Determining the effects of manganese source and level on growth performance and carcass characteristics of growing–finishing pigs

Abstract

Two experiments were conducted to determine the effects of Mn source and level on finishing pig growth performance and carcass characteristics. Dietary treatments were arranged in a 2 × 3 factorial with main effects of Mn source (MnSO₄; Eurochem, Veracruz, Mexico, or Mn hydroxychloride (IBM); Micronutrients, Indianapolis, IN) and increasing added Mn (8, 16, and 32 mg/kg of complete diet). The trace mineral premix was formulated without added Mn. Copper was added to all diets at 10 and 150 mg/kg in Exp. 1 and 2, respectively. In both experiments, 1,994 pigs (PIC; 337 × 1050; initially 34.5 ± 0.50 and 40.0 ± 0.77 kg) were used with 27 pigs per pen and 12 replicates per treatment. Diets were corn-soybean meal-distillers dried grains with solubles-based and were fed in four phases. In Exp. 1, there was a marginal Mn source × level interaction (quadratic, Ρ = 0.057) for overall feed efficiency (G:F), with a decrease then increase in pigs fed IBM, but G:F increased with increasing Mn from MnSO₄. There was no evidence for Mn source differences for average daily gain (ADG), average daily feed intake (ADFI), or body weight (BW), but pigs fed 16 mg/kg Mn, regardless of source, tended to have decreased (quadratic, Ρ < 0.05) ADG and final BW compared with other levels. For carcass yield, there was a tendency for Mn source × level interaction (quadratic, Ρ = 0.075), where carcass yield did not change by increasing MnSO₄ but was greatest for 16 mg/kg Mn from IBM. Loin depth increased (source × level, Ρ = 0.041) for pigs fed increasing Mn from MnSO₄ but decreased when Mn was increased from IBM. Pigs fed the intermediate level of Mn tended to have the lightest HCW (quadratic, Ρ = 0.071) and decreased loin depth (quadratic, Ρ = 0.044). Liver Mn concentration increased (linear, Ρ = 0.015) as added Mn increased and tended to be greater (P = 0.075) when supplied by MnSO₄ compared with IBM. In Exp. 2, there was no (P > 0.10) Mn source × level interaction observed for ADG, ADFI, and G:F. Pigs fed IBM had increased (P < 0.05) final BW, ADG, and ADFI compared with pigs fed MnSO₄. Pigs fed 16 mg/kg of Mn tended (P = 0.088) to have reduced ADFI when compared with pigs fed 8 and 32 mg/kg of Mn. In conclusion, there appears to be little benefit in growth performance by feeding more than 8 mg/kg of added Mn. When high levels of Cu were fed in Exp. 2, pigs fed IBM had improved growth performance when compared with those fed MnSO₄. Further research is needed to understand the potential benefits of Mn hydroxychloride fed in conjunction with high levels of Cu on pig growth performance.

INTRODUCTION

Manganese is an essential trace mineral that is a key component in carbohydrate, lipid, and protein metabolism, as well as playing a role in mitochondrial superoxide dismutase (MnSOD) activity and bone development (Suttle, 2010). According to the NRC (2012), the quantitative requirement for Mn for nursery and finishing diets ranges from 2 to 4 mg/kg. Assuming bioavailability is not a concern; many swine diets today meet the NRC (2012) estimated requirement for Mn from the major dietary ingredients before a trace mineral premix is added to the diet. However, due to the unknown bioavailability of the innate Mn in ingredients, swine diets typically contain added Mn through a trace mineral premix. In a survey conducted by Flohr et al. (2016), swine diet Mn levels were found to be supplemented at as low as 3.3 mg/kg and as high as 40 mg/kg throughout the entire finishing period. Therefore, there is a wide discrepancy of Mn supplementation in commercial swine diets.

Manganese is typically supplemented in swine diets as manganese sulfate; however, different mineral sources can be utilized within swine diets, such as hydroxychloride trace minerals. To our knowledge, little research has been completed to observe the effects of Mn hydroxychloride in finishing diets or how other dietary levels of trace minerals such as Cu may affect the response. Therefore, the objective of our study was to determine the effects of increasing dietary levels of Mn and source of Mn on growth performance and carcass characteristics of growing–finishing pigs.

MATERIALS AND METHODS

The Kansas State University Institutional Animal Care and Use Committee approved the protocols used in this experiment.

Animals and Diets

Two studies were conducted with a total of four barns (two barns per study) at a commercial research-finishing site in southwest Minnesota (New Horizon Farms, Pipestone, MN). Each barn was naturally ventilated and double-curtain-sided with a slatted concrete floor and deep manure storage. Each pen (3.05 × 5.49 m²) was equipped with a 5-hole stainless steel dry self-feeder (Thorp Equipment, Thorp, WI) and a bowl waterer for ad libitum access to feed and water. The first experiment was conducted from 3 October 2018 to 27 February 2019, and the second experiment was conducted from 5 December 2019 to 22 April 2020.

In each experiment, two groups of 972 pigs (1,944 total pigs, PIC 337 × 1050; initially 34.5 ± 0.50 and 40.0 ± 0.77 kg) were used in 107- or 100-d growth trial. Pigs were housed in mixed gender pens with 27 pigs per pen and 12 pens per treatment. Each dietary treatment was evenly distributed within and among the two barns. Daily feed additions to each pen were achieved by using a robotic feeding system (FeedPro; Feedlogic Corp., Wilmar, MN) able to record feed amounts for individual pens. The treatments were structured as a randomized complete block design and arranged in a 2 × 3 factorial with main effects of Mn source (MnSO₄, Eurochem, Veracruz, Mexico; or Mn hydroxychloride, IBM; IntelliBond M, Micronutrients USA, LLC, Indianapolis, IN) and increasing supplemental Mn (8, 16, or 32 mg/kg of complete diet). All treatment diets were manufactured at the New Horizon Farms Feed Mill in Pipestone, MN and were formulated to meet or exceed NRC (2012) requirement estimates for growing–finishing pigs for their respective weight ranges (Table 1). The same basal diets were fed in both experiments. Diets were fed in meal form and in four dietary phases within each experiment.

Table 1.

Composition of basal diets, Exp. 1 and 2 (as-fed basis)^a

Items	Phase 1	Phase 2	Phase 3	Phase 4
Ingredients, %
Corn	58.80	66.88	72.51	80.66
Soybean meal (46.5% CP)	26.60	18.77	13.29	15.35
DDGSb	10.00	10.00	10.00	—
Beef tallow	1.50	1.50	1.50	1.50
Limestone, ground	1.08	1.00	0.95	0.73
Monocalcium phosphate (21% P)	0.90	0.75	0.65	0.75
Salt	0.35	0.35	0.35	0.35
L-lysine-HCl	0.37	0.39	0.39	0.30
DL-methionine	0.06	0.03	0.01	0.02
L-threonine	0.09	0.09	0.10	0.10
L-tryptophan	0.02	0.03	0.03	0.03
Phytasec	0.04	0.04	0.04	0.04
Vitamin-trace mineral premixd,e	0.15	0.15	0.15	0.15
Mn sourcef	+/−	+/−	+/−	+/−
Calculated analysis
Standardized ileal digestible amino acids, %
Lysine	1.15	0.97	0.84	0.79
Isoleucine:lysine	63	61	59	60
Leucine:lysine	140	147	155	147
Methionine:lysine	31	30	29	29
Methionine and cysteine:lysine	55	55	56	56
Threonine:lysine	62	62	64	65
Tryptophan:lysine	19	19	19	20
Valine:lysine	70	70	70	70
Lysine:net energy, g/Mcal	4.62	3.82	3.26	3.05
Net energy, kcal/kg	2,486	2,539	2,574	2,594
Crude protein, %	20.8	17.8	15.6	14.4
Calcium, %	0.73	0.63	0.57	0.52
Standardized total tract digestible phosphorus, %	0.52	0.47	0.41	0.39

^aIn Exp.1, phases 1, 2, 3, and 4 were fed from 34.5 to 56.7, 56.7 to 72.6, 72.6 to 99.8, and 99.8 kg to market, respectively. In Exp. 2, phases 1, 2, 3, and 4 were fed from 40.0 to 49.0, 49.0 to 77.0, 77.0 to 104.3, and 104.3 kg to market, respectively.

^bDDGS, dried distillers grains with solubles.

^cOptiphos 2000 (Huvepharma Inc., Peachtree City, GA) provided 858.7 U of phytase FTU/kg of diet with an assumed release of 0.12 available P.

^dProvided per kg of diet: 80 mg Zn, 110 mg Fe, 0.33 mg I, 0.30 mg Se, 5,290 IU vitamin A, 1,322 IU vitamin D, 26 IU vitamin E, 1.2 mg vitamin K, 22.5 mg niacin, 7.5 mg pantothenic acid, 2.25 mg riboflavin, and 10.5 μg vitamin B12.

^eTribasic copper chloride (IntelliBond C, Micronutrients, Indianapolis, IN) provided 10 and 150 mg/kg of Cu for Exp. 1 and Exp. 2, respectively.

^fMn hydroxychloride (IntelliBond M, Micronutrients, Indianapolis, IN); or Mn sulfate (MnSO₄, Eurochem, Veracruz, Mexico).

In Exp. 1, dietary phases were fed from 34.5 to 56.7, 56.7 to 72.6, 72.6 to 99.8, and 99.8 kg to market. For Exp. 1, the grower period was from 34.5 to 72.6 kg and the finisher period was 72.6 kg to market, respectively. Experimental diets were corn-soybean meal-dried distillers grains with solubles (DDGS)-based and were formulated with a Mn, Cu, and Zn free premix. Manganese, Cu, and Zn were added to the diet by a hand-made premix, which were added in place of corn in the diet. Each hand addition contributed the desired source of Mn, MnSO₄ or IBM, and Mn level, 8, 16, or 32 mg/kg to the appropriate treatment, along with hand additions to provide 10 mg of Cu from IntelliBond C (Micronutrients, Indianapolis, IN) and 80 mg of Zn from IntelliBond Z (Micronutrients, Indianapolis, IN) per kg of the diet.

In Exp. 2, dietary phases were fed from 40.0 to 49.0, 49.0 to 77.0, 77.0 to 104.3, and 104.3 kg to market. For Exp. 2, the grower period was from 40.0 to 77.0 kg and the finisher period was 77.0 kg to market, respectively. Additions of dietary levels of Mn were the same as in Exp. 1. All diets contained 80 mg/kg of Zn (IntelliBond Z), similar to Exp. 1; however, the premix used in Exp. 2 provided 150 mg of Cu (IntelliBond C) per kg of the diet.

In both experiments, pens of pigs were weighed approximately every 14 days to determine average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F). On d 86 for Exp. 1 and d 84 for Exp. 2, the three heaviest pigs in each pen were selected and marketed. These pigs were included in the calculation of pen growth performance, but not the carcass characteristics. On the last day of Exp. 1 (d 97), final weights were obtained, and pigs were tattooed with a pen identification number and transported to a U.S. Department of Agriculture-inspected packing plant (JBS Swift, Worthington, MN) for carcass data collection. Carcass measurements included hot carcass weight (HCW), loin depth, backfat, and percentage lean. Loin depth and backfat were measured by optical probe (SFK; Herlev, Denmark) at the 10th rib. Percentage lean was calculated from a plant proprietary equation. Carcass yield was calculated by dividing the pen average HCW by the pen average final live weight obtained at the farm. For Exp. 2, because of the ongoing outbreak of COVID-19, only final weights (d 100) were obtained and no carcass data were collected.

Chemical Analysis

Representative diet samples were obtained from all feeders of each treatment by phase, then delivered to the Kansas State University Swine Laboratory, Manhattan, KS, and stored at −20°C until analysis. Samples of the diets were combined within dietary treatment, and two composite samples from each treatment were analyzed in duplicate (Cumberland Valley Analytical Services, Hagerstown, MD). Samples were analyzed for Mn, Cu, and Zn contents (Method 985.01; AOAC International, 2000).

Mineral content of the liver was determined in Exp. 1. Liver samples were collected from three random pigs per pen at the slaughtering plant from pigs marketed at the end of the study in the second group. Each liver sample was collected from the same location from the liver lobe that is attached to the gallbladder on each individual pig. The liver samples were dried at 105°C for 24 h. Liver tissues were then acid digested using trace metal grade nitric acid in preparation for heavy metal determination. Liver samples were analyzed for Mn, Cu, and Zn at the Kansas State University Veterinary Medicine Diagnostic Laboratory using an inductively coupled plasma mass spectrometry (ICP-MS; PerkinElmer, NexION 350×, Waltham, MA). Bismuth, germanium, and rhodium served as internal standards. All runs included bovine liver trace elements and methylmercury in freeze-dried muscle tissue (National Institute of Standards and Technology, Gaithersburg, MD) standards as appropriate for verification of instrument accuracy.

Statistical Analysis

Data from both experiments were analyzed as a randomized complete block design for one-way analysis of variance using the lmer function from the lme4 package in R (version 3.5.1 (2018-07-02), R Foundation for Statistical Computing, Vienna, Austria), with pen considered the experimental unit, BW as blocking factor, and treatment as fixed effect. Predetermined orthogonal contrasts were used to evaluate the interactive effects of Mn source and level. Interactions (Ρ ≤ 0.10) were evaluated linearly or quadratically within source. All results were considered significant at Ρ ≤ 0.05 and marginally significant between Ρ > 0.05 and Ρ ≤ 0.10.

RESULTS

Chemical Analysis

The analyzed dietary Mn, Cu, and Zn were consistent with calculated values used in diet formulation for Exp. 1 and 2 and followed the intended Mn titration additions (Table 2).

Table 2.

Chemical analysis of Exp. 1 and 2 diets (as-fed basis)^a

	MnSO4, mg/kg			IBM, mg/kg
Mineral, mg/kg	8	16	32	8	16	32
Experiment 1
Cu	40	31	33	33	33	40
Mn	30	36	51	30	38	51
Zn	121	117	125	122	116	121
Experiment 2
Cu	217	207	194	197	199	206
Mn	34	35	50	35	41	52
Zn	126	132	130	126	131	125

^aValues represent means from 16 composite samples (four per phase). For each treatment, samples were collected from multiple feeders, blended, subsampled, ground, and analyzed (Cumberland Valley Analytical Services, Hagerstown, MD). IntelliBond M (IBM, Micronutrients, Indianapolis, IN).

Experiment 1

There was no evidence of differences (Ρ > 0.10) for an Mn source × level interaction or main effect of source and level on any growth performance response in the grower and finisher phases (Table 3). Overall, there was a marginal Mn source × level interaction (quadratic, Ρ = 0.057) for G:F, with G:F improving as Mn increased when supplied from MnSO₄, but decreasing and then increasing when Mn was supplemented from IBM. There was no evidence for Mn source differences for ADG or ADFI, but pigs fed 16 mg/kg of Mn, regardless of source, had the poorest (quadratic, Ρ < 0.05) ADG and lightest final BW compared with 8 or 32 mg/kg.

Table 3.

Effects of Mn source and level on grow–finish pig growth performance, Exp. 1^a

	MnSO4, mg/kg			IBM, mg/kg				Source × level, Ρ =
Itemb,c	8	16	32	8	16	32	SEM	Linear	Quadratic
BW, kg
Grower	71.8	71.5	71.7	72.5	72.0	72.1	1.14	0.725	0.889
Finald	133.6	132.3	134.5	134.8	132.5	134.3	1.19	0.483	0.681
Grower
ADG, kg	0.92	0.91	0.92	0.93	0.93	0.93	0.013	0.638	0.884
ADFI, kg	1.92	1.92	1.91	1.94	1.93	1.94	0.034	0.896	0.823
G:F	0.477	0.476	0.481	0.482	0.479	0.478	0.0051	0.366	0.954
Finisher
ADG, kg	0.96	0.95	0.97	0.96	0.94	0.96	0.013	0.876	0.772
ADFI, kg	2.89	2.82	2.86	2.86	2.86	2.84	0.029	0.914	0.170
G:F	0.334	0.338	0.340	0.355	0.328	0.339	0.0038	0.952	0.117
Overall
ADG, kg	0.95	0.94	0.96	0.96	0.94	0.95	0.008	0.351	0.593
ADFI, kg	2.51	2.46	2.49	2.50	2.49	2.48	0.025	0.904	0.289
G:F	0.377	0.381	0.385	0.384	0.377	0.384	0.0030	0.348	0.057
Carcass characteristics
HCW, kg	98.4	96.9	98.4	98.9	98.3	98.6	0.77	0.600	0.329
Carcass yield, %	73.5	73.3	73.2	73.4	74.2	73.3	0.20	0.970	0.075
Backfat depth, mme	17.0	16.6	16.9	17.0	17.1	17.0	0.31	0.981	0.108
Loin depth, mme	67.7	68.0	69.2	69.2	68.3	68.9	0.45	0.035	0.633
Lean, %e	56.6	56.7	56.8	56.8	56.6	56.8	0.20	0.564	0.115

^aA total of 1,944 pigs (initial BW of 34.5 kg) were used in two groups with 27 pigs per pen and 6 replicates per treatment. Mn sources were Mn sulfate (MnSO₄, Erachem, Veracruz, Mexico) or IntelliBond M (IBM, Micronutrients, Indianapolis, IN).

^bBW, body weight; ADG, average daily gain; ADFI, average daily feed intake; G:F, gain-to-feed ratio.

^cThe grower period was from d 0 to d 42 in group 1 and from d 0 to 39 in group 2. The finisher period was from d 42 to 106 in group 1 and from d 39 to 107 in group 2.

^dQuadratic effect of Mn level (P = 0.10).

^eAdjusted using HCW as covariate.

There was a tendency for Mn source × level interaction (quadratic, Ρ = 0.075) for carcass yield, in which yield did not change by increasing MnSO₄, but was greatest for pigs fed 16 mg/kg Mn from IBM. Loin depth increased (linear, source × level, Ρ = 0.035) for increasing Mn from MnSO₄ but decreased when Mn was increased from IBM. The intermediate level of Mn had the lightest HCW (quadratic, Ρ = 0.071) and smallest loin depth (quadratic, Ρ = 0.044).

There was no evidence of difference (Ρ > 0.10) observed for concentration of Cu and Zn in the liver (Table 4). Manganese concentration increased (linear, Ρ = 0.015) as Mn supplementation increased and tended to be greater (P = 0.075) when Mn was supplied by MnSO₄ compared with IBM. Mortality and removals were also evaluated and were very low (< 1% each) and not affected by treatment (data not shown).

Table 4.

Effects of Mn source and level on grow–finish pig micromineral liver concentrations, Exp. 1^a,b

	MnSO4, mg/kg			IBM, mg/kg				Source × level, Ρ =
Item	8	16	32	8	16	32	SEM	Linear	Quadratic
Micromineral, mg/kg
Cu	38.9	38.1	40.0	38.3	39.4	38.0	4.27	0.815	0.752
Mnc,d	8.63	8.88	9.87	8.07	8.51	8.88	0.44	0.560	0.663
Zn	242.1	243.6	244.4	203.7	238.7	232.0	17.5	0.521	0.380

^aA total of 36 pens were used in the second marketed group with three pigs per pen and six replicates per treatment. Mn sources were Mn sulfate (MnSO₄, Erachem, Veracruz, Mexico) or IntelliBond M (IBM, Micronutrients, Indianapolis, IN).

^bLiver micromineral analysis done by ICP-MS.

^cManganese main effect (linear, Ρ = 0.015).

^dManganese source effect (P = 0.075).

Experiment 2

For Exp. 2, growth-promoting levels of Cu were included in all diets. In the grower period, there was an Mn source × level interaction (linear, Ρ = 0.029) observed for G:F, with G:F improving as Mn increased from IBM but decreasing with increased Mn from MnSO₄ (Table 5). There was no evidence (Ρ > 0.10) for Mn source or Mn level effects on ADG, but ADFI was greater (P = 0.034) when Mn was provided by IBM. Pigs fed 16 mg/kg Mn tended (quadratic, P = 0.052) to have decreased ADFI when compared with those fed 8 or 32 mg/kg, regardless of source.

Table 5.

Effects of Mn source and level on grow–finish pig growth performance, Exp. 2^a

	MnSO4, mg/kg			IBM, mg/kg				Source × level, Ρ =
Itemb,c	8	16	32	8	16	32	SEM	Linear	Quadratic
BW, kg
Initial	40.0	40.0	40.0	40.0	40.0	39.9	0.77	0.821	0.650
Grower	75.9	75.1	74.8	76.3	75.4	75.8	1.20	0.468	0.699
Finald	129.9	129.8	130.0	132.6	130.4	131.2	1.32	0.391	0.207
Grower
ADG, kg	0.89	0.88	0.87	0.90	0.88	0.89	0.013	0.330	0.615
ADFI, kgd,e	1.93	1.89	1.92	1.98	1.93	1.95	0.034	0.578	0.879
G:F	0.463	0.464	0.452	0.454	0.458	0.459	0.0051	0.029	0.652
Finisher
ADG, kg	0.95	0.96	0.97	0.99	0.97	0.97	0.011	0.039	0.258
ADFI, kgd	2.81	2.79	2.83	2.88	2.83	2.85	0.030	0.272	0.782
G:F	0.340	0.344	0.343	0.346	0.343	0.340	0.0041	0.267	0.402
Overall
ADG, kgd	0.93	0.93	0.93	0.95	0.93	0.94	0.007	0.284	0.213
ADFI, kgd,e	2.43	2.41	2.45	2.50	2.45	2.46	0.029	0.236	0.856
G:F	0.381	0.384	0.379	0.382	0.380	0.380	0.0038	0.801	0.332

^aA total of 1,944 pigs (initial BW of 40.0 kg) were used in two groups with 27 pigs per pen and 12 replicates per treatment. Mn sources were Mn sulfate (MnSO₄, Erachem, Veracruz, Mexico) or IntelliBond M (IBM, Micronutrients, Indianapolis, IN).

^bBW, body weight; ADG, average daily gain; ADFI, average daily feed intake; G:F, gain-to-feed ratio; HCW, hot carcass weight.

^cThe grower period was from d 0 to d 42 in group 1 and from d 0 to 38 in group 2. The finisher period was from d 42 to 97 in group 1 and from d 38 to 100 in group 2.

^dMain effect of Mn source (P < 0.05).

^eQuadratic effect of Mn level (P = 0.10).

In the finisher period, there was an Mn source × level interaction (linear, Ρ = 0.039) for ADG, with ADG improving as supplemental Mn was increased for MnSO₄ but decreasing when Mn was increased for pigs fed IBM. There was no evidence (Ρ > 0.10) of difference for Mn level to influence ADFI or ADG, but ADFI was greater (Ρ = 0.05) for pigs fed Mn from IBM compared with pigs fed Mn from MnSO₄. There was no evidence of difference (Ρ > 0.10) for an Mn source or level effect on G:F in the finisher period.

Overall, there was no evidence (Ρ > 0.10) for an Mn source × level interaction for final BW or any observed growth responses. Pigs fed Mn provided by IBM had greater (Ρ < 0.05) ADG and ADFI and heavier (Ρ < 0.05) final BW than pigs fed Mn from MnSO₄. Regardless of source, pigs fed 16 mg/kg of Mn tended (quadratic, Ρ = 0.088) to have lower overall ADFI than pigs fed 8 or 32 mg/kg of Mn. There was no evidence of difference (Ρ > 0.10) for Mn source or level effect on G:F. Mortality and removals were also evaluated and were very low (< 1% each) and not affected by treatment (data not shown).

DISCUSSION

According to the NRC (2012), the Mn requirement estimate for growing–finishing pigs is 2–4 mg/kg diet. However, the majority of the research for determining the Mn requirement was conducted more than 50 years ago. Due to the unknown bioavailability of Mn from ingredients commonly used in diets, Mn is usually added to swine diets through a trace mineral premix, frequently as MnSO₄. Manganese hydroxychloride is another source of Mn that can be added to swine diet trace mineral premixes. Hydroxychloride-based minerals are manufactured through the reaction of hydrochloric acid, high purity forms of metal, and water. The product of this reaction is hydroxychloride crystals that contain the desired metal covalently bonded to chloride and hydroxyl groups. The covalent bonds possessed by hydroxychloride minerals reduce the ability for the minerals to react with other components of the diet and potentially improve bioavailability (Cao et al., 2000).

To our knowledge, this is the first study that evaluated the effects of Mn hydroxychloride on any growth performance and carcass characteristic. In Exp. 1, there was no main effect of Mn source on any growth performance and carcass characteristics, although there was a source × level interaction on G:F and loin depth. However, in Exp. 2, pigs fed supplemental Mn from IBM had increased ADG and ADFI and heavier final BW when compared with pigs fed MnSO₄. The reason for these different effects of IBM on growth performance is not clear but could have been a result of the high levels of dietary Cu in Exp. 2. High levels of Cu (> 75 mg/kg) have been shown to improve growth performance in growing–finishing pigs (Coble et al., 2017). Copper is excreted from the body through bile; however, when dietary Mn was increased to 200 mg/kg in rats. Mercadante et al. (2016) observed a reduction in Cu levels in the bile, signifying a hepatobiliary metabolism relationship between Mn and Cu. With the potential of improved bioavailability of the Mn hydroxychloride, less Cu may have been excreted allowing for potentially greater utilization of Cu in pigs being fed Mn hydroxychloride. However, this theory warrants further investigation.

Grummer et al. (1950) reported improvement in ADG and G:F when 40 mg/kg of Mn was added to a basal diet containing 12 mg/kg of Mn but saw no further improvement in growth performance at 80 or 160 mg/kg of Mn. Grummer et al. (1950) did not test levels of Mn supplemented below 40 mg/kg but based on our results, there is no additional benefit to add more than 8 mg/kg of supplemental Mn to the diet on growth performance or carcass characteristics. Apple et al. (2004) observed no additional benefit when Mn supplementation was greater than 20 mg/kg on ADG, ADFI, or on carcass characteristics, but they did not evaluate levels less than 20 mg/kg of supplemental Mn. In additional research, Apple et al. (2004) did observe an improvement in G:F when Mn was supplemented at 320 mg/kg. In both of our experiments, regardless of source, 16 mg/kg of Mn reduced growth performance. In Exp. 1, final BW and ADG were reduced and ADFI was reduced in Exp. 2. We have no explanation for the reduction in growth and intake for the intermediate Mn level as ADG and ADFI increased to control values when pigs were fed 32 mg/kg of Mn. Therefore, there appears to be little, if any, improvement in growth performance when feeding more than 8 mg supplemental Mn/kg diet.

Our results from Exp. 1 suggest that there is no evidence of difference in HCW, yield, and carcass characteristics between pigs fed different Mn sources when growth-promoting levels of copper are not fed in the diet; however, 16 mg/kg of supplemental Mn did reduce loin depth and tended to produce lighter HCW. The reduction in HCW and loin depth appears to be directly correlated with the lighter final BW and reduced ADG that occurred for 16 mg/kg in Exp. 1. Plumlee et al. (1956) visually observed an increase in fat deposition when pigs were fed Mn-deficient diets containing 0.05 mg/kg, which indicates that Mn can affect fat deposition; however, neither source nor level of Mn influenced percentage lean or backfat depth in our studies. Similarly, Apple et al. (2004) and Sawyer et al. (2007) did not observe changes in carcass characteristics from added dietary Mn; however, they did observe improvements in fresh pork color and cooked pork tenderness when 320–350 mg/kg of Mn was fed.

Manganese absorption occurs in the small intestine and is transported into the body by the divalent metal transporter 1 (DMT1; Au et al., 2008). Our results indicate that the dietary Mn levels fed in our study, regardless of source, did not affect Cu or Zn levels in the liver. Manganese concentration in the liver increased as dietary Mn increased, which agrees with observations of Grummer et al. (1950) and Mercadante et al. (2016). In the present study, liver concentrations for Mn and Cu were greater than those observed by Gowanlock et al. (2015), probably because of the fortification levels between their study and the values herein. However, in Exp. 1, liver Mn concentrations were lower when IBM was the dietary Mn source than when MnSO₄ was used. This might suggest decreased bioavailability of IBM vs. MnSO₄; however, in Exp. 2, Mn provided by IBM increased ADG possibly suggesting the opposite. We are unaware of any data indicating bioavailability of Mn hydroxychloride. The lower level of liver Mn concentration when supplemental Mn was fed from IBM could also be the result of increased Mn utilization or increased Mn excretion for the hydroxychloride form of Mn; however, neither of these reasons were evaluated in this study.

In conclusion, our results suggest that supplementing growing–finishing diets with greater than 8 mg/kg of Mn did not lead to any improvements in growth performance and carcass characteristics. More research is needed to further understand the potential benefits of Mn hydroxychloride fed in conjunction with pharmacological levels of Cu on pig growth performance and to understand the reason a decrease in growth performance was observed at the intermediate supplementation level.