INTRODUCTION {#s1}
============

Zinc (Zn) is an essential mineral for protein, carbohydrate, and lipid metabolism due to its essentiality as a component of metalloenzymes, such as DNA and RNA synthetases, digestive enzymes, and insulin ([@CIT0013]). The dietary Zn requirement for 25 to 135 kg grow-finish pigs ranges from 50 to 60 mg/kg depending on body weight (BW) ([@CIT0013]). This requirement is usually met through the trace mineral premix, which is commonly considered the sole source of added Zn ([@CIT0012]). According to a U.S. swine industry survey, added Zn levels in grow-finish diets range from 72 to 86 mg/kg ([@CIT0005]), thus it is relatively common practice to supplement more Zn in growing pig diets than the requirement.

Traditionally, inorganic Zn sources such as Zn oxide and Zn sulfate are most commonly used in swine diets. Zinc hydroxychloride is a novel inorganic source of Zn manufactured through the reaction of high purity forms of the metal with water and hydrochloric acid ([@CIT0010]). This process forms hydroxychloride crystals that contain Zn covalently bonded to hydroxyl groups and chloride. The covalent bonds are expected to reduce reactiveness with other components of the diet and to improve bioavailability ([@CIT0002]).

Currently, there are few studies with grow-finish pigs, especially conducted under large-scale commercial conditions, evaluating dietary Zn concentrations. Therefore, the objective of this study was to determine the effects of increasing dietary Zn from Zn hydroxychloride on growth performance and carcass characteristics of grow-finish pigs housed under commercial conditions.

MATERIAL AND METHODS {#s2}
====================

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

Animals and Diets {#s3}
-----------------

The study was conducted at a commercial research facility in southwestern Minnesota. Two identical barns, naturally ventilated and double-curtain sided, were used for a total of 90 pens with 27 pigs per pen. Pens (5.5 × 3.0 m) had completely slatted floors and were equipped with a four-hole stainless-steel dry self-feeder and a cup waterer. Feed additions were accomplished and recorded by a computerized feeding system (FeedPro; Feedlogic Corp., Wilmar, MN).

A total of 2,430 pigs (PIC 337 × 1050; Hendersonville, TN; initially 30.1 ± 0.70 kg BW) were used in a 113-d growth trial. Pens of pigs were blocked by BW within barn and were randomly assigned to one of five treatments in a randomized complete block design. Treatments consisted of 50, 87.5, 125, 162.5, or 200 mg/kg added Zn from Zn hydroxychloride (IntelliBond Z; Micronutrients, Indianapolis, IN). A vitamin-trace mineral premix was formulated without Zn to ensure Zn hydroxychloride was the only source of added Zn. There were 18 replicates per treatment. Diets were manufactured at the New Horizon Farms feed mill (Pipestone, MN) and offered in five phases in meal form ([Table 1](#T1){ref-type="table"}).

###### 

Composition of the basal diets (as-fed basis)^1^

                                     Feeding phase                           
  ---------------------------------- --------------- ------- ------- ------- -------
  Ingredients, %                                                             
   Corn                              50.39           55.56   58.37   61.77   78.35
   Soybean meal, 47% crude protein   17.08           11.95   9.34    5.92    9.64
   DDGS                              30.00           30.00   30.00   30.00   10.00
   Calcium carbonate                 1.35            1.35    1.25    1.25    1.10
   Sodium chloride                   0.35            0.35    0.35    0.35    0.35
   [l]{.smallcaps}-Lysine HCl        0.50            0.50    0.45    0.45    0.30
   [dl]{.smallcaps}-Methionine       0.05            0.02    ---     ---     0.01
   [l]{.smallcaps}-Threonine         0.06            0.05    0.02    0.04    0.05
   [l]{.smallcaps}-Tryptophan        0.04            0.04    0.03    0.03    0.02
   Vitamin-trace mineral premix^2^   0.15            0.15    0.15    0.15    0.15
   Zn hydroxychloride^3^             +/−             +/−     +/−     +/−     +/−
  Total                              100.0           100.0   100.0   100.0   100.0
  SID amino acids, %                                                         
   Lysine                            1.03            0.91    0.81    0.73    0.65
   Isoleucine:lysine                 63              62      64      63      64
   Leucine:lysine                    162             172     186     196     186
   Methionine:lysine                 30              29      29      31      30
   Methionine and cystine:lysine     56              56      58      60      60
   Threonine:lysine                  61              61      61      63      66
   Tryptophan:lysine                 19              19      19      19      19
   Valine:lysine                     71              71      75      76      75
  Net energy^4^, kcal/kg             2,465           2,493   2,511   2,531   2,551
  Crude protein, %                   20.0            17.9    16.8    15.4    13.0
  Calcium, %                         0.59            0.56    0.52    0.50    0.46
  STTD P, %                          0.37            0.36    0.36    0.35    0.27

DDGS = distillers dried grains with soluble, SID = standardized ileal digestible, STTD P = standardized total tract digestible phosphorus.

^1^Phases 1, 2, 3, 4, and 5 were fed from approximately 30 to 45 kg, 45 to 64 kg, 64 to 82 kg, 82 to 104 kg, and 104 kg to marketing, respectively.

^2^The premix did not contain Zn and provided per kilogram of premix: 4,116,034 IU vitamin A; 661,387 IU vitamin D; 26,455 IU vitamin E; 1,764 mg vitamin K; 16.2 mg vitamin B12; 17,637 mg niacin; 11,759 mg pantothenic acid; 5,880 mg riboflavin; 50.7 g Fe from iron sulfate; 19 g Mn from manganese oxide; 10.8 g Cu from copper sulfate; 0.25 g I from calcium iodate; 0.2 g Se from sodium selenite; and 500,000 FTU phytase.

^3^IntelliBond Z (Micronutrients, Indianapolis, IN) was added at 0.009%, 0.016%, 0.022%, 0.029%, or 0.036% to achieve 50, 87.5, 125, 167.5, or 200 mg/kg of added Zn, respectively.

^4^Net energy values for ingredients used in diet formulation were derived from the [@CIT0013].

Pens of pigs were weighed and feed disappearance measured approximately every 2 wk to calculate average daily gain (ADG), average daily feed intake (ADFI), and gain-to-feed ratio (G:F). Daily Zn intake was calculated by multiplying overall ADFI by the inclusion of added Zn. Mortality and removals were recorded daily. On day 90, according to standard farm protocol, the three heaviest pigs in each pen were identified and transported to a packing plant (JBS Swift and Company, Worthington, MN) to be processed. At the end of the experimental period, final pen weights were recorded, and remaining pigs were tattooed with a pen identification number and were transported to a packing plant (JBS Swift and Company, 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 using an optical probe (Fat-O-Meter; SFK, Herlev, Denmark). 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.

Chemical Analysis {#s4}
-----------------

Representative samples were collected from 12 feeders per dietary treatment during each of the five phases. Samples were stored at −20 °C until analysis. Diet samples were analyzed for dry matter (method 935.29; [@CIT0001]), crude protein (990.03, [@CIT0001]), calcium (Ca) and phosphorus (P; method 985.01; [@CIT0001]), Zn, Fe, and Cu (985.01; [@CIT0001]) in duplicate at both Midwest Laboratories Inc. (Omaha, NE) and Cumberland Valley Analytical Services (Hagerstown, MD).

Statistical Analysis {#s5}
--------------------

Data were analyzed as a randomized complete block design with block nested within barn as a random effect and pen as the experimental unit. Polynomial contrasts were constructed to evaluate the linear and quadratic effects of increasing Zn on ADG, ADFI, G:F, BW, daily Zn intake, and carcass characteristics. All data were modeled as normally distributed with the exception pig removals and mortality were modeled using a binomial distribution. Data were analyzed using the GLIMMIX procedure of SAS 9.4 (SAS Institute Inc., Cary, NC). Results were considered significant at *P* ≤ 0.05 and a tendency at 0.05 \< *P* ≤ 0.10.

RESULTS AND DISCUSSION {#s6}
======================

Chemical Analysis {#s7}
-----------------

Results of proximate analysis and total Zn analysis followed formulated values ([Tables 2](#T2){ref-type="table"} and [3](#T3){ref-type="table"}). Crude protein, total Ca, and total P levels were similar across treatments within dietary phase ([Tables 2](#T2){ref-type="table"} and [3](#T3){ref-type="table"}). The average analyzed total Zn across all phases was 110, 115, 142, 185, and 204 mg/kg for diets formulated to 50, 87.5, 125, 162.5, and 200 mg/kg added Zn, respectively. The analyzed total Fe and Cu were similar across treatments ([Tables 2](#T2){ref-type="table"} and [3](#T3){ref-type="table"}) and were, on average, 150 and 151 ppm, respectively.

###### 

Chemical analysis of Phase 1, 2, and 3 diets (as-fed basis)^1^

                        Phase 1 added Zn, mg/kg   Phase 2 added Zn, mg/kg   Phase 3 added Zn, mg/kg                                                                                
  --------------------- ------------------------- ------------------------- ------------------------- ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------
  Dry matter^2^, %      87.1                      87.2                      87.2                      87.4   87.2   86.9   86.8   86.4   86.6   86.3   86.8   86.7   86.8   86.5   86.7
  Crude protein^2^, %   21.5                      21.2                      21.3                      21.5   22.3   19.6   18.9   19.3   19.7   18.9   19.4   18.6   18.2   18.5   18.7
  Ca^2^, %              0.62                      0.69                      0.64                      0.66   0.65   0.56   0.46   0.61   0.59   0.57   0.47   0.56   0.54   0.48   0.59
  P^2^, %               0.51                      0.50                      0.51                      0.53   0.52   0.47   0.47   0.47   0.47   0.46   0.49   0.47   0.49   0.48   0.48
  Zn^3^, mg/kg          99                        108                       146                       172    198    93     114    138    177    183    173    117    134    184    198
  Fe^4^, mg/kg          138                       145                       147                       184    170    133    133    185    217    160    132    120    172    135    162
  Cu^4^, mg/kg          146                       167                       163                       137    139    149    166    157    155    160    143    136    143    137    135

^1^For each treatment, samples were collected from multiple feeders, blended, subsampled, ground, and analyzed.

^2^Values represent means of two samples from Cumberland Valley Analytical Services (Hagerstown, MD) and two samples from Midwest Laboratories Inc. (Omaha, NE).

^3^Values represent means of eight samples from Cumberland Valley Analytical Services (Hagerstown, MD) and six samples from Midwest Laboratories Inc. (Omaha, NE).

^4^Values represent means of two samples from Cumberland Valley Analytical Services (Hagerstown, MD).

###### 

Chemical analysis of Phase 4 and 5 diets (as-fed basis)^1^

                        Phase 4 added Zn, mg/kg   Phase 5 added Zn, mg/kg                                                    
  --------------------- ------------------------- ------------------------- ------ ------ ------ ------ ------ ------ ------ ------
  Dry matter^2^, %      86.3                      86.4                      86.6   86.4   86.6   85.5   86.4   85.3   85.7   85.8
  Crude protein^2^, %   16.9                      17.1                      16.8   16.0   17.2   12.9   13.4   13.3   12.9   13.0
  Ca^2^, %              0.33                      0.46                      0.43   0.57   0.48   0.53   0.65   0.50   0.62   0.61
  P^2^, %               0.45                      0.45                      0.45   0.44   0.45   0.32   0.35   0.33   0.34   0.32
  Zn^3^, mg/kg          89                        116                       147    201    208    94     121    147    190    231
  Fe^4^, mg/kg          132                       184                       169    177    136    123    127    123    130    128
  Cu^4^, mg/kg          125                       146                       150    154    145    159    170    154    178    172

^1^For each treatment, samples were collected from multiple feeders, blended, subsampled, ground, and analyzed.

^2^Values represent means of two samples from Cumberland Valley Analytical Services (Hagerstown, MD) and two samples from Midwest Laboratories Inc. (Omaha, NE).

^3^Values represent means of eight samples from Cumberland Valley Analytical Services (Hagerstown, MD) and six samples from Midwest Laboratories Inc. (Omaha, NE).

^4^Values represent means of two samples from Cumberland Valley Analytical Services (Hagerstown, MD).

Growth Performance and Carcass Characteristics {#s8}
----------------------------------------------

From days 0 to 42, pigs fed diets with increasing added Zn had decreased (linear, *P* = 0.043; [Table 4](#T4){ref-type="table"}) ADFI. There was a tendency for reduced ADG (linear, *P* = 0.092) and day-42 BW (linear, *P* = 0.078) as added dietary Zn increased. From days 42 to 113, increasing added Zn resulted in a quadratic response (*P* = 0.042) for ADFI, with ADFI decreasing for pigs fed 87.5 mg/kg of Zn and then returning to control intake values as Zn increased. As a result, there was a tendency (linear, *P* = 0.056) for improved G:F with increasing added Zn. For overall growth performance (days 0 to 113), there were tendencies for quadratic responses for ADFI (*P* = 0.073) and G:F (*P* = 0.059). The lowest ADFI was observed at 87.5 mg/kg added Zn and the greatest G:F was observed at 125 mg/kg added Zn. Pigs fed diets with increasing added Zn presented a linear increase in daily Zn intake (*P* \< 0.001). There were no differences (*P* \> 0.10) in overall ADG, final BW, HCW, backfat, loin depth, and lean percentage. Pig mortality and removal were low (0.8 and 2.5%, respectively) and were not influenced by dietary Zn inclusion.

###### 

Effects of increasing added Zn on grow-finish pig performance and carcass characteristics^1^

                            Added Zn, mg/kg           Probability, *P* \<                                   
  ------------------------- ----------------- ------- --------------------- ------- ------- ------- ------- -------
  BW, kg                                                                                                    
   Day 0                    30.1              30.1    30.1                  30.1    30.1    0.70    0.947   0.895
   Day 42                   68.6              67.6    67.7                  68.0    67.1    1.28    0.078   0.752
   Day 113                  132.2             129.7   130.7                 131.7   130.9   2.50    0.830   0.314
  Days 0 to 42                                                                                              
   ADG, kg                  0.95              0.92    0.93                  0.94    0.91    0.019   0.092   0.674
   ADFI, kg                 2.01              1.94    1.94                  1.96    1.93    0.037   0.043   0.227
   G:F                      0.473             0.475   0.479                 0.479   0.474   0.007   0.600   0.182
  Days 42 to 113                                                                                            
   ADG, kg                  0.93              0.91    0.94                  0.93    0.94    0.023   0.208   0.407
   ADFI, kg                 2.93              2.85    2.88                  2.89    2.91    0.038   0.868   0.042
   G:F                      0.318             0.320   0.325                 0.323   0.323   0.004   0.056   0.155
  Days 0 to 113                                                                                             
   ADG, kg                  0.94              0.92    0.93                  0.93    0.93    0.021   0.887   0.494
   ADFI, kg                 2.58              2.50    2.52                  2.53    2.53    0.032   0.317   0.073
   G:F                      0.364             0.366   0.371                 0.369   0.367   0.005   0.154   0.059
   Zn intake, mg/d          129               215     310                   412     507     4.58    0.001   0.142
  Carcass characteristics                                                                                   
   HCW, kg                  96.7              94.8    95.3                  96.4    95.8    1.80    0.852   0.140
   Yield, %                 73.3              73.1    72.9                  73.2    73.2    0.18    0.702   0.151
   Backfat^2^, mm           17.5              17.5    17.2                  17.6    17.2    0.62    0.536   0.947
   Loin depth^2^, mm        68.6              69.0    68.0                  69.1    68.7    1.18    0.817   0.767
   Lean^2^, %               56.4              56.5    56.5                  56.4    56.6    0.51    0.470   0.851

^1^A total of 2,430 pigs were used in a 113-d study with 27 pigs per pen and 18 replicates per treatment.

^2^Adjusted using HCW as a covariate.

On the basis of our results, there was no benefit to adding more than 50 mg/kg Zn for ADG and carcass characteristics, but 125 mg/kg of added Zn improved G:F. [@CIT0003] evaluated 50, 100, and 150 mg/kg of added Zn from two sources (Zn sulfate and Zn hydroxychloride) for grow-finish pigs. In contrast to findings of the current study, it was observed that pigs fed diets with 100 mg/kg added Zn had a tendency for greater ADG and significantly greater HCW regardless of Zn source. Carcass yield increased linearly with increasing added Zn, and pigs fed Zn hydroxychloride had greater carcass yield and a tendency for heavier HCW than those fed Zn sulfate.

Although our experimental diets did not contain ractopamine hydrochloride, our results are in agreement with [@CIT0014], who observed in one trial a tendency for a linear improvement in G:F of finishing pigs with increasing added Zn from Zn oxide from 50 to 150 mg/kg in diets with ractopamine. However, the results were inconsistent, and in a second trial [@CIT0014] did not observe evidence for effects of increasing added Zn. A similar result was reported by [@CIT0006], where added Zn improved G:F for finishing pigs fed diets with ractopamine but results were not repeatable in subsequent trials. It is important to note that our diets did not contain ractopamine, thus these results may not be directly comparable

However, other research suggests there is little evidence for Zn effects on growth performance of grow-finish pigs. [@CIT0004] observed no evidence for effects of 150 mg/kg added Zn from Zn oxide on growth performance and carcass characteristics of 48- to 136-kg pigs, as well as no additive effects of Zn and Cu. [@CIT0008] evaluated organic and inorganic Zn sources ranging from 60 to 140 mg/kg added Zn for grow-finish pigs raised under restricted floor space allowance and observed no effects on growth performance and carcass characteristics. Interestingly, [@CIT0011] observed no effects of removing supplemental trace minerals (Zn, Cu, Fe, and Mn) up to 6 wk preslaughter on growth performance, although some carcass characteristics were negatively affected. Moreover, [@CIT0007] found no evidence for differences between pigs fed a basal corn--soybean meal diet without supplemental Zn, Cu, Fe, and Mn or the basal diet with 50% or 100% of the [@CIT0013] mineral requirement estimates.

The [@CIT0013] presents Zn requirement estimates as mg per kilogram of diet as well as mg per day. These values range from 50 to 60 mg/kg of diet and 90 to 139 mg per day for grow-finish pigs from 25 to 135 kg BW. In our study, pigs fed diets with 50 ppm added Zn had a daily Zn intake of 129 mg, which should meet the requirement estimated by the [@CIT0013]. Overall, available research suggests that there are no benefits of feeding higher levels of Zn to grow-finish pigs although in the current study G:F was improved with 125 mg/kg added Zn or 310 mg/d. It is important to note that most existing research evaluated other inorganic sources, such as Zn oxide and Zn sulfate, or organic Zn sources. It could be hypothesized that the contrasts observed between the available literature and our study were at least partially driven by differences in Zn availability in the Zn hydroxychloride used in our study. An important consideration is that excessive Zn excreted in the manure can pollute ground water by leaching or soil erosion ([@CIT0009]). As restrictions to nutrient excretion increase in the swine industry, the use of lower levels of added Zn close to the requirements may be beneficial.

In conclusion, our results suggest that supplementing grow-finish diets with greater than 50 mg/kg added Zn did not lead to improvements in ADG and carcass characteristics. However, there may be G:F benefits of supplementing up to 125 mg/kg added Zn. These results match some of the data found in the literature, but inconsistencies in response are apparent and differences in Zn source could at least partially explain the results observed in the current study. Further research is required to compare Zn hydroxychloride with other inorganic and organic Zn sources to determine if source of Zn has an impact on the response observed.

 {#s100}

*Conflict of interest statement*. None declared.

Contribution No. 19-180-J from the Kansas Agricultural Experiment Station.

Appreciation is expressed to Micronutrients (Indianapolis, IN) for technical and partial financial support and to New Horizon Farms (Pipestone, MN) for technical support and expertise in conducting the experiment.
