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Estimation of the optimum standardized ileal digestible total sulfur amino acid to lysine ratio in late finishing gilts fed low protein diets supplemented with crystalline amino acids

Wenfeng Ma

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

College of Animal Science and Technology, Henan University of Science and Technology, NO. 263 Kaiyuan Road, Luoyang, 471023 PR China

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Jinlong Zhu,

Jinlong Zhu

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

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Xiangfang Zeng,

Xiangfang Zeng

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

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Xutong Liu,

Xutong Liu

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

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Philip Thacker,

Philip Thacker

Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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Shiyan Qiao,

Corresponding Author

Shiyan Qiao

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

Correspondence: Shiyan Qiao, State Key Laboratory of Animal Nutrition, China Agricultural University, No.2, Yuanmingyuan West Road, Beijing 100193, China. (Email: [email protected])Search for more papers by this author

Wenfeng Ma,

Wenfeng Ma

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

College of Animal Science and Technology, Henan University of Science and Technology, NO. 263 Kaiyuan Road, Luoyang, 471023 PR China

Search for more papers by this author

Jinlong Zhu,

Jinlong Zhu

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

Search for more papers by this author

Xiangfang Zeng,

Xiangfang Zeng

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

Search for more papers by this author

Xutong Liu,

Xutong Liu

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

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Philip Thacker,

Philip Thacker

Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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Shiyan Qiao,

Corresponding Author

Shiyan Qiao

State Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China

First published: 30 June 2015

https://doi.org/10.1111/asj.12398

Citations: 2

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Abstract

A total of 90 gilts were used to investigate the effects of various standard ileal digestible (SID) total sulfur amino acid (TSAA) to lysine (Lys) ratios on the performance and carcass characteristics of late finishing gilts receiving low crude protein (CP) diets supplemented with crystalline amino acids (CAA). Graded levels of crystalline methionine (Met) (0, 0.3, 0.5, 0.8 or 1.1 g/kg) were added to the basal diet to produce diets providing SID TSAA to Lys ratios of 0.48, 0.53, 0.58, 0.63 or 0.68. At the termination of the experiment, 30 gilts (one pig per pen) with an average body weight (BW) of 120 kg were killed to evaluate carcass traits. Increasing the SID TSAA to Lys ratio increased average daily gain (ADG) (linear and quadratic effect, P < 0.05), improved feed conversion ratio (FCR) (linear and quadratic effect, P < 0.05) and decreased serum urea nitrogen (SUN) concentration (linear and quadratic effect, P < 0.05) of finishing gilts. No effects were obtained for carcass traits. The optimum SID TSAA to Lys ratios to maximize ADG as well as to minimize FCR and SUN levels were 0.57, 0.58 and 0.53 using a linear-break point model and 0.64, 0.62 and 0.61 using a quadratic model.

Introduction

Total sulfur amino acids (TSAA), including methionine (Met) and cysteine (Cys), are considered as the second or third limiting amino acid for finishing pigs fed corn – soybean meal based diets (Knowles et al. 1998; Yi et al. 2006). A reduction of dietary TSAA concentration occurs when swine producers decrease the dietary protein level by reducing the soybean content to allow for price relief and reductions of nitrogen excretion into the environment (Shriver et al. 2003). Because of the lower concentration of TSAA in corn compared with soybean meal, it is helpful to add crystalline amino acids (CAA) in these low crude protein (CP) diets to avoid reductions in pig performance (Frantz et al. 2009).

Sulfur amino acids are not only an essential amino acid for their contribution to protein synthesis but also an important component to meet the maintenance of pigs (Matthews et al. 2001). Because of the increased need for TSAA for maintenance moving toward the finishing period (Fuller et al. 1989; Wang & Fuller 1989; Hahn & Baker 1995; NRC 1998), it is important to provide the appropriate dietary level of TSAA for late finishing pigs. In addition, the TSAA requirement of pigs has mostly been represented as the TSAA to lysine (Lys) ratio (Gaines et al. 2005) and the recommended ratios have mostly been estimated by prediction models (NRC 2012). However, few empirical studies have been conducted in late finishing pigs, especially on the basis of standardized ileal digestible (SID) amino acids (AA).

For late finishing pigs, gilts appeared to have decreased feed intake and increased lean protein deposition compared with barrows (Friesen et al. 1994; Knowles et al. 1998). It is important to recognize that the optimum TSAA to Lys ratio should be evaluated separately based on the gender of pigs in order to feed the animal closer to its requirement. The current study evaluated only gilts and the experiment was conducted to determine the optimum TSAA to Lys ratio for late finishing gilts fed low CP diets supplemented with CAA.

Materials and Methods

All experimental procedures and animal care were approved by the China Agricultural University Animal Care and Use Committee (Beijing, China)

Animals, housing and dietary treatments

A total of 90 Duroc × Yorkshire × Landrace gilts, with an average initial body weight (BW) of 96.6 ± 5.6 kg, were used in a 28-day performance trial. The experiments were conducted at the Pig Research Facility at the Swine Nutrition Research Centre of the National Feed Engineering Technology Research Centre (Chengde, Hebei Province, China). Gilts were placed in partially steel-slatted concrete floored pens (2.4 m × 1.8 m) that provided 1.3 m² of floor space per pig. Each pen was equipped with a stainless steel self-feeder and a nipple drinker. The gilts were allotted to one of five treatments based on initial weight in a randomized complete block design with six replicates per treatment and three gilts per pen. Feed and water were available ad libitum. Pigs and feeders were weighed at the beginning and end of the experiment to determine average daily gain (ADG), average daily feed intake (ADFI) and feed conversion ratio (FCR).

Experimental diets

The experimental diets were formulated based on corn, wheat bran and corn starch (Table 1) and provided 5.1 g/kg SID Lys ensuring that Lys was marginally deficient for gilts from 90 to 120 kg (NRC 2012). This level was provided to avoid an underestimation of the TSAA to Lys ratio by the excess Lys level. Crystalline L-Met was added to the basal diet to formulate dietary SID TSAA to Lys ratios of 0.48, 0.53, 0.58, 0.63 and 0.68 (Table 2). With the exception of TSAA, the SID ratios of the remaining indispensable AAs to SID Lys in the experimental diets were formulated to meet at least 110% of recommendations of the NRC (2012). All experimental ingredients were analyzed for AA at the start of the experiment. The SID AA content were determined by multiplying the determined AA content in corn and wheat bran by the SID coefficients of the corresponding AA in those feedstuffs from the NRC (2012) and summing the values. The efficiency of the utilization of CAA was assumed to be 100% (Tuitoek et al. 1997). The gilts received a commercially prepared diet for the first 5 days and the commercial diet was progressively replaced by the experimental diets during the next 4 days. The experiments started on the 10th day.

Table 1. Ingredient composition of the basal diet (% as-fed)

Ingredients	Basal diet
Corn	65.33
Wheat bran	15.40
Corn starch	16.00
Limestone	0.45
Dicalcium phosphate	1.15
Salt	0.40
Vitamin-mineral premix†	0.50
L-lysine HCl (78.8%)¶	0.41
DL-methionine (99%)‡	0.00
L-threonine (99%)§	0.17
L-tryptophan (98.5%)§	0.03
L-isoleucine (98%)§	0.09
L-valine (98%)§	0.07

†Premix provided the following per kg of complete diet for finishing pigs: vitamin A, 5512 IU; vitamin D3, 2200 IU; vitamin E, 30 IU; vitamin K3, 2.2 mg; thiamine, 1.5 mg; riboflavin, 4 mg; niacin, 30 mg; pantothenic acid, 13.8 mg; pyridoxine, 3 mg; folacin, 0.7 mg; biotin, 44 μg; cobalamin vitamin B12, 27.6 μg; choline chloride, 400 mg; Mn, 40 mg; Fe, 75 mg; Zn, 75 mg; Cu, 100 mg; I, 0.3 mg; Se, 0.3 mg. ‡DL-methionine was added at 0, 0, 0.3, 0.5, 0.8 or 1.1 g/kg of the diet in place of corn and provided standard ileal digestible total sulfur amino acid (SID TSAA) to lysine (Lys) ratios of 0.48, 0.53, 0.58, 0.63 and 0.68, respectively. §Provided by Health & Nutrition of Evonik Industries (Essen, Germany). ¶Provided by DaCheng Group, ChangChun, China.

Table 2. Chemical analysis and calculated nutritional content of the experimental diets (% as-fed)†

	0.48	0.53	0.58	0.63	0.68
	SID TSAA to Lys ratios
Chemically determined values (%)
Crude protein	8.73	8.80	8.88	8.83	8.96
Isoleucine	0.35	0.35	0.35	0.35	0.35
Lysine	0.58	0.58	0.58	0.58	0.58
Methionine + cysteine	0.31	0.34	0.36	0.39	0.41
Threonine	0.48	0.48	0.48	0.48	0.48
Tryptophan	0.11	0.11	0.11	0.11	0.11
Valine	0.46	0.46	0.46	0.46	0.46
Calculated values
DE, Mcal/kg‡	3.26	3.26	3.26	3.26	3.26
NE, Mcal/kg‡	2.48	2.48	2.48	2.48	2.48
SID amino acids (%)§
Isoleucine	0.30	0.30	0.30	0.30	0.30
Lysine	0.51	0.51	0.51	0.51	0.51
Methionine + cystine	0.25	0.27	0.30	0.32	0.35
Threonine	0.37	0.37	0.37	0.37	0.37
Tryptophan	0.09	0.09	0.09	0.09	0.09
Valine	0.39	0.39	0.39	0.39	0.39

†Values are based on a composite sample obtained weekly and all chemically determined values are the results of an analysis conducted in duplicate. ‡Digestible energy (DE) and net energy (NE) content of the diets were calculated using energy values for the ingredients obtained from NRC (2012). §The standardized ileal digestible (SID) concentrations were determined by multiplying the determined AA content in corn and wheat bran by the SID coefficients of the corresponding AA in those feedstuffs from NRC (2012) and summing the values. TSAA, total sulfur amino acid.

Blood collection and animal slaughter

A previous study (Hahn et al. 1995) determined AA requirement by analyzing serum urea nitrogen (SUN) concentrations of samples collected after 12 h feed deprivation. We followed this method. At the termination of the experiment, six randomly selected gilts per treatment (one pig per pen) were bled after an overnight fast. Gilts were bled via jugular vein puncture and the samples were kept in heparin-free vacutainer tubes (Becton Dickinson Vacutainer Systems, Franklin Lakes, NJ, USA). After collection, the blood samples were placed on ice for 1 h and then centrifuged at 3000 × g (Heraeus Biofuge 22R Centrifuge, Hanau, Germany) for 10 min and the serum was obtained and frozen at −80°C until analysis.

Thirty gilts (one pig per pen) were randomly selected for slaughter at a weight of approximately 120 kg after an overnight fast on day 29 to measure carcass traits. Gilts were killed under commercial conditions by exsanguination following electrical stunning at the Beijing Yuhang Meat Processing Facility (Beijing, China) and hot carcass weight was immediately recorded following slaughter. Dressing percentage was determined from live weight and hot carcass weight. Carcass length was measured from the anterior edge of the symphysis pubis to the cranial edge of the first rib adjacent to the thoracic vertebra. The right carcass was split and then cut between the 10th and 11th ribs to allow measurement of Longissimus muscle area, fat depth, 45-min and 24-h pH (hand-held pH meter, Model 2000; VWR Scientific Products Co., South Plainfield, NJ, USA). Carcass fat-free lean gain was calculated using the equations of the National Pork Producers Council (NPPC 1994). Drip loss was calculated by hanging a loin section (100 g, Longissimus muscle sample) in a inflated and closed plastic bag for 24 h at 4°C (King et al. 2000). Loin muscle marbling score was determined according to NPPC (1994) guidelines and the CIELAB L* (lightness), a* (redness) and b* (yellowness) color was determined with a colorimeter (Chromameter, CR410; Minolta, Tokyo, Japan).

Chemical analyses

The CP content (AOAC method 984.13) in the experimental diets was analyzed according to AOAC (2003) procedures. Amino acid concentrations of ingredients and mixed diets were determined by ion-exchange chromatography using a Hitachi L-8800 AA Analyzer (Tokyo, Japan) following acid hydrolysis with 6 N HCl for 24 h at 110°C (AOAC method 999.13). Methionine and Cys were determined after cold performic acid oxidation overnight and hydrolyzed with 7.5 N HCl (AOAC method 994.12). Tryptophan was determined using reverse-phase high-performance liquid chromatography (HPLC; Waters 2690, Milford, MA, USA) after alkaline hydrolysis at 120°C for 16 h.

Serum urea nitrogen concentration was determined with a blood urea nitrogen color test kit according to the manufacturer's instructions (Nanjing Jiancheng Bioengineering Institute, Nanjing, China). Serum AA concentrations were determined by ion-exchange chromatography (S-433D Amino Acid Analyzer, Sykam GmbH, Eresing, Germany) as described by Sedgwick et al. (1991).

Statistical analyses

Data were analyzed by the GLM procedure of SAS (Statistical Analysis System, Version 6.12, 1998, SAS Institute Inc., Cary, NC, USA) using a randomized complete block design. Means are expressed as least squares means with pen as the experimental unit. An alpha level of P < 0.05 was the criterion for statistical significance. Polynomial contrasts were performed to determine linear and quadratic relationships. Estimates of AA requirements for optimum performance, SUN and carcass traits were determined by subjecting the data to least squares, broken-line methodology (y = L + U × (R − x), where (R − x) is = 0 when x > R, L = plateau, U = slope, R = breakpoint; Robbins et al. 2006) as well as determining the asymptote of the quadratically fitted line using the NLIN procedure of SAS.

Results

The lowest ADG was obtained in pigs fed the diet with the SID TSAA to Lys ratio of 0.48, which was significantly lower than those of pigs fed the SID TSAA to Lys ratios of 0.58, 0.63 and 0.68 (P < 0.05). The lowest FCR and SUN concentration was obtained in pigs fed the SID TSAA to Lys ratio of 0.58, which was significantly lower compared with those fed the diet with the SID TSAA to Lys ratio of 0.48 (P < 0.05). Increasing the SID TSAA to Lys ratio increased ADG (linear and quadratic effect, P < 0.05) and improved FCR (linear and quadratic effect, P < 0.05). Serum urea nitrogen decreased as the SID TSAA to Lys ratio increased (linear and quadratic effect, P < 0.05) (Table 3).

Table 3. Effect of standardized ileal digestible (SID) total sulfur amino acid (TSAA) to lysine (Lys) ratio in the diet on performance and serum urea nitrogen levels of finishing gilts (96 to 120 kg)†

	0.48	0.53	0.58	0.63	0.68	SEM‡	ANOVA	Linear	Quadratic
	SID TSAA to Lys ratios					SEM‡	P value§
Average daily gain (kg)	0.78^b	0.84^ab	0.90^a	0.88^a	0.90^a	0.02	< 0.01	< 0.01	< 0.01
Average daily feed intake (kg)	2.92	3.05	2.98	2.96	3.09	0.11	0.82	0.46	0.76
feed conversion ratio	3.75^a	3.64^ab	3.32^b	3.36^ab	3.43^ab	0.10	0.03	0.01	0.01
Serum urea nitrogen (mmol/L)	2.41^a	2.11^b	1.99^b	2.21^b	2.10^b	0.06	< 0.01	0.04	< 0.01

†Data are means of six replicates per treatment. ‡Standard error of mean. §Linear and quadratic effects for SID TSAA to Lysine ratios. Means in a row with different letters are different (P < 0.05).

There were no effects observed on carcass traits by increasing the SID TSAA to Lys ratio (Table 4). The serum Met concentration of pigs fed the diets with the TSAA to Lys ratios of 0.48 and 0.53 was lower than those of pigs fed the diet with the TSAA to Lys ratio of 0.60 (P < 0.05). The serum glycine concentration of pigs fed the diets with the TSAA to Lys ratios of 0.48 and 0.53 was lower than those of pigs fed the diet with the TSAA to Lys ratios of 0.58 and 0.63 (P < 0.05). Increasing the dietary SID TSAA to Lys ratio increased the serum concentrations of Met and glycine (linear and quadratic effect, P < 0.05) (Table 5).

Table 4. Effect of dietary standardized ileal digestible (SID) total sulfur amino acid (TSAA) to lysine (Lys) ratio on carcass traits of finishing gilts†

	0.48	0.53	0.58	0.63	0.68	SEM‡	ANOVA	Linear	Quadratic
	SID TSAA to Lys ratios					SEM‡		P value§
Carcass traits
Slaughter weight, kg	120.2	121.6	119.7	120.3	121.7	2.90	0.98	0.85	0.95
Carcass weight, kg	89.3	90.3	88.9	89.0	90.7	2.70	0.98	0.87	0.94
Dressing percentage, %	74.3	74.3	74.2	74.0	74.4	1.13	0.99	0.99	0.98
Carcass length, cm	88.5	90.3	86.8	92.8	89.0	1.61	0.14	0.54	0.80
Tenth rib fat depth, cm	3.2	3.1	3.1	3.2	3.2	0.20	0.99	0.91	0.85
Longissimus muscle area, cm²	41.4	41.7	43.4	41.0	42.4	2.32	0.95	0.86	0.96
Fat-free lean gain, g/day	190.7	196.7	216.6	223.0	212.9	25.88	0.89	0.37	0.60
Muscle quality
Marbling	2.7	2.7	2.7	2.8	2.7	0.16	0.99	0.70	0.89
Drip loss %	4.2	3.9	4.0	3.9	4.5	0.31	0.66	0.57	0.36
pH_45min	6.1	6.1	6.2	5.9	6.1	0.12	0.44	0.82	0.97
pH_24h	5.7	5.6	5.6	5.6	5.6	0.08	0.85	0.34	0.57
L* Light	43.6	44.4	45.1	46.4	45.7	1.27	0.58	0.12	0.27
a* Redness	5.9	5.2	5.8	5.3	4.9	0.46	0.51	0.16	0.38
b* Yellowness	3.3	3.2	4.0	3.4	3.4	0.39	0.65	0.75	0.70

†Data are means of six replicates per treatment. ‡Standard error of mean. §Linear and quadratic effects for SID TSAA to Lysine ratios. Means in a row with different letters are different (P < 0.05).

Table 5. Effect of dietary standardized ileal digestible (SID) total sulfur amino acid (TSAA) to lysine (Lys) ratio on the serum amino acid concentrations of finishing gilts†

	0.48	0.53	0.58	0.63	0.68	SEM‡	ANOVA	Linear	Quadratic
	SID Thr to Lys ratio						P value§
Serum essential amino acids, nmol/mL
Arginine	199	176	174	182	159	20	0.72	0.22	0.47
Histidine	113	82	88	91	90	11	0.35	0.31	0.24
Isoleucine	116	113	95	98	129	11	0.20	0.71	0.11
Leucine	216	213	185	176	216	13	0.14	0.49	0.12
Lysine	285	346	391	355	339	41	0.49	0.38	0.19
Methionine	33^b	33^b	49^ab	57^ab	60^a	7	0.01	< 0.01	< 0.01
Phenylalanine	100	95	98	88	96	8	0.91	0.61	0.82
Threonine	191	149	236	206	176	24	0.16	0.77	0.62
Tryptophan	13	12	13	12	12	2	0.97	0.51	0.79
Valine	354	342	354	293	366	19	0.13	0.81	0.50
Serum non-essential amino acids, nmol/mL
Alanine	637	709	685	651	759	58	0.62	0.29	0.56
Asparate	16	16	17	18	20	2	0.49	0.08	0.17
Glutamine	302	294	251	243	315	36	0.56	0.88	0.36
Glycine	1164^b	1198^b	1819^a	1755^a	1616^ab	132	< 0.01	< 0.01	< 0.01
Cystine	39	33	31	24	33	8	0.80	0.43	0.52
Proline	1221	1403	1485	1465	1317	142	0.68	0.59	0.30
Serine	157	144	206	192	187	23	0.33	0.15	0.29
Tyrosine	64	63	62	59	61	7	0.99	0.68	0.91

†Data are means of six replicates per treatment. ‡Standard error of mean. §Linear and quadratic contrasts for SID Thr (threonine) to Lys ratios. Means in a row with different letters are different (P < 0.05).

The linear broken-line model estimated the optimum dietary SID TSAA to Lys ratio as 0.57, 0.58 and 0.53 to maximize ADG, minimize FCR and SUN, respectively. The quadratic model estimated the optimum SID TSAA to Lys ratio as 0.64, 0.62 and 0.61 to maximize ADG, minimize FCR and SUN, respectively (Figs 1-3).

**Figure 1**
Open in figure viewer PowerPoint

Fitted broken line analysis (—) and quadratic model (---) of average daily gain plotted against standardized ileal digestible sulfur amino acids to lysine ratios.

**Figure 2**
Open in figure viewer PowerPoint

Fitted broken line analysis (—) and quadratic model (---) of feed conversion ratio plotted against standardized ileal digestible sulfur amino acids to lysine ratios.

**Figure 3**
Open in figure viewer PowerPoint

Fitted broken line analysis (—) and quadratic model (---) of serum urea nitrogen plotted against standardized ileal digestible sulfur amino acids to lysine ratios.

Discussion

Replacing a portion of the dietary crude protein content by supplementing crystalline AA is a useful approach to improve feed efficiency, reduce feed cost and also to obtain environmental benefits (Russell et al. 1986; Tuitoek et al. 1997). Previous studies have indicated that pig performance is not affected by reducing the crude protein content by 3–4% while supplementing with crystalline AA (Kerr et al. 1995; Zhang et al. 2013). Sulfur amino acid has been considered to be the second or third limiting AA for pigs fed corn-soybean meal-based diets and the requirement is typically presented relative to Lys (Knowles et al. 1998; Loughmiller et al. 1998; Dean et al. 2007). A better understanding of the optimum TSAA to Lys ratio plays a very important role when supplementing crystalline AA to a low CP diet (Frantz et al. 2009).

Amino acid to Lys ratios were typically calculated using the ideal AA profile. The TSAA to Lys ratio of 0.50 was estimated by ARC (1981) using the carcass AA concentrations as the primary criteria, which could not reflect the effects on AA requirement caused by an increase in animal weight and the associated increased AA requirements for maintenance. The AA profile was then improved to separate the AA requirement for maintenance and protein accretion and the profile estimates of the ratio of TSAA to Lys ranged from 0.52 to 0.63 for finishing pigs (Wang & Fuller 1989; Boisen et al. 2000).

Previous studies showed that the AA requirement is more applicable when using digestible AA basis rather than total AA values to estimate the available AA in feedstuffs. The AA requirement could be expressed on the basis of apparent ileal digestibility (AID), true ileal digestibility (TID), or SID AA (Stein et al. 2007a,b). In addition, SID AA allows for additivity by taking into account basal endogenous AA losses and is preferred in feed formulations for producers. Based on SID AA, the NRC (2012) and BSAS (2003) recommended a SID TSAA to Lys ratio of 0.58 or 0.59 for late finishing pigs, while a SID TSAA to Lys ratio of 0.63 was recommend by the National Swine Nutrition Guide (NSNG) (2010). However, the recommended ratios were estimated using a prediction model and limited empirical studies have been conducted to validate the prediction model on the basis of SID AA (NRC 2012). Thus, this study was conducted to determine the optimum TSAA to Lys ratio for late finishing gilts fed low CP diets supplemented with CAA.

To express the TSAA requirement as an AA to Lys ratio, Lys should be at a suboptimal level to avoid an underestimation of the determined AA (Boisen 2003). In the current study, the optimum TSAA to Lys ratio was determined by setting a marginally deficient SID Lys of 0.51% in the experimental diets (about 20% lower than the NRC's recommendation for late finishing gilts), while the other indispensable AAs (except for Met) were provided at a level of 110% of the SID requirements relative to Lys for all other AA as recommended by the NRC (2012).

The choice of statistical method used to interpret the data in AA does-response studies may influence the estimated value of the requirement. The most commonly applied model included both the linear-broken line analysis and quadratic model (Kendall et al. 2007). For the purpose of comparison, both models were represented in the present study. The quadratic model can effectively estimate increases and decreases, and estimated the AA requirement to reach 100% of the maximum response and it often overestimates the nutritional requirement (Baker 1986). In contrast, broken-line regression analysis provides a break point which corresponds to the minimum requirement for pigs and avoids the bias of arbitrarily to select the AA requirement (Baker 1986). Thus, the appropriate SID TSAA to Lys ratio was obtained by the linear-breakpoint model in this study.

The optimum TSAA to Lys ratio has mostly been determined using performance and carcass traits as the response criteria (Knowles et al. 1998; Loughmiller et al. 1998; Matthews et al. 2001). For performance, ADFI was the primary response variable in AA dose response studies, which might affect the daily TSAA intake and then influence pig growth (Chung et al. 1989; Loughmiller et al. 1998; Gaines et al. 2005; Roth et al. 2006). Roth et al. (2006) reported that piglets prefer a diet adequate for Met to a Met-deficient diet with the same dietary level of Cys. However, no significant effect of ADFI was obtained in this study, which may indicate that the feed intake of late finishing pigs is less susceptive compared with younger pigs. In the current study, linear and quadratic effects were obtained in ADG and FCR, while no effect was observed on carcass traits. Thus, the ADG and FCR were used to evaluate the optimum TSAA to Lys ratio in the present study.

Serum AA concentrations were also detected to determine the optimum TSAA to Lys ratio. Keith et al. (1972) concluded that no changes could be observed in plasma AA level until an ascending curve was produced as the optimal dietary level increased in the experimental diets. The serum AAs which were affected in this study were Met and glycine, in which the changes of serum Met were expected to correspond to the changes of the dietary Met level in experimental diets. In addition, the changes of the dietary Met might influence glutathione synthesis and subsequently influence the serum level of glycine because serum glycine was also related to that metabolism (Wu et al. 2004). However, the concentrations of serum AA in the present study did not directly reflect the optimum TSAA to Lys ratio.

In addition, SUN could be a strong indicator in TSAA dose-response studies, which directly responds to the TSAA changes in the experimental diets (Loughmiller et al. 1998; Matthews et al. 2001). Previous studies showed that the lowest concentration of SUN indicated the maximum N utilization, indicating that the first limiting AA was balanced (Brown & Cline 1974; Kohn et al. 2005). Thus, SUN concentration could also be a very valid response criteria in this study.

Therefore, in the present study, the appropriate SID TSAA to Lys ratio was dependent on ADG, FCR or SUN and was used as the response criteria, in which the estimate of 0.58 using FCR as response criteria is more acceptable because of its economic superiority in improving feed efficiency for finishing pigs, and none for ADG or SUN.

Conclusions

The appropriate SID TSAA to lysine ratio was estimated as 0.58 for 96 to 120 kg for gilts fed low CP diets using FCR as the response criteria, which is in accordance with the recommend ratio by the NRC (2012).

Acknowledgments

This study was supported by Modern Agro-Industry Technology Research System of Beijing and the National Basic Research Program of China (2012CB124704). The authors thank the DaCheng Group, (ChangChun China) and Health & Nutrition of Evonik Industries (AG Germany) for providing supplemental amino acids.

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Citing Literature

Volume87, Issue1

January 2016

Pages 76-83

Estimation of the optimum standardized ileal digestible total sulfur amino acid to lysine ratio in late finishing gilts fed low protein diets supplemented with crystalline amino acids

Abstract

Introduction