Plasma very low density lipoproteins (VLDL) of gilts were separated into two sub fractions according to their affinity for heparin. The proportion of VLDL present as subfraction 2 (higher affinity for heparin) varied, according to the genetic line of the pigs, between 0·21 and 0·78 in virgin gilts. The proportions were related to the variation in piglet survival in the same nine genetic lines by a quadratic equation, which predicted that greater than 90% survival to weaning was to be found in piglets born to gilts having between about 0·3 and 0·7 of their VLDL as subfraction 2. This observation suggests a simple measurement that could be used in the selection of sows for a breeding programme. The proportion of subfraction 2 fell throughout pregnancy in each of three genetic lines measured and only returned to pre-pregnant values after weaning: these changes did not correlate with the changes in the lipid composition of plasma VLDL measured during pregnancy and lactation. The findings suggest a role for the VLDL subfractions in controlling the nutrition of the neonatal pig.

Sixty entire male pigs between 7 and 28 days and 60 entire male pigs between 28 and 56 days of age were given ad libitum basal diets using peanut meal and wheat gluten with or without dried milk as the protein supplements. The diets contained between 7·0 and 10·7 g lysine per kg. These diets were supplemented with graded additions of synthetic L-lysine hydrochloride.

It was found that the lysine required for maximum performance and nitrogen retention was at least 12·7 g/kg dry matter (DM) between 7 and 28 days of age and not greater than 12 g/kg DM between 28 and 56 days of age. These values are 4·7 and 5·3 g/kg dietary crude protein for the two ages or 0·81 and 0·79 g/MJ digestible energy.

A method is discussed for the calculation of the most economical level of lysine supplementation. This is dependent on the cost of the basal diet and lysine, and the value of the weight gain.

The concentration of free lysine in the blood plasma did not give a clear indication of lysine requirement of the pigs.

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.

Fleece weight and shedding score, a measure of premature fleece loss, were examined at first shearing and in older female sheep of three hilt breeds and their reciprocal crosses. Starting from a non-inbred base, the sheep were mated, mostly by younger-parent × offspring, for four generations, to produce inbreeding coefficients from 0·25 to 0·59. Crosses of inbred lines were also produced within breed type. The experimental design allowed the effects of inbreeding of the individual to be separated from the effects of maternal inbreeding.

Inbreeding of the individual significantly and linearly reduced fleece weight. This effect was still apparent after adjustment for body weight. Maternal inbreeding significantly reduced only the weight of first fleeces, but the trends were similar at the later ages, especially among the purebreds. There was no significant interaction of purebred/crossbred status with level of inbreeding. Inbreeding did not significantly affect shedding score. The pure breeds and the crosses did not differ, on average, in fleece weights or shedding scores, but within the purebred and within the crossbred classes breed variation was significant. Fleece weight declined and the incidence of shedding increased with increasing age. The larger the number of lambs born in the year of shearing the lower the fleece weight and the greater the extent of fleece shedding.

The effects of inbreeding could not be fully explained, statistically, in terms of dominance alone. Therefore, it seems probable that epistasis also plays a role in producing the observed changes with inbreeding in these traits.

In a 2 × 2 × 2 × 2 factorial experiment, the effects on reproductive performance of active immunization against androstenedione and of two sward heights in two periods, August to October (period 1) and October to November (period 2), were investigated in 144 Beulah Speckled-face and 146 Brecknock Cheviot ewes. Sward heights of either 5 to 6 cm (2200 kg dry matter (DM) per ha) (high) or 2 to 3 cm (650 kg DM per ha) (low) were achieved by adjustment of stocking rate during period 1. Two groups of ewes were balanced for body condition score across breed and sward height and the ewes of one group were actively immunized against androstenedione by injection with Fecundin® at 8 and 5 weeks before mating at the second synchronized oestrus after progestagen pessary withdrawal. Ewes were reallocated to the high (at 15 ewes per ha) and low (at 20 ewes per ha) swards at the start of period 2 by breed and immunization treatment, balanced for live weight, body condition score and sward height in period 1. After mating in mid November, all ewes were run at 19 ewes per ha on the residual pasture of the high sward until slaughtered either at return to service or at 4 weeks after first mating when corpora lutea and embryos were counted.

There were no effects or interactions due to sward height in period 1 on the reproductive responses to sward height in period 2. Ovulation and potential lambing rates were greater in the Beulah breed than in the Cheviot, and on the high sward than on the low in period 2. Ovulation rates were higher in immunized ewes than in control ewes but the effects of immunization and sward height on potential lambing rate were not independent. Potential lambing rates were higher in immunized ewes than in control ewes at the low sward height but not at the high sward height.

Adjustment of stocking rate during period 1 served to increase the range in ewe body condition at the start of period 2 in mid October. Reproductive response to body condition at this time differed between the breeds, the relationship being strongly positive in the more prolific Beulah control ewes but less so in the less prolific Cheviot control ewes. The response to immunization in both breeds was relatively greater at lower levels of body condition, thereby reducing the effect of body condition in immunized ewes.

The use of Fecundin® is therefore unlikely to produce much advantage when pre-mating nutrition or body condition are high but there may be some advantage in using it when nutritional resources are limited in the autumn, particularly in ewes in poorer body condition.

The effects of the proportion of hay in the diet (18 or 40%), implantation of diethylstilboestrol and the type of housing on the performance of male calves was examined over the growth period from 250 to 500 kg live weight in a 2×2×2 factorial experiment. The dry-matter consumption per 100 kg of live weight was lower and the live-weight gain and efficiency of conversion of crude metabolizable energy was higher with the diet containing 18% of hay than with that containing 40%. Diethylstilboestrol implantation had only a small effect on performance. Animals that were allowed yard space in addition to pens gained less rapidly, were less efficient in the conversion of energy into live weight, and were somewhat leaner, than those restricted to pens.

The present study was carried out on 1243 kids (379 Black Bengal; 568 Jamunapari × Black Bengal and 296 Beetal × Black Bengal) born between 1981 and 1986. Data were analysed to study the influence of genetic group, sex, season of birth, type of birth, birth weight, dam's weight at kidding, filial generation and parity of dam (as covariate) on survivability of kids during the pre-weaning period (0 to 90 days of age). The overall survival rates during 0 to 15, 16 to 30, 31 to 60, 61 to 90 and 0 to 90 days of age were 81·68 (s.e. 2·89), 95·55 (s.e. 2·00), 90·77 (s.e. 2·33), 93·31 (s.e. 2·59) and 69·94 (s.e. 3·65) % respectively. Variation in survival rates from 0 to 15 days of age was significant due to genetic group (P < 0·01), birth weight (P < 0·01), dam's weight at kidding and parity of dams as a covariate (P < 0·05). Season of birth (P < 0·05), weight of kids (P < 0·01) and dam's weight at kidding (P < 0·01) had significant effects on survival rate during 0 to 90 days of age. Sex and type of birth had no significant effect on survival rates. The interaction effects of genetic groups with season of birth and sex on pre-weaning survival rates were not significant in all ages except the effect between genetic group and sex during 31 to 60 days of age. An improvement in survival rates during the period of study was noticed with the increase in filial generation number from Fj to F3, although the difference was not significant except from 61 to 90 days of age (P < 0·05). Birth weight of kids had a positive linear relationship with survivability during the pre-weaning period. Summer-born kids had a significantly lower survival rate than those born in the winter and monsoon periods from 16 to 30, 61 to 90 and from 0 to 90 days of age. A negative linear relationship was observed between dam's weight at kidding and survival rates of kids from 0 to 90 days of age except the 25 to 30 kg dams deviate. Heritability estimates of survival rates during pre-weaning periods were not significantly different from zero, indicating that improvement by simple selection is probably limited.

Body composition in the pig, and its variation, is mostly referred to in terms of body protein and lipid content of the whole body. This study was made to check for animal-intrinsic variation in the partitioning of body protein into protein pools and of body lipid into lipid depots. Results from serial slaughter trials on 316 Danish Landrace and 76 Danish Yorkshire pigs were used to estimate additive genetic and litter-associated variance components for several traits. These traits were total body protein and lipid mass (TOTPROT and TOTLIPD), the proportions of total body protein that are present in the muscles (PROTMUS) or in the (sub-)cutaneous tissue plus bones (connective tissue protein, PROTCON), and the proportions of total body lipid that are present in the (sub-)cutaneous tissue (LIPDSUB), in the muscles (inter- and intramuscular fat, LIPDMUS), or in the bones (LIPDBON). TOTPROT and TOTLIPD were adjusted by regression for body weight; PROTMUS and PROTCON were adjusted for PROTCON, and LIPDSUB, LIPDMUS and LIPDBON were adjusted for TOTLIPD. The pooled estimates (± s.e.) of the degree of genetic determination (the sum of the additive genetic and litter-associated variance components, which approximates the repeatability) of these traits were 0·48 ± 0·19 for TOTPROT, 0·56 ± 0·20 for TOTLIPD, 0·56 ± 0·12 for PROTMUS, 0·57 ± 0·15 for PROTCON, 0·32 ± 0·10 for LIPDMUS, 0·33 ± 0·12 for LIPDSUB, and 0·22 ± 0·10 for LIPDBON. It is concluded that there is animal-intrinsic variation in partitioning of body protein and lipid.

Thirty-six wether lambs were given food hourly consisting of mature chopped oaten hay, sprayed with 10 g urea per kg hay. A pelleted supplement was given containing 0 or 75 g/day fish meal and 0 or ca. 1·5 MJ/day gross energy as acetate or propionate in a 2 × 3 factorial design. Dry matter (DM) digestibility, nitrogen retention and urinary allantoin excretion were determined over 7 days. Tissue protein kinetics were estimated using 15N-glycine. Net fluxes of acetate, bicarbonate and glucose were estimated with continuous infusion of 14C-acetate, 14C-bicarbonate and 3H-glucose respectively.

Intake of oaten hay (g DM per kg live weight per day) was reduced by supplementation. Live-weight gain was lower in lambs supplemented with propionate than in lambs given the basal diet only. Supplementation with fish meal increased DM digestibility, live-weight gain and wool growth. Daily excretion of urinary allantoin did not differ between groups of lambs. Nitrogen retention and protein flux were reduced in lambs supplemented with acetate or propionate and were increased in lambs given fish meal. Plasma acetate concentrations were higher for lambs supplemented with acetate. However, net flux of acetate did not differ between groups of lambs. The net flux of glucose was increased by supplementation with propionate and with fish meal. Plasma levels of β-hydroxybutyrate were reduced in lambs supplemented with propionate.

Availability of propionate and glucose were not limiting live-weight gain in these lambs. Also, a high ratio of acetate to propionate did not reduce the apparent efficiency of nutrient use.

Zeolite, cellulose acetate phthalate (CAP) and maize starch were used as the coating materials for encapsulating pancreatic protease, which was proposed as an additive to enhance the food efficiency and growth rate of weanling pigs. In this study, microcapsules were formed by the process of coacervation in liquid paraffin media, where the deposition time was in the range of 20 to 60 min, and the concentrations of CAP, zeolite and maize starch were 20, 40 and 120 g/kg respectively. Hardening by n-hexane was achieved with 15 min of stirring. Microcapsules generated by this method were 20 to 30 mesh in diameter. They were stable for at least 3 h in simulated gastric conditions (pH 3·97), but disintegrated rapidly under simulated intestinal conditions (pH 6·82).

Responses of selection for milk production and secondary traits were predicted in open nucleus schemes using a deterministic model. Secondary traits considered were: traits recorded during lactation (e.g. mastitis resistance; calving ease); traits recorded in the nucleus only (e.g. food intake); traits recorded early in life (e.g. growth rate); and traits recorded late in life (e.g. longevity). Also, genotype × environment interactions between nucleus and commercial herds and predictors of merit in juveniles were considered.

Extension of the breeding goal to include an uncorrelated secondary trait, which was recorded at each lactation, had the same heritability as milk production (assumed throughout to be 0·25) and half its economic value, increased total economic gain by a factor of 0·12. This increase was only 0·04, if the heritability of the secondary trait was 0·1. The situation for traits of low heritability was not improved by progeny testing of young bulls due to the short optimized generation intervals. Gain increased only by a factor of 0·04, if the economic value was 0·25.

Including a secondary trait of heritability 0·25 and a genetic correlation with yield of 0·5 in the index, only increased economic response rates by a factor of 0·04. However, when the genetic correlation was –0·5 the benefits were greater with increases of 0·09, 0·10 and 0·22 for heritabilities of 0·05, 0·10 and 0·25, respectively. Hence, including traits with low heritability but with strong negative correlations with yield, which might apply to fertility and disease resistance, increased rates of gain moderately.

If an uncorrelated secondary trait was recorded in the nucleus only, e.g. food intake, and had half the economic value of milk production, total gains increased by a factor of 0·10. Hence, recording of secondary traits can be restricted to the nucleus with only minor loss of gain. The extra economic benefit was greatest from secondary traits measured early in life compared with late in life, e.g. longevity, with benefits increased by factors of 0·24 and 0·06, respectively.

Open nucleus schemes are robust in the presence of genotype × environment interactions between nucleus and commercial herds, if the breeding value estimation method accounts for these interactions, juvenile indicator traits of milk production may increase rates of gain by a factor of 0·11, if the heritability of the indicator trait is 0·25 and the correlation with milk production is 0·5.

The effects of milking on the overall productivity of N'Dama cattle kept under village conditions were studied using three groups of cows during a 27-month period. One group of 46 cows in 16 herds in Nioro Jattaba village was not milked for human consumption and their calves had access to all of their milk (group 1). Another group of 46 cows in the same herds was partially milked for human use once a day while the calves had access to the residual milk (group 2), the normal husbandry practice in this village. A third group of 70 cows in six herds, managed similarly in a similar production environment in the nearby village of Keneba, was milked twice daily with the residual milk for their calves, the normal husbandry in this village. A mean extraction of 296 (s.d. 110·7) kg milk per cow occurred in group 2 during a lactation period of 517 days while the figure for group 3 was 450 (s.d. 177·7) kg per cow over 430 days. At the end of the observation period, calf mortality to 12 months averaged 6% and was similar in all groups. Calf weights at 12 months in group 1 (98·1 kg) exceeded that of groups 2 and 3 by proportionately 0·26 and 0·38 (P < 0·001) respectively. Mean calving rate was 53% in groups 1 and 2, and 72% in group 3. Post-partum body-weight losses were most severe in group 1, followed in order by groups 2 and 3. A herd productivity index (incorporating calf weight, live weight equivalent of milk extracted for human use, cow and calf viability and calving rate) was 100·7 kg calf weight per 100 kg metabolic weight of cow per year for group 1, 110·2 for group 2 and 166·1 kg for group 3. These values suggest that recommendations to small-holder farmers for enhancing livestock productivity should include strategies to improve milk yields in general rather than advocating no milk extraction or changing the milking regime currently practised.

Correlation between circulating thyroid hormones and live-weight gain during the first year of life were observed in four trials on cattle of the two main Belgian breeds: Belgian Blue and Friesian. Thyroid hormones assayed and considered as potential predictors of growth rate were: thyroxine (T4), triiodothyronine (T3) and free thyroxine (FT4). Triiodothyronine uptake (T,U), thyroid stimulating hormone (TSH) and free thyroxine index (FTI) were also estimated.

No significant correlation was found between live-weight gain and T4 or T3 in 13 Belgian Blue heifers (trial 1). In 49 bulls submitted to a performance test, T4 and FTI increased from 3 to 12 months whilst T3U decreased (trial 2). Significant correlations between live-weight gain and T4 and between live-weight gain and FTI were obtained from samples taken between 66 and 95 days. Live-weight gain was correlated with T3U when animals were between 96 and 125 days, and 126 to 155 days of age. It is possible that the interaction between total T4 and T3U is important in controlling growth. In 13 young calves, beef merit, expressed as live-weight gain in 1 year, was highly correlated with T4 concentration at 8 to 10 days of age (trial 3). T3 and FT4 did not seem to be directly associated with the growth processes. In trial 4, live-weight gains of 42 young animals housed on a commercial farm were negatively correlated with T4 and with FTI. The conflicting results encountered may perhaps be explained by the lack of information on the use of anabolic preparations in field conditions. T4 concentration and FTI may be considered as potential parameters for the estimation of beef merit but the results are insufficient to draw definite conclusions.

Dried grass pellets of high density (1·3 g/ml) were fed ad libitum to British Friesian male calves 5 to 7 mo old (Experiment 1) or 2 to 5 mo old (Experiment 2). All eight animals in Experiment 1 and eight out of 12 in Experiment 2 were fitted with rumen fistulae. In some subperiods of both experiments, pellets were broken down in water and administered as a slurry directly into the rumen at a level equivalent to approximately one-third that of voluntary intake. Intake per os was significantly reduced by this treatment but total intake (per os+per rumen) was not affected.

In Experiment 3 pellets of high (1·24 g/ml) or medium (1·14 g/ml) density were fed ad libitum to 40 9-mo-old Suffolk × Scottish Halfbred wether sheep, either alone or with access limited to 3 hr a day, in which case chopped hay was always available. Pellet density did not affect total intake, but when offered as supplements to hay the consumption of medium-density pellets was higher than that of high-density pellets. In all three experiments animals required 2 or 3 weeks to reach maximum intake of pellets. Over the 42 days of Experiment 3 the proportion of pellets in the feed consumed by animals given pellets as supplements to hay increased from 60 to 80%.

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.

One hundred and five Holstein cattle, with a mean body weight of 668 kg, were superovulated using a total dose of 18 mg FSH. Five animals did not have corpora lutea (CL). No ova or embryos were recovered from 12 of the 100 animals which had CL. A total of 810 ova/embryos were collected from the remaining 88 animals and 432 of these were of transferable quality. The mean (range) measurements for subcutaneous fat deposition in samples taken at slaughter from the back, rump and ventral abdominal wall were 14 mm (3 to 34), 7 mm (0·7 to 33) and 7 mm (0·8 to 32), respectively. The number of transferable embryos and fertilized ova decreased as fat levels increased, but the differences between the groups were not significant (P > 0·05). There was a positive relationship between the number of CL and (a) the number of fertilized ova, and (b) the number of transferable embryos (r = 0·53, P < 0·001 and r = 0·48, P < 0·001). The correlation between the number of fertilized embryos and the number of transferable embryos was r = 0·91, P < 0·001.

Field data from four breeds (Charolais, Hereford, South Devon and Welsh Black) comprising a total of 145, 68, 262 and 197 records respectively and obtained from 30, 24, 38 and 25 herds were used. Station data from two breeds (Charolais and Hereford) comprising 268 and 687 records respectively, from cattle originating from 27 and 44 herds respectively were also used. The data were analysed to identify environmental factors which significantly affected bull performance traits. In performance traits, herd of origin, in field and station data, accounted on average for 0·50 and 0·18 respectively of the total variance. Year, season and dam's age accounted for 0·02, 0·03 and 0·02 respectively of the total variance, the last being higher in pre-weaning than in post-weaning traits.

This experiment examined the effects of sustained levels of undernutrition applied to grazing Brahman heifers on subsequent lifetime reproductive performance. Weaned heifers were raised to a target weight of 270 kg at three different stocking rates to obtain contrasting weight gains (0·097 kglday, L = low; 0·215 kg/day, M = medium; 0·259 kg/day, H = high). They were then subjected to common grazing in a low-quality Brachiaria humidicola pasture, where mean weight changes ranged between -0.100 and +0·300 kg/day depending upon season and physiological condition. Differences in age at first calving between L and the other treatments (200 days) were maintained in subsequent calvings. Calf weaning weights were higher in L and M than in H. The mean weight of calf weaned per cow per year was higher in M than in either L of H. Cow weight at conception increased with parity, and was higher in L than in the other treatments. Similar trends were observed for the dam's weight preand post calving, and at weaning. This trend was associated with higher weight loss during lactation in I..

We conclude that in tropical grazing systems that allow only low growth rates, differences established early in reproductive life may become perpetuated for the lifetime of the animal. Management implications in the context of extensive tropical grazing systems are discussed.

Although the conventional in situ ruminal degradability method is a relevant tool to describe the nutritional value of ruminant feeds, its need for rumen-fistulated animals may impose a restriction on its use when considering animal welfare issues and cost. The aim of the present work was to develop a ruminal degradability technique which avoids using surgically prepared animals. The concept was to orally dose a series of porous bags containing the test feeds at different times before slaughter, when the bags would be removed from the rumen for degradation measurement. Bags, smaller than those used in the conventional nylon bag technique, were made from woven nylon fabric, following two shape designs (rectangular flat shape, tetrahedral shape) and were fitted with one of three types of device for preventing their regurgitation. These bags were used in two experiments with individually housed non-pregnant, non-lactating sheep, as host animals for the in situ ruminal incubation of forage substrates. The bags were closed at the top edge by machine stitching and wrapped in tissue paper before oral dosing. Standard times for ruminal incubation of substrates in all of the tests were 4, 8, 16, 24, 48, 72 and 96 h before slaughter. The purpose of the first experiment was to compare the effectiveness of the three anti-regurgitation device designs, constructed from nylon cable ties (‘Z-shaped’, ARD1; ‘double Z-shaped’, ARD2; ‘umbrella-shaped’, ARD3), and to observe whether viable degradation curves could be generated using grass hay as the substrate. In the second experiment, three other substrates (perennial ryegrass, red clover and barley straw) were compared using flat and tetrahedral bags fitted with type ARD1 anti-regurgitation devices. Non-linear mixed-effect regression models were used to fit asymptotic exponential curves of the percentage dry matter loss of the four substrates against time of incubation in the reticulorumen, and the effect of type of anti-regurgitation device and the shape of nylon bag. All three devices were highly successful at preventing regurgitation with 93% to 100% of dosed bags being recovered in the reticulorumen at slaughter. Ruminal degradation data obtained for tested forages were in accordance with those expected from the conventional degradability technique using fistulated animals, with no significant differences in the asymptotic values of degradation curves between bag shape or anti-regurgitation device. The results of this research demonstrate the potential for using a small bag technique with intact sheep to characterise the in situ ruminal degradability of roughages.

Energy expenditure (EE) can be estimated based on respiratory gas exchange measurements, traditionally done in respiration chambers by indirect calorimetry (IC). However, the 13C-bicarbonate technique (13C-BT) might be an alternative minimal invasive method for estimation of CO2 production and EE in the field. In this study, four Shetland ponies were used to explore the effect of feeding on CO2 production and EE measured simultaneously by IC and 13C-BT. The ponies were individually housed in respiration chambers and received either a single oral or intravenous (IV) bolus dose of 13C-labelled sodium bicarbonate (NaH13CO3). The ponies were fed haylage 3 h before (T−3), simultaneously with (T0) or 3 h after (T+3) administration of 13C-bicarbonate. The CO2 produced and O2 consumed by the ponies were measured for 6 h with both administration routes of 13C-bicarbonate at the three different feeding times. Feeding time affected the CO2 production (P<0.001) and O2 consumption (P<0.001), but not the respiratory quotient (RQ) measured by IC. The recovery factor (RF) of 13C in breath CO2 was affected by feeding time (P<0.01) and three different RF were used in the calculation of CO2 production measured by 13C-BT. An average RQ was used for the calculations of EE. There was no difference between IC and 13C-BT for estimation of CO2 production. An effect of feeding time (P<0.001) on the estimated EE was found, with higher EE when feed was offered (T0 and T+3) compared with when no feed was available (T−3) during measurements. In conclusion, this study showed that feeding time affects the RF and measurements of CO2 production and EE. This should be considered when the 13C-BT is used in the field. IV administration of 13C-bicarbonate is recommended in future studies with horses to avoid complex 13C enrichment-time curves with maxima and shoulders as observed in several experiments with oral administration of 13C-bicarbonate.