The effect of standardized ileal digestible isoleucine:lysine in diets containing 20% dried distillers grains with solubles on finishing pig performance and carcass characteristics (2024)

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J Anim Sci
v.100(9); 2022 Sep
PMC9486908

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J Anim Sci. 2022 Sep; 100(9): skac234.

Published online 2022 Jul 1. doi:10.1093/jas/skac234

PMCID: PMC9486908

PMID: 35776121

David A Clizer, Blair J Tostenson, Sam K Tauer, Ryan S Samuel, and Paul M Cline

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Abstract

In order to determine the standardized ileal digestible (SID) Ile:Lys requirement in finishing diets containing 20% DDGS, a 56-d study was conducted utilizing 2,268 pigs (DNA 600 x Topigs Norsvin 70, initially 82.3 kg). A total of six dietary treatments were fed, including a standard corn-soybean meal (SBM) diet and five diets containing 20% DDGS with SID Ile:Lys ratios of 55%, 60%, 65%, 70%, and 75%. Dietary treatments were assigned to pens, incompletely balancing for previous treatment, with each treatment being replicated 14 times. Pair-wise comparisons were used to evaluate the impact of dietary treatment on performance and carcass traits, whereas single degree of freedom orthogonal polynomials were used to evaluate dose response of SID Ile:Lys in 20% DDGS diets. Increasing the SID Ile:Lys ratio in diets containing 20% DDGS did not impact pig growth performance criteria in a quadratic or linear fashion (P ≥ 0.18). However, increasing the SID Ile:Lys ratio in 20% DDGS diets resulted in decreased back fat (BF; Quadratic, P = 0.01), increased loin depth (Quadratic, P = 0.03), and tended to increase percent lean (Quadratic, P = 0.07) with optimal carcass parameters occurring when 65% SID Ile:Lys was supplied in 20% DDGS diets. Pigs fed the corn-SBM diet had a similar final body weight (BW; P = 0.26) and cumulative average daily gain (ADG; P = 0.12) compared to pigs fed a 20% DDGS diet containing 70% SID Ile:Lys ratio and 3% greater cumulative average daily feed intake (ADFI) compared to pigs receiving diets with SID Ile:Lys ratios of 65% and 75% (P ≤ 0.01). In conclusion, these results suggest that when feeding 20% DDGS in late finishing swine diets, a SID Ile:Lys ratio of 70% should be utilized when attempting to achieve similar overall growth performance relative to a corn-SBM diet.

Keywords: branched chain amino acids, dried distillers grain with solubles, finishing pig, isoleucine, leucine

Introduction

The amino acids (AA) Val and Ile are typically considered the 5th or 6th limiting AA after Lys, Thr, Met, and Trp in low crude protein (CP) swine diets (Liu et al., 2000; Lordelo et al., 2008). Figueroa et al. (2003) suggested that Val is limiting before Ile in common corn-SBM diets. However, the inclusion of by-products protein sources can affect the order of limitation of AA (Lordelo et al., 2008). The inclusion of corn by-products, such as DDGS, ultimately leads to higher concentrations of dietary corn protein which can affect the order of limitation of AA. As the proportion of dietary CP contributed by corn protein increases, the concentration of dietary Ile decreases at a faster rate compared to Val, resulting in Ile to become the 5th limiting AA prior to that of Val. The higher concentrations of corn protein can also result in greater concentrations of dietary Leu which has been demonstrated to impact the Ile requirement (Htoo et al., 2017). An antagonistic relationship exists between the branched-chain amino acids (BCAA; Val, Ile, and Leu) because they share the same degrading enzymes in the first two steps of their catabolism (Hutson et al., 2005). As a result, the excess of any one of the BCAA can result in the increased catabolism of all the BCAA, leading to a potential AA deficiency (Harper et al., 1984). Therefore, accurate estimates of optimal SID Ile:Lys requirements are necessary to ensure adequate performance when diets contain high amounts of corn protein.

The NRC (2012) estimates the SID Ile:Lys requirement at 53.4% for the 75- to 100-kg pig and 54.1% for the 100- to 135-kg pig. The published literature on the Ile requirement reports a wide range of optimal levels ranging from less than 50% (Barea et al., 2009) to 62% (Fu et al., 2006) relative to Lys. Previous research in nursery and early growing pigs has estimated the SID Ile:Lys requirement between 51% and 54% when dietary concentrations of Leu are not in excess (Waguespack et al., 2012; Htoo et al., 2014; Soumeh et al., 2014), whereas others have estimated the SID Ile:Lys requirement at 58% or 59% when Leu is in excess (Wiltafsky et al., 2009; Htoo et al., 2017). Although these studies continue to indicate that the Ile requirement needs to be adjusted according to dietary Leu concentrations, this might not be the only factor impacting the Ile requirement. Research by Zier-Rush et al. (2018) suggested that the SID Ile:Lys requirement may be closer to 60% or 61% for the late finishing pig. This estimated that Ile requirement was in agreement with the empirical estimates of 60% to 62% for the 90-kg barrow by Kendall et al. (2004). Comparing the SID Ile:Lys ratios estimated in the nursery pig versus ratios for finishing pigs may suggest that the optimal SID Ile:Lys ratio might increase as BW of pigs increase. A study conducted by Kerkaert et al. (2021) indicated that, in finishing, the negative effects of high dietary Leu can be mitigated by increased supplementation of Ile. When compared with the BCAA/large neutral amino acids prediction model of Cemin et al. (2019), the predicted ADG response of their Ile treatment was overestimated in the growing period and underestimated in the finishing period (Kerkaert et al., 2021). Kerkaert et al. (2021) suggested that pigs may be more sensitive to an Ile deficiency relative to Leu during the finishing period compared to the growing period. Therefore, the objective of this study was to determine the SID Ile:Lys requirement of finishing pigs fed diets containing excess dietary Leu through the inclusion of DDGS.

Materials and Methods

The South Dakota State University Institutional Animal Care and Use committee approved the protocol used in this study (2001-002E). The study was performed in a double long, curtain sided commercial research facility owned and operated by Christensen Farms (Sleepy Eye, MN). Each pen (3.2 × 5.6 m) contained a 4-slot stainless steel dry feeder (Hog Slat Inc., Newton Grove, NC) and two cup waterers, allowing ad libitum access to feed and water. Individual pens received daily feed rations through an automated feeding system (DryExact Pro; Big Dutchman Inc., Holland, MI) capable of measuring and mixing feed. Following the completion of the previous 28-d study that evaluated valine in early finish pigs and prior to the start of the current study, pigs were fed a corn-SBM based diet containing 20% DDGS which met or exceeded NRC (2012) nutrient requirements for a total of 14 d.

A total of 2,268 pigs (DNA 600 x TopigsNorsvin 70) were used in a 56-d finishing study. Pens contained 26 or 27 pigs with approximately equal number of barrows and gilts. Each treatment was replicated 14 times and pens were blocked based on pen location within the barn. Pens were assigned to one of six dietary treatments, incompletely balancing for previous treatment. Dietary treatments consisted of a corn-SBM diet (CS) or diets containing 20% DDGS with a SID Ile:Lys ratio of 55%, 60%, 65%, 70%, or 75%. All diets were provided in the meal form and were fed in two phases with the dietary phase change occurring on 28 d. Three main diets were milled, which included the CS diet and two diets containing 20% DDGS with a SID Ile:Lys ratios of 55% and 75% (Table 1). The high and low SID Ile:Lys diets containing 20% DDGS were blended on site using the automated feeding system to achieve SID Ile:Lys ratios of 55%, 60%, 65%, 70%, and 75%. Diets were formulated to supply SID Lys at 95% of requirement (PIC, 2016) to ensure that the Ile requirement was not underestimated. All diets were formulated to contain similar dietary net energy and SID Lys concentrations within phase. Threonine levels were increased in diets containing 20% DDGS to ensure that Thr did not limit performance due to greater fiber content relative to the corn-SBM diet (Wellington et al., 2018). Similarly, to limit any impact large neutral AA concentrations might have on pig performance, the dietary Trp levels were increased in diets containing 20% DDGS (Clizer et al., 2020). The SID Val:Lys ratio was set at 78% in diets containing 20% DDGS based off the prediction equation of Cemin et al. (2019) as levels above 78% suggested to have little benefit to performance. The addition of crystalline L-Ile was utilized to achieve the 75% SID Ile:Lys ratio. Crystalline L-Ile, L-Val, and L-Trp were measured out by hand and delivered to the mill to ensure accurate inclusion rates.

Table 1.

Ingredient and calculate nutrient composition of diets (as-fed basis)

Item	Common¹	Phase 1 (days 0 to 28)			Phase 2 (days 28 to 56)
		SID Ile:Lys, %		CS	SID Ile:Lys, %		CS
		55	75	CS	55	75	CS
Ingredients, %
Corn	62.45	72.52	72.38	85.15	74.89	74.76	86.42
Soybean meal	14.18	3.66	3.66	11.85	1.42	1.42	10.86
DDGS	20.00	20.00	20.00	—	20.00	20.00	—
Choice white grease	0.75	1.20	1.20	0.35	1.25	1.25	0.40
Calcium carbonate	1.09	1.05	1.05	0.87	0.94	0.94	0.81
Salt	0.48	0.41	0.41	0.57	0.41	0.41	0.57
Monophosphate 21%	—	—	—	0.31	—	—	0.15
VTM premix²	0.10	0.10	0.10	0.10	0.10	0.10	0.10
Magnesium oxide 54%	0.30	0.30	0.30	0.30	0.30	0.30	0.30
Lysine HCL	0.42	0.46	0.46	0.31	0.44	0.44	0.25
L-Threonine	0.13	0.13	0.13	0.11	0.13	0.13	0.09
L-Methionine	0.05	0.03	0.03	0.07	0.01	0.01	0.04
L-Tryptophan	0.05	0.07	0.07	0.02	0.06	0.06	0.01
L-Valine	—	0.07	0.07	—	0.05	0.05	—
L-Isoleucine	—	—	0.15	—	—	0.13	—
Calculated analysis
NE, Kcal/kg	2,384	2,464	2,464	2466	2,481	2,481	2,479
CP, %	16.87	12.83	12.96	11.7	11.9	12.02	11.23
NDF, %	10.47	10.90	10.89	6.37	10.91	10.90	6.39
Ca, %	0.50	0.45	0.45	0.45	0.40	0.40	0.40
P, %	0.38	0.33	0.33	0.33	0.33	0.33	0.30
ATTD P, %³	0.30	0.28	0.28	0.27	0.27	0.27	0.24
SID amino acid, %
Lys	0.95	0.73	0.73	0.73	0.66	0.66	0.66
Met:Lys	33	32	32	34	30	30	32
(Met+Cys):Lys	59	58	58	58	58	58	58
Thr:Lys	66	67	67	64	69	69	66
Trp:Lys	21	21	21	18	21	21	18
Val:Lys	72	78	78	67	78	78	72
Ile:Lys	61	55	75	56.5	55	75	60
Leu:Lys	150	161	161	133	170	170	144
Ile:Leu	41	34	47	42	32	44	42
(Val+Ile):Leu	89	83	95	93	78	90	91

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Common diet was fed from days −14 to 0.

Provided per kilogram of the diet: 600 FTU Axtra Phytase Gold 65G, 60.4 g Danisco® Xylanase 60000 G, 2,527 IU vitamin A, 1,134 IU vitamin D₃, 32.4 IU vitamin E, 0.97 mg vitamin K₃, 32.4 mg niacin, 13.2 mg pantothenic acid, 4.5 mg riboflavin, 21 ug vitamin B₁₂, 40 mg Mn from manganous oxide, 50 mg Zn from zinc hydroxychloride, 100 mg Fe from ferrous sulfate, 10 mg Cu from copper sulfate, 1.0 mg I from ethylenediamine dihydroiodide, and 0.30 mg Se from sodium selenite.

A 0.14% apparent total tract digestible (ATTD) P release credit was provided for phytase.

Diet samples were collected from every batch milled and delivered for both dietary phases. Samples were stored in a freezer (−20 °C) until subsamples were pooled together within phase by the three milled dietary treatments and sent for analysis. The University of Missouri Chemical Laboratories (University of Missouri, Columbia MO) determined the complete AA, crude protein, fat, and fiber contents of the diets. Dietary samples for the three milled diets were pooled together and sent to the North Dakota State University Veterinary Diagnostic Laboratory (North Dakota State University, Fargo, ND) for analysis of mycotoxins.

Weigh periods were conducted every 2 wk to calculate ADG, ADFI, and gain to feed (G:F). Pigs were weighed by pen via a pen scale. Feed disappearance was determined by feed delivery data reported by the automated feeding system minus the feed amount remaining in the feeder on the weigh day. The weight of feed remaining in feeders was determined using a regression equation which utilized feed height in the calculation. Two regression equations were developed to account for differences in feed density between the corn-SBM diet and diets containing 20% DDGS. Groups of pigs were marketed in two cuts, with the initial cut occurring on 28 d of the study and the remaining pigs marketed on 56 d. Pen inventory was standardized within block during the initial cut. Pigs in the initial cut were determined to be the heaviest pigs in each pen by visual evaluation and represented approximately 15% of the barn inventory. Prior to shipment to a USDA-inspected packing plant (Triumph Food LLC, St. Joseph, MO), pigs selected for market were weighted by pen groups via pen scale. All pigs had hot carcass weight (HCW), BF, loin depth, and percent lean captured at the USDA-inspected packing plant.

Data were analyzed as a randomized complete block design with pen as the experimental unit and pen location as the blocking factor. Previous treatment was included in the model to test for any interactions between previous and current treatments. Analysis failed to detect any significant interactions between previous and current treatments. However, analysis did reveal that previous treatment was significant for initial BW. Therefore, initial BW was utilized as a covariate for the remaining statistical analyzes. This model was determined to be the most appropriate approach based of statistical model comparison using the Bayesian information criterion (Milliken and Johnson, 2009). Analysis of variance was performed using the GLIMMIX procedure of SAS 9.4 (SAS Inst. Inc., Cary, NC). Single degree of freedom orthogonal polynomials were used to evaluate SID Ile:Lys dose response in diets containing DDGS. Contrast coefficients for single degree of freedom orthogonal polynomials were based on equally spaced treatments. Pair-wise comparisons were used to evaluate the treatment response of CS diets relative to diets containing DDGS with differing SID Ile:Lys ratios for growth performance and carcass characteristics. Results were considered significant at P ≤ 0.05 and marginally significant at 0.10 ≥ P > 0.05 for the main effects of increasing the SID Ile:Lys in 20% DDGS diet and overall impact of diet. Significance was determined at P|t| ≤ 0.05 for comparisons between the CS diet and DDGS diets with graded levels of SID Ile:Lys, following a significant effect of diet.

Results

Diet analysis

Chemical analysis of diets verified that levels of CP, fat, fiber, and free AA levels of Trp, Thr, Met, Val, Ile, and Lys were within 5% to 10% of values used in formulation. Christensen Farms (Sleepy Eye, MN) supplied the AA values utilized for diet formulation in this study by providing a historical profile of major ingredients from source specific manufactures. Mycotoxin analysis of complete diets indicated that concentrations of deoxynivalenol (DON) were below 600 ppb and other mycotoxin levels were at or below detectable concentrations (Table 2).

Table 2.

Mycotoxin concentrations of dietary treatments (as-fed basis, ppb)¹

Item	SID Ile:Lys, %		CS
Item	55	75	CS
Alfatoxin B1	< 20	< 20	< 20
Alfatoxin B2	< 20	< 20	< 20
Alfatoxin G1	< 20	< 20	< 20
Alfatoxin G2	< 20	< 20	< 20
Fumonisin B1	< 200	< 200	< 200
Fumonisin B2	< 200	< 200	< 200
HT-2 Toxin	< 200	< 200	< 200
T-2 Toxin	20	< 20	< 20
Ochratoxin A	< 20	< 20	< 20
Sterigmatocystin	< 20	< 20	< 20
Zearalenone	< 100	< 100	< 100
Deoxynivalenol (DON)	540	571	308

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Representative samples of each dietary treatment for both phases were pooled and analyzed at North Dakota State University Veterinary Diagnostic Laboratory (Fargo, ND) by liquid chromatography and tandem mass spectrometric detection (LC/MS/MS).

Growth performance

Table 3 presents the growth performance and carcass characteristics of pigs 20% DDGS diets with increasing SID Ile:Lys and the CS diet. Increasing the SID Ile:Lys ratio in diets containing 20% DDGS failed to impact the growth performance of pigs in a linear or quadratic fashion (P > 0.17) over the course of the study. During the first 28 d, pigs fed the CS diet had greater BW, ADG, and ADFI (P|t| ≤ 0.05) compared to pigs fed 20% DDGS diets, regardless of SID Ile:Lys level. The G:F of pigs from days 0 to 28 was not impacted due to dietary treatment (P = 0.91). From days 28 to 56, dietary treatment tended (P = 0.07) to impact the ADG of pigs, but the ADFI and G:F of pigs during this period was not impacted due to dietary treatment (P ≥ 0.17). For the overall period (days 0 to 56), pigs fed the CS diet and 20% DDGS diets containing a 70% SID Ile:Lys ratio had equal final BW (P|t| = 0.26). The CS fed pigs also had similar (P|t| = 0.06) final BW relative to pigs receiving 20% DDGS diets with a SID Ile:Lys ratio of 60%. The ADG of pigs fed the CS diet was 3% to 5% greater (P|t| < 0.04) compared to pigs fed diets containing 20% DDGS with SID Ile:Lys ratios of 55%, 60%, 65%, or 75%. However, when pigs were fed 20% DDGS diets with a SID Ile:Lys ratio of 70%, ADG was not different (P|t| = 0.12) relative to pig receiving CS diets. Pigs consuming the CS diets had approximately 3% greater ADFI (P|t| ≤ 0.01) in comparison to pigs fed 20% DDGS with a SID Ile:Lys ratio of 65% or 75%. The G:F response of pigs tended to be impacted by dietary treatment (P = 0.10) for the cumulative period.

Table 3.

Performance response and carcass characteristics of pigs fed increasing levels of SID Ile:Lys in diets containing 20% DDGS compared to a corn-SBM diet (CS)

Item	Dietary treatments						Main effect of diet		Ile:Lys in DDGS diets
	SID Ile:Lys, %					CS	Main effect of diet		SEM	P
	55	60	65	70	75	CS	SEM	P	SEM	Lin	Quad
BW, kg
Initial	82.4	82.6	82.7	82.3	81.7	82.3	0.64	—	0.66	—	—
d 28	107.8^b	108.0^b	107.6^b	107.8^b	107.5^b	108.8^a	0.41	0.030	0.34	0.513	0.719
Final	129.4^bc	129.4^abc	128.4^c	129.9^ab	128.2^c	130.6^a	0.63	0.002	0.61	0.389	0.683
d 0 to 28
ADG, kg	0.923^b	0.931^b	0.920^b	0.924^b	0.915^b	0.963^a	0.015	0.028	0.014	0.544	0.678
ADFI, kg	2.89^b	2.91^b	2.88^b	2.87^b	2.86^b	2.98^a	0.034	0.006	0.028	0.272	0.772
G:F	0.319	0.320	0.320	0.322	0.320	0.323	0.003	0.910	0.003	0.720	0.719
d 28 to 56
ADG, kg	0.901	0.906	0.874	0.929	0.885	0.921	0.020	0.068	0.016	0.883	0.910
ADFI, kg	3.05	3.07	2.97	3.08	3.02	3.05	0.046	0.173	0.033	0.634	0.712
G:F	0.296	0.295	0.294	0.301	0.294	0.302	0.004	0.247	0.004	0.751	0.658
d 0 to 56
ADG, kg	0.913^bc	0.919^bc	0.899^c	0.925^ab	0.901^c	0.943^a	0.010	0.003	0.018	0.979	0.188
ADFI, kg	2.96^ab	2.98^ab	2.92^b	2.97^ab	2.93^b	3.01^a	0.032	0.048	0.025	0.317	0.971
G:F	0.308	0.308	0.308	0.312	0.308	0.313	0.002	0.104	0.006	0.709	0.175
Carcass characteristics:
HCW, kg	94.2	94.8	94.3	94.3	93.5	95.5	0.58	0.174	0.58	0.245	0.253
Back Fat, mm	16.5^a	16.4^a	15.8^b	16.0^ab	16.3^a	16.3^a	0.22	0.031	0.20	0.146	0.011
Loin Depth, mm	58.0^cd	57.9^d	58.7^b	58.5^bc	58.0^cd	59.4^a	0.30	< 0.001	0.21	0.411	0.031
Percent Lean, %	53.4	53.4	53.7	53.6	53.5	53.6	0.14	0.057	0.11	0.128	0.073

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Means within a row lacking common superscript differ significantly, P ≤ 0.05.

BW, body weight; ADG, average daily gain; ADFI, average daily feed intake; G:F, gain-to-feed ratio; HCW, hot carcass weight; Lin, linear; Quad, quadratic.

Carcass characteristics

Increasing the SID Ile:Lys ratio in diets containing 20% DDGS did not impact HCW in a linear or quadratic fashion (P > 0.25). However, increasing the SID Ile:Lys ratio in 20% DDGS diets decreased BF (Quadratic, P = 0.01) and increased loin depth (Quadratic, P = 0.03) with the 65% SID Ile:Lys ratio providing the lowest BF and greatest loin depth. Increasing the SID Ile:Lys ratio in diets containing 20% DDGS also tended to increase percent lean (Quadratic, P = 0.07) and providing a ratio of 65% SID Ile:Lys resulted in the greatest percent lean. The HCW of pigs that received the CS diet was not different (P = 0.17) compared to pigs fed 20% DDGS, regardless of SID Ile:Lys. Pigs fed the CS diet had 1% to 2.5% greater loin depths (P|t| ≤ 0.02) compared to 20% DDGS diets, regardless of SID Ile:Lys ratio. The BF of pigs fed a 20% DDGS diet with a SID Ile:Lys ratio of 65% was 3% lower (P|t| = 0.02) compared to pigs that received the CS diet. However, the BF of CS fed pigs was equal (P|t| ≥ 0.23) to pigs fed SID Ile:Lys ratios of 55%, 60%, 70%, and 75% in 20% DDGS diets. The percent lean of pigs tended (P = 0.06) to be impacted by dietary treatment.

Discussion

The published literature on the Ile requirement is somewhat variable and may appear to be a multifaceted equation that includes both concentrations of other BCAA and the BW of pigs. Previous research has demonstrated that the dietary Leu concentration can impact the optimal Ile requirement for growth performance (Wiltafsky et al., 2009). It has been suggested that Leu levels are in excess when dietary concentrations are greater than 130% SID Leu:Lys (Htoo et al., 2014). Recent research by Htoo et al. (2017) has indicated that when the Leu:Lys ratio increases from 110% to 160%, the optimal Ile:Lys ratio changed from 54% to 58%. In the present study, diets provided 161% and 170% SID Leu:Lys. Based off the research of Htoo et al. (2017), the optimal SID Ile:Lys ratio in our study would be approximately 58%. Although linear or quadratic trends were not detected in this study, providing a SID Ile:Lys ratio of 60% in DDGS diets with a SID Leu:Lys ratio of 161% resulted in the greatest numerical ADG between DDGS diets during the first 28 d. This would be in reasonable agreement with the estimates of Htoo et al. (2017) when dietary Leu:Lys 160%. However, during the last 28 d, supplying a SID Ile:Lys ratio of 70% resulted in the greatest numerical ADG of pigs fed 20% DDGS. These results are similar to that of Kerkaert et al. (2021), who observed supplying a 68% SID Ile:Lys ratio in 20% DDGS diets during the finishing period resulted in similar ADG relative to pigs fed the high SBM positive control diet. The increase in dietary SID Leu:Lys from 161% to 170% in the current study may have resulted in the increased catabolism of Ile and thus increased the optimal SID Ile:Lys requirement from around 60% during the first period to approximately 70% in the second period.However, previous research has also suggested that the optimal SID Ile:Lys ratio may be closer to 60% or 61% for the finishing pigs (Kendall et al., 2004; Zier-Rush et al., 2018), rather than approximately 54% estimated by the NRC (2012). The lack of scientific literature on the Ile requirement in late finishing pigs makes it difficult to distinguish between whether pigs become more sensitive to an Ile deficiency relative to Leu during late finish, as suggested by Kerkaert et al. (2021), or whether the Ile requirement of finishing pigs is higher than previously suggested. The inability to detect linear or quadratic trends in the present study did not allow for the determination of an optimal SID Ile:Lys requirement in finishing pigs. The continued publication of literature on the Ile requirement in finishing pigs with and without excess dietary Leu concentrations will help us to clarify whether the Ile requirement increases solely due to dietary Leu or if the BW of pigs also contributes to a change in the Ile requirement of pigs.

When 20% DDGS diets with graded levels of SID Ile:Lys were compared to a corn-SBM diet, providing a corn-SBM diet to pigs during the first 28 d resulted in the greater ADG and ADFI relative to DDGS diets. The reduced ADFI of pigs feed DDGS explains the reduction in ADG observed for this period, as the feed efficiency of pigs was not impacted. The difference in ADFI could be explained by the feed intake of DDGS fed pigs being limited due to the bulk volume of feed in the intestinal tract (Nyachoti et al., 2004; Li and Patience, 2017). Alternatively, the reduced ADFI of DDGS fed pigs could also be a result of feed contaminants, such as mycotoxins. The analyzed concentration of DON in diets containing 20% DDGS was almost double compared to the corn-SBM diet; however, these levels are lower than the concentrations previously demonstrated to negatively impact performance (Accensi et al., 2006; Ensley and Radke, 2019). This would suggest that mycotoxins are not an explanation for the differences in ADFI observed between DDGS diets and the corn-SBM diet. The similarities of final BW and cumulative ADG between pigs fed a 20% DDGS diet with a SID Ile:Lys ratio of 70% and a pigs that received a CS diet indicates that Ile is a factor that is responsible for the differences in growth performance of pigs when fed DDGS or a standard CS diet. This is in agreement with the suggestions of Kerkaert et al. (2021), as described previously. However, the weak statistical similarities observed in the present study may suggest other dietary components of corn-SBM diets are also important to the growth performance of pigs.

Finding a quadratic response in BF and loin depth with the 65% Ile:Lys treatment having the greatest numeric response is a novel finding. However, this response was not large enough to have a significant impact on the performance criteria and would not be supported by prior work. Previous research from Dean et al. (2005) showed no impact of increasing the Ile level in late finishing diets on pig carcass characteristics, with the exception a linear increase in kg of fat free lean. However, in the current study, increasing the SID Ile:Lys ratio in DDGS diets led to a quadratic decrease in BF and quadratic increase in loin depth with the optimal SID Ile:Lys level between the 65% and 70% treatment groups. This improvement in carcass traits would suggest that the SID Ile:Lys requirement is above 65% when feeding 20% DDGS during the finishing period. However, compared to pigs fed the corn-SBM diet, pigs receiving the diets containing 20% DDGS had decreased loin depths, with the 65% and 70% treatments numerically closest to the corn-SBM treatment. In addition, when comparing BF of DDGS fed pigs to corn-SBM fed pigs, there was only a significant difference in reduction of BF for the 65% treatment group. Taken together, this would suggest that the lean tissue deposition of pigs fed diets containing 20% DDGS was limited, although improved by adjusting the Ile:Lys ratio. Further research is required to determine if other dietary components, such as SBM inclusion level, could be the explanation of difference observed in carcass characteristics between corn-SBM and DDGS diets.

In conclusion, observed similar final BW and cumulative ADG, ADFI, and G:F indicate that when feeding pigs 20% DDGS during the finishing stage of production, a SID Ile:Lys ratio of 70% should achieve similar overall growth performance compared to pigs fed a corn-SBM diet. In addition, the observed impacts on carcass characteristics would support that the optimal SID Ile:Lys ratio is between 65% and 70% when pigs are fed diets containing 20% DDGS. Further research is required to accurately define the optimal SID Ile:Lys ratio in late finishing swine diets at varying DDGS inclusion levels and to better quantify the mechanisms impacting the SID Ile:Lys requirement.

Acknowledgments

We wish to thank Christensen Farms for their technical support, use of resources, and funding of this study, along with Triumph Foods for their assistance with collection of carcass data. This work was also supported by the USDA National Institute of Food and Agriculture, Hatch project SD00H682-19.

Glossary

Abbreviations

AA	amino acids
ADG	average daily gain
ADFI	average daily feed intake
BCAA	branched chain amino acids
BF	back fat
BW	body weight
CP	crude protein
DDGS	dried distillers grains with solubles
DON	deoxynivalenol
G:F	gain-to-feed ratio
HCW	hot carcass weight
SBM	soybean meal
SID	standardized ileal digestible

Contributor Information

David A Clizer, Christensen Farms, Sleepy Eye, MN 56085, USA.

Blair J Tostenson, Christensen Farms, Sleepy Eye, MN 56085, USA.

Sam K Tauer, Christensen Farms, Sleepy Eye, MN 56085, USA.

Ryan S Samuel, Department of Animal Science, South Dakota State University, Brookings, SD 57007, USA.

Paul M Cline, Christensen Farms, Sleepy Eye, MN 56085, USA.

Conflict of Interest Statement

The authors declare no real or perceived conflicts of interest.

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