1. Nutritional factors affecting the urinary excretion of acid-soluble peptides (ASP) were studied in rats. The ratio, total urinary nitrogen: ASP-form leucine + valine was lowest in the rats fed on a protein-free diet and increased as retained N: absorbed N decreased. The ratio was not affected by dietary protein level when the level was below the National Research Council (1978) recommended requirement, but increased greatly when it exceeded the recommended requirement.

2. The excretion of ASP-form leucine + valine per kg body-weight was significantly lower in the protein- deficient rats than in those fed on protein-adequate diets. This variable decreased during the stage of rapid growth, but did not change as markedly after the onset of adolescence. It increased again when the rats became older. The patterns of change in the rate of excretion of ASP-form amino acids during growth and that of Nr-methylhistidine were different.

3. When labelled amino acids were injected into rats, the largest amount of the ASP-form label was excreted on the 1st day of injection. From the 2nd day the excretion of ASP-form label decreased exponentially.

4. The findings suggest that the rate of urinary excretion of ASP-form amino acids can be employed as an index of protein metabolism, particularly as a simple index of the assessment of the status of protein nutrition.

1. Concentrations of amino acids (AA) in blood obtained from arterial and portal permanent catheters were measured together with the portal hepatic blood flow-rate during a post-prandial period of 8 h, in nine conscious pigs (initial mean body-weight 49.3 (SEM 1.8) kg) receiving experimental meals (500, 600 or 1000 g) at 3–4 d intervals from 6–8 to 20 d after the surgical implantation of the catheters and electromagnetic flow probe. The semi-synthetic starch-based diets contained variable amounts of fish meal giving crude protein (nitrogen x 6.25; CP) concentrations (g/kg) of 80 (seven meals), 120 (seven meals), 160 (five meals) and 240 (three meals).

2. During the post-prandial period, variations in the individual blood AA concentrations were parallel to those of total amino-N (Rérat et al. 1988) to a greater or lesser extent according to the AA considered. Portal concentrations, which always exceeded arterial ones (except for glutamic acid and glutamine), increased quickly and, after a peak, returned slowly to reach initial values (small intake) or above (large intake) after 8 h.

3. Relations between amounts of each AA appearing during 8 h after the meal and amounts ingested were characterized by a highly significant linear regression (with the exceptions of glutamic acid and cystine). There were also close and significant relations between amounts of AA absorbed during the first 2 and 4 h after the meal and the amounts ingested.

4. For a mean intake of 90 g CP, aromatic AA showed the highest hourly absorption coefficients (about 010 /h), and sulphur-AA (0.07/h), lysine (007/h) and arginine (0.056/h) the lowest ones. Alanine was synthesized (amounts absorbed within 8 h exceeding those ingested) at the expense of glutamic acid (absorption coefficient 001/h).

5. For a given period of time, the AA abscrption coefficients decreased with increasing intake, but not in the same proportions for all AA, resulting in an enrichment (lysine, arginine, serine, proline) or depletion (branched- chain AA, histidine) of the absorbed mixture.

6. Some substances of the urea cycle were synthesized in rather large amounts in the gut wall (for a mean level of intake of 90 g CP: citrulline 2.41 g/8 h, ornithine 1.09 g/8 h). Blood glutamine was taken up by the gut wall in larger amounts (4.28 g/8 h).

1. Protein synthesis was estimated in vivo in breast (superficial pectoral) and tibia (gastrocnemius) muscles, liver, kidney, pancreas, crop, duodenum, jejunum and ileum, using L-[U-14C]lysine injection. The effect on incorporation of [14C]lysine 1 and 2 h after injection was examined in five chickens adapted or not adapted to intermittent feeding.

2. Incorporation of [14C]lysine into tissue decreased in magnitude in the following descending order: pancreas > jejunum, duodenum > ileum, crop, liver > kidney > tibia, breast muscle and blood plasma.

3. The incorporation of [14C]lysine into muscle protein was higher in chicks after 24 h of refeeding than after 24 h of food deprivation. These differences were higher in adapted than in non-adapted birds. On days of refeeding the rate of incorporation exceeded that found in chickens fed ad lib.

4. Bound 14C from lysine in the intestinal segments was less than in control birds after food deprivation and greater after refeeding in non-adapted chicks only.

5. A negative relation was observed between bound and free 14C in muscles and in other tissues.

6. Short- and long-term adaptations to feeding regimens are discussed.

1. Six non-anaesthetized pigs (mean body-weight 57.0 kg) were used to study the intestinal absorption of amino acids (AA) from either an enzymic hydrolysate of milk (PEP) containing a large percentage of small peptides (about 50% with less than five AA residues) and very few free AA (8%), or from a mixture of free AA with an identical pattern (AAL) infused intraduodenally in one of two amounts (55 or 110 g). Concomitant insulin and glucagon production rates were estimated.

2. Each pig was previously fitted, under anaesthesia, with an electromagnetic flow probe. around the portal vein, with permanent catheters in the portal vein, the carotid artery and the duodenum. Each infusion was performed after an 18 h fasting period and each pig received each infusion. The observation period lasted for 5 h.

3. The absorption of AA was greater, more rapid and more homogeneous after PEP infusion than after AAL infusion, independent of the amount infused.

4. For the majority of AA considered individually, the absorption coefficient was higher after infusion of PEP than after that of AAL. The exceptions were methionine with a higher absorption coefficient after AAL infusion, and isoleucine, aspartic acid + asparagine and glutamic acid + glutamine with identical coefficients for both infusions.

5. Some AA, such as asparagine, ornithine, citrulline and taurine, while absent in the infusates, appeared in the portal vein in appreciable amounts after the infusion of both solutions. While a small proportion of taurine may arise from recycling of taurine-containing bile salts, it seems that the gut wall is able to synthesize all four AA.

6. Insulin production did not differ according to the nature or amount of solutions infused. Glucagon production was greater after PEP infusion.

1. Ryegrass (Lolium perenne cv. Cropper) silage was given to four Friesian steers (initial live weight (LW) 172 kg) alone or with a fishmeal supplement (150 g fresh weight/kg silage dry matter (DM)) in a balanced two- period change-over design. The dietary components were the same as those used in a recent experiment by Gill et al. (1987). All diets were offered hourly at 24 g DM/kg LW.

2. Fishmeal supplementation increased dietary nitrogen intake (P < 0.01) and significantly increased the flow of total N (P < 0.01), non-ammonia N (NAN) (P < 0.01) and amino acids (P < 0.05) at the duodenum. The increased supply of NAN to the duodenum was due largely (67%) to increased flow of undegraded dietary protein.

3. Microbial protein production was estimated simultaneously with 15N, diaminopimelic acid (DAPA) and a novel technique using L-[4,5-3H]leucine. Estimates varied with the marker and source of microbial isolate but mean values indicated that microbial N flow was significantly increased by fishmeal supplementation (P < 0.05). The use of L-[4,5-3H]leucine as a microbial marker is justified and its possible advantages over other markers are discussed.

4. The efficiency of microbial protein synthesis was significantly increased from 30.8 g N/kg organic matter apparently digested in the rumen (OMADR) to 54.3 gN/kg OMADR by fishmeal supplementation (P < 0.01). However, this indicates that relatively high efficiencies can be achieved with unsupplemented high quality silage supplied continuously. Rumen degradable N (RDN) supply was significantly increased by fishmeal supplementation (P < 0.05) but apparent efficiency of capture of RDN by rumen microbes was not significantly increased.

5. Attempts were made to investigate the source of N utilized by the microbes on the two diets by intrarumen infusions of (15NH4)2SO4 and L-[4,5-3H]leucine but these were confounded by rumen-mixing problems. Findings obtained suggest that a lower proportion of microbial N may have been derived from rumen ammonia when the silage was supplemented with fishmeal but no differences in the extent of direct incorporation of leucine into microbial protein were observed. This could indicate an increase in microbial peptide uptake on the fishmeal-supplemented diet. However, evidence was also obtained suggesting that the improvement in microbial protein synthetic efficiency with supplementary fishmeal was also due to the provision of a more continuous supply of nitrogenous substrates for microbial growth, as a result of hourly feeding.

6. The results are related to the increased growth responses attained by fishmeal supplementation of this silage in the experiment of Gill et al. (1987).

1. Results of 138 nitrogen balance studies from experiments with forty male Limousin and nineteen male Ile de France preruminant lambs were used to estimate total N requirements and to assess the effects of dietary tricaproin inclusion, protein concentration, lysine and methionine supplementation and the age of lambs on protein retention. In addition, energy balances were obtained in twenty-four of the Limousin lambs from birth up to 3 weeks of age by means of a slaughter technique.

2. For milk substitute unsupplemented with amino acids, tricaproin inclusion increased N balance slightly (P < 0.001) when a medium-protein concentration (260 g/kg dry matter (DM)) was used, in 2-week-old lambs (+4.3%) and in 5-week-old Limousin lambs (+5.3%), or very markedly when a high-protein concentration (320 g/kg DM) was used, in 2-week-old lambs (+14.5%) and in 5-week-old lambs (+18.6%). Protein requirements decreased from 300 g/kg DM in 3-week-old lambs to 270 g/kg DM in 5-week-old lambs for the control milk containing tallow-coconut oil mixture (0.67:0.33 by wt) and was increased above 300 g/kg DM at all ages in the case of experimental milk containing tallow-coconut oil-tricaproin mixture (0.33:0.33:0.33, by wt).

3. For milk containing either a medium- or a high-protein concentration and supplemented with lysine and methionine, inclusion of tricaproin increased N balance (P < 0.01) to a similar extent (+8.5 up to +14.3 %) in 1 to 2- and 3-week-old Limousin lambs. An increase in N concentration in the milk had no effect in 1 - and 2-week-old lambs, but led to a decreased N balance in 3-week-old lambs. Protein requirements decreased from 310 g/kg DM in 1-week-old lambs to 190 g/kg DM in 3-week-old lambs for the milk containing tallow-coconut oil or from 300 g/kg DM in 2-week-old lambs to 210 g/kg DM in 3-week-old lambs for the milk containing tallow-coconut oil-tricaproin.

4. For the Ile de France lambs given milk supplemented with lysine and methionine, tricaproin inclusion did not alter N balance. Protein requirements decreased from 370 g/kg DM in 2-week-old lambs to 270 g/kg DM in 4-week-old lambs for milk containing both kinds of fat mixtures.

5. Between birth and 3 weeks of age, efficiency of metabolizable energy (ME) utilization in the Limousin lambs was not dependent on the nature of the milk fat or the protein concentration of the diet (ME efficiency for tissue deposition varying from 0.672 to 0.698) whilst ME efficiency for protein deposition was 0.52 and ME efficiency for lipid deposition was close to 1; daily maintenance expenditure was 553 kJ/kg body-weight0.75

1. Pure swards of perennial ryegrass (Loliwn perenne cv. Melle) as a primary growth (May), trimmed primary growth (early June) and regrowth (late June), and white clover (Trifolium repens cv. Blanca) as a mature primary growth (July) and vegetative regrowth (August), were grazed by twelve Friesian steers (mean body-weights throughout experiment 152–231 kg) at daily allocation rates of forage which provided dry matter (DM) intakes ranging from 28 to 36 g/kg body-weight).

2. Total nitrogen contents of the three ryegrasses declined with season (37–20 g/kg DM), but in vitro organic matter (OM) digestibilities were relatively constant (0.76–0.79). The clovers had higher N (average 366 g/kg DM) and lower fibre and water-soluble carbohydrate contents than the grasses, and in vitro OM digestibilities of 0.70 and 076 respectively.

3. Duodenal digesta samples were obtained, using a portable sampling apparatus from the animals grazing the pasture, and estimates of the flow of nutrients into the small intestine were derived using two indigestible markers which were continuously infused into the rumen using a portable infusion apparatus. Forage intakes were calculated from estimates of faecal output of indigestible OM and the predicted in vivo OM digestibilities of the forages consumed. Coefficients of variation for OM flow to the small intestine and OM intake were 11.8 and 10.9 % respectively.

4. The apparent digestion of OM in the rumen ranged from 722 to 741 g/kg digestible OM intake and from 681 to 711 g/kg digestible OM intake for the grass and clover diets respectively. Substantial losses of ingested N before the small intestine were measured on all diets except the regrowth ryegrass. Losses were 0.30 and 040 g/g N intake on the primary growth ryegrass and the regrowth clover respectively; N contents were 37 and 39 g/kg DM respectively.

5. Comparison of the values obtained at pasture with that obtained when similar diets were offered to housed cattle (Beever et al. 1985) indicated that combined relations relating duodenal OM and non-ammonia-N (NAN) flows to OM and N intakes respectively could be established for each diet. For two diets (primary-growth grass and regrowth clover) the relations were curvilinear; for the remaining diets, the derived relations were linear.

6. The effects of forage species, stage of maturity and variations in the stem: leaf ratio in the grasses, on nutrient supply in grazing animals are discussed, and a combined relation for all grazed and harvested forages examined in this and in a previous study reported by Beever et al. (1985) is established. The relation Y = 1.430 - 0.0169X (r072, residual sd 0.140). where Y is NAN flow/N intake (g/g) and X is N content in the crop (gfkg OM), indicates that net losses of N across the rumen will occur on diets which contain more than 25.5 g N/kg OM.

7. Possible limitations in the techniques available to measure nutrient digestion and supply in grazing ruminants are discussed.

1. Daily injection of the β2-adrenergic agonist clenbuterol (1 mg/kg body-weight) increased weight gain by 12% in young (35 d) male rats and by 18% in castrated rats, but had no effect on energy intake, expenditure or efficiency in either group.

2. Body fat content was not affected by clenbuterol or castration, but water and protein content were significantly increased by clenbuterol treatment in both intact and castrated rats. The ratio, body protein:fat was increased by 13 and 16% in these two groups compared with their respective, untreated controls.

3. Bilateral surgical adrenalectomy (ADX) of young (45 d) male rats significantly reduced body-weight, and energy intake, expenditure and efficiency. Carcass energy and fat contents were also reduced in ADX rats compared with age-matched controls.

4. Clenbuterol injections stimulated weight gain (% increase: intact 15, ADX 35), and increased body protein content (% increase: intact 12, ADX 8) and the ratio, carcass protein:fat (% increase: intact 34, ADX 23).

5. These findings demonstrate that the effects of clenbuterol on body-weight gain and composition in male rats occur in the absence of either gonadal or adrenal hormones. Together with other studies, these results provide further evidence to suggest that clenbuterol probably exerts its effects by a direct action on lean body mass.

1. Studies have been made on the effects of dietary copper on the iron and Cu distribution in rats and on the metabolic activity and absorptive capacity of intestines perfused both vascularly and luminally.

2. Rats maintained for 4–5 weeks on a Cu-deficient diet (0.4μg Cu/kg) had significantly lower plasma, liver and intestinal Cu concentrations and significantly reduced plasma caeruloplasmin and liver cytochrome c oxidase (EC 1.9.3.1) activity compared with controls receiving a Cu-supplemented diet (5 μg Cu/kg). Disturbances in Fe metabolism in Cu-deficient rats were evident as shown by a mild anaemia, significantly elevated hepatic Fe concentrations and hypoferraemia.

3. Intestinal glucose uptake from both the luminal perfusion medium (LPM) and vascular perfusion medium (VPM) was unaffected by Cu deficiency despite a significant (25–30 %) reduction in oxygen consumption. This was associated with a 40% decline in mucosal cytochrome c oxidase activity.

4. In studies of Fe absorption, Fe uptake from the LPM was unaffected by Cu deficiency while transfer of Fe to VPM was significantly reduced (50%) compared with control preparations. Addition of apotransferrin (1 g/l) to the VPM was without effect in preparations from control rats but significantly increased the transfer of Fe to the VPM in preparations from Cu-deficient rats without affecting Fe uptake from the LPM.

5. The addition of either human or porcine caeruloplasmin (together with apotransferrin) to the VPM, such that the resultant ferroxidase (EC 1.16.3.1) activity of the VPM supernatant fraction was four to five times that of normal rat plasma, was without effect on either Fe uptake, tissue retention or Fe transfer to the VPM by preparations from either Cu-deficient or control rats.

6. These findings offer no evidence in support of the proposed role for caeruloplasmin with its associated ferroxidase activity in Fe absorption in the rat.

1. Blood from chronic indwelling catheters in the caudal aorta and anterior mesenteric, gastrosplenic and hepatic portal veins was used to measure blood flow and net flux of oxygen, glucose and nitrogenous compounds across hepatic portal-drained viscera (PDV), post-stomach (anterior mesenteric-drained viscera (MDV)) and stomach tissues of two beef steers (390 kg mean live weight).

2. Steers were fed in sequence on (1) chopped lucerne (Medicago sativa) hay (twelve meals/d), (2) chopped lucerne hay (two meals/d) and (3) a pelleted concentrate diet containing 780 g ground maize/kg (two meals/d). The lucerne hay and concentrate contained 26.5 and 16.8 g nitrogen/kg respectively.

3. Five measurements of net flux (blood flow multiplied by venous-arterial concentration differences (VA)) were obtained hourly on 2 d for each dietary regimen, beginning 0.5 h before feeding at 08.00 hours. Blood flow was measured by downstream dilution of p-aminohippurate (PAH).

4. Blood flow across MDV averaged 42% of PDV blood flow (665 litres/h).

5. Net use of O2 across MDV accounted for 51 % of net PDV use of O2 (920 mmol/h). This disproportionate use of O2 in relation to blood flow was due to greater VA for O2 across MDV than across stomach tissues. Dietary regimen had no effect on the proportions of PDV blood flow and net O2 consumption attributable to MDV or stomach tissues.

6. When lucerne was given, net glucose use across MDV represented 69% of PDV use (35 mmol/h). When concentrate was given, MDV glucose use switched to net absorption (29 mmol/h), reducing net PDV glucose use to I mmol/h.

7. When concentrate was given, net MDV absorption of α-amino-N (AAN) increased from 98 to 190 mmol/h, yet net PDV absorption (101 mmol/h) was not affected. Net stomach AAN flux increased from -7 to -69 mmol/h when concentrate was given, negating the increase in net MDV absorption.

8. Net absorption of ammonia-N across MDV represented 28 and 52% of net PDV absorption when lucerne and concentrate were given respectively. Net NH3-N absorption across PDV was lower when lucerne was given than when concentrate was given (295 v. 154 mmol/h), reflecting lower dietary N intake (153 v. 83 g/d). Net MDV absorption of NH3-N was not affected by diet. Net removal of blood urea-N (BUN) across PDV (101 mmol/h) was not affected by diet. Across MDV, BUN removal was lower when concentrate was given than when lucerne was given (32 v. 77 mmol/h). In beef steers, MDV tissues account for substantial portions of net flux of non-protein-N across PDV and are responsible for essentially all PDV absorption of AAN.

1. Two experimental approaches were employed to assess the relation between food consumption rate and maintenance requirements in male weanling rats. The first approach involved restricting food intake in rats previously given free access to food from weaning to 59 d of age. The second approach involved restriction of food intake to various levels after weaning. Maintenance requirements (g foodid per g body-weight (W)) were estimated by dividing the rate of food consumption by the resulting equilibrium W (EBW) for each animal. In addition, food consumption was partitioned into growth-independent (maintenance) and growth-dependent (gain) components by alternately setting W and specific growth rate (W') to zero in an equation relating food intake rate to W and W. Coupling coefficients representing maintenance consumption (g food/d per g W) and gain consumption (g food/g gain) were estimated for each animal by least squares.

2. Both techniques for estimating maintenance consumption provided similar estimates within and across experiments, and regardless of when food restriction was imposed or its severity, consumption for maintenance was about 5% W/d.

3. The EBW to which animals in each treatment group aspired was directly proportional to that group's food intake rate.

4. Coventional measures of growth efficiency were also related to food intake; efficiency decreased with decreasing food intake. Partitioning food consumption into maintenance and gain components revealed that as the rate of food intake decreased, the proportion of total intake consumed for maintenance increased. The results suggest that growth efficiency declines during food intake restriction because proportionately more of total intake is used for maintenance, leaving less available for gain.

1. Net flux of volatile fatty acids (VFA), D-β-hyroxybutyrate (BOHB) and L-lactate across post-stomach (anterior mesenteric-drained viscera (MDV)), stomach and total hepatic portal-drained viscera (PDV) tissues was measured in two beef steers (mean live weight 390 kg).

2. Net flux was measured while steers were fed, in sequence, on (1) chopped lucerne (Medicato sativa) (twelve meals/d), (2) chopped lucerne (two meals/d) and (3) a pelleted concentrate diet containing 780 g ground maize/ kg (two meals/d).

3. Five hourly net flux measurements were obtained on 2 d for each dietary regimen, beginning 0.5 h before a meal delivered at 08.00 hours. Net flux was calculated as venous-arterial concentration differences (VA) multiplied by blood flow (measured by downstream dilution of p-aminohippurate (PAH)).

4. Stomach tissues accounted for 85 to over 100% of net VFA and BOHB appearance across PDV. Net appearance across stomach tissues represented 74% of net PDV L-lactate appearance.

5. When lucerne was given, there was net utilization of arterial acetate and BOHB across MDV. This MDV utilization may reflect acetate and BOHB use as an energy source or their incorporation into mesenteric fat.

6. When concentrate was given, more n-butyrate and n-valerate and less L-lactate appeared across PDV and less 3-methylbutyrate appeared across stomach tissues than when lucerne was given. Postprandial increases in VFA, BOHB and L-lactate net flux across PDV followed meal-feeding of lucerne.

7. On a net basis, the relative contribution of MDV tissues to total PDV net appearance of VFA and BOHB was small (< 15%) in these steers.

1. The total water turnovers of grazing ewes and their lambs were estimated on days 9, 23, 44, 69 and 86 of lactation from the dilution of tritiated water injected into the dam and deuterium oxide injected into the offspring. The contribution of milk to the total water turnover of the lambs was estimated at the same times from the accumulation of tritium in their body water.

2. Mean total water turnover in the ewes was 6.5 litres/d over the entire period. In lambs, total turnover rose from 1459 ml/d at 9 d to 2791 ml/d at 86 d, and was closely related to live weight (r2 0.760, P < 0.001). The corrections to total water turnover, which were required because of the increasing body water pool size of the lambs during each measurement period, fell from +10.6% at 9 d to +3.7% at 86 d. All corrections were significant (P < 0.001).

3. The intake of water as milk fell throughout the study, from 1501 ml/d at 9 d to 471 ml/d at 86 d. Pool-size corrections were significant (P < 0.001). Milk intakes calculated from these results were 1816, 1054, 862, 742 and 588 ml/d at 9, 23, 44, 69 and 86 d of lactation. The rapid decline in milk intake reflected undernutrition of the ewes in early lactation.

4. The level of live-weight gain in early lactation was closely related to, and at a level expected from, the estimated milk intakes. From comparisons of estimated milk intakes with published estimates, it is concluded that the combined use of deuterium oxide and tritiated water results in accurate estimates of milk intake by the lamb throughout the ewe's lactation.

1. [3H]leucine infused directly into the rumen of sheep labelled microbial protein and, when compared with the specific activity of the rumen free-leucine pool, indicated that 50% of the bacterial protein leucine originated from the rumen free-leucine pool.

2. The lower limit for bacterial protein turnover in the rumen was 0.37/d when calculated as the difference between the specific rate of disappearances of labelled bacteria from the rumen and the liquid-phase dilution rate.

3. Intravenously infused leucine also labelled the rumen bacteria. The build-up of specific activity in the rumen bacteria was sigmoidal and did not resemble that of the salivary protein which suggested that the rumen epithelium was a major endogenous protein input to the rumen. Additionally, bacteria isolated from the rumen epithelium had high radioactivity indicating that they were ingesting the rumen epithelial cells.

1. Sweet-potato (Ipomoea batatas L. Lam.) strips (SPS) mixed with maize powder (CP) in proportions 10:0, 9:1, 8:2, and 7:3 were ensiled for 1, 2 or 3 months.

2. Trypsin inhibitor activity (TIA) decreased during ensilage in samples of all treatments while the SPS–CP mixture (7:3, w/w) ensiled for 3 months contained the lowest TIA.

3. SPS–CP (8:2, w/w) dried or ensiled for 2 months, or ensiled for 2 months and dried, were each mixed with twice the amount of control diet (1:2, w/w) to make three diets. These three diets together with the control diet were used for a feeding experiment with rats to evaluate the nutritive value.

4. General composition analysis (including metabolizable energy), fatty acid composition and amino acid analysis (including percentage of essential amino acids) of the samples did not change during ensilage to an extent which could explain the improved performance of rats fed on ensiled diets.

5. Rats fed on diets containing dried SPS–CP (8:2, w/w) showed significantly lower (P < 0.05) body-weight gain than rats fed on the control diet or ensiled SPS diets, at the end of the 8th week. They also showed enlargement of the pancreas. The adverse effect of SPS was associated with TIA which seemed to be prevented to some extent by ensilage.

6. The possibility that the starch of SPS may also contribute to the adverse effect cannot be excluded at present.

1. In growing rats, the time-course effects of giving a normal-protein diet (200 g casein/kg; NP) for 52 d, a low-protein diet (20 g casein/kg; LP) for 52 d and a LP diet for 26 d followed by balanced refeeding (200 g casein/kg; BR) for 26 d, on the fatty acid composition of liver total lipids and microsomal phospholipids were investigated together with Δ6- and Δ5-microsomal desaturase activities.

2. The oleic acid content (mg/g tissue) of liver total lipids increased progressively with the LP diet, while linoleic acid was increased only at days 7 and 52. 20:3ω6, 20:4ω6, 22:5ω6 and 22:6ω3 fatty acids decreased during the period on the LP diet. BR for 7 d was sufficient to restore the fatty acid composition of total lipids to control values. Changes in the fatty acid composition of liver microsomal L-α-phosphatidylcholines were observed only after 52 d on the LP diet; the proportions (% w/w total fatty acids) of 18:0, 20:3ω6 and 20:4ω6 fatty acids decreased while oleic acid increased. The fatty acid composition of L-α-phosphatidylethanolamines was less affected.

3. Δ6- and Δ5-desaturase activities decreased to 20–30% of their original values after 2 d on the LP diet; a smaller deficit prevailed after 14 d but disappeared after 25 d, to appear again after 52 d. As early as day 2 of BR, desaturase activities were greatly recovered and returned to control values at day 13.

4. The present work shows that modifications in microsomal Δ6- and Δ5-desaturase activities are not strictly paralleled by the changes in the composition of fatty acids of liver total lipids and microsomal phospholipids.

1. Lean (Fa/–) and obese (fa/fa) Zucker rats were adrenalectomized or sham-operated at 18 d of age (3 d before weaning). After weaning the rats were fed ad lib. on semi-synthetic diets containing either a low (8 g/kg) or a high (178 g/kg) proportion of fat. Other groups of sham-operated rats were given the same amount eaten by adrenalectomized animals (restricted intake). Rats were killed at 40 d of age.

2. Adrenalectomy reduced the body lipid content of lean and obese rats compared with intact animals fed ad lib. or given a restricted intake. Adrenalectomized obese rats contained more body lipid than intact or adrenalectomized lean rats.

3. Sham-operated obese rats given a restricted intake had less body protein than similarly treated lean animals and this phenotypic difference was abolished by adrenalectomy.

4. There were no effects of diet on growth or body composition of intact or adrenalectomized rats.

5. It is concluded that preweaning adrenalectomy prevented development of the obese phenotype when rats were fed on either diet. Comparison of these results with a previous study, in which adrenalectomized Zucker rats were fed on a stock diet (Fletcher, 1986b), showed, however, that feeding either of the semi-synthetic diets caused greater deposition of body lipid in obese rats.

1. Polysaccharide depolymerases and glycoside hydrolases involved in the breakdown of plant structural polysaccharides (hemicellulose and pectins) were monitored in three fractions of the liquid phase of horse caecum digesta: acellular fluid (AF), bacteria (B) and protozoa plus bacteria (PB).

2. Both bacteria and protozoa were found to be involved in the decomposition of pectic substances, with two enzymic activities: depolymerase (polygalacturonase, EC 3.2.1.15; and pectin lyase, EC 4.2.2.10) and esterase (pectinesterase, EC 3.1.1.11). The activity of the PB fraction was higher than that of B.

3. With hemicellulosic substrates, all three fractions showed a significant xylan endo-1,3-β-xylosidase (EC 3.2.1.32) activity. Mannan was hardly broken down.

4. Galactomannan and arabinogalactan were broken down more extensively by the PB fraction than by the B fraction. Glycosidase activities (xylan 1,4-β-xylosidase, EC 3.2.1.37 and α-L-arabinofuranosidase, EC 3.2.1.55) were also observed.

1. The basic assumptions involved in one- and two-compartment models with age-independent distributed residence times (exponential, G1) for describing digesta flow are reviewed as the bases for describing families of one- and two-compartment models which assume age-dependent distributions (Gn) of residence times.

2. The two-compartment, age-independent model with exponentially distributed residence times (GIGI) yielded estimates of essentially equal rate parameters when fitted to faecal values for all four cows receiving a diet of 500 g coarsely chopped, sodium hydroxide-treated straw /kg and one of four cows receiving the same diet but with ground and pelleted straw. The incorporation of progressively higher orders of age dependency (G2-G6, Gn) into the faster turnover compartment of two-compartment models (GnG1) resulted in a resolution of equal rate parameters estimated by the G1G1 model and a reduction in standard errors for the rate and the initial concentration parameters.

3. The occurrence of equal rate parameters in two-compartment models indicated an age-dependent process; a process which could equally well be described by a one-compartment, age-dependent compartment having an order of age dependency equal to the sum of these orders in the two-compartment model with equal rate parameters.

4. The age-independent models overestimated time of first appearance in the faeces of a meal's particles. The association of age dependency with the faster turnover compartment resulted in earlier estimates for first appearance of the marked particles; estimates which were more consistent with observed first appearance.

5. The faecal excretion pattern from cows fed on the ground and pelleted straw diet exhibited an age-independent distribution of longer residence times which dominated approximately 80% of the later residence times. Age-dependent, one-compartment models gave a poor fit to such data from these cows fed on ground and pelleted straw. In contrast, age-dependent, one-compartment models provided an excellent fit to data from cows fed on chopped straw; data which indicated that age-independent distributions of residence times were much delayed in appearing or were totally absent.

6. The mean residence time for the slower turnover, age-independent compartment estimated from faecal excretional of stained particles from either diet was similar to that estimated from duodenal concentrations of the stained particles. This suggests that the slower turnover model compartment was confined to preduodenal sites.

7. The mean residence time for the faster turnover, age-dependent compartment estimated from duodenal data was 58–62 % that estimated from faecal data and suggests that the site of this model compartment was both pre- and post-duodenal.

8. It is emphasized that the slow and imperfect mixing of particulate matter that occurs in reticulo-rumen digesta is inconsistent with the assumptions of instantaneous and homogeneous mixing made by models having age-independent distributions of residence times. The use of age-dependent distributed residence times can accommodate such imperfect mixing and is consistent with the existence of age-discriminating processes involved in particle flow from the reticulo-rumen. Age dependency also offers improved precision in estimating parameters of digesta flow via processes having inherent uncertainty in their mixing and age-discriminating mechanisms.