Energy and nitrogen metabolism were examined at weekly intervals during lactation weeks 1 to 8 in Holstein-Friesian cows (no. = 9) offered a diet of hay, maize pellets (whole plant) and concentrates, (barley, maize and soya bean; forage : concentrate ratio 65 : 35), with feeding levels close to ad libitum.

After calving, the cows lost body weight until week 7, with peak milk yield (35 kg/day) recorded during week 3. Dry-matter intakes increased progressively to week 4 then remained relatively constant. Apparent digestibility of dietary energy was unaffected by stage of lactation but the overall value was low (0·653) indicative of the quantity and quality of long hay in the diet. Metabolizable energy intakes ranged between 163 and 202 MJ/day, with little between-week variation after that between weeks 1 and 2 (P < 0·01). Milk energy output was relatively stable during weeks 1 to 4 but then declined progressively for each remaining week, whilst heat energy output was relatively constant throughout. Estimates of body energy retention indicated the cows were in negative energy balance at all times, being greater in week 1 (64 MJ/day, P < 0·01) than weeks 2 to 4 (mean, 35 MJ/day) or weeks 5 to 8 (22 MJ/day). Digestible nitrogen (N) intake was reduced in week 1, whilst apparent N digestibility declined significantly , with little between-week variation after that between weeks 1 and 2 < 0·05) as lactation progressed. The cows were in negative N balance (–19 g/day) during week 1, with zero (week 2) or positive balances noted thereafter.

It is concluded that during early lactation, the extent of body tissue mobilization in average yielding cows can be substantial and prolonged, whilst attainment of positive body N status occurred earlier and was not related to the energy status of the cows.

Two N balance experiments using growing pigs were conducted to study the effect of essential:total (E:T) N ratio on N retention and utilization. Purified diets contained casein and crystalline amino acids as the sole sources of N. E:T values ranged from 0·25 to 0·86 while either the concentration of total N (Expt. 1) or essential N (Expt 2) was kept constant. At a constant concentration of total dietary N, N retention and total N utilization were maximized with an E:T value of approximately 0·6, while essential N utilization gradually decreased as E:T increased. At a constant level of essential N, N retention remained unchanged until the E:T value reached 0·48 and then decreased. In Expt 2, maximum total N utilization was attained with an E:T value of 0·66 while N excretion and essential N utilization decreased with increased E:T value. These results suggest that under conditions of optimal protein utilization, essential amino acids are partially degraded and used for the synthesis of non-essential amino acids.

The effect of body fat content on the protein metabolism of energy-restricted sheep has been studied in two experiments. In the first experiment, six Suffolk-cross wether sheep, three weighing about 39 kg and three of about 61 kg, were given progressively increasing amounts of casein-N from 0 to 3000 mg N/kg metabolic body weight (W0·75) daily with constant energy, 91 kJ/kg W0·75 daily, from a high-propionic acid mixture of volatile fatty acids (VFA). In the second experiment, two lean and two fat sheep of similar body weights were given progressively increasing amounts of casein with the same VFA mixture. All the animals attained a positive N balance when they were in negative energy balance. N balance was not affected by body fatness of the magnitude studied, although lean animals utilized increasing levels of standard VFA (acetate-propionate-butyrate 65:25:10, molar proportions) infusion for N accretion more efficiently than fat animals. Endogenous energy was utilized for protein accretion with an efficiency of 0·56. Supply of glucogenic VFA equivalent to 28 mmol glucose/kg W0·75 reduced fasting N excretion by 39%. Fasting heat production decreased from 335 to 300 kJ/kg W0·75 with the infusion of casein and glucogenic VFA. It is argued that fasting induces additional heat losses due to raised protein metabolism and is unsuited as a baseline for dietary assessment.

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.

Utilization of endogenous and exogenous energy for protein accretion during energy undernutrition has been studied. Nine lambs nourished by intragastric infusion were given either progressively increasing or decreasing amounts of casein-N up to 2550 mg/kg metabolic weight (W0·75), with or without 250 kJ/kg W0·75 of volatile fatty acids daily. Energy balance (respiration calorimetry) and N balance were measured. While all experimental animals were in negative energy balance, N balance increased curve-linearly with the increase in casein-N infusion and attained positive N balance. Endogenous energy (presumably body fat) was found to meet the energy needs for protein accretion during energy undernutrition. It is concluded that body fat can be effectively utilized to support lean-tissue growth during energy undernutrition, so that the classical nutritional concept of dietary energy:protein ratio is only meaningful when both endogenous and exogenous energy are considered.

The relationship between essential fatty acids (EFA) deficiency and the utilization of dietary protein, growth rate and survival of offspring was investigated in rats during development and reproduction. EFA deficiency was induced by feeding a 200 g casein/kg-based diet containing 70 g hydrogenated coconut oil (HCO)/lkg as the only source of fat. The conversion efficiency of dietary protein was assessed as net protein utilization (NPU), using a 10 d comparative carcass technique. Consumption of the deficient diet during the 10 d assay period induced biochemical changes characteristic of mild EFA deficiency in humans (triene:tetraene 0·27 (SD 0·04) compared with 0·026 (SD 0·004) for wn-deficient controls), but there were no significant changes in growth rate or protein utilization. These variables were also unchanged when the deficient diet was fed for an additional 7 d before the assay, although triene: tetraene increased to 0.8 (SD 0·02). Feeding the deficient diet for 63 d before assay produced severe EFA deficiency (triene:tetraene 1.4 (SD 0·3) v. 0·036 (SD 0·005) for controls), a fall in growth rate (25% during assay period), and NPU (31.5 (SD 0·63) v. 39.0 (SD 0·93) for controls). These severely-EFA-deficient animals had a 30% higher fasting-resting rate of energy metabolism than that of age-matched controls. However, there was no change in the rate of endogenous N loss. Voluntary energy consumption was increased in animals fed on deficient diets, either with 200 g protein/kg, or protein free. The reduced efficiency of protein utilization could be entirely accounted for by the restricted amount of energy available for growth and protein deposition. Consumption of an EFA-deficient diet during pregnancy and lactation resulted in high mortality (11% survival rate at weaning compared with 79% for controls) and retarded growth in the preweaning offspring. It is concluded that animals are particularly sensitive to EFA deficiency during reproduction and pre- and post-natal stages of development. However, after weaning only severe EFA deficiency retarded growth, primarily through changes in energy balance.

Using a high protein diet (483 g/kg), two trials were carried out to assess any potential benefit which may result from the supplementation of diets for fingerling and grower class gilthead seabream with pyridoxine at levels which were below, matched and far exceeded the minimum dietary requirement. At the level of supplementation below the minimal requirement (lowest level) the responses from the practical diet almost matched the responses from the diet containing an amount of pyridoxine corresponding to the published minimum requirement for the species and no significant improvement in performance was recorded in response to increasing supplement level. However, despite the absence of a marked effect on overall performance, a small potential for increased activity of alanine aminotransferase was recorded amongst grower class fish given diets containing the lowest supplement. The proximate composition of the grower class fish was unaffected by the level of pyridoxine supplementation though marginal increases in the lipid content of the fingerlings were observed. At the haematological level, haematocrit, total haemoglobin and the plasma concentrations of glucose and protein were also unaffected. However, on the application of an acute stressor (repeated netting), significant alterations in haematocrit and plasma glucose concentration reflecting dietary pyridoxine supplement were recorded.

An experiment was designed to test the prediction that two genetically-very-different kinds of pigs would retain the same amounts of protein in their bodies when given the same allowances of the same feed for the same period of time, when these allowances were limiting for both. An allowance of 33·957 kg of a feed with 12·76 MJ metabolizable energy (ME) and 189 g crude protein (N × 6·25)/kg feed was given to Large White × Landrace (LW ×) and Chinese Meishan (CM) female pigs over three different periods of time: (1) ad lib. (AL) with the time taken by individual pigs to consume the allowance being a variable, (2) over a period of 7 weeks (H) and (3) over a period of 9 weeks (L). In addition, in a fourth treatment, pigs of both breeds received the same allowance but supplemented with starch also over a period of 7 weeks (HS). The performance of the pigs on treatment AL was affected by pig breed, with CM pigs gaining protein at a slower, and lipid at a faster, rate than LW × pigs. On treatments L, H and HS the average amounts of protein retained were 2·693 and 2·655 kg for the LW × (n 15) and CM (n 15) pigs respectively (SED 0·106 kg). There was a statistical indication that the LW × pigs may have been more efficient on L, and less efficient on HS, than the CM pigs but we have been unable to propose any biological reason for such an effect, if it was in fact a real one. Thus, the efficiency with which ideal protein was utilized was close to being constant, and apparently at its maximum, for the two breeds. However, although CM pigs had the same protein gain, and the same live weights, on the same feed allowances as the LW × pigs, they gained significantly more lipid. This was attributed in part to their digesting their feed better and in part to their having a lower energy requirement for maintenance through a lower level of physical activity. Given that these two very different kinds of pigs use limiting protein with the same efficiency, it is suggested that it is safe to make the assumption in models of pig growth that the material efficiency of using limiting protein is constant across genotypes of pig.

Twenty crossbred (Large Wliite × Piétrain) boars were used to investigate the effect of energy intake (0·71, 0·80, 0·90, 1·00 ad libitum) and body weight (BW) on growth performance, and rates of protein (PD) and lipid (LD) deposition between 40 and 100 kg BW. Daily crude protein intake was kept constant. Total PD and LD were measured according to the comparative slaughter technique (CST) during total experiment and according to nitrogen and energy balance technique at 45, 65, 80 and 94 kg mean BW. Average daily gain increased linearly with metabolizable energy (ME) intake (+36 g/MJ ME) whereas food conversion ratio was not affected by energy level (28·0 M] ME per kg BW gain). Daily PD increased from 126 to 171 g/day, and LD from 70 to 187 glday between 0·71 ad libitum and ad libitum ME intake. Maintenance ME requirement was constant when expressed per kg BW'60 (992 kJ/day). PD varied with ME intake above maintenance (MEp) according to a linear-plateau relationship. The slope decreased with BW (+11·5 g/MJ MEp at 65 kg BW and +9·0 g/MJ MEp at 94 kg BW). LD was linearly related to MEp (+16 g/M} MEp). The LD: PD ratio in marginal empty BW gain was constant within BW class, but increased from 1/5 to 1/9 between 65 and 94 kg BW.

A study was undertaken to ascertain the effect of pyridoxine in diets of varying protein to energy ratio (P: E) for Mediterranean gilthead seabream fry (Sparus aurata) with respect to protein utilization. Diets were formulated to provide three levels of added pyridoxine (0·5, 5·0 and 50·0 mg/kg), at two protein levels (320 or 470 g/kg) whilst maintaining a similar gross isocalorific profile at 20 MJ/kg. Feeding level was proportionately 0·02 of the fish body weight per day, resulting in the following effects after a 70-day feeding trial. Low P: E ratio diets (15 g/MJ) did not perform as well as high P: E diets (24 g/MJ), with respect to growth related indices. Higher inclusions of pyridoxine into diets of equal P: E ratios, caused elevations in food conversion efficiency, protein efficiency ratio and apparent net protein utilization. A marked improvement of growth rate concomitant with pyridoxine level only occurred in the high P: E diets. It was concluded that incremental additions of pyridoxine in the diet, increased the efficiency of protein utilization for all diets tested. However, the protein sparing effect oflipid was not achieved for the low protein diets indicating the relatively high requirement for protein in the growth of the juvenile seabream.

The objective of the present experiment was to define the form of the relationship between varying levels of energy intake at constant, high protein intake and the performance of young pigs. By doing so it was expected that we could distinguish between four models that predict the pig's responses to its diet as rates of protein and lipid retention. Forty young pigs were assigned at 12 kg live weight either to an initial slaughter group (n 8) or to one of four allowances of starch intake at a constant intake of a high-protein feed (with 345 g crude protein (nitrogen x 6.25)/kg food). Half the pigs were killed after 4 weeks and half after 8 weeks on the treatments; at each slaughter point on each treatment half the pigs were entire males and half were females. Increasing the intake of starch (energy) resulted in significant increases in the rates of live weight, empty-body, protein and lipid gains of pigs slaughtered at both stages. There was no minimum positive lipid: protein ratio in the gain of the pigs. Male pigs deposited more protein and less lipid than females but this effect of sex on protein and lipid retention was seen only on the two highest allowances of starch intake. The calculated efficiency of protein utilization increased with increasing starch intake up to a maximum of 0.81, when probably the energy: protein in the diet became non-limiting. The results led to the rejection of two of the models that predict the rates of protein and lipid retention as responses to protein and energy intake, but the two remaining models could not be rejected, at least qualitatively.

The objective of the experiment was to define the form of the relationship between varying levels of protein and energy intake and the performance of young pigs. Forty-four young pigs were assigned at 12 kg live weight for 6 weeks either to an initial slaughter group (n 8) or to one of the nine feeding treatments (n 4); three allowances of a high-protein food with 355 g crude protein (nitrogen × 6.25; CP)/kg (P1, P2, P3) at three levels of feeding (L, M and H). Each feeding level was met by supplementing the allowance of feed P with the appropriate amount of starch and each treatment had two males and two females. The rate of protein deposition was not affected by feeding level at the two lowest allowances of basal feed P (P1 and P2), but it increased with increasing the feeding level for the pigs on treatment P3. Males deposited more protein than females, but this effect was more pronounced with treatment P3. The rate of lipid deposition increased with each increase in the level of feeding and decreased with increasing the allowance of feed P. The calculated efficiency of protein utilization (ep) was expressed as a function of the energy: protein ratio in the feed (MJ metabolizable energy/kg digestible CP). The best model to describe the relationship was a linear-plateau model, with the maximum value for ep of 0.814 at 73 MJ/kg. This relationship provided the basis of a model that could predict the response of a growing pig to its diet as rates of protein and lipid retention

Three series of nitrogen-balance experiments were carried out on growing rats fed on purified isonitrogenous diets (16 g N/kg) to study the importance of non-essential N and the essential:total N (E:T) ratio for attaining maximum N balance (NB) and biological value (BV) of protein. Minimum dietary levels of asparagine, proline and glutamic acid required for maximum NB and BV were estimated to be 1.0, 2.0 and 5.0 g/kg respectively. In an essential amino acid-based diet, the levels of individual amino acids were successively reduced to 110% of the requirement. Reducing the level of arginine, lysine or methionine + cystine resulted in a significant increase in NB and BV while the response of rats given the isoleucine-reduced diet significantly decreased. Addition of asparagine, proline and glutamic acid in the estimated minimum amounts to an essential amino acid-based diet resulted in a significant increase in NB and BV. A further significant increase was found when the levels of arginine, lysine and methionine + cystine in the diet were reduced to 110 % of the requirement. The performance of rats fed on the latter diet was similar to that of rats given a diet with the optimum E:T ratio. It is concluded that the optimum protein utilization may be influenced by the presence of some non-essential amino acids and by the surplus of some essential amino acids rather than by the E:T ratio per se.