Impact of Glycine Supplementation to Dietary Crude Protein Reduction in Broiler Chickens

Abstract

A 21-day experiment was conducted to evaluate the effect of dietary reduction of crude protein (CP) concentrations with graded levels of supplemental glycine (Gly) on growth performance of broiler chickens. Day-old chicks (n = 250) were randomly divided into five treatment groups which were divided into five replicates of ten chicks each in a completely randomized design. The treatments were as follows: T1 comprised of the control group with a standard CP diet (SCPD; 3100 kcal ME/kg and 22% CP) while T2, T3, T4 and T5 comprised of groups fed reduced CP diets (RCPD; 3100 kcal ME/kg and 19% CP) with supplemental Gly at 0.2, 0.4, 0.6 and 0.8% graded levels, respectively. Weight gain (WG), feed intake (FI) and feed conversion ratio (FCR) data was collected on a weekly basis. Final body weight and weight gain of birds fed control and 0.8% Gly diets were similar and higher (P < 0.05) than those fed other treatment diets. A similar FCR was recorded among birds fed control, 0.6% and 0.8% Gly diets but lower (P < 0.05) than other treatment groups. Therefore, a minimum level of 0.6% Gly supplementation is necessary to optimized performance of broilers (21-d old) fed RCPD.

1. Introduction

The mounting demand of animal proteins for an expanding global population in the face of limited natural resources shall be guided by the responsibility to increase productivity while minimizing environmental impact. Leaving conventional animal feeding methods in the past and shifting to well establish modern dietary strategies could play a substantial role in securing a smaller ecological footprint from animal production. This means lowering dietary crude protein (CP) while supplementing essential amino acids (AA) to cover the nutritional requirements of the broilers. Growing emphasis on environmental regulation requires global animal production to adopt strategies like feeding low CP diets to minimize nitrogen (N) excretion. Furthermore, N excretion declines by approximately 14% providing strong environmental incentives to successfully reducing CP in broiler feeds by greater than 30 g/kg [1, 2, 3]. The formulation of these diets is typically based on decreases in soybean meal and increases in feed grain (maize or wheat) contents coupled with elevated inclusions of non-bound (crystalline and synthetic) amino acids to meet requirements. Real benefits for sustainable chicken-meat production using less resources will stem from the successful development of such diets. There is a genuine quest to develop effective, reduced crude protein (CP)-diets for broiler chickens because their acceptance would generate several material advantages. These advantages range from reduced nitrogen and ammonia emissions, improved litter quality and enhanced bird welfare to less undigested protein passing into the hind gut to fuel the proliferation of potential pathogens [4, 5]. Furthermore, a reduction in dietary CP may improve flock health by reducing the risk of necrotic enteritis (NE) caused by the proliferation of Clostridium perfringens in the hind gut [6, 7]. The economic benefits of reducing dietary CP stem from reductions in energy expenditure on excreting excess N as uric acid and sparing of matrix space in feed formulation for inclusion of less energy dense ingredients, potentially reducing feed costs [8]. Besides, reducing dietary CP has a particular advantage for producers in tropical and subtropical regions especially sub-sahara Africa; where heat stress is a common problem in poultry production, and causes major economic losses annually [9, 10]. Given the fact that heat increment of CP is the highest, compared to fat and carbohydrates [11], it has been proposed that the adverse effects of heat stress on poultry performance can be alleviated by reducing CP [12, 13].

However, in some of the animal feeding studies, lowering dietary CP beyond a certain level showed undesirable effects on growth performance and carcass quality of broilers [14, 15, 16, 17]. Whilst greater reductions of dietary CP (40 to 50 g/kg) invariably compromise broiler performance and increase lipid deposition, a limited number of studies have investigated both aspects [16, 18, 19]. Thus, inferior FCR and increased fat deposition epitomize the challenges to successfully reducing dietary CP using substantial levels of non-bound amino acids. Such findings imply that there is a threshold to CP reductions that can be accommodated by broiler chickens. If the factors contributing to this threshold were to be identified it should be possible to put corrective strategies in place so that tangibly reduced-CP diets, with their attendant advantages, will meet acceptance.

A number of explanatory approaches or reasons have been advanced or debated as the possible consequences of tangibly lowering dietary CP on broiler performance [20]. The difference in the optimal ratio of essential AA between experimental diets [21], specific non-essential AA [22] and utilization of free AA compared to peptide bound AA [16] are among the approaches mostly discussed. Considering the sum of nonessential amino acids probably is not sufficient because specific metabolic processes can become limiting [23]. This leads to the implication that single nonessential amino acids are important to avoid unfavorable effects of low crude protein feed on the growth of broiler chickens [24, 25]. Single nonessential amino acids have been supplemented to low crude protein feed in several studies. Supplementing free glutamic acid, aspartic acid, proline, and alanine consistently did not prevent from reduced growth caused by feeding low crude protein feed [26, 27]. However, growth-increasing effects were determined when free glycine was supplemented. Two studies showed that supplementing feed with a crude protein concentration of 16% with free glycine to the level of about 22% crude protein control feed prevented reduction of growth compared to the control feed [27, 28, 29].

However, the concentration of glycine in nutrition of broiler chickens cannot be considered alone because studies revealed that serine in the feed has the same effect on the growth as glycine [30]. Animals can convert glycine into serine and vice versa. On a molar basis, serine is considered to be as effective as Gly for various functions in poultry due to the inter-conversion between the two amino acids [30]. Owing to their assumed unlimited metabolic inter-conversion, Gly and Ser are usually assessed simultaneously when determining the physiological value of diets. Most studies use the sum of both Gly and Ser concentrations, usually termed ‘Gly + Ser’ to capture the analogous effect of these AAs. Therefore, this study was carried out to evaluate the impact of reducing crude protein concentration with feed-grade glycine supplementation in corn-soybean based diet of broiler chicken on growth performance during the period of 1–21 days of age.

2. Materials and method

2.1 Husbandry and treatments

A total of 250 day old mixed-sex broilers (Arbor acre) with comparable initial body weights were raised at 10 chicks/replicate in a deep litter pens under standard environmental and hygienic practices from day 1 to 35. They were acquired from a reputable commercial hatchery and immunized at the hatchery following a vaccination regime for Arbor acre strain. Birds had free access to mash feed and freshwater during the course of the trial. Feeds (Table 1) based on corn and soybean for starter (1–21 days) phase were prepared according to the recent NRC broiler chicken’s nutritional requirements except for CP. The experiment was performed as a completely randomized design with five dietary treatments arranged in five replications of 10 chicks each. The treatments were as follows: T1 comprised of the control group with a standard CP diet (SCPD) while T2, T3, T4 and T5 comprised of groups fed reduced CP diets (RCPD) with supplemental Gly at 0.2, 0.4, 0.6 and 0.8% graded levels respectively. Diets (mash) and water were provided ad libitum throughout the experiment. All experimental diets were made isocaloric to contain 3100 kcal ME/kg; whereas, SCPD was formulated to contain 22% CP and the RCPD were isonitrogenous to contain 19% CP for T2 – T5. The feed formulation and nutritional composition of the starter (1–21 days of age) diets are shown in Table 1. Dietary treatment groups were set up in an alternating pen pattern within the facility. All birds were housed in the Poultry Research Facility of the Department of Animal Production and Health Technology at the Federal College of Wildlife Management, New Bussa, Niger state, Nigeria. All animals were maintained according to the guidelines specified by the Research Committee Council on Animal Care, and protocols were approved by the Federal College of Wildlife Management Animal Care and Use Committee.

Ingredients	SCPDb	RCPDc + 0.2% Gly	RCPD +0.4% Gly	RCPD +0.60%Gly	RCPD +0.80%Gly
Maize	51.19	60.00	60.00	60.00	60.00
Soybean	38.58	30.45	30.45	30.25	30.23
Soya-oil	4.90	4.00	4.00	4.00	3.82
DCP	1.95	2.00	2.00	2.00	2.00
Limestone	1.38	1.40	1.40	1.40	1.40
Salt	0.50	0.50	0.50	0.50	0.50
Premixa	0.50	0.50	0.50	0.50	0.50
DL-methionine	0.25	0.30	0.30	0.30	0.30
L-Lysine	0.15	0.30	0.30	0.30	0.30
L-Glycine	0.00	0.20	0.40	0.60	0.80
L-Threonine	0.00	0.15	0.15	0.15	0.15
Inert Filler	0.60	0.20	0.00	0.00	0.00
Total	100	100	100	100	100
Nutrients level
Crude protein	22.09	19.40	19.40	19.31	19.30
MEd	3193.72	3198.15	3198.15	3192.75	3177.09
Lysine	1.12	1.12	1.12	1.12	1.12
Methionine	0.56	0.59	0.59	0.59	0.59
Cystine	0.30	0.28	0.28	0.28	0.28
Met+Cys	0.85	0.87	0.87	0.86	0.86
Threonine	0.90	0.93	0.93	0.93	0.93
Arginine	1.27	1.10	1.10	1.09	1.09
Isoleucine	0.94	0.82	0.82	0.82	0.82
Leucine	1.92	1.81	1.81	1.80	1.80
Valine	0.97	0.87	0.87	0.86	0.86
Histidine	0.97	0.83	0.83	0.82	0.82
Phenylalanine	1.13	1.01	1.01	1.01	1.01
Glycine	0.90	0.99	1.18	1.37	1.56
Serine	1.13	1.00	1.00	1.00	0.99
Gly + Ser	2.03	1.98	2.18	2.36	2.56

Table 1.

Ingredients composition and nutrient levels of experimental diets (% as fed).

^a

Vitamin/Mineral Premix supplied per kg of the diet: Vit A: 10,000iu; Vit D: 28000iu; Vit E: 35,000iu; Vit K: 1900 mg; Vit B12: 19 mg; Riboflavin: 7000 mg; Pyridoxine: 3800 mg; Thiamine: 2200 mg; Pantothenic acid: 11000 mg; Nicotinic acid: 45,000 mg; Folic acid: 1400 mg; Biotin: 113 mg; Cu: 8000 mg; Mn: 64000 mg; Zn: 40, 000 mg; Fe: 32000 mg; Se: 160 mg; Iodine: 800 mg; Cobalt: 400 mg; Choline: 475000 mg.

^b

SCPD: Standard crude protein diet.

^c

RCPD: Reduced crude protein diet.

^d

ME: Metabolizable energy.

2.2 Sampling and measurements

Body weight (BW) was measured weekly for each pen. Feed intake was determined as the difference between the amount of feed offered and the amount unconsumed in starter and grower phase. The daily feed intake (DFI) was calculated by dividing each pen’s consumed feed on starter and grower phase by actual total number of birds. The feed conversion ratio (FCR: g feed/g body weight gain) was calculated by dividing daily feed intake by daily body weight gain. On day 35, two birds per replicate with a BW close to the pen average weight were chosen, and the blood for biochemical analyses was taken from the jugular vein and centrifuged at 3000 rpm for 10 min to separate serum that was stored at -80°C. The serum concentrations of total protein, albumin, glucose, triglyceride, and creatinine were determined by utilizing enzymatic colorimetric kits as specified by the manufacturer. Serum globulin was estimated accordingly by subtracting albumin from total protein.

2.3 Statistical analysis

The statistical processing of the results was done using general linear model (GLM) of SPSS, version 20.0 (SPSS Inc., Chicago, IL, USA). When comparing treatments means, post hoc Tukey’s multiple range test was carried out to assess any significant differences for the measured parameters. Differences were considered significant at p < .05. Replicate-pen was used as the experimental units for the analysis.

3. Result and discussion

Growth performance of the broiler chicks fed RCPD containing graded levels of supplemental glycine are shown in Table 2. At day 21, the results of the present study showed that glycine supplementation did not affect (P > 0.05) the initial body weight and feed intake, however, significant differences (P < 0.05) were observed on final body weight, weight gain and FCR of the broiler chicks. Chicks fed control diet had higher (p < 0.05) final body weight and weight gain than the chicks fed RCPD with 0.2, 0.4 and 0.6% supplemental glycine levels. Similar (p > 0.05) final body weight and weight gain were observed among chicks fed control diet and 0.8% Gly supplemented RCPD. Birds fed control, 0.6 and 0.8% Gly diets have similar (p > 0.05) feed conversion ratio, and were lower (p < 0.05) than those on RCPD with 0.2 and 0.4% Gly diet. The present study showed that increasing levels of supplemental Gly increased weight gain and decreased feed conversion ratio among the experimental birds. Gly addition at 0.2 and 0.4% levels which provided 1.98 and 2.18% total Gly + Ser, respectively in the diets failed to completely overcome the adverse impact of dietary reduction of CP by 3% on the performance of the broiler chickens at 21 d old. This observation corroborates with the findings of Awad et al. [30] who concluded that the provision of 2.02–2.22% dietary Gly + Ser during starter period failed to support optimal growth performance in broiler chickens raised under tropical climate. However, providing 2.36 and 2.56% total Gly + Ser concentration in the RCPD through increased supplementation of Gly at 0.6 and 0.8% levels were observed to restored the FCR to equal those group fed control diet.

Treatment	Initial weight (g/bird)	Final body weight (g/bird)	Feed intake (g/bird)	Weight gain (g/bird)	FCR
T1	42.54	893.26a	1111.67	850.72a	1.31a
T2	42.73	723.87d	1195.63	681.14d	1.76a
T3	42.89	794.11c	1164.75	751.22c	1.55b
T4	42.58	825.14b	1156.63	782.56b	1.48^ab
T5	42.55	871.82a	1148.22	829.27a	1.38a
SEM	1.65	12.12	25.86	10.93	0.08
p-values	0.141	0.032	0.311	0.016	0.002

Table 2.

Growth performance of broiler chicken (1–21 d old) fed reduced crude protein diets with graded levels of supplemental glycine.

^a

Means within column with no common superscripts differ significantly (p < 0.05).

^b

Means within column with no common superscripts differ significantly (p < 0.05).

^c

Means within column with no common superscripts differ significantly (p < 0.05).

^d

Means within column with no common superscripts differ significantly (p < 0.05).

T1: Standard crude protein diet (Control).

T2: Reduced crude protein diet +0.2% glycine.

T3: Reduced crude protein diet +0.4% glycine.

T4: Reduced crude protein diet +0.6% glycine.

T5: Reduced crude protein diet +0.8% glycine.

SEM: Standard Error Mean.

This present finding is in agreement with the reports of previous researchers [18, 27, 31, 32] who confirmed that maintaining a minimum level of 2.32% in diets via Gly supplementation allowed to decreased dietary CP concentration up to 3% or more without compromising the accumulative growth performance of broiler chickens (1–21 d old). Also, our result is in accordance with previous reports which suggested that maintenance of optimal amino acid ratios for essential amino acids and sufficient total Gly + Ser levels appear most important considerations in formulating broiler diets with reduced CP concentrations [2, 25, 33, 34, 35]. According to Kamely et al. [35], feeding low protein diets formulated to provide higher Gly + Ser and meet digestible amino acid requirements could be an efficient way to reduce nitrogen excretion to the environment and decrease feed cost without impacting growth performance. Also, the current result concur with the report of Siegert et al. [22], who conducted a meta-analysis of 10 studies and concluded that sufficient supply of dietary Gly + Ser had significant positive effects on weight gain and feed conversion efficiency of birds fed low CP diets. Understanding the active roles play by Gly in several number of non-protein pathways can further account for the reasons why it can potentially improve the performance of broiler chickens supplied with RCPD [36, 37]. Gly plays significant function for methionine recycling and cysteine biosynthesis, threonine catabolism, uric acid and creatine synthesis [38]. Moreover, Gly represents the main part of the gut mucin glycoproteins [39]. In essence, Gly can promote the metabolic and nutritional efficiency of EAA as well as gut functionality and consequently growth performance [13, 40, 41]. In view of these key metabolic roles of Gly, although it is notionally a non-essential amino acid; Gly may become conditionally limiting in reduced CP diets formulated based on vegetable ingredients [41, 42, 43]. Provision of higher dietary level of Gly + Ser via increased Gly supplementation may be essential to achieve greater CP reduction without compromising the growth performance of growing broiler chickens [13, 32, 40].

4. Conclusion and recommendation

In the present study, increasing dietary levels of glycine supplementation resulted in an increased body weight gain and improved feed conversion ratio of broiler chicks fed diets containing reduced crude protein concentration. Maintaining a minimum of 0.6% supplemental Gly that provided 2.36% Gly + Ser level has shown to support the reduction of crude protein concentration from 22 to 19% in diet of broiler chickens at 21 days of age, without undermining their accumulative growth performance and concomitantly minimizing the impact of broiler production on the environment. Therefore, higher Gly supplementation (0.6–0.8% inclusion) can be recommended for the initial stages of growth of broiler chickens up to 21 d of age, based on weight gain and feed conversion responses raised under tropical environment.

As Gly have the potential to limit growth performance of broiler chickens, this amino acid, ideally on a digestible basis, should appear in recommendations suitable for facilitating low CP diets, because such diets are expected to become more important in the future. Growing evidence in the present study shows that a sufficient provision of supplemental glycine is necessary for the optimal growth of chickens. Thus, ideal protein diets for poultry must supply all physiologically and nutritionally amino acids to maximize their growth performance and productivity. This nutritional strategy is expected to facilitate the formulation of low-protein diets and precision nutrition through the addition of low-cost supplemental amino acids or their alternative sources of animal proteins. In regions where free crystalline glycine is prohibited or not approved, an adequate dietary Gly + Ser supply can only be achieved by inclusion of feedstuffs of animal origin which represent good source of glycine, to prepare balanced low protein diets for chickens and help sustain the global animal agriculture for increased food productivity. Thus, in our study, dietary crude protein was reduced with supplemental glycine fortification up to 3% without any adverse effects on broiler performance. So, in future, when amino acid industry expanded and all nutritional amino acids distributed as feed grade supplements for animal use, it could be possible reduce crude protein up to 6% which will be more economic. If progress in these directions can be actualized, then the prospects of reduced protein diets contributing to sustainable chicken-meat production are promising and becomes increasingly real.

Acknowledgments

The authors wish to acknowledge the support of the management of Federal College of Wildlife Management, New Bussa, Nigeria, towards the success of this research project.

Conflict of interest

The authors declare that they have no conflicts of interest associated with this manuscripts.