The effects of phytase and phytic acid on the loss of endogenous amino acids and minerals from broiler chickens

1. The effects of myo-inositol hexaphosphate (IP6) and phytase (EC 3.1.3.26) on the excretion of endogenous compounds were investigated using growing broiler chickens. 2. A total of 32 female Ross broilers were used in a precision feeding assay involving a 2 x 2 factorial arrangement of treatments. The materials administered were glucose, glucose + 1000 units of phytase activity (FTU), glucose + 1 g of IP6 and glucose + 1 g of IP6 + 1000 FTU. Excreta were collected quantitatively over a 48-h period following intubation of the test materials. The excretion of nitrogen, amino acids, minerals, sialic acid and phytate phosphorus was determined. 3. The ingestion of 1 g of IP6 by broilers increased the excretion of endogenous nitrogen, amino acids, iron, sodium, sulphur and sialic acid compared with birds fed on glucose. Supplementation of IP6 with exogenous phytase reduced the excretion of endogenous amino acids, calcium, sodium, phytate phosphorus and sialic acid compared with birds fed IP6. 4. It can be concluded that IP6 increases the excretion of endogenous minerals and amino acids in broiler chickens. Part of the beneficial effects of the addition of exogenous phytases to the diets of poultry appears to be mediated through a reduction in endogenous losses of these nutrients.     

The influence of environmental temperature, age and sex on the digestibility of amino acids in growing broiler chickens

The ability of the broiler chicken to metabolise energy and to digest and absorb amino acids increased from 30 to 50 d of age. Although sex had no major effect on metabolisable energy or amino acid digestibilities at these ages, the influence of environmental temperature on amino acid digestibilities appeared to be sex-related, there being decreased digestibilities of most amino acids at higher temperatures in female but not male birds.     

The effects of dietary protein independent of essential amino acids on growth and body composition in genetically lean and fat chickens

1. Growth performance between 28 and 49 d of age and carcase composition at 49 d in genetically lean (LL) and fat (FL) broilers fed on diets varying in non-essential amino acid (NEAA) concentrations were compared in 2 experiments. In experiment 1, 3 crude protein (CP) contents (133, 155, and 178 g/kg) were compared. In experiment 2, 4 CP levels (131, 150, 170 and 189 g/kg) were compared. All diets were supplemented with synthetic amino acids to cover the EAA requirement of the LL birds. 2. Weight gains of FL chickens were not affected by dietary treatments, while those of LL increased when CP level increased. 3. Reducing CP content always increased body lipids, abdominal fat and food conversion ratio in both lines in both experiments; however, the effect on abdominal fat was more pronounced in the FL birds. 4. Reducing CP concentration always decreased breast muscle proportion in both lines in both experiments, even when growth rate was not affected by CP. 5. It is concluded that LL chickens require diets more concentrated in NEAA than fat chickens and that there seems to be an effect of NEAA on breast muscle development.     

Graded methionine dietary inclusions influence growth performance and apparent ileal amino acid digestibility coefficients and disappearance rates in broiler chickens

Graded quantities of 1.38,2.76 and 4.14 g/kg L-methionine were included in a control diet formulated to contain 3.07 g/kg digestible methionine.Each of the 4 dietary treatments was offered to 6 replicate cages(initially 8 birds per cage)from 1 to 21 d postehatch.The parameters assessed included growth performance,nutrient utilisation(apparent metabolisable energy[AME],AME:GE ratios,N retention,Ncorrected apparent metabolisable energy[AMEn]),apparent digestibility coefficients and disappearance rates of amino acids in the distal ileum.They also included free amino concentrations in systemic plasma(brachial vein)at 20 d postehatch and in hepatic tissue at 14 and 21 d postehatch.Graded L-methionine inclusions quadratically influenced weight gain(r=0.688;P=0.001)and FCR(r=0.780;P<0.001).It may be deduced from the quadratic regressions that 3.43 g/kg L-methionine supported maximum weight gain of 1,036 g/kg and 3.50 g/kg L-methionine minimum FCR of 1.193,from 1 to 21 d postehatch.The control diet contained specified levels of 3.07 g/kg digestible methionine and 13.0 g/kg digestible lysine.Thus,an inclusion of 3.465 g/kg L-methionine corresponded to a total of 6.535 g/kg methionine or a methionine-to-lysine ratio of 50.3,which is higher than standard recommendations.The implications of this and other outcomes of the present study are reported and discussed.     

Dietary effects of commercial probiotics on growth performance,digestibility, and intestinal morphometry of broiler chickens

This study compared five commercially available probiotics vis-à-vis antibiotic growth promotant (AGP) supplementation and absence of feed additive based on efficiency, intestinal morphometry, and energy digestibility in improving broiler chicken production. A total of 630 straight run (Cobb) day-old broiler chicks were distributed to seven treatments following a completely randomized design, with ten replicates per treatment and nine birds per replicate per cage. Dietary treatments consisted of basal diet in combination with the following: without probiotics and AGP supplementation (treatment 1); 75 ppm each of chlorotetracycline (CTC) and Zn bacitracin (treatment 2); probiotic A, Bacillus subtilis (treatment 3); probiotic B, Bacillus subtilis (treatment 4); probiotic C, Enterococcus faecium (treatment 5); and probiotic D, Bacillus subtilis (treatment 6); probiotic E, Enterococcus faecium, Bifidobacterium spp., Pediococcus spp., and Lactobacillus spp. (treatment 7). At day 42, energy digestibility was determined by fasting three randomly selected birds from each treatment for 12 h and then subjecting them to their corresponding dietary treatments. Excreta were collected and pooled after 24 h of feeding. Pooled excreta were weighed, oven-dried, and subjected to energy analyses after 3-day collection. Apparent total tract metabolizable energy was then computed. At day 47, three birds were randomly selected per treatment for intestinal morphometry (villi height and crypt depth) of the duodenum, jejunum, and ileum. Dietary supplementation using probiotics showed no significant effect on overall body weight, weight gain, feed consumption, feed efficiency, dressing percentage, mortality, harvest recovery, carcass quality parameters (e.g., meat to bone ratio and abdominal fat content), intestinal morphometry, and energy digestibility. Birds under treatment 7 (basal feed + probiotic E) generated the highest income over feed and chick cost.

     

Effect of amino acid deficiency on precaecal amino acid digestibility in broiler chickens

In this study, we determined whether deficient dietary amino acid (AA) concentrations influence the precaecal (pc) AA digestibility when determined using the regression approach. We mixed two basal diets. Basal diet 1 was deficient in essential AAs, whereas adequate AA concentrations were ensured in basal diet 2 by adding free AAs. Rapeseed cake and full‐fat soya beans as test ingredients were included in the basal diets at levels of 100 and 200, and 150 and 300 g/kg, respectively, at the expense of maize starch. Each diet was tested with six replicates of 10 broiler chickens each. The feed intake of the chickens that were fed diets based on basal diet 2 was similar, whereas the feed intake of the chickens that were fed diets based on basal diet 1 differed considerably. The numerical differences in pc AA digestibility determined with basal diet 1 or 2 ranged from 2.6 percentage points to 20.8 percentage points in rapeseed cake and from 0.5 percentage points to 15.2 percentage points in soya beans. Across all measured AAs, the average differences were 10.1 percentage points and 5.4 percentage points in rapeseed cake and soya beans, respectively. The differences in the estimated pc AA digestibility between the basal diets were probably caused by different basal endogenous AA losses in the digesta between treatments as a consequence of different feed intake. Adequate AA concentrations and test ingredient levels that are specifically adjusted to avoid a negative effect on feed intake are recommended for future studies.     

Effect of dietary lignocellulose on ileal and total tract digestibility of fat and fatty acids in broiler chickens

The study was conducted to determine the effect of a lignocellulose supplemented diet on apparent ileal and total tract digestibility of fat and fatty acids (FA) in broiler chickens. A total of 48 21‐day‐old male Ross 308 chickens were divided into four treatment groups (n = 12) with six replicates per treatment. From 21 to 42 days of age, the broilers were fed experimental diets varied in the amount of lignocellulose: 0%, 0.25%, 0.5% and 1%. Total excreta were gathered during the last 3 days of the feeding trial and digesta was collected from the ileum at 42 days of the bird age. Digestibility was determined by the indicator method. The ether extract content in diet/digesta/excreta was determined by the gravimetric method, and fatty acid methyl esters were analysed by gas chromatography–mass spectrometry. Fat digestibility measured to the end of the small intestine and in the whole gastrointestinal tract in birds was high and exceeded 90% and 87% respectively. Addition of lignocellulose (1%) increased (p < 0.05) ileal fat digestibility but had no significant effect on total tract fat digestion. Absorption of total fatty acids (TFA) as well as myristic (C14:0), palmitoleic (C16:1) and α‐linolenic (C18:3n?3) acids, estimated by both methods, was significantly higher in birds fed the diets supplemented with lignocellulose, especially at a dose of 1%. Total tract absorption of some dietary polyunsaturated fatty acids (PUFA) (C20:2, C20:4n?6) was significantly lower from diet supplemented with 0.5% and 0.25% lignocellulose. There was observed a decrease in apparent digestibility of fat and most examined fatty acids, when measured between terminal ileum and total gastrointestinal tract. The results suggest that lignocellulose can affect digestion and FA absorption in broilers but, as the effect of lignocellulose was not studied previously, further investigations are necessary to confirm the results of the present experiment.     

Influence of starch sources and dietary protein levels on intestinal functionality and intestinal mucosal amino acids catabolism in broiler chickens

Background: There is growing interest in carbohydrate and protein nutrition to enhance the efficiency of animal production. Reduced-crude protein diets depress environmental pollution and feeding cost, but the challenge to their adoption is maintaining digestive function and growth performance of birds. The present study was conducted to evaluate the influence of different dietary starch sources and protein levels on intestinal functionality and mucosal amino acid catabolism.Methods: Six dietary treatments, based on maize and soybean meal, were offered to 360 AA+male chicks from 6 to 35 d post-hatch as a 3 × 2 factorial array. Either waxy rice or amylose was added to a conventional maize-soy diet to provide three sources of starch with different digestion rates and relatively high and low dietary protein levels. Growth performance, parameters of intestinal functionality and concentrations of free amino acid in the portal circulation were determined.Results: In the grower phase, starch source influenced(P 0.02) weight gain as diets containing amylose supported significantly higher weight gains than waxy rice. Significant increase of ileal ATP concentrations and Na+/K+-ATPase activity were found in amylose treatment. Also, amylose decreased BrdU positive cell numbers and down-regulated m RNA expression for CASP-3. GOT activity in the ileum was higher(P 0.01) in birds offered low protein diets and there was a trend(P = 0.057) for waxy rice as a starch source to increase ileal GOT activities. There was a significant influence on the concentration of seventeen amino acids in the portal circulation with tryptophan the one exception. Waxy rice as a starch source generated 13.6% and 22.4% numerically higher concentrations of non-essential amino acids than maize and amylose, respectively.Conclusions: Amino acid catabolism in the gut mucosa is subject to nutritional regulation. Given that amino acids can be spared from catabolism in the gut mucosa by supplementation of amylose, it follows their post-enteral availability would be improved and intestinal energy would be derived more efficiently from glucose.     

Combined effects of clenbuterol and various concentrations of protein on performance of broiler chickens

1. The effects of the beta-adrenergic agonist, clenbuterol (1 mg/kg diet) on the growth and muscle composition in female broiler chickens (14 to 32 d of age) fed on diets containing various concentrations of protein (220, 240 or 260 g protein/kg) were examined. 2. Body weight gain over the 18 d period increased linearly with increasing protein intake. The rate of gain was significantly higher in clenbuterol-treated chickens than in control birds. 3. Dietary clenbuterol increased thigh muscle weight and protein concentration of breast and thigh muscle, regardless of dietary protein content. Protein/DNA ratio in thigh muscle was enhanced by clenbuterol feeding, and the magnitude of difference of the ratio was higher in chickens fed on the 240 and 260 g CP/kg diets than in those fed the 220 g CP/kg diet. 4. It was concluded that clenbuterol-treated chickens require increased dietary protein to maintain maximal growth, and that increased protein consumption is an important factor in improving growth in clenbuterolfed broilers.     

The influence of dietary protein and antimicrobial feed additives on salmonella carriage by broiler chickens

Salmonella carriage in broiler chickens was not apparently influenced by the protein concentration of the ration, pelleting or by the coccidiostat monensin. The inclusion of penicillin in the diet was associated with an increase in salmonella shedding, particularly in the first half of the rearing period, but did not influence the lactobacillary count in the crop or the pH of the contents of the crop, gizzard and caecum. Furazolidone medication (150 mg/kg food) for the first 10 days had no effect on salmonella carriage at the time of slaughter (seven weeks) and studies into the influence of growth promoting antibiotics (avoparcin, bacitracin and virginiamycin) on salmonella shedding were inconclusive. Circumstantial evidence indicated that the food was the most probable source of infection for the birds studied in these experiments.     

Effects of citric acid and microbial phytase on amino acid digestibility in broiler chickens

1. Two experiments with growing chickens were carried out to study the effects of the inclusion of a microbial phytase (Natuphos 5000) and citric acid (CA) in maize-soybean-based diets on the performance and apparent ileal digestibility (AID) of crude protein (CP) and amino acids (AA). In both experiments the diets were formulated to contain the same amounts of energy and protein. 2. In the first experiment, data were analysed as a 2 x 2 factorial arrangement with two concentrations of available phosphorus (AP) from one day to 3 weeks of age (3.5 and 2.2 g/kg) and for 3 to 6 weeks (2.7 and 1.4 g/kg), and two inclusions of commercial phytase (0 and 500 FTU/kg) in each period. The AID of CP and dispensable and indispensable AA were not modified by the AP content of the diet. Addition of phytase improved the AID of CP and dispensable and indispensable AA only at low AP levels. 3. In the second experiment, data were analysed as a 3 x 2 factorial arrangement with three concentrations of citric acid (0, 20 and 50 g/kg) and two inclusions of commercial phytase (0 and 750 FTU/kg). Diets were formulated with deficient contents of AP (2.5 g/kg). Performance was not affected by commercial phytase addition. The addition of CA reduced the weight gain but did not modify the feed intake and gain:feed. In general, the AID of CP and dispensable and indispensable AA were not affected by CA addition. Commercial phytase increased the apparent ileal digestibility of crude protein but had no effect on AID of dispensable and indispensable AA. 4. In conclusion, the present work showed that microbial phytase enhanced AA digestibility in maize-soy-based diet only at very low AP concentrations, and that CA had no affect on the AID of CP and dispensable and indispensable AA. No synergism between CA and microbial phytase was detected.     

The effects of dietary oxidized fat and selenium source on performance,glutathione peroxidase,and glutathione reductase activity in broiler chickens

Normal or elevated selenium status of broilers, which is influenced by dietary selenium sources, improves the bird’s ability to overcome the adverse effects of reactive oxygen metabolites. The objective of this study was to evaluate the effects of feeding graded levels of peroxidized poultry fat on blood and hepatic glutathione peroxidase (GSH-Px), and hepatic glutathione reductase activity in broiler chickens fed either inorganic sodium selenite (SEL) or organic selenium enriched in the organic selenium yeast product Sel-Plex (SP). Nine starter diets, varying in levels of oxidized fat (0, 3, and 6 mEq/kg) and dietary selenium sources, were fed to 360 male chicks from hatch to 21 d of age. Sel-Plex or SEL was added to the basal diet to provide either 0 or 0.2 ppm of supplemental selenium in the diets. Blood and hepatic samples were obtained for each treatment group at 21 d of age. Neither peroxidized fat nor selenium source significantly altered the activity of hepatic glutathione reductase (P ≤ 0.05). Blood GSH-Px was influenced significantly by both fat and selenium source (P ≤ 0.05), but the fat × selenium source interaction was not significant (P ≥ 0.3). A selenium source effect on the hepatic GSH-Px activity (P ≤ 0.05) was evidenced by higher GSH-Px activity, even in the basal diet with no added peroxidized fat. An increase in GSH-Px activity was seen in the erythrocyte and hepatic samples in both the SEL and SP treatments when peroxidized fat was given at 3 mEq/kg, but in the erythrocytes and in the hepatic tissues from SEL-supplemented birds, there was an apparent inhibition of GSH-Px activity. This inhibition was not seen in the hepatic tissue samples from SP-fed birds. Because elevated GSH-Px activity is indicative of oxidative stress, it was concluded that dietary SP supplementation resulted in better selenium and redox status in broilers than did SEL. These results indicate that the dietary selenium supplied in an organic form (selenium yeast as SP) improved the selenium and redox status in broilers, leading to greater resistance to oxidative stress than when the inorganic form of selenium (SEL) was fed.     

Apparent and standardised ileal digestibility of amino acids in wheat,triticale and barley for broiler chickens at two different ages

ABSTRACT

1. The influence of the age of broiler chickens on ileal amino acid (AA) digestibilities in three cereals was examined. The digestibility estimates, including both apparent (AID) and standardised (SID) coefficients, for wheat, triticale and barley samples were determined using 14 and 28 d old broilers of mixed sex, fed diets containing the test grains as the only source of protein.

2. Each assay diet was offered to five replicate cages of 14 and eight birds respectively, for the 14 and 28 day samplings. On both sampling days, the contents from the lower half of the ileum were collected for determination of the AID coefficients. The SID coefficients were calculated using ileal endogenous amino acid (EAA) losses previously determined from birds fed an N-free diet.

3. The average AID for essential AA (EAA) in triticale and barley were significantly higher at 28 day compared to those at 14 days. In contrast, the magnitude of AID coefficients of AA in wheat was found to be in larger in the younger chickens.

4. Correction of AID for EAA losses increased the values of digestibility coefficients by an average of about 7%. The SID values of AA in wheat was not influenced by the age of broilers. The SID of most AA in triticale, and of all the AA in barley, were higher in 28-day-old chickens compared with their 14-day-old counterparts. The results suggested that the SID coefficients of EAA in wheat determined at day 28 could be applied to feed formulations for 14-day-old broilers, but in the case of triticale and barley, the values obtained in older chickens were not applicable in younger birds and vice versa.     

Effects of dietary threonine and crude protein on growth performance, carcase and meat composition of broiler chickens

1. The aim of the study was to investigate the effects of dietary threonine (Thr) and crude protein (CP) in maize-soybean meal based diets on the growth performance, carcase traits and meat composition of broiler chickens and to determine the dietary Thr requirement for optimum performance (weight gain and feed conversion efficiency (FCE)) at 0 to 3 weeks and 3 to 6 weeks of age. 2. Two basal diets that differed in CP (191.3 or 179.7 and 176.7 or 165.4 g/kg at 0 to 3 and 3 to 6 weeks, respectively) were formulated to have identical contents of Thr (6.0 and 5.4 g/kg), energy (12.97 and 13.39 MJ ME/kg) and other essential amino acids except for Gly + Ser. Basal diets were supplemented with L-Thr from 0.6 to 1.8 g/kg in 0.6 g/kg increments. Broiler chicks (540) were randomly allocated to 9 dietary treatments with 6 replicates of 10 (5 female, 5 male) chicks. 3. A significant interaction between dietary CP and Thr was found for feed intake, body weight (BW) gain and FCE. Increasing Thr supplementation improved feed intake, BW gain and FCE, especially in high CP diets in both feeding periods. 4. Incremental increases in dietary Thr increased breast yield at both CP levels and drumstick yield only on high CP diets. The proportion of thigh decreased with Thr concentration. Liver weight was significantly reduced by Thr supplementation; abdominal fat was not affected. 5. Estimated Thr requirements for FCE increased as dietary CP increased according to an exponential model. This model indicated higher Thr requirements than those of broken-line models for growth performance.     

Effects of inulin on growth performance,nutrient digestibility and metabolisable energy in broiler chickens

1. A 35 d feeding trial with 480 d-old male broiler chickens (Cobb) was conducted to evaluate the effects of graded levels of inulin (0, 5, 10, 15 and 20 g/kg diet) in a maize-soybean meal based diet on growth performance, AME_N and apparent ileal digestibility (AID) of crude protein, amino acids, starch, crude fat and fatty acids. A commercial product (Inulina 95®) containing 914?0 g/kg of inulin and a degree of polymerisation (DP) ranging from DP 1–10 to DP 51–60 was used.

2. The performance of birds (feed intake, BW gain and feed to gain ratio) was not affected by the different dietary inclusions of inulin.

3. Dietary inulin significantly improved the AID coefficient of crude protein and crude fat. Also, there was a significant effect of inulin on the digestibility of most amino acids (10 out of 15) and major fatty acids (oleic and linoleic acids). The AID coefficient of starch and the AME_N content of diets were not affected by inulin inclusion.

4. In conclusion, the current study showed that inulin supplementation improved the digestibility of protein and fat in a maize-soybean meal based diet, but had no effect on the performance of broiler chickens.     

The effects of supplementary bacterial phytase on dietary energy and total tract amino acid digestibility when fed to young chickens

1. Four diets were offered to broiler chickens from 7 to 17 d of age; these included a phosphorus-adequate positive control (PC) (4·7 g/kg available P), a sub-optimal P negative control (NC, 2·5 g/kg available P) with (500 and 12500 FTU/kg) and without phytase. Dietary apparent metabolisable energy (AME), dietary net energy for production (NEp), the efficiency of AME retention (Kre), heat production and total tract amino acid digestibility coefficients were determined. The determination of NEp involved a comparative slaughter technique in which growing chickens were fed the experimental diets ad libitum. 2. Feed intake, weight gain and feed conversion efficiency increased significantly in a dose dependent manner in response to dietary phytase activity. Overall, the NEp of the phytase supplemented diets significantly improved by approximately 15·6% compared with the negative control, while dietary AME was unaffected. Although phytase did not affect AME, the large increase in the NEp demonstrated that dietary phytases improves energy utilisation, i.e. diverting more energy, not accounted for in the AME procedure, for production. This is largely a result of the stimulatory effect that phytase has on feed intake rather than on digestibility of the diet. 3. Overall, the diet supplemented with 12500 FTU had 6·4% significant improvement in total tract digestibility coefficients of the total amino acids compared with the negative control. With regard to individual amino acids, the impact of phytase was far more pronounced for threonine, an important component of the gastrointestinal mucin, than for other amino acids. 4. Dietary NEp was more highly correlated with performance criteria than dietary AME and seems to be a more sensitive way to evaluate broiler response to phytase supplementation.     

Responses of broiler chickens to dietary protein: effects of early life protein nutrition on later responses

1. A study was conducted with modern broiler chicks to test the effects of early life protein nutrition and sex on responses in growth and body composition to dietary protein at a later age. Effects on the incidence of metabolic disorders were also evaluated. 2. From 11 to 26 d of age (EXP1), birds were given 8 diets varying in balanced protein to energy ratio (BPE ratio) between 0.575 and 1.100 g digestible lysine per MJ AMEn. Birds from two treatment groups in EXP1 (BPE ratio of 0.725 and 1.025 g/MJ, respectively) were subsequently used in a test from 26 to 41 d of age (EXP2). In EXP2, 8 diets were used, varying in BPE ratio between 0.500 and 1.025 g/MJ. 3. Responses in weight gain and feed conversion to BPE ratio in EXP2 changed considerably when BPE ratio in EXP1 was modified, irrespective of sex. Up to 10% improvement in both weight gain and feed conversion in EXP2 was observed if BPE ratio in EXP1 was 0.725 compared with 1.025 g/MJ. With males, however, the effect of treatment in EXP1 on weight gain in EXP2 was present only at high BPE ratios. 4. For the relative gain of breast meat and abdominal fat, but not for carcase, the responses of male broilers to BPE ratio in EXP2 were altered by the BPE ratio in EXP1. With females, responses in composition of the gain to diet in EXP2 were independent of BPE ratio in EXP1. 5. The incidence of metabolic disorders was low, irrespective of treatment in EXP1. The lower BPE ratio in EXP1 increased mortality in EXP2 from 0.8 to 3.6%. 6. Our findings show that broiler responses to dietary protein depend on previous protein nutrition and sex. Effects of early life protein nutrition on incidence of metabolic disorders were not observed. The results strongly suggest that protein levels in grower and finisher diets should not be optimised independently, but simultaneously.     

The effect of dietary energy:protein ratio,protein quality and food allocation on the efficiency of utilisation of protein by broiler chickens

1. Various theories have been proposed to explain the reduced performance of broilers when given feeds excessively high in protein, but a satisfactory solution to this problem had, up to now, not been found. Three experiments were conducted to test the hypothesis that the efficiency of utilisation of protein (e_p) is a linear–plateau function of the ratio between the feed apparent metabolisable energy and digestible crude protein contents (AMEn:DCP) and that dietary protein quality, feed allocation and sex do not influence this relationship.

2. A ‘linear–plateau’ model successfully described the efficiency of protein utilisation (e_p) as a function of AMEn:DCP in all three experiments. In Experiment 1, with both sexes being both ad libitum and control fed, the breakpoint was at 58.6 MJ AMEn/kg DCP. In Experiment 2, both sexes were fed balanced and unbalanced protein series, and at different rates, the slopes of the ascending part of the linear–plateau relationships for the different treatments were the same for all treatments (0.0204), and the inflection point was at 71 MJ AMEn/kg DCP. Using similar treatments in Experiment 3, the breakpoint for the balanced protein was 72 MJ/kg and for the unbalanced, 64, with a combined slope of 68 MJ AMEn/kg DCP.

3. The three experiments provide adequate evidence that e_p is a linear–plateau function of the dietary AMEn:DCP ratio with a breakpoint of around 66.2 ± 1.98 MJ AMEn/kg DCP. Below this critical ratio, food intake declines as does protein and lipid retention.

4. That broilers, like pigs, exhibit an energy-dependent phase when high-protein feeds are offered is of practical importance when formulating pre-starter feeds for broilers and starter feeds for turkey poults as the e_p of such feeds may well fall below the maximum due to the lack of dietary energy required to process the high dietary protein contained in such feeds resulting in poorer performance than expected.