Growth performance, caecal microbiome profile, short-chain fatty acids, and litter characteristics in response to placement on reused litter and combined threonine, arginine and glutamine supplementation to juvenile male broiler chickens

BACKGROUND

Exposure of broilers to litter microbiome may increase specific amino acid (AA) requirements towards activated immune responses. This may challenge the generality of the ideal protein (IP) concept, in which dietary essential AA to lysine ratios aimed to mimic presumably constant AA to lysine ratios in whole bird requirements. Therefore, we tested the effect of threonine, arginine and glutamine (TAG) supplementation to IP-based control diets (C) on performance, caecal microbiome composition, short-chain fatty acids and litter characteristics of broiler chickens placed on reused litter.

RESULTS

Thirty-two pens with ten male broiler chickens each were used in a 2 × 2 factorial arrangement of two diet treatments (with or without TAG supplementation) and two litter treatments (placement on clean or reused litter) for 21 days (n = 8). Caecal contents were analysed for microbiome profile using percent guanine + cytosine (%G + C profile) method and short chain fatty acids. TAG-supplemented birds underperformed compared to C birds (P = 0.002), whereas birds placed on reused litter outperformed those on clean litter (P = 0.047). Diet, reused litter and their interaction impacted the %G + C profile at different ranges. Whilst TAG supplementation reduced bacterial abundance at %G + C 51-56 (P < 0.05), reused litter placement tended to reduce %G + C 23-31 and increase %G + C 56-59 (P < 0.10). However, TAG supplementation reduced bacterial abundance at %G + C 47-51 (P < 0.05) and increased caecal branched chain fatty acids on clean litter only (P = 0.025). Greater levels of propionic acid were observed for C birds placed on reused litter only (P = 0.008). Litter pH was greater for reused litter pens than clean litter pens at day 21 (P < 0.001). In addition, litter moisture content was less for TAG birds and reused litter pens compared to C birds (P = 0.041) and clean litter pens (P < 0.001), respectively.

CONCLUSIONS

These data support the view that irrespective of performance benefits arising from bird placement on reused litter, TAG supplementation to IP-formulated baseline rations impaired growth, supported by the lowered abundance of caecal bacteria known to dominate in well-performing birds and greater levels of caecal branched chain fatty acids.

Effects of Total Sulfur Amino Acids on Growth Performance, Immunity, and Meat Yield in Broilers Fed Diets With and Without Antibiotics

Two experiments were conducted to evaluate the effects of digestible sulfur amino acids (SAA) on performance, carcass yield, immunity, and amino acid transporters in broilers fed diets with or without an antibiotic growth promoter (AGP). In experiment 1, a total of 250 1-day-old Cobb500 male chicks were assigned to battery cages with two levels of AGP (0 and 0.05% bacitracin) and five levels of SAA (0.7, 0.8, 0.9, 1.0, and 1.1%) for 21 d. In experiment 2, a total of 900 1-day-old Cobb500 male chicks were assigned to floor pens with two levels of AGP and three levels of SAA for the starter (0.7, 0.8, and 0.9%) or finisher phase (0.52, 0.62, and 0.72%) for 42 d. In experiment 1, from 0 to 7 d, the body weight gain (BWG) was the lowest for birds fed 0.7% SAA. The AGP significantly decreased the feed conversion ratio (FCR), and birds fed 0.9 and 1.1% SAA had significantly lower FCR than 0.7% SAA. From 8 to 14 d, for the AGP-fed birds, the lowest BWG was observed in the 0.7% SAA group. In birds not fed AGP, birds fed 0.8% SAA had higher BWG than 0.7 and 1.1% SAA. Birds fed 0.7% SAA diet had lower feed intake (FI) than 0.8% SAA and higher FCR than 0.8, 0.9, and 1.0% SAA. In experiment 2, from 0 to 21 d, the lowest BWG and the highest FCR were observed in birds fed 0.7% SAA, whereas birds fed 0.9% SAA had the highest BWG and lowest FCR. From 22 to 42 d, FCR was lower for birds fed AGP, and for birds fed 0.72%. Interactions between the factors were found for FI and BWG. The whole thigh and wing weights were the highest for 0.62% SAA, and the pectoralis major weight was higher for birds fed 0.62% SAA than those fed 0.52% SAA. There was an interaction between SAA and AGP for Lat1 (large neutral amino acid transporter) expression, and AGP-fed birds had higher expression of ileal interleukin 1β (Il−1β gene). The interleukin 10 (Il-10) expression was upregulated in the ileum. There was an interaction between factors for sodium-dependent neutral amino acid transporter B [0] AT1 (SLC6A19) expression. The results suggested that both AGP and SAA supplementation would affect the growth performance of the broilers.

Dietary glycine equivalent and standardized ileal digestible methionine + cysteine levels for male broiler chickens fed low-crude-protein diets

This study was conducted to determine the optimum dietary glycine equivalent (Glyequi) level in low-crude-protein diets of 181 g·kg−1 containing varied concentrations of standardized ileal digestible (SID) methionine + cysteine (Met + Cys) for broiler chicks (1–21 d old). A total of 1275, 1-d-old Cobb-Vantress® male broilers were distributed in a 5 × 3 factorial arrangement with a completely randomized design of 15 treatments with five replicates of 17 birds each. Treatments consisted of five levels of dietary Glyequi (14.9, 16.4, 17.9, 19.4, and 20.6 g·kg−1) and three concentrations of SID Met + Cys (7.70, 9.0, and 10.3 g·kg−1). Interactions between Glyequi and SID Met + Cys levels were observed for feed:gain (P = 0.055) and breast meat yield (BMY) (P = 0.017). In 7.7 and 9.0 g SID Met + Cys·kg−1 diets, optimal feed:gain and increased BMY were observed at Glyequi levels not lower than 17.9 g·kg−1. In 10.3 g SID Met + Cys·kg−1 diet, a lower feed:gain was achieved at 19.4 g Glyequi·kg−1. Therefore, a minimum dietary level of 17.9 g Glyequi·kg−1 is needed to increase growth of broilers fed diets containing 7.7 or 9.0 g·kg−1 SID Met + Cys, whereas 19.4 g·kg−1 Glyequi is necessary in diets containing 10.3 g·kg−1 SID Met + Cys for optimum growth.

Cyst(e)ine fortification in low crude protein diet improves growth performance of broilers by modulating serum metabolite profile

This study was aimed to explore the metabolomical mechanisms for the potentially ameliorative effect of cyst(e)ine (Cys) fortification on growth performance of broilers fed low crude protein (CP) diet. A total of 432 1-d-old broilers were randomly divided into 6 groups, each of which received one of the following diets: normal-CP diet (positive control, PC), low-CP diet (negative control, NC), NC diet fortified with 0.05%, 0.1%, 0.15% or 0.2% of Cys. Samples were collected on d 42. Results showed that increasing Cys fortification quadratically elevated (P < 0.05) the accumulative growth performance and leg muscle yield of broilers fed NC diet, with 0.1% being the optimal dose. Thus, samples from PC, NC and NC plus 0.1% Cys (NCC) groups were selected for further analysis. Both dietary CP reduction and fortification of 0.1% Cys in NC diet caused complex changes (P < 0.05) in serum amino acids and some other metabolites primarily involved in lipid metabolism. Multiple lipogenesis-related pathways were regulated (P < 0.05) following Cys fortification in NC diet, which could at least partially interpret the benefit of Cys fortification in NC diet on broiler performance. In conclusion, fortifying low-CP diet with 0.1% Cys promoted the growth performance of broilers probably through modulating serum metabolite profile.

Changes in cecum microbial community in response to total sulfur amino acid (TSAA: DL-methionine) in antibiotic-free and supplemented poultry birds

The effect of essential total sulfur amino acids (TSAA) like methionine and cysteine on the cecal microbiome of broilers was investigated at 2 different time points (days 21 and 42) of broiler rearing. A total of 360-day-old Cobb male broiler chicks were randomly distributed to 6 dietary treatments in a 2 × 3 factorial arrangement, with 2 levels of antibiotic growth promoters (AGP: 0 and 0.05%) and 3 levels of TSAA (DL-methionine) either for starter (0.7, 0.8, and 0.9%) or finisher chicks (0.52, 0.62, and 0.72%), labeled as diets 1 to 6. Cecal digesta from each replicate (n = 10) were sampled on days 21 and 42. DNA was extracted for the amplification of the V4 region of bacterial 16S rRNA genes and subjected to Illumina sequencing. Bioinformatic analyses were performed using QIIME, Mothur, and ad hoc tools and functional profiles of the inferred metagenome were analyzed using PICRUST. Statistical difference was determined by 2-way ANOVA and PERMANOVA. Clustering of cecal communities using PCoA showed clear separation of microbial communities based on age (P < 0.05) of birds and between low and medium/ high levels of TSAA (DL-methionine). At day 21, bacterial richness and diversity were higher than at day 42 where Clostridium cluster XI and Lactobacillus were found most abundant. No variability in taxonomic richness at the genus level was observed with AGP and DL-methionine supplementation. Interbird variation for richness was greater at day 42 compared to day 21. The mean fold difference of richness was greater (1.5 mean fold) with diets 1 and 6, suggesting interactive effects of AGP and TSAA (DL-methionine) in the diet. KEGG function profiles calculated by PICRUST suggest that the cecal microbiome increased glycolysis and energy generation correlated with increased dietary TSAA (DL-methionine) supplementation levels during the late broiler growth period (day 42). This study increases our knowledge of microbial dynamics and functions that are relevant to host nutrition and performance that may help us tailoring alternative strategies for raising poultry birds under antibiotic-free conditions.

Broilers fed a low protein diet supplemented with synthetic amino acids maintained growth performance and retained intestinal integrity while reducing nitrogen excretion when raised under poor sanitary conditions

The present study investigated the effects of supplementing a low protein (LP) diet supplemented with key essential amino acids (AA) to broilers on growth performance, intestinal tract function, blood metabolites, and nitrogen excretion when the animals were maintained under various sanitary conditions for 35 D after hatching. Three hundred eighty-four one-day-old male broilers (Ross 308) were randomly allotted to groups that received one of 6 dietary treatments in a 2 × 3 factorial arrangement (i.e., 2 environmental conditions and 3 dietary treatments) to give 8 replicates per treatment. Broilers were challenged with 2 environmental conditions (sanitary vs. poor sanitary). The dietary treatments were (1) high protein (HP) diet, (2) LP diet, and (3) LP diet with synthetic key essential AA (LPA): the LP diet was supplemented with synthetic AA up to the required levels for broilers. On day 14, birds consumed the LP diet impaired growth performance compared with those fed the HP diet, while the average daily weight gain-to-feed conversion ratio of birds fed the LPA diet improved to the level of birds fed the HP diet under poor sanitary conditions (P < 0.05). Broilers raised under poor sanitary conditions and fed the LP diet displayed higher (P < 0.05) zonula occludens (ZO-1) expression on day 14 than broilers fed either the HP or LPA diet. Under sanitary conditions, birds fed HP and LPA diets showed higher villus height and crypt depth compared with those of broilers fed the LP diet on day 35. Moreover, broilers raised in the poor sanitary environment had higher (P < 0.05) serum endotoxins than those raised in the sanitary environment. Broilers fed the LPA diet showed reduced (P < 0.05) nitrogen excretion on days 14 and 35 compared with those fed the LP and HP diets independent of the environment. In conclusion, the LPA diet did not impair growth performance under poor sanitary conditions for 14 D after hatch while resulting in lower nitrogen excretion in any environment conditions throughout the experiment.

Feeding broiler chicks diets containing keto- and hydroxy-acids: performance and carcass weight

This study evaluated reduced dietary CP and supplementing amino acid analogs to sustain growth and carcass weight in 0- to 21-day-old Cobb × Avian-48 male broiler chicks. A total of 6 diets with 3 levels of CP (22.5, 19.5, and 16.5%) and 2 sources of AA analogs, either synthetic amino acids (SA) or keto-/hydroxy-acids (KA), were assigned randomly to 36 cages (8 chicks/cage) in a 3 × 2 factorial design. For SA diets, DL-Met, DL-Met + L-Ile, and D-Met + L-Ile + L-Val were used to supplement 22.5, 19.5, and 16.5% CP diets, respectively, and for corresponding KA diets, DL-Met was replaced with methionine hydroxy analog (MHA), L-Ile was replaced with keto-Ile, and L-Val was replaced with keto-Val. Water and all isocaloric diets (3,050 kcal ME/kg) were given ad libitum. Lowering dietary CP to 16.5% reduced BW at 7, 14, and 21 D (P ≤ 0.0001) and feed intake at 8 to 14, 15 to 21, and 0 to 21 D (P ≤ 0.001). Body weight gain (BWG) was reduced and feed-to-gain ratio (FGR) was increased (P ≤ 0.003 to 0.0001) at all times for chicks fed 16.5% CP; however, chicks fed 22.5 and 19.5% CP had comparable performance. Differences in 0 to 7 D BWG (SA, 122.9 vs. KA, 113.9 g/bird; P ≤ 0.04), a 0 to 21 D FGR cumulative effect (1.45 vs. 1.51; P ≤ 0.02), and a 15 to 21 D (P ≤ 0.04) and 0 to 21 D (P ≤ 0.05) CP × AA interaction were also observed. Greater liver weight among 16.5 vs. 19.5 or 22.5% CP fed chicks was found at 14 and 21 D (P ≤ 0.0001 and P = 0.06, respectively). Lower dietary CP reduced spleen weight on day 21 birds (P ≤ 0.0005) with lighter spleens among 16.5 and 19.5% vs. the 22.5% CP fed group (0.090, 0.095, 0.119 g/100 g BW, respectively). Breast weight at 21 D was significantly less for 16.5 vs. 22.5% CP fed chicks. Fat pad weight on day 21 was heaviest among 16.5% chicks (P ≤ 0.0004). Overall, lowering dietary CP to 16.5% had a negative effect, but keto-acid supplementation supported 0 to 21 D broiler growth compared to SA; however, transamination efficiency of KA may be lower for 0 to 7D old chicks compared to older birds.

Threonine supplementation reduces dietary protein and improves lipid metabolism in Pekin ducks

1. This study was conducted to investigate the efficiency of threonine (Thr) supplementation on reducing dietary crude protein (CP) content and the effects of Thr on lipid metabolism in Pekin ducks. The effects of dietary CP concentration (160, 190 and 220 g/kg) and Thr supplemental concentration (0, 0.7, 1.4, 2.1 and 2.8 g/kg) on growth performance, carcass, liver lipid and plasma profiles were determined in Pekin ducks from 1-21 d of age. 2. A total of 720-d-old male Pekin ducks were randomly allotted to 1 of 15 dietary treatments with 6 replicate cages of 8 birds per cage for each treatment according to average body weight. 3. Dietary Thr supplementation improved growth performance and breast muscle percentage at all CP diets, and ducks fed Thr-supplemented diets had higher plasma concentrations of some plasma amino acids. Thr supplementation reduced the concentrations of total lipid, triglyceride, cholesterol in liver, and plasma low density lipoprotein cholesterin concentration at 160 and 190 g/kg CP, whereas it increased triglyceride concentration at 160 g/kg CP. 4. Thr requirements based on quadratic broken-line model estimation were 6.6 and 7.0 g/kg for optimal average daily gain (ADG), and 6.7 and 7.3 g/kg for breast muscle percentage of Pekin ducks from 1-21 d of age at 190 and 220 g/kg CP, respectively. The dietary Thr requirements and estimated ADG (55.18 vs. 55.86 g/d/bird) and breast muscle percentage (2.79% vs. 2.75%) of Pekin ducks did not differ between 190 and 220 g/kg CP according to the t-test results. 5. Dietary CP level could be reduced to 190 g/kg in Pekin ducks from 1-21 d of age with Thr supplementation to balance dietary amino acids, and Thr supplementation prevented excess liver lipid deposition in this instance.

Individual and combined effects of crude protein, methionine, and probiotic levels on laying hen productive performance and nitrogen pollution in the manure

The present study aimed to investigate the effect of the dietary levels of protein, methionine (Met), as well as probiotic on productive performance, feed utilization, and environmental pollution by N in Lohmann Brown laying hens. A total number of 160 Lohmann Brown laying hens at 20 weeks of age were randomly divided into 8 treatment groups using a 2 × 2 × 2 factorial design experiment. The experiment involved two levels of crude protein (16 and 18 %), two levels of Met (0.45 and 0.50 %), and two concentrations of probiotic (0 or 1 g/kg diet, with a concentration of 10¹⁰ CFU/g of Lactobacillus acidophilus) within 20-42 weeks of age. Results revealed that egg production parameters were significantly (P < 0.01) improved for hens fed diets of 18 % CP comparing with that of 16 % protein within the period from 26 to 30 weeks of age. Protein utilization and feed efficiency values were enhanced with 0.67 and 0.72 % Met during the period of 26-30 weeks of the age. For the N pollution, results showed that increasing crude protein in the diet from 16 to 18 % caused significant (P < 0.01) increase in the excreted N from 0.349 to 0.492 g/d. The methionine level of 0.72 % recorded the highest values of total consumed N being 3.98 g/d and excreted N being 0.527 g/d comparing with the other levels. It could be concluded that the best productive performance could be given by using 0.72 % total sulfur amino acids (TSAA). Furthermore, the dietary level of 18 % CP with 0.72 % Met is preferred in feeding laying hens through the whole experimental period. Ecologically, reducing the level of crude protein in layer diets to be 16 % along with the supplementation of Met can play an important role in minimizing the pollution with N from poultry excretion.

Manipulation of dietary methionine+cysteine and threonine in broilers significantly decreases environmental nitrogen excretion

The intensification of livestock have increased the emission of pollutants to the environment, leading to a growing interest in seeking strategies that minimise these emissions. Studies have shown that it is possible to manipulate diets by reducing CP levels and thus reducing nitrogen (N) excretion, without compromising performance. However, there is no knowledge of any study that has focused on reducing N excretion and relating this reduction to individual amino acids. This study investigated the effect of dietary methionine+cysteine (MC) and threonine (THR), the two most limiting amino acids for broiler production, on nitrogen excretion (NE) and nitrogen deposition (ND) and determined the efficiency of utilisation of both amino acids for protein deposition. Six trials were conducted to measure the NE and ND in broiler chickens during three rearing phases in response to dietary amino acid. The efficiency of utilisation of the amino acids was calculated by linear regression of body protein deposition and the amino acid intake. Despite the differences between sexes and phases, the efficiency of utilisation was the same, being 0.60 and 0.59 for MC and THR, respectively. The rate of NE behaved exponentially, increasing with amino acid intake, and can exceed 50% of N intake, being higher than ND. On average, for a reduction in intake of each unit of MC or THR (mg) there is a reduction of 0.5% of NE. Although this reduction seems low, considering that it corresponds to changes in one amino acid only, the impact on a large scale would be significant. Knowledge of how animals respond to NE and ND/protein deposition according to amino acid dietary content may represent new efforts towards reducing the impact on environment.

Evaluation of protein concentration and limiting amino acids including lysine and met + cys in prestarter diet on performance of broilers

Four experiments were conducted, in two stages, to evaluate protein and limiting amino acids' (lysine and methionine + cystine) levels in pre-starter diets on broilers' performance. In each experiment of Stage 1, 640 new-born male Ross 308 cockerels were randomly allocated to eight dietary treatments with a 2 × 4 factorial arrangement. In experiment 1-1, two levels of crude protein (CP: 21% and 23.2%) and four levels of Lys (1.2, 1.3, 1.4, and 1.5%) and in experiment 1-2, two levels of CP (21 and 23.2%) and four levels of Met + Cys (0.85, 0.90, 0.95, and 1.00%) were used. In Stage 2, the optimum levels of Lys and Met + Cys obtained from Stage 1 (1.3 and 1.5% Lys, 0.90 and 1.00% Met + Cys in experiment 1-1 and 1-2, resp.) with two levels of CP (21 and 23.2%) were used in two separate simultaneous experiments with a 2 × 2 factorial arrangement for male and female birds. The levels of CP significantly influenced BWG and FCR in experiment 1-1. Dietary levels of Lys affect BWG (experiment 1-1) and FI (experiments 1-1 and 2-1) significantly. In experiments 1-2 and 2-2, the different levels of Met + Cys did not affect BWG, FI, and FCR of male or female broilers. The results of these experiments indicated that the optimal level of dietary protein and Lys were 23.2% and 1.5%, respectively. Diets with 1% Met + Cys caused optimal performance.

Gompertz-Laird model prediction of optimum utilization of crude protein and metabolizable energy by French guinea fowl broilers

This study was conducted to assess the influence of dietary CP and ME on growth parameters of the French guinea fowl, a meat-type variety. In a 2 x 3 x 3 factorial arrangement, 297 one-day-old French guinea keets (162 females and 135 males) were randomly assigned to experimental diets comprising 3,050, 3,100, and 3,150 kcal of ME/kg, each containing 21, 23, and 25% CP from hatch to 4 wk of age (WOA), and 3,100, 3150, and 3,200 kcal of ME/kg, each containing 19, 21, and 23% CP at 5 to 8 WOA. Using BW and G:F data from hatch to 8 WOA, the Gompertz-Laird growth model was employed to estimate growth patterns of the French guinea fowl. Mean differences in exponential growth rate, age of maximum growth, and asymptotic BW among dietary CP and ME levels were not significant. However, instantaneous growth rate and weight at inflection point were significantly higher (P < 0.05) in birds on the 25% CP diet than those on the 21% CP diet at hatch to 4 WOA (1.12 kg/wk and 0.79 kg vs. 1.04 kg/wk and 0.74 kg, respectively). The exponential growth rate was also higher (P < 0.05) in birds fed the 3,050 kcal of ME/kg diet with either 23 or 25% CP than those fed diets containing 3,050 kcal of ME/kg and 21% CP. Mean G:F was higher (P < 0.05) in birds fed diets containing 3,050 kcal of ME/kg and either 21 or 23% CP than those in other dietary treatments. Therefore, based on the Gompertz-Laird growth model estimates, feeding 21 and 23% CP and 3,100 kcal of ME/kg at hatch to 4 WOA and 19 and 21% CP with 3,150 kcal of ME/kg at 5 to 8 WOA can be recommended as adequate for growth for the French guinea fowl broilers.

Glycine supplementation to low protein, amino acid-supplemented diets supports optimal performance of broiler chicks

Six experiments were conducted to determine the effects of low CP in diets for broilers and to evaluate limiting essential and nonessential amino acids (AA) in these diets. All experiments were conducted with Ross x Ross broilers in brooder batteries from 0 to 17 or 18 d posthatch. Treatments were replicated with 6 pens of either 5 or 6 broilers each. In Experiment (Exp.) 1, corn-soybean meal diets were formulated to 16.18, 17.68, 19.18, 20.68, or 22.18% CP. The 22.18% CP diet provided 1.23% Lys and 0.89% TSAA, met or exceeded all nutrient requirements of young broilers, and served as the positive control (PC) diet in all experiments. Increasing dietary CP linearly increased final BW, daily gain (ADG), and gain:feed (G:F) (P < 0.005). In Exp. 2, additions of crystalline essential (EAA) or nonessential AA (NEAA) were added to the low CP diet to simulate the AA profile of the PC. Daily gain, final BW, and G:F were decreased (P < 0.01) when CP was reduced, but the addition of the NEAA increased final BW, ADG, and G:F (P < 0.07) to the level of broilers fed the PC. Addition of EAA alone was without effect. In Exp. 3, chicks fed diets with supplemental Glu, Ala, Asp, or Pro had reduced daily feed intake (ADFI), ADG, and final BW (P < 0.05) compared with the PC diet. Addition of Gly or the combination of Gly, Glu, Asp, Ala, and Pro to the low CP diet increased G:F (P < 0.01) compared with chicks fed PC, and ADG was not different from that of broilers fed the PC diet. In Exp. 4, chicks were fed either the PC diet, the low CP diet with Gly + Ser concentrations of 1.23, 1.35, 1.47, 1.59, 1.71, 1.83, 1.95, or 2.07%, or a 10th diet that contained 1.23% Gly + Ser and with Glu to equal the N concentration of the 2.07% Gly + Ser diet. Final BW, ADG, and G:F were increased linearly (P < 0.001) as the concentration of dietary Gly + Ser was increased. Chicks fed the low CP diet with 2.07% Gly + Ser had growth performance that was not different from that of chicks fed the PC. The addition of Glu to the low CP diet was without effect. In Exp. 5, chicks were fed the PC with additions of 0, 0.15, or 0.30% Gly or the low CP diet containing 1.60, 1.72, 1.84, 1.96, 2.08, 2.20, or 2.32% Gly + Ser. Glycine addition to the PC had no effect, but Gly addition to the low CP diet increased G:F linearly (P < 0.001). Growth performance of chicks fed the low CP diet with 2.32% Gly + Ser was equal to that of chicks fed the PC diet. In Exp. 6, chicks were fed the PC or the low CP diet containing 1.80, 1.95, 2.10, 2.25, 2.40, 2.55, 2.70, 2.85, or 3.00% Gly + Ser. Glycine addition to the low CP diet increased G:F linearly (P < 0.001). In summary, low CP diets result in optimal growth of broilers with Gly + Ser levels of 2.44%.