Current feed evaluation systems for ruminants include very simple models of intermediary metabolism – either single, static coefficients or simple equations to describe the efficiency with which the energy and nitrogen available for absorption are converted into milk and meat. For example, the revised UK system ‘Feed into milk’ defines a fixed efficiency of utilization of metabolizable protein (MP) for milk production of 0·68 (Thomas, 2004). Such a simple ‘efficiency’ coefficient is the tip of the tip of a rather large iceberg, representing the metabolism of the end-products of digestion between the point of their absorption into the animal and their use for the synthesis of milk and meat. The complex biochemistry of metabolism in the tissues of the gut (the portal-drained viscera, PDV), the liver and other internal organs, the adipose tissue, muscle and mammary gland: all reduced (a term used nonpejoratively) to single coefficients or simple equations. Simple and very useful – up to a point.

A Working Party established by the Agricultural and Food Research Council (AFRC) Technical Committee on Responses to Nutrients wrote, in 1998, ‘The present state of knowledge on the nutritional and hormonal interactions “in intermediary metabolism” is considerably inferior to the knowledge which exists with respect to the processes of ruminal and intestinal digestion, and thus to develop a satisfactory model of metabolism requires research not only in model construction but also in the physiology and biochemistry of animals.’ (AFRC, 1998). Perhaps contrary to the common perception of a contracting research base in farm animal science, the required research in physiology and biochemistry is continuing, research which is summarized and put in context by two review articles in this issue of Animal Science.

Kristensen (2005) focuses on a key aspect of the ‘energy economy’ of the cow: the metabolism of volatile fatty acids (VFA) in the tissues of the digestive tract and the liver. Using novel techniques, Kristensen and colleagues have re-examined (and overturned?) one of the key paradigms of ruminant physiology, that a large proportion of the VFA generated by fermentation in the rumen is metabolised during absorption.

Researchers from Canada and the UK are untangling factors affecting the partitioning of absorbed amino acids between use for protein synthesis and catabolism and this topic is reviewed by Lapierre et al. (2005). This paper emphasizes the important concept that ‘efficiency of utilization’ of absorbed amino acids is as much a function of demand (in the lactating cow, principally from the mammary gland) as it is of supply.

The statement from AFRC (1998), quoted above, demands two activities, physiological research and model construction. These authors continue: ‘It is likely, however, that the best progress will be achieved if both types of research are undertaken in parallel.’ In a third review article in this volume, Hanigan (2005) summarizes the ‘state of the art’ in the simulation of PDV and hepatic metabolism, demonstrating how a computer model is a quantitative library in which to store new knowledge as well as a driver of future research.

All three papers are based on presentations given to the 2004 annual meeting of the British Society of Animal Science. They demonstrate ‘the current state of the art’ for these important topics in ruminant physiology, offering a below the water-line look at these rather large and often forbidding icebergs. The reviews should serve as a valuable summary for those already working in this field of research, a concise and convenient introduction for those entering the field, and a window onto the future of practical systems for predicting the responses of dairy cows to the feedingstuffs they are offered.

Volatile fatty acids (VFA) are quantitatively important substrates for dairy cows and other ruminants. It has been a central dogma in the nutritional physiology of ruminants that the ruminal epithelium metabolizes a large fraction of VFA during theirabsorption and consequently a relatively small fraction of VFA is available for peripheral tissues including the mammary gland. New data on splanchnic metabolism of VFA indicate that the ruminal epithelium metabolizes none or small amounts of acetate and propionate absorbed from the rumen. However, the ruminal epithelium has a large fractional uptake of butyrate and valerate during their absorption from the rumen. The liver takes up proportionately 0·9 or more of the absorbed propionate, however multiple factors are involved in regulation of hepatic metabolism and propionate does not determine glucose availability to the cow per se. In light of the quantitative importance of VFA to the dairy cow it is important that future research attempts to bridge the gap between the biology of food degradation/digestion in the gastro-intestinal tract and availability of specific nutrients to the cow which impact intermediary metabolism and nutrient utilizationin productive tissues.

A database reviewing the metabolism of nitrogen (N) compounds from absorption to milk has been compiled from 14 published and unpublished studies (33 treatments) that measured the net flux of N compounds across the splanchnic tissues in dairy cows. Apparent N digestibility averaged 0·65, with this then partitioned between 0·34 excreted in urine and 0·31 secreted as milk.

Nitrogen metabolites are absorbed from the lumen of the gut into the portal vein, mainly as free amino acids (AA) and ammonia; these represented 0·58 and 0·57 of digested N, respectively. All of the ammonia absorbed was removed by the liver with, as a result, a net splanchnic flux of zero. Detoxification of ammonia by the liver and catabolism of AA results in production of urea as an end-product. Hepatic ureagenesis is a major cross-road in terms of whole body N exchange, being the equivalent of 0·81 of digested N. Therefore, salvage of a considerable part of this ureagenesis is needed to support milk protein synthesis. This salvage occurs via transfer of urea from the blood circulation into the lumen of the gut. On average, 0·47 of hepatic ureagenesis was returned to the gut via the portal-drained viscera (equivalent to 0·34 of digested N) with 0·56 of this then used for anabolic purposes e.g. as precursor N for microbial protein synthesis. On average, 0·65 of estimated digestible AA was recovered in the portal vein. This loss (0·35) is due to oxidation of certain AA across the gut wall and non-absorption of endogenous secretions. The magnitude of this loss is not uniform among AA and varies between less than 0·05 for histidine to more than 0·90 for some non-essential AA, such as glutamine.

A second database (six studies, 14 treatments) was constructed to further examine the subsequent fate of absorbed essential AA. When all AA are aggregated, the liver removed, on average, 0·45 of portal absorption but this value hides the considerable variation between individual AA. Simplistically, the AA behave as two major groups: one group undergoes very little hepatic removal and includes the branched-chain AA and lysine. For the second group, removal varies between 0·35 and 0·50 of portal absorption, and includes histidine, methionine and phenylalanine. For both groups, however, the efficiency of transfer of absorbed AA into milk protein decreases with increasing supply of protein. This loss of efficiency is linked directly with increased hepatic removal of AA from the second group and, probably, increased catabolism by peripheral tissues, including the mammary gland, of AA from the first group. Therefore, we must stop using fixed factors of conversion of digestible AA to milk in our predictive schemes and acknowledge that metabolism of AA between delivery from the duodenum and conversion to milk protein will vary with nutrient supply. New information evolving from re-analysis of the literature and recent studies will allow better models to be devised for the prediction of nutrient-based responses by the lactating cow. Consideration of biological efficiency, rather than maximal milk yield, will lead to systems that are economically more sensible for the farmer and that have better environmental impacts.

Rations for dairy cattle are currently balanced to meet needs for energy, protein, vitamins, and minerals. While individual vitamins and minerals are considered, energy and protein are generally treated in aggregate even though entities within those aggregates can affect milk yield and composition. Significant efforts have been undertaken to describe ruminal metabolism in detail, but descriptions of post-absorptive metabolism assume constant fractional conversions of energy and protein to milk. A quantitative understanding of nutrient metabolism by the post-absorptive tissues is required, and the splanchnic tissues are critical components of the post-absorptive system as they mediate absorption of nutrients and play a rôle in regulation of metabolite availability.

Glucogenic precursor supply can significantly affect endocrine status as well as splanchnic release of glucose, acetate, lactate, ketones, and the non-essential amino acids. Although the relative affinities of the splanchnic tissues for the essential amino acids (AA) are low as compared with the udder, net clearance on a daily basis represents approximately 2/3 of the net supply to the animal due largely to recycling of AA back to the tissue bed. This could be significantly reduced by stimulating removal and use by the udder as splanchnic affinities are much lower than mammary affinities. Additionally, the essential AA composition of absorbed protein is significantly modified by these tissues due to differing affinities for each of the AA. The extent of that modification is not a fixed constant but rather a function of several factors including milk yield. The accuracy of our current feeding systems could be improved if such variable rates of substrate removal replaced current static estimates.

A study seeking to estimate phenotypic and genetic parameters for sow body condition around lactation and their relationship with piglet growth and survival is described. It also investigates the genetic relationship between piglet growth and survival, including both direct and maternal effects for both traits. Records were available from 24 549 Swedish Yorkshire piglets born in 2198 litters. Sows had records of weight and backfat depth at farrowing and at weaning. Piglets had individual records for weight at birth and weaning and cause of death for those that failed to survive during lactation. Mixed linear bivariate models were used to estimate correlations between traits. The models for the sow traits took into account the random effects of permanent environment and the genetic effect of the sow, whereas the models for the piglet traits included a litter effect as well as direct and maternal genetic effects. Estimated heritabilities for sow weight and backfat at farrowing and change of weight and backfat during lactation were low to moderate (0·10 to 0·47). We found significant genetic correlations between change of weight and backfat during lactation and piglet survival and growth, indicating that sows with the genetic capacity for rapid early piglet growth and high survival rate may lose more body reserves during lactation. Negative direct-maternal correlations for early piglet growth and survival imply that both the piglet and the sow trait should be included in a genetic evaluation for these traits. Genetic correlations between piglet survival and growth were not clearly favourable. Selection for the direct effect of piglet survival may lead to a decrease in early growth rate. We conclude that in a selection programme aiming at improving piglet survival and growth, attention should be paid to the sow's body condition during lactation. A high enough level of body reserves needs to be maintained in the sow if the incidence of reproductive problems and involuntary culling is not to increase.

The objective of this work was to establish economic values (EVs) of mastitis and lameness in order to enhance the current UK dairy profit index (£PLI) by including these health traits. The EVs of traits currently in £PLI were also re-evaluated to account for changes in costs/returns over time and to determine their sensitivity to changes in some of the basic assumptions used in their derivation.

Predicted transmitting abilities (PTAs) for mastitis are not available in the UK. Instead, PTAs for somatic cell count (SCC), which has a strong genetic correlation with clinical mastitis, were used to predict clinical mastitis. Similarly, PTAs for locomotion and (for bulls with no locomotion PTA) the ‘legs and feet’ composite were used to predict lameness.

The EV of mastitis was estimated at £0·83 per percent incidence, giving an index weight for SCC PTA of £0·20. The EV of lameness was estimated at £0·99 per percent incidence, giving an index weight for locomotion PTA of £1·28. The associated index weight for the ‘legs and feet’ composite was estimated to be £1·50. Economic values for all traits (production, lifespan, mastitis and lameness) were found to be sensitive to their associated price assumption but not to price assumptions of other traits in the index or to other production parameters in the model.

Better information is needed on the influence of cow age (parity) on incidence of disease and on the probability of involuntary culling to determine the appropriate balance between the EVs for longevity and health. Currently, 16% of the weight in £PLI is attributable to non-production traits. In our revised index this weight increased to 23%. Even so, selection using this index is still predicted to result in an increase in mastitis and lameness, albeit at a very low rate. This situation may be changed by the introduction of fertility into £PLI and through better information about health traits. Incorporation of consumer preference into £PLI may require traits associated with health and welfare of the cow to receive more weight than their EV would suggest in order to maintain or improve health traits in national selection programmes.

The effect of reducing the frequency of official milk recording and the number of recorded samples per test-day on the accuracy of predicting daily yield and cumulative 305-day yield was investigated. A control data set consisting of 58 210 primiparous cows with milk test-day records every 4 weeks was used to investigate the influence of reduced milk recording frequencies. The accuracy of prediction of daily yield with one milk sample per test-day was investigated using 41 874 testday records from 683 cows. Results show that five or more test-day records taken at 8-weekly intervals (A8) predicted 305-day yield with a high level of accuracy. Correlations between 305-day yield predicted from 4-weekly recording intervals (A4) and from 8-weekly intervals were 0.99, 0.98 and 0.98 for milk, fat and protein, respectively. The mean error in estimating 305-day yield from the A8 scheme was 6.8 kg (s.d. 191 kg) for milk yield, 0.3 kg (s.d. 10 kg) for fat yield, and −0.3 kg (s.d. 7 kg) for protein yield, compared with the A4 scheme. Milk yield and composition taken during either morning (AM) or evening (PM) milking predicted 24-h yield with a high degree of accuracy. Alternating between AM and PM sampling every 4 weeks predicted 305-day yield with a higher degree of accuracy than either all AM or all PM sampling. Alternate AM-PM recording every 4 weeks and AM + PM recording every 8 weeks produced very similar accuracies in predicting 305-day yield compared with the official AM + PM recording every 4 weeks.

Eighty-seven Manchego sucking lambs raised exclusively on maternal milk were slaughtered. The carcasses were weighed immediately after dressing and classified according to hot carcass weight (HCW): low HCW (<5.5 kg), medium HCW (5.5 to 6.5 kg) and high HCW (≥ 6.5 kg). The effects of this classification system on carcass and meat quality were studied. Carcass conformation improved as carcass weight increased; carcasses became more compact. Carcass fatness increased (P < 0.001) as carcasses became heavier. With regard to the joints, leg (P < 0.001) and shoulder (P < 0.05) proportions decreased as carcass weight increased, whereas loin-rib (P < 0.001) and flank (P < 0.001) proportions increased. The proportion of fat in the leg clearly increased in the heavier carcasses (P < 0.001), while that of bone decreased (P < 0.001) and that of muscle was not affected. pH at 24 h in the m. longissimus dorsi and m. semitendinosus decreased as carcass weight increased. The only meat colour parameter affected by carcass weight was the yellowness index; lighter-weight carcasses displayed lower index (P < 0.05). Cooking losses were unaffected as carcass weight increased, whereas with regard to tenderness, the shear force value tended to be higher in the low carcass weight group. No differences between carcass weight groups were found in the sensory analysis. Fatty acid composition did not show any significant effect due to carcass weight except in stearic fatty acid (C18:0); C18:0 proportion decreased (P < 0.01) as carcasses became heavier. Carcass weight affected carcass quality (all carcass quality parameters studied were higher in the heavier carcasses), whereas meat quality varied little as a result the small variation between carcass weights in sucking lambs.

The magnitude of the discrepancy between conventional regression estimates of protein retention efficiency and theoretical estimates of synthesis efficiency indicates a major contribution ascribable to protein turn-over in the generally accepted estimates. As protein turn-over is known to be influenced by diet, feeding level and degree of maturity, this suggests the development of an estimator of protein efficiency that can be adapted for such differences. Therefore, based on generally accepted formulas for growth description, a method of estimating protein retention efficiency was developed which is flexible enough to accommodate different diets, feeding levels and degrees of maturity. Moreover, a formula was derived to convert one type of estimate to the other by regarding constant efficiency as equivalent to variable efficiency at the mid point of the estimation interval. Increase in scientific depth to this descriptive approach is provided by a theoretical consideration of a possible mechanism of hormonal control of protein synthesis and breakdown, ultimately expressed as proportionalities to powers of whole body protein (P). Molecular considerations on cellular synthesis and breakdown indicate a difference between breakdown and synthesis powers equal to (2/9)Q. The factor (2/9) is indicated by an argument based on insulinlike growth factor derived activator diffusion attributes by nucleus and body tissue geometries, while Q is equal to the proportion of nuclei activated by insulin-like growth factor. This proportion is likely to be a function of the concentration of growth factor in the blood. Hence, a linear relationship between intake and blood insulin-like growth factor concentration suggests that Q can be represented by a scaled transformation of intake, 0 ≤ Q ≤ 1, such that a value of Q = 1 represents ad libitum intake on a suitable diet and Q = 0 intake at the maintenance requirement. The quantification of breakdown and synthesis power differences by (2/9)Q leads to kP = {1 + [1 − (P/α)(2/9)Q]−1/6}−1, for turn-over related protein retention efficiency (kP), with α the limit value of P at maturity, so that 0 ≤ (P/α) ≤ 1. Experimental estimates, derived from direct estimates of whole body protein synthesis and breakdown at predetermined levels of intake, are in excellent agreement with the theoretical (2/9)Q in the power associated with (P/α) in kP. Furthermore, conventional multiple regression retention efficiencies satisfactorily approximate the turn-over related retention efficiency that can be calculated at a given level of intake for the mid point of the interval covered by the regression estimates.

During winter in southern Queensland, eight rusa deer stags aged 4 years were given ad libitum lucerne (Medicago sativa) hay and confined in individual metabolism pens for 26 days. Stags ate 2·04 kg dry matter (DM) per day and drank 6·4 kg water per day, while the drinking water: food DM ratio was 3·3 l/kg. In experiment 2, seven rusa stags were given ad libitum lucerne hay or oaten (Avena spp.) hay with or without barley grain supplementation (200 g/day) for 56 days (four periods). This experiment was conducted from 26 July to 19 September 2001, when the stags were exhibiting the behaviour characteristic of the rut. Rusa stags ate 1·19 and 1·17 kg DM per day of lucerne and oaten hay respectively. Rusa stags given oaten hay drank slightly more water than those that received lucerne hay (5·34 and 4·47 kg/day, respectively). The drinking water: food DM ratios were 3·81 and 4·67 kg/kg for lucerne and oaten hay, respectively. Barley grain supplementation (200 g/day) had no influence on total food or water intakes of the rusa stags.

The nutritional cost of both the acquisition and maintenance of immunity to gastro-intestinal nematodes was investigated using immunologically naïve 5-month-old lambs and immunologically competent 17-month-old ewes. Within each age cohort, animals were either infected with the equivalent of 80 L3 Trichostrongylus colubriformis larvae per kg live weight (LW) per day (IF), similarly infected and concurrently immuno-suppressed with weekly injections of 1·3 mg/kg LW of the glucocorticoid methylprednisolone acetate (ISIF), immuno-suppressed only (IS) or remained as controls (C). Body composition of all animals was estimated using X-ray computer tomography on days -14 and 76 relative to the start of infection. Body weight and faecal nematode egg counts (FEC; eggs per gram of fresh faeces (e.p.g.)) were taken weekly and blood samples for serum proteins and antibodies were obtained every 2 weeks. FEC in IF lambs peaked at 1250 e.p.g. before a typical decline as immunity developed to less than 100 e.p.g. by day 75. FEC of less than 100 e.p.g. in IF ewes indicated immunity was maintained. Successful immuno-suppression in ISIF lambs and ewes was indicated by FEC of 4000 e.p.g. on day 75 and was confirmed by comparative worm burdens and serum antibody titres. The typical reduction in voluntary food intake (VFI) as a consequence of infection was observed in IF lambs (proportionately 0·30, P < 0·001) but not in IF ewes, ISIF lambs or ISIF ewes. Gross efficiency of use of metabolizable energy for net energy deposition was reduced by proportionately 0·20 in lambs during acquisition of immunity and by 0·16 in ewes maintaining an established immunity. Infection in immuno-suppressed animals reduced efficiency by 0·05 and 0·15 for lambs and ewes, respectively. These findings allowed the hypothesis that the reduction in VFI and loss in performance in young parasitised sheep is caused by physiological signalling associated with the acquisition phase of the host immune response to infection, rather than simply the damage caused by the parasite per se.

An experiment was carried out to investigate the effects of increasing the digestible undegradable protein (DUP) concentration of concentrates on colostrum production and lamb output from prolific ewe breed types. Sixty triplet-bearing ewes with a mean condition score of 3.75 were allocated to one of five treatments (no. =; 12) for the final 6 weeks of pregnancy. Four of the treatments were offered a restricted grass diet (0.6 kg dry matter (DM) per day) plus one of four isoenergetic and isonitrogenous concentrates (0.56 kg DM per day) that were formulated to provide 25 (P25); 40 (P40); 75 (P75) and 95 (P95) g DUP/kg DM. Control ewes were offered a grass-only diet with the aim of achieving an equivalent total DM intake (1.16 kg DM per day). Concentrate composition had no effect on total diet DM, crude protein or metabolizable energy intakes, but the intake of DUP increased with increasing concentrate DUP level (P < 0.001). Supplementing grass with concentrates led to significant reductions in plasma β-hydroxybutyrate (P < 0.05) and non-esterified fatty acids(P < 0.001) concentrations. Plasma urea concentration tended to be negatively and linearly related to the concentrate DUP level (P < 0.1). Colostrum production and composition were not affected by concentrate type, except for the concentration of gross energy which was highest for P40 ewes. Concentrate feeding led to significant (P < 0.05) increases in colostrum yield 18 h post lambing and colostral outputs of ash, crude protein, total solids and gross energy but resulted in a lower (P < 0.05) colostral immunoglobulin G concentration. Lamb birth weight and total litter weight were not affected by late pregnancy feeding regime. However lamb survival to weaning was reduced significantly (P < 0.05) for P25 and control ewes, resulting in lower levels of lamb output at weaning. The results from this study indicate that offering rumen undegradable rather than rumen degradable protein has no effect on colostrum production and lamb performance when ewes are offered high quality grass-based diets in late pregnancy.

Two isonitrogenous, isocaloric and isofibrous diets for finishing beef cattle differing only in their roughage constituents were compared in a feeding trial with Limousin bulls. A control maize silage (chop length = 9 mm) and wheat straw were the roughage sources of the former diet representing 0·23 and 0·09 of the dietary dry matter (DM) respectively. These roughage sources were fully replaced in the latter by the inclusion of a coarsely chopped maize silage (19 mm) at a level of 0·45 of the total dietary DM. Two balanced groups of 20 Limousin bulls (425·8 ± 20·3 kg live weight) each allotted in four pens were given the two diets ad libitum for a finishing period of 138 days. The different dietary roughages did not affect either bull growth performance (average daily gain = 1·39 ± 0·25 kg/day) or food conversion ratio. However, the diet containing the coarse silage significantly (P < 0·05) reduced the intake of both DM (8·30 v. 9·02 kg/day) and neutral-detergent fibre (NDF) (2·64 v. 2·92 kg/day). Although total ruminating time was similar for both diets, bulls given the coarse maize silage diet spent more time ruminating 1 kg of DM or NDF, probably due to the higher percentage of dietary forage NDF. No food selection activity towards specific chemical constituents was observed in either diet. Slaughter measurements and meat quality were satisfactory and unaffected by the type of dietary roughage. Based on cattle growth performance, feeding behaviour, and meat quality, the use of coarse maize silage as a dietary source of roughage can be considered a feasible alternative in beef cattle finishing.