54 were offered different protein and fat intakes using nine different milk replacers. The protein concentrations were 200, 240 and 280g/kg dry matter (DM) and those of fat were 200, 240 and 280 g/kg DM. Animals were maintained on experiment until they were 60 days old. All were slaughtered on day 61. Nitrogen (N) balance trials were performed during the last 8 days of the 1st and 2nd months. Body composition of the animals slaughtered at birth and at 61 days were determined. Rates of energy retained as protein and as fat were determined (kJ/kg M0·75 per day) and the corresponding rates of metabolizable energy intake as protein and as fat (kJ/kg M0·75 per day) estimated.

Once the relationships between the rates ofN retained and those of digestible N ingested had been established, it was evident that by increasing the protein content of the diet the efficiency of protein retention was decreased. In contrast, increasing the fat content of the milk replacer increased the efficiency of protein retention. The latter effect was noted for the milk replacers containing the high and medium levels of protein but not for those that contained the lowest level of protein, indicating that the level of protein was then the limiting factor. Having recorded this protein-sparing effect of the fat, the results obtained from the slaughter trials were used to develop generalized equations expressing the rates of energy retention in the form of protein or fat as a function of the rates of metabolizable energy intake achieved as both protein and fat. From the analysis of these equations conclusions are drawn about the variable contribution to protein retention in these animals of energy ingested as fat. This contribution depended on the energy intake achieved both in the form of protein and in the form of fat.

The aim of this study was to assess the apparent digestibility of broken rice using total collection of feces and the pepsin-cellulase in vitro technique to provide updated and more accurate digestion coefficients for this by-product when fed to horses. The in vivo digestibility trial was consecutively performed, using five adult geldings, weighing 555.6 kg on average. First, hay was given as the only feedstuff, while second, the experimental diet consisted of the same hay plus broken rice at a forage-to-concentrate ratio of 70/30 (on dry matter (DM) basis). Feces were collected over 6 days preceded by a 14-day adaptation period. The digestibility trial was carried out to determine the digestion coefficients for DM, organic matter (OM), CP and gross energy in both diets, while apparent digestion coefficients for the same parameters were calculated for broken rice alone, using the difference between the two sets of results. At the same time, an in vitro trial was carried out using pepsin-cellulase technique on the samples of hay and broken rice tested during the in vivo trial. As expected, supplementation with broken rice increased digestibility according to all the parameters used. The high OM digestion coefficients of broken rice were confirmed both by the calculated in vivo method and by the predicted results of pepsin-cellulase technique (92.6% and 87.1%, respectively), underlining the high digestibility of this by-product when fed to horses.

The ruminal activity, daily live-weight gain and feed conversion efficiency of zebu calves inoculated with whole rumen contents from a mature cow were compared with those of calves maintained under the same conditions except that they were ciliate-free and isolated from other ruminants.

A thriving mixed population of ciliate protozoa became established in the rumen of calves two weeks after inoculation. Entodinium was the first genus to appear and was followed by Isotricha, Polyplastron, Ophryoscolex, Diplodinium and Dasytricha.

The ruminal volatile fatty acid and ammonia concentrations in the inoculated calves showed a pronounced rise above the values found in the uninoculated controls.

The inoculated calves grew faster and had a higher feed conversion efficiency than the uninoculated controls.

Phenotypic variances within pairs of monozygotic and dizygotic twin heifers and also genetic variances and heritabilities were calculated for 12 linear body measurements at a sequence of eight ages up to two years old. The 60 pairs of fraternal and 60 pairs of identical twins used were reared as part of a larger uniformity trial in which feeding was effectively ad libitum throughout.

Size differences between members of DZ twin pairs were found to be approximately normally distributed with about the same variance for all breeds and crosses. The variance within DZ pairs increased strongly with age, with a marked increase between 9 and 12 months of age and with most body measurements showing a broadly similar trend. On a logarithmic scale DZ variances increased roughly linearly with degree of maturity and at about the same rate in each body measurement. Coefficients of variation within DZ pairs corrected for measuring error had an average value of 2%. They did not change greatly with age, and were roughly the same for most body measurements although width measurements tended to be more variable than average.

Coefficients of variation within MZ pairs had a corresponding overall average of 1·4%; they declined rapidly with age from 2·0% to 1·1%, were roughly the same for all body measurements, but at early ages tended to be greater in late than in early maturing body parts. However, they showed no association with the earliness of maturing of a body part provided variation was measured at the same degree of maturity for each body part.

Genetic variation increased rapidly with age in all body measurements. The rate of increase with age was greater for late than for early maturing parts. The rate of increase with degree of maturity, however, was about the same for all body measurements. Coefficients of genetic variation increased slowly with age; they had an average value of 1·6%.

Estimates of heritability are given at a sequence of eight ages for each of 12 body measurements. They increased strongly with age from 0·14 on average at three months of age to 0·67 on average at two years of age. At any fixed age, early maturing body parts tended to have higher heritabilities than later maturing body parts. However, if heritability was measured at the same degree of maturity in each body part, early and late maturing parts had about equal heritabilities.

The present results are compared with those obtained from twin cattle studies in New Zealand, Sweden and Wisconsin, U.S.A.

Inferences from twins about genetic variances and heritabilities for unrelated animals are discussed.

Serum α1-acid glycoprotein (AGP) is elevated during late gestation and at birth in the pig and rapidly declines postnatally. In contrast, the pig is born with minimal lipid stores in the adipose tissue, but rapidly accumulates lipid during the first week. The present study examined if AGP can affect adipose tissue metabolism in the neonatal pig. Isolated cell cultures or tissue explants were prepared from dorsal subcutaneous adipose tissue of preweaning piglets. Porcine AGP was used at concentrations of 0, 100, 1000 and 5000 ng/ml medium in 24 h incubations. AGP reduced the messenger RNA (mRNA) abundance of the lipogenic enzymes, malic enzyme (ME), fatty acid synthase and acetyl coA carboxylase by at least 40% (P<0.001). The activity of ME and citrate lyase were also reduced by AGP (P<0.05). Glucose oxidation was reduced by treatment with 5000 ng AGP/ml medium (P<0.05). The 14C-glucose incorporation into fatty acids was reduced by ~25% by AGP treatment for 24 h with 1000 ng AGP/ml medium (P<0.05). The decrease in glucose metabolism by AGP appears to function through an inhibition in insulin-mediated glucose oxidation and incorporation into fatty acids. This was supported by the analysis of the mRNA abundance for sterol regulatory element-binding protein (SREBP), carbohydrate regulatory element-binding protein (ChREBP) and insulin receptor substrate 1 (IRS1), which all demonstrated reductions of at least 23% in response to AGP treatment (P<0.05). These data demonstrate an overall suppression of lipogenesis due to AGP inhibition of lipogenic gene expression in vitro, which the metabolic data and SREBP, ChREBP and IRS1 gene expression analysis suggest is through an inhibition in insulin-mediated events. Second, these data suggest that AGP may contribute to limiting lipogenesis within adipose tissue during the perinatal period, as AGP levels are highest for any serum protein at birth.

1. Twenty newborn Ayrshire male calves were used in a comparison of milk substitute diets containing two different margarine fats. The liquid diets were offered in two feeds daily ad libitum and the calves were slaughtered at 118 kg live weight.

2. The two diets consisted of reconstituted spray-dried skim milk powder containing 200 g margarine fat/kg. One fat contained coconut and palm oils (CP) and the other coconut, palm and groundnut oils (CPG). The essential fatty acid content of margarine CP was less than half that of margarine CPG.

3. The incidence of diarrhoea and of a high rectal temperature, and the severity of lung lesions tended to be higher, the incidence of nasal discharge was higher and daily dry-matter intake and liveweight gain were lower for calves given margarine CPG.

4. Digestibility and balance trials were made at 1, 4 and 10 weeks of age. Nitrogen balance and apparent digestibility of the fat and other nutrients were not affected by the source of margarine, but the digestibility of dry matter and of fat was highest at 4 weeks of age, and of protein was lowest at 1 week of age. Apparent absorption of ash and calcium declined with age and absolute calcium retention was highest at 4 weeks. Total plasma lipids at 13 weeks of age did not differ between treatments.

5. Age at slaughter was greater for calves given margarine CPG but dressed-carcass weight, killing-out percentage, perirenal fat deposition and kidney weight did not differ between treatments. Skin weight tended to be lower for the calves given margarine CPG. 6. It is suggested that the apparent increased susceptibility to infection of calves given margarine CPG was associated with its higher unsaturated fatty acid content.

Accounting for environmental heteroscedasticity and genetics by environment interaction (G×E) in genetic evaluation is important because animals may not perform predictably across environments. The objectives of this study were to evaluate the presence and consequences of heteroscedasticity and G×E on genetic evaluation. The population considered was crossbred lambs sired by terminal sires and reared under commercial conditions in the UK. Data on 6325 lambs sired by Charollais, Suffolk and Texel rams were obtained. The experiment was conducted between 1999 and 2002 on three farms located in England, Scotland and Wales. There were 2322, 2137 and 1866 lambs in England, Scotland and Wales, respectively. A total of 89 sires were mated to 1984 ewes of two types (Welsh and Scottish Mules). Most rams were used for two breeding seasons with some rotated among farms to create genetic links. Lambs were reared on pasture and had their parentage, birth, 5 week, 10 week, and slaughter weights recorded. Lambs were slaughtered at a constant fatness, at which they were ultrasonically scanned for fat and muscle depth. Heteroscedasticity was evaluated in two ways. First, data were separated into three subsets by farm. Within-farm variance component estimates were then compared with those derived from the complete data (Model 1). Second, the combined data were fitted, but with a heterogeneous (by farm) environmental variance structure (Model 2). To investigate G×E, a model with a random farm by sire (F×S) interaction was used (Model 3). The ratio of the F×S variance to total variance was a measure of the level of G×E in the population. Heterogeneity in environmental variability across farm was identified for all traits (P<0.01). Rank correlations of sire estimated breeding value between farms differed for Model 1 for all traits. However, sires ranked similarly (rank correlation of 0.99) for weight traits with Model 2, but less so for ultrasonic measures. Including the F×S interaction (Model 3) improved model fit for all traits. However, the F×S term explained a small proportion of variation in weights (<2%) although more in ultrasonic traits (at least 10%). In conclusion, heteroscedasticity and G×E were not large for these data, and can be ignored in genetic evaluation of weight but, perhaps, not ultrasonic traits. Still, before incorporating heteroscedasticity and G×E into routine evaluations of even ultrasonic traits, their consequences on selection response in the breeding goal should be evaluated.

In the Molise region (Italy), some autochthonous populations are still bred and, between them, some wild horses named ‘Pentro horses.’ The breeding area is a natural pasture. It is 2200 ha extended including a broad plane surrounded by wooden hills. The aim of this research was to determine the nutritional characteristics of this area over a 2-year period to improve the management of the herd and to define the stocking rate in relation to the forage production in terms of production and quality. The forage samples were collected over two successive years during the grazing period (May to October) from five experimental areas and analysed for dry matter (DM), organic matter (OM), crude protein (CP), crude fibre (CF), neutral-detergent fibre (NDF), acid-detergent fibre (ADF), acid-detergent lignin (ADL) and gross energy (GE). Horse feed units (HFU) and horse-digestible crude protein (HDCP) were also predicted. Data were analysed with a one-way ANOVA test using month and area as factors. The DM, HFU and HDCP total production was determined to be compared with the total nutrient requirements of the herds from May to October. The results show that seasonal and yearly climatic variations significantly affect chemical composition and nutritive value of the pasture. The parameters most influenced were DM, CP, ADF and to a less extent NDF, while OM, ADL and GE show smaller differences during the observed period. The results show a low production per ha; nevertheless, because of the low stocking rate (0.3 to 0.6 head per ha), nutrient production meets the nutrient requirements of the horses regarding DM and energy. The differences among the areas have to be ascribed to the different botanical compositions and to the different draining capacity of the soil, and also in this case the greatest variations are for DM, CP and ADF.

Hierarchical (each dam mated to one sire) and factorial (each dam mated to several sires) designs involving juvenile selection in a closed nucleus breeding programme were compared for rates of genetic response and inbreeding using stochastic simulation. Numbers of sires and dams selected and herd sizes varied. Generations were discrete. Sires and dams were selected at 15 months on the basis of an estimated breeding value (EBV) calculated using selection index which considered information on close relatives. Selection was for total merit index. Matings were carried out assuming use of multiple emulation and embryo transfer (MOET) or in vitro embryo production (1VEP) technologies, over a range of reproductive parameters. Reproductive rates assumed a fixed number of potential offspring per collection with IVEP and MOET. Comparison of mating designs using MOET showed that rates of genetic response and inbreeding with factorial designs voere from 88 to 431% and 36 to 111% of those with the hierarchical design. Generally rates of genetic response were increased and inbreeding decreased by increasing the number of mates per dam due to a reduction of correlations among EBV. Reduced rates of genetic response were occasionally observed in factorial designs involving the mating of dams to large numbers of sires. This increased the generation interval due to the time to carry out the matings. Factorial designs were found to be most efficient for increasing genetic response and or decreasing inbreeding, for breeding programmes involving the selection of equal numbers of sires and dam. For the breeding programmes considered, rates of genetic response with IVEP equalled the best of the MOET designs, achieved at highest MOET rates, at the lowest level of IVEP. But at the equivalent rate of genetic response, rate of inbreeding was increased by 8%. The increased rate of inbreeding was due to the shorter generation interval with IVEP, and the increased probability of selecting related individuals assuming all donors respond to IVEP.

The feeding value of moist barley (approximately 27% moisture) which had been treated with 1·3 % of a mixture of formic (70 %) and propionic (30 %) acids was compared with that of dried barley for growing pigs from 23 to 85 kg live weight. Two experiments were undertaken; 96 pigs were fed ad libitum on one of three diets containing dried or acid-treated moist barley and another 48 pigs were individually fed on one of four diets containing dried or acid-treated moist barley in controlled amounts. These amounts were governed by an allowance of dry matter at a certain live weight.

In neither experiment were significant differences found between the treatments in growth rate or efficiency of utilization of food dry matter. Pigs fed ad libitum showed no significant difference in intake of dry matter, but those given the diet containing dried barley had a higher content of fat in their carcass than those on the acid-treated barley. A total of 21 pigs, 16 fed ad libitum and all receiving diets containing acid-treated barley had a discolouration of fat in the carcass.

1. Paired lactation records of milk yield and fat percentage from experimental twins and commercial twins, full-sisters and half-sisters are used in a study of the influence of contemporaneity, genetic variation and herd differences.

2. The design of the investigation was such that members of a pair were alike in:

(i) both time of calving and lactation number,

(ii) time of calving only,

(iii) lactation number only, or

(iv) neither time of calving nor lactation number.

3. Intra-class correlations between pair members in respect both of yield and fat percentage were highest for contemporary records. Differences in time of calving and in lactation number produced, in aggregate, a decline in correlation of about 20%. However, the separate estimates varied considerably between the different types of pair.

4. The within-pair and between-pair variance components independently confirm these effects of contemporaneity. As the degree of contemporaneity of pair members declines the within-pair variance increases in size and the between-pair component decreases.