Long-term subclinical infection with Trichostrongylus colubriformis: effects on food intake, diet selection and performance of growing lambs.

Growing lambs (mean liveweight 22.4 kg) were used to test for the effects of long-term subclinical infection with Trichostrongylus colubriformis (2500 larvae per day) on the voluntary food intake (VFI), diet selection and performance of sheep given a choice between two foods of different protein content (low versus high). Sheep were either uninfected controls or parasitised for 27 weeks (Group 27P). Additional sheep were infected for either 9 or 18 weeks (before and after the development of complete resistance to parasites, as assessed by faecal egg counts) and then treated with anthelmintic to remove the parasites (Groups 9P and 18P). Group 18P remained uninfected after anthelmintic treatment whereas Group 9P was infected again for the last 9 weeks after a 9 week worm-free period. The VFI declined or remained static from Week 5 up to Week 11/12 of continuous infection, for the 27P and 18P groups. For 9P sheep, VFI recovered within a few days after the anthelmintic administration and was unaffected during re-infection over the last 9 weeks of the experiment. Diet selection changed in such a pattern that the proportion of the high protein food (prop H) in the selected diet started to increase from Week 6 of the infection. The prop H selected remained higher over the interval 10-18 weeks of the continuous infection for 27P and 18P groups than the uninfected controls. There were no significant differences in diet selection among any of the groups beyond Week 18 of the experiment. The infection caused a growth check from Week 7 to approximately Week 12/14 of dosing; these was no evidence of compensatory growth following the development of resistance or recovery. The results are consistent with the view that sheep given a choice between two appropriate foods will modify their diet selection in order to moderate the adverse effects of subclinical nematode infection. The performance and diet selection of all sheep beyond Week 18 of infection suggest that no obvious nutritional penalties are incurred with the continuation of the infection once the animals have become resistant to parasites.

The effects of physical form of feed, carbohydrate source, and inclusion of sodium bicarbonate on the diet selections of sheep.

We proposed an hypothesis that ruminants attempt to select a diet that promotes high levels of feed intake by maintaining optimal ruminal conditions. Three tests of the hypothesis considered whether the diet selection of sheep given either a choice of two high energy density (ED) feeds or a choice between a high and a low ED feed is affected by 1) sodium bicarbonate inclusion (NaHCO3; 1, 2, and 4% [wt/wt]) in the high [ED] feed); 2) dietary carbohydrate source (barley-based, B and sugar beet/ barley, S) of the high ED feed; or 3) physical form (alfalfa: pelleted, ALFP and long chop, ALFL) of the low ED feed. To conduct these, 42 lambs were used in seven 6 x 6 Latin squares, which were either foods ALFL and ALFP offered alone and paired with feed B or S and their NaHCO3 derivatives as a choice, or foods B and S offered alone and paired with their NaHCO3 derivatives as a choice. Each Latin square period lasted for 3 wk. For the choices between a NaHCO3-supplemented high ED feed and either a high or a low ED one, the inclusion of NaHCO3 increased feed intake. Its level of inclusion had a marked effect on the proportion of unsupplemented high ED feed selected (P < .05) but not on the proportion of low ED feed (alfalfa) selected. More alfalfa was selected when the high ED feed was based on barley (B) rather than sugar beet/barley (S) (P < .05). The physical form of the alfalfa affected diet selection because its selection was greater (P < .001) when ALFP (pelleted) was offered in the feed choice rather than ALFL (long-chop). The significant effects of NaHCO3 level, dietary carbohydrate, and physical form on the diet selection and feed intake of the sheep are consistent with the proposed experimental hypothesis.

A design for flexible intestinal cannulas.

A design for a cannula made by fixing silicone rubber tubing to rigid internal and external flanges is described. These cannulas have been used to fit simple duodenal, simple ileal and abomasal fistulas in adult lactating cattle for periods up to 1 year and have caused minimal tissue reaction at the site of fistulation.

The efficiency of conversion of metabolisable protein into milk true protein over a range of metabolisable protein intakes.

The aim of this work was to test the robustness of the 0.68 estimate of the efficiency of conversion of metabolisable protein into true milk protein (Agriculture and Food Research Council (AFRC), 1993) for protein-limiting diets and to determine whether a different value is appropriate for practical rationing. Seventy-two multiparous cows were blocked on the basis of milk energy output per unit of dry matter intake (DMI), and allocated at random to one of four treatments. Treatments supplied metabolisable energy (ME) at a fixed level to individuals within a block, but varied metabolisable protein (MP) supply from 25% below the estimated requirements, through -12.5% and +12.5% up to 25% above requirements for the average performance of animals within blocks at the start of the study. Cows were offered diets to meet their predicted ME requirements for each 3-week period with measurements performed in the last week of each period. Milk protein output was regressed against the estimated MP available for production for each cow and the efficiency of conversion of MP into milk true protein was calculated, assuming a maintenance requirement according to the MP system. The efficiency of conversion of MP into milk true protein decreased with the increasing supply of MP from 0.77 to 0.50. Using an iterative approach to determine the best fit of the data when supply matched requirement resulted in a range of efficiency values between 0.62 and 0.64 g of true milk protein per g of MP.

Protein-energy interrelationships in dairy cows.

In dairy cows two distinct and important aspects of the interrelationship between protein and energy-yielding nutrients can be identified. First, a change of protein input can influence performance by changing overall plane of nutrition. To a large extent this results from changes of digestibility and associated intake of ration ingredients. Within this context it appears that at high levels of feeding rumen microbial demand for nitrogen per unit fermentable organic matter is high; initial failure to meet this need for nitrogen probably accounts for many responses to protein supplementation in practice through stimulation of ruminal digestion. Second is that changing "protein" supply to tissues can alter the pattern and efficiency of absorbed nutrient use. In early lactation protein supplementation favors partition of available nutrients toward mammary secretion. There are effects on glucose and fatty acid metabolism, all interacting with the endocrine system within a metabolic framework that tends to conserve those nutrients (amino acids, glucose) that are in shortest supply relative to demand. Response to increased input of amino acids depends both on physiological state of the cow and balance of all nutrients absorbed from the gastrointestinal tract.

Food intake and diet selection in sheep: the effect of manipulating the rates of digestion of carbohydrates and protein of the foods offered as a choice.

An experiment was designed to investigate whether the degree of synchrony between the rates of digestion of carbohydrates and N of foods offered as a choice would have an effect, through their consequences, on the short- and long-term diet selection of sheep. Four foods (RL, RH, SL and SH) with the same high metabolizable energy, and similar high metabolizable protein contents were made into pellets. Foods RL and RH were based on a rapidly fermentable carbohydrate source and foods SL and SH on a slowly fermentable carbohydrate source; within each source one food (RL or SL) had a low, and the other (RH or SH) a high, rumen-degradable protein (RDP) content. The foods within a carbohydrate source were offered either singly or as a choice (RL/RH or SL/SH) to eleven rumen-fistulated mature sheep. The design was two 3 x 3 Latin squares (replicated once) with 5-week periods; squares consisted of two single foods and their respective choice. Weeks 1, 3 and 5 were considered to be controls, and weeks 2 and 4 used for rumen infusions of either urea or fructose infused over 4 h (10.00-14.00 hours). Food intake (FI) and diet selections (DS) were recorded daily and every 2 h (08.00-16.00 hours) on days 2-5 of each week; rumen pH and NH3 concentrations were also measured during these time intervals of day 5. As expected, feeding treatment affected significantly the rumen measurements: rumen NH3 concentrations were higher on foods RH and SH, and rumen pH lowest on RL. Daily FI was lowest on treatments SL, and choice SL/SH. The mean daily proportion of the low-RDP food in the selected diet was lower when the carbohydrate source was rapidly (choice RL/RH) rather than slowly fermentable (choice SL/SH); this was consistent with the experimental hypothesis. Short-term infusions affected further rumen variables (in the expected directions), irrespective of feeding treatment. However, DS over the 4h infusion period were unaffected; these short-term DS were consistent with the ones selected over the longer term (daily). The results suggest that the long-term (daily) diet selection of sheep may be affected by the degree of synchrony of energy and protein to the rumen. The fact that diet selections were not altered further by short-term manipulations of these supplies might reflect inadequacies of the methodology (infusions) adopted here.

Measurement of the rate of flow of dry matter in digesta passing through the duodenum of sheep.

1. A total of seven sheep fitted with re-entrant cannulas in the proximal duodenum were used in three experiments to measure the passage of dry matter (DM) in duodenal digesta. 2. Fifty-three 24 h measurements and ten 48 h measurements of DM flow were made in Expt 1. Coefficient of variation (CV) of these measurements was 22-1% which was similar to variability reported for chromic oxide 'corrected' flows and for an estimate of day-to-day variation. DM flow on day 1 was 103-2% (SE = 8-94) of flow on day 2 in 48 h measurements. 3. In two 3 X 3 Latin-square design experiments, duodenal DM flow was measured continuously over 72 h and on days 1, 2 and 3 DM flow was 100-4, 102-6 and 96-9% (SE = 5-82) of mean flow (Expt 2) and 95-8, 101-2 and 103-0% (SE = 6-70) of mean flow (Expt 3) respectively. 4. It was concluded that no depression in duodenal DM flow occurred during the first 24 h of collection in these experiments and that the variability of repeated 24 h measurements of flow was within day-to-day variation.

Prediction of responses to required nutrients in dairy cows.

Nutritional science is developing from the calculation of requirements toward prediction of responses to nutrients. Responses are defined as the consequences of the animal's interaction with its feed and other aspects of its environment. For dairy cows the important responses to predict are yields of major milk constituents (fat, protein, and lactose), rates of changes of body fat and protein, and voluntary feed consumption. In order to make predictions of cow responses, equal weight needs to be given to quantitative descriptions of cow characteristics, feed characteristics, and relevant aspects of the environment. This paper presents a framework that allows quantitative descriptions of these various attributes to be combined for the purposes of response prediction. The nutrients of importance are those major end products of digestion, which act as substrates for the generation of animal products. Quantitative aspects of genotype include description of potential milk constituent yield, mature body size and composition, and rate of maturing, including a description of "labile" protein mass and animal capacities for ingestion of potentially constraining feed attributes. A variety of approaches is available to simulate nutrient supply and use for purposes of response prediction. The synthesis of such models adds both intellectual richness to the process of developing nutritional science and improved frameworks for testing theories through experiment.

Attitude change in response to information that male homosexuality has a biological basis.

We assessed 28 college students' attitudes toward homosexuals immediately before and 1 week after the presentation of either an article suggesting that male homosexuality has a biological component or a control article. Change in attitude toward homosexuality for subjects exposed to the biological article depended on subjects' memory of the article and on their college major. Subjects undecided in their major and those who scored below average on a memory test of the article's content had the most positive attitude change, while biological science majors and those who scored above average on the memory test became more negative toward homosexuals. The results suggest biological information about homosexuality can have both beneficial and detrimental effects on attitudes toward homosexuals.

Diet selection in sheep: the ability of growing lambs to select a diet that meets their crude protein (nitrogen x 6.25) requirements.

To test the proposition that sheep are able to select a diet that meets their crude protein (N x 6.25; CP) requirements, feeds L, A, B, C and H with the same energy content (11 MJ metabolizable energy/kg feed) but different CP contents (78, 109, 141, 172 and 235 g CP/kg fresh feed respectively) were formulated. In addition, feed U, which was feed L plus 21.4 g urea/kg (CP content 132 g/kg), was also made. The feeds were offered ad lib. either singly (n 4 per treatment) or as a choice between feed H and another feed (pairs LH, AH, BH, CH and UH; n 9 per feed pair) to individually penned Suffolk x Scottish mule wether lambs, over the live-weight range 25-45 kg. On the single feeds the rates of live-weight gain were 273, 326, 412, 418, 396 and 407 g/day (SE of difference (SED) 34; P < 0.01) and protein (excluding wool) gain were 27, 32, 44, 45, 41 and 39 g/d (SED 4; P < 0.001) for feeds L, A, B, C, H and U respectively. When sheep were given a choice between a feed below (L or A) and a feed above their CP requirements (H; as judged by the single-feeding treatments) the CP concentration selected was not different between the two pairs: 131 (SE 4) v. 133 (SE 4) g CP/kg feed for pairs LH and AH respectively. On the choices BH and CH (a choice between two feeds above requirements) the feed lower in CP was constantly preferred (874 (SE 33) and 910 (SE 33) g feed B and C respectively per kg total feed intake; CP selected was 157 and 178 g CP/kg respectively). However, this was not the case with the UH choice on which sheep consumed only 599 (SE 61) g feed U/kg total feed intake, resulting in a selection of a higher CP in their diet (173 g CP/kg). The live-weight gains of the animals given a choice between two feeds were 416, 387, 415, 410 and 383 g/d (SED 37) and protein gains were 45, 40, 46, 50 and 43 (SE 7) for pairs LH, AH, BH, CH and UH respectively, which were comparable with the best performance achieved on a single feed. The results suggest that sheep were able to select a diet that meets their CP requirements and avoid, at least to a certain extent, excess of protein intake.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of abomasal infusion of casein on acetate, palmitate and glucose kinetics in cows during early lactation.

Four mature Friesian cows were used in a Latin square design experiment to measure the effects of abomasal casein infusion on milk production and the kinetics of glucose, acetate and palmitate in jugular venous blood. The basal ration contained 110 g crude protein/kg dry matter and was offered at the rate of 4 kg hay, 3 kg alkali-treated straw cubes plus 9 kg concentrates/d. Treatments were infusion of casein into the abomasum at the rate of 0, 240 or 460 g/d. The experiment started 1 week post partum. Experimental periods lasted 21 d. [6-3H]glucose, [9, 10-3H]palmitate and [U-14C]acetate were continuously infused into one jugular vein for measurements of whole-body flux rates. The yield of milk protein was increased significantly (P less than 0.05) by the first increment of casein infusion, with no further increases. Casein infusion had no effect on blood concentrations of glucose, acetate, palmitate or growth hormone. Mean flux rate of glucose increased with each increment of casein. The effect at 460 g/d was higher (P less than 0.1) than with no infusion. Flux rates of palmitate and acetate were increased (P less than 0.1) by the first increment of casein. The proportion of blood bicarbonate derived from acetate increased (P less than 0.05) with the first increment of casein. Protein status of cows early in lactation influences the dynamics of glucose and fatty acid metabolism.

Glucose and urea kinetics in cows in early lactation.

1. Four mature Friesian cows were used. For 2 weeks after calving they were fed 3 kg hay (147 g crude protein (nitrogen X 6.25; CP)/kg dry matter (DM) plus 9 kg concentrates (177 g CP/kg DM) per d and thereafter 4 kg hay plus 12 kg concentrates/d. 2. At approximately the 2nd, 4th and 9th weeks post-partum each cow was given a single intravenous dose of [14C]urea (1 mCi) and [6-3H]glucose (1.5 mCi). Following this, jugular venous blood samples were withdrawn up to 26 h post-injection. 3. The log (specific activity) v. time curves were not linear for either metabolite. Glucose irrversible loss rates (IRL) were calculated by a stochastic procedure. The linear part of the urea log (specific activity) v. time curve gave the same estimate for urea IRL rate as a stochastic method of calculation, but urea pool sizes were overestimated so that stochastic analysis of results was preferred. 4. Mean milk yields at the time of the three measurements were 27.1, 30.8 and 27.9 kg milk/d. 5. Urea IRL was significantly lower (P < 0.05) in the first 4 weeks of lactation than in the 9th, and blood urea concentration was lower (P < 0.05) in the 4th than in the 2nd and 9th week, but there was no close correlation between urea IRL and concentrations. 6. Glucose IRL rose (P < 0.05) between the 2nd and 9th weeks post-partum, perhaps due to the increase in feed intake. The results were generally consistent with the relationship between milk yield and glucose IRL established by Paterson & Linzell (1974). 7. Urea IRL (y) and milk protein-N (x) production values were used to calculate the efficiency of protein utilization (EPU) as x divided by (x+0.35 y). This assumes that 0.35 of urea IRL was due to protein catabolism. EPU was found to be 0.59-0.80 (mean 0.69). 8. The possible contribution of catabolized amino acids to glucose IRL was calculated by assuming that 0.35 of urea IRL was due to protein catabolism and 0.2 of catabolized protein-C passed through the glucose pool. On this basis the contribution of protein-C top glucose-C was < 25 g/kg glucose IRL.

Formaldehyde-treated proteins for dairy cows--effects on blood hormone concentrations.

Seven Friesian heifers in mid-lactation were used in an experiment to measure the effects of including casein, formaldehyde-treated casein or formaldehyde-treated soya-bean meal in the diet on performance and on the concentrations of certain metabolites and hormones in the blood. 2. Milk yield and the concentration of fat, protein and lactose in milk were not affected by the treatments. 3. The concentration of growth hormone (GH) in jugular venous blood was higher with both formaldehyde-treated proteins than it was with untreated casein (P less than 0.05). Diet had no effect on insulin, prolactin or thyroxine concentrations in blood. 4. Urea concentration tended to be higher and non-esterified fatty acids lower in blood from heifers offered the diet containing untreated casein, but these differences were not statistically significant. Blood glucose concentration was not affected by dietary treatment. 5. It was concluded that blood GH concentration can be increased by offering protein supplements which will increase total amino acid supply to the intestines in lactating cattle. This appears to be a direct effect on GH status independent of effects on milk yield.

Maternal protein reserves and their influence on lactational performance in rats.

To determine the contribution of tissue protein reserves to lactational performance, multiparous female Sprague-Dawley rats were mated, caged individually and offered a diet high in protein (215 g crude protein (N x 6.25; CP)/kg dry matter (DM); H) ad lib. until day 12 of gestation. Subsequently half the rats continued to receive diet H while the remainder were offered a diet low in protein (65 g CP/kg DM; L) until parturition. This treatment aimed to produce a difference in carcass protein at parturition. On day 1 of lactation females were allocated to either diet H or a low-protein diet (90 g CP/kg DM; L2) offered until day 13 of lactation, giving four lactation treatment groups HH, HL2, LH and LL2. Groups of females were slaughtered on days 2 and 12 of gestation and days 1 and 13 of lactation and carcass and major organs were analysed. Weight gain of standardized litters was used as an indicator of lactational performance. Maternal carcass protein contents at parturition were 43.5 (SE 1.2) and 38.7 (SE 0.8) g (P < 0.01) for diets H and L respectively. During lactation there was little change in carcass protein content of HH rats while LH rats appeared to replenish their depleted reserves. Food intake or lactational performance did not differ between these two groups. HL2 and LL2 rats lost carcass protein with HL2 rats losing more than LL2 rats (P < 0.05). Intake and lactational performance were reduced compared with that on diet H (P < 0.05) but for the first 6 d of lactation were both greater (P < 0.05) for diet HL2 than for diet LL2. All four groups showed a considerable loss of body fat during lactation which was not affected by diet. The ability of HL2 rats to catabolize more protein and consume more food allowed them to sustain a greater lactational performance. Previous maternal protein depletion had no influence on lactational performance as long as an adequate supply of dietary protein was provided.

Maternal protein reserves and their influence on lactational performance in rats. 3. The effects of dietary protein restriction and stage of lactation on milk composition.

The effects of severe protein restriction following parturition on the changes in rat milk composition during lactation were investigated using multiparous female Sprague-Dawley rats caged individually following mating and offered a high-protein diet (H; 215 g crude protein (N x 6.25; CP)/kg dry matter (DM)) ad lib. until parturition. Following parturition, half the females continued to receive diet H, whilst the remainder were offered a diet low in protein (L; 90 g CP/kg DM) ad lib. On days 2, 4, 8 and 12 of lactation groups of females from both dietary treatments were used to provide a milk sample. Milk samples were analysed for their lactose (enzymically), protein (binding to Coomassie blue), lipid (gravimetrically) and mineral (spectrophotometrically) contents. The milk lactose concentration of group H increased with stage of lactation (r2 0.85, P < 0.001). Such an increase was prevented by diet L, and from day 8 of lactation the milk lactose of group L was lower (P < 0.05) than in group H. Group H milk protein concentration did not change during lactation and averaged 90.7 mg/g. Dietary protein restriction reduced the milk protein concentration of group L so that on days 2, 4 and 12 of lactation it was lower (P < 0.05) than that of group H. On day 8 of lactation the milk protein concentration of group L had increased (P < 0.05) and was comparable with that of group H. For group H, milk lipid averaged 166.8 mg/g and was generally unchanged during lactation. Diet L increased (P < 0.01) the milk lipid concentration (205.5 mg/g) compared with diet H and this was also significant on days 4 and 8 of lactation (P < 0.05). Group L milk lipid concentration also increased between days 4 and 8 of lactation (P < 0.05). Milk Na concentration declined during lactation in both dietary groups (P < 0.01) but was unaffected by dietary treatment. Both milk Ca and P concentrations increased (P < 0.01) during lactation in both dietary groups, whilst protein restriction also increased the Ca and P concentrations (P < 0.05). Milk K and Mg concentrations were unaffected by dietary treatment or stage of lactation. This significant alteration in the milk composition of severely protein-restricted dams, while possibly favouring the disposal of greater quantities of energy-yielding nutrients, suggests that equations developed for the estimation of milk production in rats cannot be used under such conditions.

Mammary sensitivity to protein restriction and re-alimentation.

The present study tested the influence of protein undernutrition and re-alimentation on mammary gland size and secretory cell activity in lactating rats. During gestation, female Sprague-Dawley rats were offered a high-protein diet (215 g crude protein (N x 6.25; CP)/kg DM; H); litters were standardized to twelve pups at parturition. During lactation, two diets were offered ad libitum, diet H and a low-protein diet (90 g CP/kg DM; L). Lactational dietary treatments were the supply ad libitum of either diet H (HHH) or diet L (LLL) for the first 12 d of lactation, or diet L transferring to diet H on either day 6 (LHH) or 9 (LLH) of lactation. On days 1, 6, 9 and 12 of lactation, rats from each group (n > or = 6) were used to estimate mammary dry mass, fat, protein, DNA and RNA; the activities of lactose synthetase (EC 2.4.1.22) enzyme and Na+,K(+)-ATPase (EC 3.6.1.37) were also measured. Rats offered a diet considered protein sufficient (H) from day 1 of lactation showed a decrease in mammary dry mass and fat but an increase in DNA, RNA and protein on day 6, after which there was no further change, except for mammary protein which continued to increase. However, rats offered diet L showed a steady loss in mammary mass and fat throughout the 12 d lactation period and no change in mammary DNA, RNA or protein. Rats previously protein restricted for either the first 6 or 9 d of lactation had their mammary dry mass and mammary fat loss halted and showed a rapid increase in mammary DNA, RNA and protein on re-alimentation. Lactose production in group HHH, as measured by lactose synthetase activity, was similar on days 1 and 6 of lactation, after which a significant increase was seen. Protein-restricted rats showed no change in lactose synthetase activity during the 12 d experimental period. Changing from diet L to diet H led to a significant increase in lactose synthetase activity to levels comparable with those offered diet H from day 1. These results show that rats offered a protein-restricted diet during lactation suffer mammary underdevelopment, but this may be rapidly reversed by re-alimentation with a high-protein diet.

Interactions between major nutrients in the diet and the lactational performance of rats.

The effect on lactational performance of replacing feed carbohydrate with fat at two different protein levels was studied. Lactating Sprague-Dawley rats with a standardized litter size of thirteen pups were allocated one of eight feeds containing either 300 or 150 g protein/kg organic matter (OM) and ranging in fat content from 100 to 550 g/kg OM from day 2 until day 14 of lactation. Daily food intake, live-weight gains, and changes in body composition of both dams and litters were measured. Feeds of low protein content resulted in a significant decline (P < 0.001) in lactational performance despite a significant increase (P < 0.001) in maternal protein mobilization. Maternal lipid mobilization was not significantly affected by feed composition. Litter lipid gain was significantly increased (P < 0.05) as fat replaced carbohydrate in the high-protein feeds, due to an increase in maternal energy intake. In contrast, lactational performance was severely depressed (P < 0.001) as fat replaced carbohydrate in the low-protein feeds. This interaction between feed components on lactational performance was in accordance with the hypothesis that the heat production of lactating rats is maximal and, hence, constraining intake.

Simulation of the metabolism of absorbed energy-yielding nutrients in young sheep: efficiency of utilization of acetate.

A previously described mathematical model that simulates the metabolism of absorbed nutrients was used to examine factors influencing the efficiency of utilization of acetate in a sheep weighing 25 kg. A significant component of the model was a function representing substrate cycles that degraded ATP when its concentration reached twice initial levels. Acetate utilization was simulated when up to 2 g mol/d were added to four diets (representing forages and concentrates) that supplied varying proportions of absorbed volatile fatty acids, glucose and protein. The basal diets provided either 4.375 (near energy balance) or 8.75 MJ/d absorbed energy. The predicted net efficiency of utilization (kf) of the energy in added acetate was high (0.58-0.70) for all combinations of absorbed nutrients at the low level of energy absorption, whereas at the higher level, it ranged from 0.16 (forage) to 0.49 (high protein concentrate). Low kf values were always associated with a considerable flux of ATP through the degradation pathway. The increase in ATP concentration that caused this pathway to operate was linked to an inhibition in the utilization of acetyl-CoA for fatty acid synthesis and an increase in its oxidation. The simulated addition of exogenous NADPH or its precursors (particularly glucose and propionate) to these diets repartitioned acetyl-CoA flux towards fatty acid synthesis instead of oxidation, decreased the flux of ATP through the degradation pathway and increased the kf of added acetate. Although a negative relationship was predicted between kf of added acetate and NADPH production for diets with increasing protein content, kf still depended on ATP flux through the degradation pathway. Addition of glucose to the high protein diets decreased this flux by decreasing acetyl-CoA oxidation and increasing fatty acid synthesis. The predictions suggest that the efficiency of acetate utilization in ruminants may be influenced by NADPH availability when a considerable amount of absorbed energy is derived from acetate. Increased absorption of glucose or propionate enhanced NADPH production, but the major effect on acetate utilization of increasing dietary protein was not through NADPH production.

Simulation of the metabolism of absorbed energy-yielding nutrients in young sheep: efficiency of utilization of lipid and amino acid.

A previously described mathematical model that simulates the metabolism of absorbed nutrients in a sheep weighing 25 kg was used to examine the effects of altering lipid and amino acid absorption, and the potential of the animal to deposit protein on the efficiency of utilization (kf) of metabolizable energy. The predicted kf of energy from lipid added to several diets ranged from 0.65 to 0.74 and was influenced by both the protein and glucose content of the diet. The highest efficiency occurred when body protein deposition was limited by amino acid absorption. Lower values occurred with high protein diets when the proportion of body energy deposited as fat declined and, with low protein diets, when NADPH supply limited the synthesis of fatty acids from acetyl-CoA. Predicted kf of energy from amino acid added to several diets ranged from 0.003 to 0.47. Low values occurred when protein synthesis was limited by amino acid absorption; an increase in amino acid supply increased ATP utilization for protein synthesis and substantially reduced fat deposition with little resulting change in energy retention. The higher values occurred when most of the additional amino acid was oxidized. Glucose supply increased the kf of added amino acid primarily by reducing the synthesis of glucose from amino acids. Predicted kf of a whole diet generally declined as the proportion of protein in the diet increased. However, when the balance of absorbed nutrients was such that fat synthesis was limited by the supply of NADPH, kf increased with increasing dietary protein. Predicted kf of a diet also declined when the proportion of body energy deposited as protein increased, except when fat synthesis was limited by the supply of NADPH. The predictions suggest that kf is determined primarily by the energetic efficiency of biochemical reactions for maintenance and growth. However, most kf values less than 0.5 were associated with a flux of more than 1 g mol/d of ATP through the degradation pathway (representing substrate cycles) that occurred when NADPH supply limited the synthesis of fatty acids from acetyl-CoA.

Nutrient supply and growth of cattle offered straw-based diets.

An experiment was conducted using steers cannulated at the rumen, duodenum and ileum to study the effects of increasing the levels of barley and fishmeal in straw-based diets. Diets A, B, C and D contained ammonia-treated straw, barley and fishmeal in the ratios, 67:33:0, 66:23:11, 53:47:0 and 52:36:12 (by weight) and were offered in daily amounts of 3.9, 3.9, 4.8 and 4.8 kg dry matter. The effects of barley were attributable to increased intakes of digestible organic matter and consequently to increased flows of microbial matter to the duodenum. There were no modifications in the balance of energy to nitrogen-yielding nutrients available for absorption. Introducing fishmeal into diets improved digestibility of cellulose and xylose by up to 6.7 and 4.7% respectively, and shifted digestion towards the large intestine. Second, it increased amino acid N supply to the small intestine which averaged 52.2, 63.2, 68.8 and 84.0 g/d with diets A, B, C and D. Some changes were also noted in the balance of amino acids absorbed. Consequently, the contribution of amino acids to metabolizable energy intake increased with the proportion of fishmeal in diets (0.17, 0.20, 0.18 and 0.21 for diets A, B, C and D). Growth rates measured in heifers amounted to 259, 431, 522 and 615 g/d for diets A, B, C and D. They appeared to be related to intestinal amino acid supply.