The effects of a temporary lysine restriction in newly weaned pigs on growth performance and body composition

Abstract

A serial slaughter study was conducted to determine the effects of a temporary Lys restriction immediately following weaning on growth performance and body composition. One hundred forty-four Yorkshire × Landrace × Duroc pigs (initial BW: 6.9 ± 0.2 kg) were randomly allocated to one of three dietary treatments (six pens per treatment with eight pigs per pen; four barrows and four gilts). For a 3-wk restriction period, pigs were fed diets that were 110% (Control) of the estimated required standardized ileal digestible (SID) Lys for nursery pigs or 20% (Lys20) or 40% (Lys40) below the estimated required SID Lys. Thereafter, all pigs were fed a common grower diet containing 120% of the estimated required SID Lys for 6 wk (recovery period). During the restriction period, ADG and G:F decreased with decreasing dietary Lys concentration (linear; P < 0.01). At the end of the restriction period, BW and whole-body protein concentrations decreased (linear; P < 0.01) and carcass lipid concentrations increased (linear; P < 0.01) with decreasing dietary Lys concentration. During the first 3 wk of the recovery period, ADG and G:F increased (linear; P < 0.05 and P < 0.01, respectively) and whole-body protein concentration at week 3 of the recovery period decreased (linear; P < 0.01) with decreasing dietary Lys concentration. There were no dietary treatment differences in whole-body lipid concentration after 3 wk of the recovery period. During the second half of the recovery period (weeks 7 through 9), there were no differences in ADG or G:F; after week 6, there were no differences in final BW (50.3 ± 0.5 kg) or whole-body protein (16.9 ± 0.2%) or lipid (14.9 ± 0.7%) concentrations. In conclusion, newly weaned pigs previously fed a Lys-limiting diet for 3 wk immediately after weaning achieved full compensatory growth with no differences in BW or body composition after a 6-wk recovery period. Reducing dietary Lys concentration early after weaning is a potential means to reduce the cost of (early) nursery diets without impacting overall growth and carcass composition.

INTRODUCTION

Feeding high-protein diets to pigs early in life was traditionally thought essential for ensuring optimal growth in the nursery and thereafter (Campbell and Dunkin, 1983). In addition to being expensive, high protein diets are often associated with greater incidence of post-weaning diarrhea in pigs due to proliferation of pathogenic bacteria and the resulting production of toxic metabolites (e.g., ammonia, branched-chain volatile fatty acids, putrescine), while a reduction in dietary protein may improve overall gut health and feed costs (e.g., Ball and Aherne, 1987; Nyachoti et al., 2006; Wellock et al., 2008). The latter, however, also carries the consequence of limiting protein deposition (Pd) and reducing growth.

Compensatory growth may be an effective means to improve nutrient utilization and decrease overall feed costs. It is well established that following a period of protein or AA restriction, pigs can achieve a body composition that is not different from unrestricted controls by altering the rates of tissue deposition during a recovery period (i.e., compensatory growth; Whang et al., 2003; Martinez-Rameriz et al., 2008b,Taylor et al., 2015). Notably, the primary focus was given to research examining protein and (or) AA restrictions in the growing phase but not in the nursery phase per se (i.e., when energy intake limits Pd; de Greef et al., 1992). The objective of this study was to determine the effects of a temporary dietary Lys restriction immediately post-weaning, followed by feeding a high-Lys diet during a recovery period on growth performance and body composition. We hypothesize that growth performance (i.e., ADG and G:F) will be reduced during Lys restriction due to a repartitioning of dietary energy toward lipid deposition (Ld) vs. Pd. Subsequently, after feeding a high Lys diet, pigs will achieve compensatory growth by utilizing body energy (lipid) stores to maximize Pd and achieve a similar body composition as unrestricted pigs.

MATERIALS AND METHODS

The study was conducted at the University of Guelph’s Arkell Swine Research Station in Guelph, ON, Canada. The experimental protocol was approved by the University of Guelph Animal Care committee prior to the commencement of the study, and all pigs were cared for according to the Canadian Council on Animal Care guidelines on the care and use of farm animals (CCAC, 2009).

Animals and Housing

One hundred forty-four newly weaned Yorkshire × Landrace × Duroc pigs (Yorkshire × Landrace dams, Duroc sires; ~21 d of age) from 26 different litters were utilized for the study. For the first 4 wk of the study, pigs (initial BW 6.9 ± 0.2 kg [mean ± SD]) were housed in two identical, environmentally controlled nursery rooms (26°C). Each room contained nine pens with plastic coated, expanded metal floors (1.2 × 3.0 m). Eight pigs were housed per pen (four barrows and four gilts), initial BW was balanced among pens, and littermates were delegated to separate pens. To ensure growing pigs had sufficient space, all pigs were relocated from their respective nursery rooms to two identical grower/finisher rooms at the beginning of study week 5; pigs remained grouped by pen. Each grower/finisher room was environmentally controlled (21°C) with fully slatted floors (1.5 × 4.0 m). Pig space allowance met the recommendations of the Canadian Code of Practice for the Care and Handling of Pigs (NFACC, 2014). For the duration of the study, all pigs were fed ad libitum and had free access to water via a nipple drinker.

Dietary Treatments

At weaning (study day 0), pens were allocated to one of three dietary treatments: 1) 110% of the estimated required standardized ileal digestible (SID) Lys (Control) or 2) 20% (Lys20) or 3) 40% (Lys40) below the estimated required SID Lys for nursery pigs (NRC, 2012). Due to the high inclusion level of corn relative to SBM in the Lys-restricted diets, the energy contents among dietary treatments were not constant. Therefore, diets were formulated on the basis of the SID Lys: net energy (NE) ratio, where both SID Lys and the SID Lys:NE ratio were reduced in a step-wise manner (Table 1). All diets were prepared and pelleted at the University of Guelph Arkell Feed Mill (Guelph, ON, Canada). Diets for the Lys20 treatment were prepared by blending the control and Lys40 diets at a ratio of 40:60. Pigs were fed the experimental diets for 3 wk during the restriction period in two phases (I: week 1, II: week 2 to 3; Table 1). There were six replicate pens per dietary treatment and three replicate pens per room.

Table 1.

Ingredient composition and nutrient concentrations of experimental diets (as-fed)¹

	Control I	Lys20 I	Lys40 I	Control II	Lys20 II	Lys40 II	Grower I	Grower II
Ingredient (%)
Corn	6.92	20.62	29.76	30.13	41.92	49.78	40.65	64.96
Soybean meal	22.90	9.16	–	23.30	11.60	3.80	30.1	28.50
Barley	25.00	25.00	25.00	25.00	25.00	25.00	20.00	–
Oat groats	10.00	10.00	10.00	–	–	–	–	–
Whey	20.00	20.00	20.00	8.00	8.00	8.00	–	–
Fish meal	5.00	5.00	5.00	3.00	3.00	3.00	–	–
Blood plasma	4.50	4.50	4.50	2.00	2.00	2.00	–	–
Blood meal	–	–	–	2.00	2.00	2.00	2.00	–
Mono/dicalcium phosphate	0.37	0.56	0.68	0.83	0.99	1.10	1.09	1.34
Vitamin and mineral premix2	0.60	0.60	0.60	0.60	0.60	0.60	0.60	0.6
Limestone	0.83	0.86	0.88	0.80	0.82	0.84	1.10	1.25
Salt	0.10	0.12	0.13	0.40	0.42	0.43	0.73	0.21
Fat, animal/vegetable blend	2.50	2.50	2.50	2.50	2.50	2.50	2.50	2.50
l-Lys·HCl	0.28	0.18	0.12	0.35	0.22	0.14	0.47	0.39
Dl-Met	0.15	0.07	0.02	0.17	0.07	0.01	0.20	0.14
l-Thr	0.05	0.02	–	0.12	0.05	–	0.16	0.11
Calcium formate	0.40	0.40	0.40	0.40	0.40	0.40	0.20	–
Calcium propionate	0.40	0.40	0.40	0.40	0.40	0.40	0.20	–
Calculated nutrient composition3
NE, kcal/kg	2,486	2,557	2,605	2,462	2,510	2,557	2,438	2,533
CP, %	25.1	19.5	15.8	23.1	18.2	15.0	22.4	19.5
Total Lys, %	1.78	1.33	1.03	1.66	1.24	0.96	1.62	1.31
SID Lys, %4	1.59	1.18	0.90	1.49	1.10	0.84	1.46	1.18
SID Thr, %	0.92	0.70	0.56	0.88	0.65	0.50	0.85	0.70
SID Trp, %	0.30	0.23	0.18	0.26	0.19	0.15	0.25	0.20
SID Met + Cys, %	0.88	0.68	0.55	0.82	0.62	0.49	0.79	0.67
SID Lys:NE	6.38	4.63	3.46	6.04	4.38	3.28	5.99	4.65
Ca, %	0.90	0.90	0.90	0.84	0.84	0.84	0.79	0.77
P, %	0.67	0.65	0.63	0.64	0.62	0.61	0.62	0.65
Analyzed nutrient composition, %
DM	89.0	88.8	89.3	89.1	88.3	88.18	86.81	87.30
CP	23.9	18.4	16.3	23.2	18.2	15.6	21.3	18.8
Total Lys	1.69	1.31	1.16	1.69	1.28	1.12	1.62	1.30
Ca	1.04	1.04	1.22	0.86	1.06	1.26	0.98	0.81
P	0.74	0.76	0.73	0.74	0.73	0.73	0.70	0.69

¹Control I, Lys20 I, and Lys40 I fed for 1 wk after weaning; Control II, Lys20 II, and Lys40 II fed in weeks 2 through 3 after weaning; Grower I fed in weeks 4 through 6 after weaning; Grower II fed in weeks 7 through 9 after weaning.

²Supplied per kg of diet: vitamin A, 12,000 IU as retinyl acetate; vitamin D3, 1,200 IU as cholecalciferol, vitamin E, 48 IU as D,L-α-tocopherol acetate; vitamin K, 3 mg as menadione; vitamin B12, 0.03 mg; d-pantothenic acid, 18 mg; riboflavin, 6 mg; choline, 600 mg; folic acid, 2.4 mg; niacin, 30 mg; thiamin, 18 mg; pyridoxine, 1.8 mg; biotin, 200 µg; Cu, 18 mg as CuSO₄·5H₂O; Fe, 120 mg as FeSO₄; Mn, 24 mg as MnSO₄; Zn, 126 mg as ZnO; Se, 0.36 mg as FeSeO₃; I, 0.6 mg as KI (DSM Nutritional Products Canada Inc., Ayr, ON, Canada).

³Calculated using ingredient values on an as-fed basis according to the NRC (2012).

⁴SID: standardized ileal digestible (NRC 2012).

Following the restriction period, all pigs were fed common grower diets for 6 wk during the recovery period in two phases (grower I: week 4 through 6; grower II: week 7 through 9). The grower diets were formulated 20% above the NRC- (2012) estimated SID Lys requirements for growing pigs. During the entire study, feed refusals were collected and weighed weekly (Defender 3000 bench scale, OHAUS Corporation, Parsippany, NJ) for determination of ADFI. At the same time, individual pig BW were recorded (weeks 1 to 4: Defender 3000 bench scale, OHAUS Corporation, Parsippany, NJ; weeks 4 through 9: Model 450 floor scale, GSE, Livonia, MI).

Serial Slaughter

Prior to implementing dietary treatments, eight pigs (four barrows, four gilts) were slaughtered to determine initial body composition. At the end of weeks 3, 6, and 9, two pigs (one barrow, one gilt) per pen with BW similar to the pen average were slaughtered for physical and chemical body composition analyses; pigs were not fasted prior to slaughter. During slaughter days, live BW were recorded prior to electrical stunning, after which pigs were exsanguinated via severing major blood vessels in the neck; blood was collected and weighed. Following this, visceral organs [heart, lungs, kidneys, liver (empty gallbladder), spleen, pancreas, empty bladder, and reproductive tract] were removed and weighed together. The gastrointestinal tract (GIT) was weighed separately after the gut contents were removed. The visceral organs (combined with the empty GIT) and the carcass were placed in separate plastic bags per pig and stored at −20°C for a minimum of 2 wk before grinding.

Once frozen, the carcass (including head, skin, hair, feet, and hooves) and viscera were removed from the freezer and reweighed. Whole carcass and viscera were ground separately three times each using a commercial meat grinder (model B-801, Autio Company, Astoria, OR) and a 12.5 (carcass) or 6 (viscera) mm die. After the final grinding, two subsamples (approximately 200 g each) from each carcass and viscera were collected, weighed, and stored at −20°C. One subsample was subsequently freeze-dried with weights recorded before and after drying to account for water loss. Freeze-dried subsamples were used for chemical analyses.

Chemical Analyses

Pelleted feed was subsampled from each 25-kg bag at manufacturing, pooled within batch and diet, and homogenized using a commercially available coffee grinder before analysis. Approximately 100 g of subsampled feed from each diet were shipped to Degussa AG (Hanua, Germany) to determine AA contents by ion-exchange chromatography coupled with post-column derivatization with ninhydrin (Llames and Fontanie, 1994; AOAC, 2006; Method 982.30). Feed samples were also sent to SGS Agrifood Laboratories (Guelph, ON, Canada) for DM (AOAC, 1997; Method 930.15), P and Ca (AOAC, 1997; Method 985:01), and N (via total combustion; LECO-FP 428; LECO Instruments Ltd., Mississauga, ON, Canada; AOAC, 1997; Method 990.03) analyses.

All analyses for carcass and viscera samples were conducted in duplicate. For DM and ash analyses, 2 g of freeze-dried sample was used, while 0.5 g of freeze-dried sample was used for lipid analysis. The DM content was determined via forced air oven drying (Model 737F, Fisher Scientific, Hampton, NH) at 100°C for 24 h. After drying, samples were combusted in a muffle furnace (Model 650-126 Fisher Scientific, Hampton, NH) at 500°C for 12 h for determination of ash concentration. The lipid concentration was determined via the high-temperature solvent extraction method (AOCS, 2017), using the ANKOM XT20 (ANKOM TECHNOLOGY, Macedon, NY, USA) lipid extractor. Nitrogen concentration was measured at Agrifood Laboratories (Guelph, ON, Canada) using combustion analysis (LECO-FP 428; LECO Instruments Ltd., Mississauga, ON, Canada) according to AOAC (1997; Method 990.03).

Calculations and Statistical Analysis

To determine whole-body chemical composition, the protein, lipid, ash, and water contents were summed for carcass and viscera. Although blood was collected and weighed during slaughter, weights were variable. Therefore, the blood weight used for whole body calculations was assumed to be 6% of empty body weight (i.e., excluding GIT contents; Upton, 2008). The chemical composition of blood was based on values from Flecknell (1979).

Daily whole body Pd and Ld were calculated as the difference in protein or lipid mass of pigs slaughtered at the end and beginning of the given period, divided by the period length. In addition, the Ld:Pd ratio of the carcass and viscera was calculated as the Ld (g/d) divided by the Pd (g/d), while the lipid-to-protein ratio (L:P) was calculated as the lipid mass (kg) divided by the protein mass (kg).

The apparent efficiency of dietary SID Lys utilization (kLys) for whole-body protein retention was calculated assuming that the Lys content of protein gain was 7.10 g per 100 g of whole-body protein gain (NRC, 2012). The total SID Lys consumed was calculated based on feed intake, analyzed dietary Lys concentration, and the estimated SID Lys digestibility for each diet (NRC, 2012). This calculation also accounted for Lys losses from the intestine and skin and hair based on DM intake and metabolic BW, respectively (NRC, 2012). A detailed description of this equation has been described elsewhere (Mansilla, 2017).

All data were analyzed as a randomized complete block design using the PROC GLIMMIX function of SAS (v.9.4, SAS Institute Inc., Cary, NC), with pen as the experimental unit. In this model, dietary treatment, block (i.e., room), and their interaction were considered fixed effects. Since the interaction was not significant for any outcome (P > 0.10), a reduced model was used. Pig within pen was considered a random effect. For growth performance data, initial BW was used as a covariate and for body composition data, sex was used as a covariate. Week was treated as a repeated measure. Differences among least square treatment means were assessed using the Tukey Honest Significance Test and linear contrasts were constructed to examine responses to incremental reductions in dietary SID Lys. Probability levels < 0.05 were considered significant, whereas 0.05 ≤ P ≤ 0.10 was considered a trend and P > 0.10 was considered not significant.

RESULTS

The chemical analyses of nursery diets were generally comparable to calculated values for CP, total Lys, and P (Table 1). For Lys40 I and Lys40 II diets, total Lys contents were 12% and 14% greater, respectively, than anticipated. These errors may reflect an underestimation of the Lys concentration in some of the dietary ingredients used. Furthermore, analyzed Ca concentrations were typically greater than formulated, which may be attributed to the underestimation of Ca concentrations in some ingredients or a systematic error during Ca analysis.

In the recovery period, one pig from the control treatment and one pig from the Lys20 treatment were removed from the trial due to Streptococcus suis infection. Based on subjective visual observations, the remaining pigs appeared to be in good health, with no evident diet effects on health (e.g., rough hair coat).

Growth Performance

Initial BW did not differ among dietary treatment groups (Table 2). During the restriction period (weeks 1 through 3 post-weaning), ADG, G:F, and final BW decreased (linear; P < 0.01) with decreasing dietary Lys concentration. There were no differences in ADFI during the restriction period, but average daily Lys intake (ADLysI) decreased (linear; P < 0.05) with decreasing dietary Lys concentration. During the first half of the recovery period (weeks 4 through 6 post-weaning), ADG and G:F increased (linear; P < 0.05) but final BW decreased (linear; P < 0.05) with decreasing dietary Lys concentration. There were no differences in ADFI or ADLysI during the first half of the recovery period. During the second half of the recovery period (weeks 7 through 9 post-weaning), there were no differences in ADG, G:F, ADFI, or ADLysI, and by week 9, there were no differences in BW. Overall there were no differences in ADG, ADFI, and G:F for the entire 9-wk experimental period (restriction + recovery periods); however, ADLysI decreased (linear; P < 0.01) with decreasing dietary Lys concentration.

Table 2.

Effects of a short-term Lys restriction after weaning on pig growth performance during the restriction (weeks 1 through 3), recovery (weeks 4 through 6 and weeks 7 through 9), and combined restriction and recovery periods (weeks 1 through 9)

	Dietary treatment1					Contrast
Trait	Control	Lys20	Lys40	SEM	P-value	Linear
Restriction (weeks 1–3)
Initial BW, kg	6.9	6.9	6.9	0.2	0.985	0.893
Final BW, kg	15.6a	14.7b	13.6c	0.2	<0.001	<0.001
ADFI, g, DM basis	457	464	457	15	0.940	0.977
ADLysI, g2	7.2a	5.4b	4.6c	0.2	<0.001	<0.001
ADG, g	411a	373b	319c	8	<0.001	<0.001
G:F	0.91a	0.81b	0.71c	0.01	<0.001	<0.001
Recovery (weeks 4–6)
Final BW, kg	30.8 a	30.4 ab	29.6 b	0.3	0.026	0.009
ADFI, g, DM basis	1,080	1,048	1,050	16	0.286	0.178
ADLysI, g2	15.9	15.5	15.5	0.2	0.286	0.177
ADG, g	722	750	759	13	0.112	0.044
G:F	0.67b	0.72a	0.73a	0.01	0.007	0.003
Recovery (weeks 7–9)
Final BW, kg	50.8	50.2	50.0	0.5	0.446	0.225
ADFI, g, DM basis	1,823	1,774	1,786	30	0.497	0.407
ADLysI, g2	21.3	20.7	20.9	0.4	0.497	0.408
ADG, g	1,007	983	1,025	16	0.161	0.407
G:F	0.56	0.56	0.58	0.01	0.425	0.256
Overall (weeks 1–9)
ADFI, g, DM basis	1,120	1,095	1,098	145	0.413	0.290
ADLysI, g2	14.8a	13.9b	13.6b	0.2	<0.001	<0.001
ADG, g	714	702	701	8	0.469	0.262
G:F	0.64	0.64	0.64	0.01	0.810	0.698

¹Restriction period: Pigs fed 110% or 20% or 40% below the estimated required SID Lys (NRC, 2012; weeks 1–3). Recovery period: all pigs fed 120% of the estimated SID Lys requirements (NRC, 2012; weeks 4–9). n = 6 pens per treatment.

²Average Daily Lysine Intake (DM basis).

^a,b,cDifferent letters in the same row represent statistically significant differences between treatments (main effect of treatment; P < 0.05).

Physical and Chemical Body Composition

At the end of the restriction period, carcass weights decreased (linear; P < 0.01) with decreasing dietary Lys concentration (Table 3). This was accompanied by decreased (linear; P < 0.01) carcass protein and water concentrations, and increased (linear; P < 0.05) carcass lipid and ash concentrations and L:P with decreasing dietary Lys concentration. Dietary Lys concentration did not influence total viscera weight at the end of the restriction period, but the size of the empty GIT as a percentage of the total viscera was increased (linear; P < 0.01) with decreasing dietary Lys concentration. Visceral water concentration was not affected by dietary Lys concentration, but the protein and ash concentrations decreased (linear; P < 0.05) and the lipid concentration and L:P ratio increased (linear; P < 0.01) with decreasing dietary Lys concentration. Empty BW and whole body protein, and water concentrations decreased (linear; P < 0.05) and whole-body lipid and ash concentrations and the whole body L:P increased (linear; P < 0.001, P = 0.054, P < 0.001, respectively) with decreasing dietary Lys concentration.

Table 3.

Effects of a short-term Lys restriction post-weaning on pig physical and chemical characteristics at the end of the restriction period (week 3)

	Dietary treatment1					Contrast
Trait	Control	Lys20	Lys40	SEM	P-value	Linear
Carcass composition
Carcass weight, kg	11.6a	11.0ab	10.3b	0.3	0.023	0.007
Protein, %	16.5a	16.1a	15.4b	0.2	0.003	0.001
Lipid, %	8.5c	11.5b	13.4a	0.4	<0.001	<0.001
Ash, %	3.3	3.7	3.7	0.1	0.042	0.029
Water, %	71.1a	68.8b	66.9c	0.3	<0.001	<0.001
L:P2	0.52c	0.71b	0.87a	0.02	<0.001	<0.001
Viscera composition
Viscera weight, kg	2.5	2.4	2.4	0.1	0.305	0.186
Empty GIT, %3	59.3a	61.2ab	63.0b	0.7	0.007	0.002
Protein, %	14.7a	14.3ab	14.1b	0.1	0.006	0.002
Lipid, %	4.1b	4.2b	4.8a	0.1	0.004	0.002
Ash, %	1.2a	1.2ab	1.2b	0.01	0.035	0.018
Water, %	80.1	80.2	80.1	0.2	0.945	0.999
L:P2	0.27b	0.30b	0.34a	0.01	<0.001	<0.001
Whole body composition4
Empty BW, kg5	15.0a	14.3ab	13.5b	0.4	0.036	0.011
Protein, %	16.4a	16.0a	15.4b	0.2	0.001	<0.001
Lipid, %	7.3a	9.6b	11.1c	0.3	<0.001	<0.001
Ash, %	2.8	3.1	3.1	0.1	0.067	0.054
Water, %	73.5a	71.4b	70.4b	0.4	<0.001	<0.001
L:P2	0.45c	0.60b	0.72a	0.02	<0.001	<0.001

¹Restriction period: Pigs fed 110% or 20% or 40% below the estimated required SID Lys (NRC, 2012; weeks 1–3). Recovery period: all pigs fed 120% of the estimated SID Lys requirements (NRC, 2012; weeks 4–9). n = 6 pens per treatment.

²Lipid-to-protein ratio for designated composition trait.

³Empty gastrointestinal tract as a percent of total viscera (includes: heart, lungs, kidneys, liver, empty gall bladder, spleen, pancreas, empty bladder, and reproductive tract).

⁴Whole body is the sum of the carcass, viscera, and blood composition for protein, lipid, ash, and water, respectively.

⁵Empty BW = live BW – gut fill.

^a,b,cDifferent letters in the same row represent statistically significant differences between treatments (main effect of treatment; P < 0.05).

After 3 wk of the recovery period, carcass weight and carcass and whole-body protein concentrations decreased (linear; P = 0.09, P < 0.01, P < 0.01, respectively) and the L:P ratio in carcass and whole body increased (linear; P < 0.05 and P = 0.07, respectively) with decreasing dietary Lys concentration (Table 4). After 6 wk of the recovery period, there were no differences in carcass or viscera weights or chemical composition (Table 5).

Table 4.

Effects of a short-term Lys restriction post-weaning on pig physical and chemical composition after 3 wk of the recovery period (week 6)

	Dietary treatment1					Contrast
Trait	Control	Lys20	Lys40	SEM	P-value	Linear
Carcass composition
Carcass weight, kg	24.9	24.0	23.7	0.5	0.211	0.092
Protein, %	17.7a	17.3ab	17.0b	0.2	0.021	0.006
Lipid, %	11.6	12.0	12.6	0.5	0.317	0.138
Ash, %	3.3	3.5	3.5	0.1	0.398	0.234
Water, %	67.0	67.1	66.6	0.5	0.769	0.589
L:P2	0.66	0.69	0.75	0.03	0.132	0.049
Viscera composition
Viscera weight, kg	4.5	4.6	4.4	0.1	0.524	0.381
Empty GIT, %3	57.9	58.3	57.9	0.6	0.820	0.974
Protein, %	13.3	13.2	13.4	0.2	0.687	0.514
Lipid, %	6.0	6.1	6.1	0.2	0.883	0.650
Ash, %	1.1	1.1	1.1	0.02	0.507	0.309
Water, %	79.6	79.6	79.4	0.2	0.596	0.371
L:P2	0.45	0.46	0.46	0.01	0.903	0.834
Whole body composition4
Empty BW, kg5	31.3	30.4	29.8	0.6	0.249	0.103
Protein, %	17.2a	16.8ab	16.6b	0.1	0.022	0.007
Lipid, %	10.1	10.4	10.9	0.4	0.368	0.170
Ash, %	2.9	3.0	3.0	0.1	0.424	0.238
Water, %	69.8	69.8	69.4	0.4	0.749	0.497
L:P2	0.59	0.62	0.66	0.03	0.180	0.070

¹Restriction period: Pigs fed 110% or 20% or 40% below the estimated required SID Lys (NRC, 2012; weeks 1–3). Recovery period: all pigs fed 120% of the estimated SID Lys requirements (NRC, 2012; weeks 4–9). n = 6 pens per treatment.

²Lipid-to-protein ratio for designated composition trait.

³Empty gastrointestinal tract as a percent of total viscera (includes: heart, lungs, kidneys, liver, empty gall bladder, spleen, pancreas, empty bladder, and reproductive tract).

⁴Whole body is the sum of the carcass, viscera, and blood composition for protein, lipid, ash, and water, respectively.

⁵Empty BW = live BW – gut fill.

^abcDifferent letters in the same row represent statistically significant differences between treatments (main effect of treatment; P < 0.05).

Table 5.

Effects of a short-term Lys restriction post-weaning on pig physical and chemical composition after 6 weeks of the recovery period (week 9)

	Dietary treatment1					Contrast
Trait	Control	Lys20	Lys40	SEM	P-value	Linear
Carcass composition
Carcass weight, kg	40.5	40.3	39.6	0.8	0.737	0.456
Protein, %	16.8	17.4	17.1	0.2	0.179	0.298
Lipid, %	16.8	17.4	17.0	0.8	0.880	0.845
Ash, %	3.4	3.3	3.4	0.1	0.816	0.902
Water, %	62.7	62.0	62.1	0.6	0.731	0.546
L:P2	1.01	1.00	1.00	0.06	0.992	0.902
Viscera composition
Viscera weight, kg	6.4	6.4	6.1	0.1	0.338	0.279
Empty GIT, %3	54.6	54.0	54.5	0.8	0.843	0.933
Protein, %	14.1	14.4	14.1	0.2	0.337	0.856
Lipid, %	8.4	8.2	8.3	0.4	0.960	0.867
Ash, %	1.1	1.1	1.0	0.01	0.099	0.131
Water, %	76.5	76.2	76.6	0.4	0.776	0.798
L:P2	0.60	0.57	0.60	0.03	0.836	0.989
Whole body composition4
Empty BW, kg5	49.8	49.7	48.7	1.0	0.640	0.406
Protein, %	16.6	17.1	16.9	0.2	0.192	0.298
Lipid, %	14.7	15.1	15.0	0.7	0.922	0.837
Ash, %	3.0	2.9	2.9	0.1	0.770	0.898
Water, %	65.7	64.9	65.2	0.6	0.648	0.589
L:P2	0.89	0.89	0.89	0.05	0.997	0.951

¹Restriction period: Pigs fed 110% or 20% or 40% below the estimated required SID Lys (NRC, 2012; weeks 1–3). Recovery period: all pigs fed 120% of the estimated SID Lys requirements (NRC, 2012; weeks 4–9). n = 6 pens per treatment.

²Lipid-to-protein ratio for designated composition trait.

³Empty gastrointestinal tract as a percent of total viscera (includes: heart, lungs, kidneys, liver, empty gall bladder, spleen, pancreas, empty bladder, and reproductive tract).

⁴Whole body is the sum of the carcass, viscera, and blood composition for protein, lipid, ash, and water, respectively.

⁵Empty BW = live BW – gut fill.

^abcDifferent letters in the same row represent statistically significant differences between treatments (main effect of treatment; P < 0.05).

Whole Body Protein and Ld

During the restriction period, the kLys increased (linear; P < 0.05) with decreasing dietary Lys concentration. Carcass, viscera, and whole body Pd decreased (linear; P < 0.05) and carcass and whole body Ld increased (linear; P < 0.01) with decreasing dietary Lys concentration (Table 6). The Ld:Pd in the carcass, viscera, and whole body increased (linear; P < 0.01) with decreasing dietary Lys concentration.

Table 6.

Effects of a short-term Lys restriction post-weaning on apparent efficiency of dietary standardized ileal digestible Lys utilization (kLys) and carcass, viscera, and whole body protein (Pd) and lipid (Ld) deposition during the restriction (weeks 1 through 3) and recovery periods (weeks 4 through 9)

	Dietary treatment1					Contrast
Trait	Control	Lys20	Lys40	SEM	P-value	Linear
Restriction
kLys, %2	64.8b	75.8a	74.6ab	2.9	0.028	0.027
Carcass
Pd, g/d	51a	45ab	36b	3	0.003	<0.001
Ld, g/d	9b	22a	28a	3	0.002	<0.001
Ld:Pd	0.16c	0.47b	0.76a	0.05	<0.001	<0.001
Viscera
Pd, g/d	12a	11ab	11b	0.5	0.044	0.020
Ld, g/d	3	3	4	0.2	0.052	0.032
Ld:Pd	0.25b	0.28b	0.35a	0.01	<0.001	<0.001
Whole body
Pd, g/d	64a	56ab	47b	3	0.003	<0.001
Ld, g/d	12b	25a	32a	3	0.002	<0.001
Ld:Pd	0.18c	0.43b	0.66a	0.04	<0.001	<0.001
Recovery (weeks 4–6)
kLys, %	59.9	59.7	60.3	2.1	0.988	0.919
Carcass
Pd, g/d	118	113	115	5	0.745	0.660
Ld, g/d	90	77	78	6	0.225	0.141
Ld:Pd	0.77	0.67	0.65	0.05	0.305	0.150
Viscera
Pd, g/d	11	12	12	1	0.393	0.304
Ld, g/d	8	8	7	0.5	0.343	0.297
Ld:Pd	0.76a	0.67ab	0.61b	0.04	0.032	0.010
Whole body
Pd, g/d	129	125	127	5	0.876	0.802
Ld, g/d	98	86	85	6	0.251	0.142
Ld:Pd	0.77	0.67	0.64	0.05	0.252	0.117
Recovery (weeks 7–9)
kLys, %	44.9	54.4	51.8	2.9	0.088	0.113
Carcass
Pd, g/d	114	136	133	8	0.146	0.116
Ld, g/d	189	196	182	20	0.871	0.798
Ld:Pd	1.76	1.48	1.40	0.18	0.327	0.159
Viscera
Pd, g/d	14	15	13	1	0.297	0.441
Ld, g/d	13	12	12	1	0.842	0.588
Ld:Pd	0.86	0.82	0.95	0.13	0.765	0.634
Whole body
Pd, g/d	128	151	146	9	0.171	0.162
Ld, g/d	202	208	194	21	0.880	0.782
Ld:Pd	1.64	1.40	1.35	0.15	0.392	0.206
Overall
kLys, %	53.6b	59.4a	57.9ab	1.6	0.043	0.065
Whole body
Pd, g/d	107	111	107	3	0.544	0.883
Ld, g/d	104	106	103	7	0.942	0.897
Ld:Pd	0.97	0.96	0.96	0.06	0.994	0.944

¹Restriction period: Pigs fed 110% or 20% or 40% below the estimated required SID Lys (NRC, 2012; weeks 1–3). Recovery period: all pigs fed 120% of the estimated SID Lys requirements (NRC, 2012; weeks 4–9). n = 6 pens per treatment.

²Apparent Lys utilization efficiency for protein retention above maintenance.

^abcDifferent letters in the same row represent statistically significant differences between treatments (main effect of treatment; P < 0.05).

During the first 3 wk of the recovery period (weeks 4 through 6), the Ld:Pd in viscera decreased (linear; P = 0.01) with decreasing dietary Lys concentration, but kLys and Pd and Ld in carcass and viscera were not impacted. During the final 3 wk of the recovery period (weeks 7 through 9), kLys and Pd and Ld in the carcass, viscera, and whole body were not influenced by dietary Lys concentration. Over the entire experimental period (restriction + recovery), there were no differences in Pd or Ld in the carcass, viscera (data not shown), or whole body (Table 6), but kLys tended to increase (linear; P = 0.065) with decreasing dietary Lys concentration fed in the restriction period.

DISCUSSION

The aim of the current study was to determine the effects of a temporary dietary Lys restriction immediately post-weaning, followed by feeding a high Lys diet during a recovery period on growth performance and body composition. Indeed, we demonstrated that following a 3-wk dietary Lys restriction, pigs weighed less and had greater whole-body lipid concentration but lower whole-body protein concentration than the unrestricted controls. After receiving a diet with 120% of estimated Lys requirements, previously restricted pigs achieved the same body weight and chemical body composition after a 6-wk recovery period via compensatory growth and without increased feed intake. Therefore, compensatory growth may be a viable way to reduce feed costs by reducing total dietary Lys concentration in the early post-weaning period, without jeopardizing overall growth performance and body composition.

Previous work exploring compensatory growth in the nursery period after an AA restriction is relatively limited, likely due to the commonly accepted notion that young pigs require diets high in AA to support Pd and to optimize growth performance in the nursery and beyond (Campbell and Dunkin, 1983). Younger pigs, however, have increased likelihood of achieving compensatory growth because they are still in the energy-dependent phase of Pd and have not attained their maximum genetic potential for Pd (de Greef et al., 1992). Conversely, compensatory growth is not always evident for grower pigs after a dietary restriction, which is likely due to differences in timing, duration, type, and severity of restriction (Taylor et al., 2013). The duration of compensatory growth is determined by the amount of time that is required for pigs to achieve the “target” whole body L:P. Following this, Pd and Ld will return to levels similar to that of the control pigs (Reynolds and O’Dohetry, 2006). This implies that constraints on the L:P rather than the Ld:Pd will determine the composition of growth and also infers genetic and stage-of-growth aspects to a pig’s ability to achieve compensatory growth.

In the current study, the temporary Lys restriction altered the partitioning of energy between protein and lipid in the carcass, viscera, and whole body, such that Pd was decreased and Ld was increased in Lys-restricted pigs. This repartitioning of energy can be attributed to dietary Lys-limiting Pd for the pigs fed the restricted diets, and consequently, energy that could have been utilized for Pd was instead partitioned toward Ld. Indeed, the maximum (apparent) efficiency of Lys utilization for protein retention was achieved during the restriction period for pigs fed Lys-restricted diets (NRC, 2012), indicating that Lys was limiting whole body Pd. As a result, ADG, G:F, and whole-body protein concentrations were less, and whole-body lipid concentrations were greater for Lys-restricted pigs at the end of the restriction period, which are in agreement with other studies that evaluated protein and Lys restrictions for both nursery and grower pigs (de Greef et al., 1994; Chiba et al., 1999; Taylor et al., 2015).

It appeared that the main compensatory growth response occurred primarily in the first 3 wk of the recovery period (weeks 4 through 6 post-weaning) in the current study, as there were no differences in ADG or G:F during the second half of the recovery period (weeks 7 through 9). Others have shown that most compensatory growth occurred in the first 2 wk of the recovery period when a dietary Lys restriction was removed for grower pigs (Reynolds and O’Doherty, 2006). In the current study, the viscera was the first pool to achieve compensatory Pd as the Ld:Pd ratio was reduced (i.e., greater Pd vs. Ld) with decreasing dietary Lys concentration in the first half of the recovery period (i.e., weeks 4 through 6) and visceral chemical composition was not different among treatment groups by the midpoint of the recovery period (i.e., week 6). The responsiveness of the viscera to dietary AA supply is likely a function of the rapid protein turnover rates and smaller pool size of the viscera vs. those of the carcass (Martinez-Ramirez et al., 2008a). Complete compensatory growth for the whole body was not achieved until the second half of the recovery period (i.e., weeks 7 through 9). This was because the whole body L:P ratio tended to increase with decreasing dietary Lys concentration at week 6, which was largely due to the fact that carcass protein concentrations were still reduced with decreasing dietary Lys concentration. Therefore, the length of the recovery period is important to ensure that both the carcass and whole body completely compensate, as well as the viscera.

During the recovery period in the current study, feed intake was not impacted by previous Lys restriction, which indicates that pigs were still limited by physical gut capacity (Nyachoti et al., 2004) and (or) the increased body lipid concentration suppressed appetite (Skiba, 2005). Therefore, compensatory growth was due to a redistribution of energy between Pd and Ld. This corroborates the observations of Whittemore and Kyriazakis (2006) whereby pigs that become fatter than a given “desired” body composition during a nutritional restriction corrected the deviation by altering the composition of growth.

Despite achieving body composition that was not different among treatment groups by the end of the recovery period, we were unable to detect differences in Pd and Ld in carcass and viscera in either the first or second half of the recovery period. It is plausible that small changes in Pd and Ld were sufficient to compensate body composition, but may not have been detected by the serial slaughter method. Others demonstrated an increase in Pd and a reduction in Ld after the removal of a nutritional restriction (de Greef, 1993; Fabian et al., 2004; Skiba, 2005) in order for the pig to correct a nutritionally derived difference in body composition from a “predetermined” target body composition (Whitermore and Kyriazakis, 2006). Moreover, in the current study, nursery pigs had not attained the upper genetic limit for Pd (e.g., Martinez-Ramirez et al., 2008a), which likely allowed previously restricted pigs to accelerate Pd to achieve a final whole-body protein concentration not different from unrestricted pigs.

Overall, it is noteworthy that pigs subjected to a Lys restriction during the early post-weaning period were capable of achieving the same body composition in the same amount of time as the controls but using less dietary Lys (i.e., increased Lys utilization efficiency and no change in feed intake). Using current commodity prices, we estimate a cost saving of $1.05 and 1.18 per pig during the entire nursery period for pigs fed Lys20 and Lys40 in the restriction period (vs. CON). We also acknowledge that the control pigs were oversupplied dietary Lys, which hinders a direct comparison to commercial scenarios. Cost savings would be less if control pigs were fed Lys similar to estimated requirements. Thus, in this scenario, we estimate a cost saving of $0.35 and 0.49 per pig during the entire nursery period for pigs fed Lys20 and Lys40 in the restriction period vs. pigs that are fed diets with Lys concentrations similar to estimated requirements. We also recognize that further research is also needed to determine whether there are any other lasting effects of temporary dietary Lys restriction early in the nursery phase on carcass and meat quality at slaughter.

In conclusion, implementing a dietary Lys restriction (20 and 40% below estimated SID Lys requirements) for nursery pigs during the first 3 wk after weaning, reduced ADG and Pd and increased Ld during the restriction period. During a 6-wk recovery period, when offered diets with 120% of estimated Lys requirements and without any alterations in feed intake, previously restricted pigs were able to adjust the distribution of whole body Pd and Ld to achieve a body composition and body weight not different from pigs that experienced no Lys restriction. Consequently, restricted pigs achieved the same amount of Pd within the same timeframe but with less dietary Lys. Therefore, a dietary Lys restriction during the early nursery period may be a feasible method to reduce per pig nursery feed costs. Further research is needed to determine whether such a restriction will impact carcass and meat quality at slaughter, prior to the commercial application of a compensatory growth feeding strategy.

Conflict of interest statement.

None declared.