Effects of feeding diets containing low crude protein and coarse wheat bran as alternatives to zinc oxide in nursery pig diets

Abstract

Two experiments were conducted to determine the effects of crude protein (CP) level in diets containing coarse wheat bran (CWB) with or without pharmacological levels of Zn (provided by zinc oxide: ZnO) on growth performance and fecal DM of nursery pigs. In experiment 1, 360 barrows (Line 200 × 400, DNA, Columbus, NE, initially 5.6 kg) were allotted to 1 of 6 dietary treatments from d 0 to 21 after weaning with 5 pigs per pen and 12 pens per treatment. Treatments included a positive control diet (21% CP) with 3,000 mg/kg Zn in phase 1 and 2,000 mg/kg in phase 2; negative control (21% CP) with 110 mg/kg added Zn, and 4 diets containing 4% CWB and 110 mg/kg added Zn formulated to contain 21%, 19.5%, 18%, or 16.5% CP. The 2 control diets and 21% CP CWB diet contained 1.40% standardized ileal digestible (SID) Lys in phase 1 and 1.35% SID Lys in phase 2, while the 19.5%, 18%, and 16.5% CP diets contained 1.33, 1.25 and 1.20% Lys, respectively, in both phases. Pigs fed the positive control diet containing pharmacological ZnO had increased (P < 0.05) ADG and G:F compared with the negative control and the 21% CP CWB diet. Reducing CP (concurrently with SID Lys) in diets containing CWB decreased ADG and G:F (linear, P = 0.002); however, fecal DM increased (linear, P = 0.005). In experiment 2, two groups of 300 and 350 pigs, initially 7.0 and 6.2 kg, respectively, were used with 5 pigs per pen and 26 pens per treatment. The objective was to determine if adding back essential AA would improve growth performance of pigs fed the low CP diets. All dietary treatments were fed for 13 days, contained 4% CWB, and consisted of: (1) positive control with 2,000 mg/kg of Zn and 21% CP (1.35% SID Lys); (2) no ZnO and 21% CP; and 3 diets with no ZnO formulated to 18% CP and (3) 1.2% SID Lys; (4) 1.35% SID Lys by the addition of feed grade amino acids (AA), and (5) diet 4 with non-essential amino acids (NEAA; Gly and Glu). Pigs fed 21% CP with ZnO had increased (P = 0.001) ADG compared to those fed 18% CP (1.35% SID Lys) with high levels of feed grade amino acids or those fed the reduced SID Lys (1.2%) diet. Overall, G:F was improved (P < 0.001) for pigs fed 21% CP diets and those fed the 18% CP diet with NEAA compared to pigs fed 1.2% SID Lys and pigs fed high levels of feed grade amino acids. Fecal DM was increased for pigs fed the reduced SID Lys diet. In summary, pharmacological levels of Zn improve pig growth performance, but reducing CP (and subsequently SID Lys) decreased nursery pig growth performance.

Introduction

The weaning process is often described as the most stressful event of a pig’s life due to abrupt dietary, physiological, social, and environmental changes (Pluske, 2016). Pigs frequently experience a period immediately after weaning termed the postweaning growth lag, which results from low feed intake initiated by weaning stress and coupled with an immature digestive system. Reductions in feed intake can be attributed to a dietary change from a highly digestible liquid diet from the sow to a solid, starch-based diet, leading to intestinal morphology implications such as intestinal villous atrophy and crypt hyperplasia (Heo et al., 2013). This results in reduced enzymatic and absorptive capacity ultimately impairing the digestion of nutrients (Spreeuwenberg et al., 2001). Newly weaned pigs have increased susceptibility to gastrointestinal issues and enteric diseases such as postweaning diarrhea (PWD), primarily occurring in the first 2 wk after weaning associated with the proliferation of β-hemolytic Escherichia coli (Pluske et al., 1997).

Nursery swine diets often include pharmacological doses of Zn to mitigate PWD and to improve growth performance (Carlson et al., 1999). High dietary concentrations of Zn from ZnO (2,000 to 3,000 mg/kg) have been observed to possess antimicrobial effects while decreasing intestinal mucosal inflammation (Ou et al., 2007). However, the inclusion of high Zn levels or antimicrobials in swine diets has been banned or restricted in some countries due to environmental concerns and the risk of antibiotic resistance (Heo et al., 2013). Due to concerns of similar regulations on pharmacological levels of Zn in the United States, an alternative nutritional strategy offering similar growth and physiological benefits to nursery pigs that improves the transition of weaning is desired for the swine industry.

Weanling pigs struggle to digest high protein diets due to underdeveloped proteolytic enzymes, therefore resulting in undigested protein entering the large intestine and serving as a substrate for bacterial fermentation and proliferation (Jeaurond et al., 2008). The fermentation of undigested protein by intestinal microbiota has been determined as a significant contributing factor to PWD (Pluske et al., 2002). As protein fermentation occurs, by-product concentrations of polyamines, ammonia, indole, and phenol products are increased and can be potentially toxic to the pig (Halas et al., 2007). Consequently, colonic pH is increased with a high protein diet, creating a neutral environment which provides favorable conditions for enterotoxigenic species such as Bacteroides and Clostridium (Jensen, 2001). Furthermore, an osmotic imbalance occurs drawing water into the lumen, leading to diarrhea.

To promote fecal consistency and intestinal health, feeding a low protein diet supplemented by crystalline amino acids in the postweaning period reduces protein fermentation in the gastrointestinal tract (GIT) and lowers the incidence of PWD (Yue and Qiao, 2008; Wang et al., 2018). However, negative implications on growth performance have been observed when dietary CP is reduced more than 4 percentage points, thought to result from deficiencies in amino acids, either essential or nonessential to the pig (Nyachoti et al., 2006; Gloaguen et al., 2014) or from excessive amino acid catabolism occurring in the liver (Sun et al., 2020). Another method to improve postweaning performance is by promoting the proliferation of commensal microbiota with the inclusion of fibrous ingredients that block the attachment of bacteria to the intestine (Becker et al., 2009). As fiber increases in the diet, beneficial microbial populations proliferate and produce volatile fatty acids such as butyrate, the primary oxidative fuel source of intestinal enterocytes (Kim et al., 2012). Molist et al. (2010) presented evidence that 4% coarse wheat bran (CWB) decreased the ability of E. coli to attach to the intestinal mucosa of E. coli-challenged pigs. With 4% added CWB, Molist et al. (2011) observed proliferation and increased counts of fiber degrading bacteria species. Thus, the insoluble fiber portion of CWB perhaps offers a similar antimicrobial effect as ZnO by limiting colonization of enterotoxigenic bacteria (McDonald et al., 2001). There is limited data showing the effects of combining different ZnO-replacement strategies on pig performance and fecal dry matter. Therefore, the objective of experiment 1 was to determine the effect of adding CWB with reduced CP level on the growth performance and fecal dry matter of nursery pigs in diets without pharmacological levels of zinc oxide. As a follow-up to this, experiment 2 was conducted to determine whether growth performance could be maintained when feeding diets with similar standardized ileal digestible (SID) Lys levels combined with the supplementation of either feed grade essential or nonessential amino acids.

Materials and Methods

The Kansas State University Institutional Animal Care and Use Committee approved the protocols used in these experiments. The studies were conducted at the Kansas State University Segregated Early Weaning Facility and the Kansas State University Swine Teaching and Research Center in Manhattan, KS. Each pen contained a 4-hole, dry self-feeder, and cup waterer for ad libitum access to feed and water.

Experiment 1: crude protein (CP) level and CWB

In experiment 1, a total of 360 barrows (Line 200 × 400, DNA, Columbus, NE, initially 5.6 kg) were used in a 45-d growth trial. Pigs were weaned at ~21 d of age and following arrival to the research facility, were randomized to pens and then allotted to 1 of 6 dietary treatments with 5 pigs per pen and 12 pens per treatment. Treatment diets were offered in 2 dietary phases (phase 1 fed from days 0 to 7 and phase 2 from days 7 to 21 postweaning; Tables 1 and 2). A post-treatment period with a common diet was fed from days 21 to 45 (Table 3). Dietary treatments included a positive control diet (21% CP) with 3,000 mg/kg Zn from ZnO in phase 1 and 2,000 mg/kg in phase 2, a negative control diet (21% CP) with 110 mg/kg added Zn, and 4 treatment diets that contained 110 mg/kg added Zn and 4% CWB and were formulated to contain either 21%, 19.5%, 18%, or 16.5% (CP). Diets were formulated to a maximum SID Lys:digestible CP level of 6.35% based on Millet et al. (2018). Therefore, SID Lys was lowered in order to reduce CP level in diets with wheat bran to limit amino acid deficiencies relative to Lys and to maintain the ratio. The 2 control diets and the 21% CP wheat bran diet contained 1.40% SID Lys in phase 1 and 1.35% SID Lys in phase 2, while the 19.5%, 18%, and 16.5% CP diets contained 1.33%, 1.25%, and 1.20% SID Lys, respectively, in both phases. All diets were formulated to obtain a similar ratio of essential SID amino acids to SID Lys with feed grade amino acids. Ingredient nutrient values as well as their SID coefficients used in diet formulation were derived from NRC (2012). The first phase was fed in pellet form and the following phases were fed as meal. There were no feed-grade antibiotics used in diet formulation.

Table 1.

Phase 1 diet composition, experiment 1 (as-fed basis)¹

	Control, 21% CP		CP (%) with CWB
	Added ZnO	No added ZnO	21	19.5	18	16.5
Ingredient, %
Corn	44.35	44.80	41.45	45.40	48.90	52.30
Soybean meal, 46.5% CP	18.10	18.10	17.45	13.45	9.75	5.45
Dried whey	25.00	25.00	25.00	25.00	25.00	25.00
Fishmeal	4.50	4.50	4.50	4.50	4.50	4.50
CWB	—	—	4.00	4.00	4.00	4.00
Enzymatically treated soybean meal2	3.75	3.75	3.75	3.75	3.75	3.75
Soybean oil	1.50	1.50	1.50	1.50	1.50	1.50
Calcium carbonate	0.25	0.25	0.30	0.30	0.35	0.35
Monocalcium phosphate, 21% P	0.30	0.30	0.20	0.25	0.30	0.35
Salt	0.30	0.30	0.30	0.30	0.33	0.33
l-Lysine	0.43	0.43	0.44	0.47	0.49	0.56
dl-Methionine	0.23	0.23	0.22	0.21	0.20	0.21
l-Threonine	0.21	0.20	0.20	0.20	0.21	0.24
l-Tryptophan	0.07	0.07	0.07	0.08	0.08	0.10
l-Valine	0.14	0.14	0.14	0.15	0.17	0.20
l-Isoleucine	---	---	---	---	---	0.05
Trace mineral premix3	0.15	0.15	0.15	0.15	0.15	0.15
Vitamin premix4	0.25	0.25	0.25	0.25	0.25	0.25
Phytase5	0.08	0.08	0.08	0.08	0.08	0.08
Zinc oxide	0.40	---	---	---	---	---
Total	100	100	100	100	100	100
Calculated analysis
Standardized digestible (SID) amino acids, %
Lysine	1.40	1.40	1.40	1.33	1.25	1.20
Isoleucine:lysine	56	56	56	54	52	52
Leucine:lysine	109	110	108	107	106	102
Methionine:lysine	38	38	37	37	38	38
Methionine and cysteine:lysine	58	58	58	58	58	58
Threonine:lysine	65	64	64	64	64	64
Tryptophan:lysine	21.0	21.0	21.1	21.0	21.0	21.1
Valine:lysine	70	70	70	70	70	70
Total lysine, %	1.54	1.54	1.54	1.45	1.37	1.31
Metabolizable energy, kcal/kg	3,421	3,434	3,392	3,397	3,397	3,403
Net energy kcal/kg	2,578	2,589	2,552	2,574	2,594	2,620
SID lysine:net energy, g/Mcal	5.42	5.40	5.47	5.13	4.81	4.57
CP, %	21.0	21.0	21.0	19.5	18.0	16.5
Calcium, %	0.65	0.65	0.65	0.65	0.66	0.66
Phosphorus, %	0.64	0.64	0.65	0.64	0.63	0.63
Available phosphorus, %	0.55	0.55	0.54	0.55	0.55	0.56
STTD P, %6	0.56	0.56	0.56	0.56	0.56	0.56

¹Phase 1 diets were fed from days 0 to 7 (~5.6 to 6.7 kg).

²HP 300 (Hamlet Protein, Findlay, OH).

³Provided per kilogram of premix: 73 g Zn from Zn sulfate; 73 g Fe from iron sulfate; 22 g Mn from manganese oxide; 11 g Cu from copper sulfate; 0.2 g I from calcium iodate; 0.2 g Se from sodium selenite.

⁴Provided per kilogram of premix: 3,527,399 IU vitamin A; 881,850 IU vitamin D; 17,637 IU vitamin E; 1,764 mg vitamin K; 15.4 mg vitamin B12; 33,069 mg niacin; 11,023 mg pantothenic acid; 3,307 mg riboflavin.

⁵HiPhos 2700 (DSM Nutritional Products, Parsippany, NJ) provided an estimated release of 0.10% STTD P.

⁶Standardized total tract digestible phosphorus.

Table 2.

Phase 2 diet composition, experiment 1 (as-fed basis)¹

	Control, 21% CP		CP (%) with CWB
	Added ZnO	No added ZnO	21	19.5	18	16.5
Ingredient, %
Corn	55.65	55.90	52.75	56.55	60.40	64.35
Soybean meal, 46.5% CP	30.20	30.15	29.35	25.30	21.30	17.00
Dried whey	10.00	10.00	10.00	10.00	10.00	10.00
CWB	—	—	4.00	4.00	4.00	4.00
Calcium carbonate	0.93	0.93	0.98	0.98	0.98	0.98
Monocalcium phosphate, 21% P	0.90	0.90	0.80	0.85	0.90	1.00
Salt	0.55	0.55	0.55	0.55	0.58	0.58
l-Lysine	0.47	0.47	0.48	0.58	0.61	0.68
dl-Methionine	0.22	0.22	0.21	0.23	0.23	0.23
l-Threonine	0.20	0.21	0.21	0.25	0.26	0.28
l-Tryptophan	0.06	0.06	0.06	0.08	0.08	0.10
l-Valine	0.13	0.13	0.13	0.18	0.20	0.23
l-Isoleucine	—	—	—	—	0.03	0.08
Trace mineral premix2	0.15	0.15	0.15	0.15	0.15	0.15
Vitamin premix3	0.25	0.25	0.25	0.25	0.25	0.25
Phytase4	0.08	0.08	0.08	0.08	0.08	0.08
Zinc oxide	0.27	—	—	—	—	—
Total	100	100	100	100	100	100
Calculated analysis
Standardized digestible (SID) amino acids, %
Lysine	1.35	1.35	1.35	1.33	1.25	1.20
Isoleucine:lysine	57	57	56	52	52	52
Leucine:lysine	114	114	112	106	105	101
Methionine:lysine	37	37	36	37	37	37
Methionine and cysteine:lysine	58	58	58	58	58	58
Threonine:lysine	64	64	64	64	64	64
Tryptophan:lysine	21.2	21.2	21.3	21.1	21.0	21.1
Valine:lysine	70	70	70	70	70	70
Total lysine, %	1.49	1.49	1.49	1.45	1.37	1.31
Metabolizable energy, kcal/kg	3,267	3,276	3,236	3,243	3,245	3,249
Net energy, kcal/kg	2,420	2,429	2,394	2,418	2,440	2,464
SID lysine:net energy, g/Mcal	5.57	5.55	5.63	5.47	5.11	4.86
CP, %	21.0	21.0	21.0	19.5	18.0	16.5
Calcium, %	0.75	0.75	0.76	0.75	0.75	0.75
Phosphorus, %	0.62	0.62	0.63	0.62	0.61	0.61
Available phosphorus, %	0.47	0.47	0.46	0.46	0.47	0.48
STTD P, %5	0.51	0.51	0.51	0.51	0.51	0.51

¹Phase 2 diets were fed from days 7 to 21 (~6.7 to 10.9 kg).

²Provided per kilogram of premix: 73 g Zn from Zn sulfate; 73 g Fe from iron sulfate; 22 g Mn from manganese oxide; 11 g Cu from copper sulfate; 0.2 g I from calcium iodate; 0.2 g Se from sodium selenite.

³Provided per kilogram of premix: 3,527,399 IU vitamin A; 881,850 IU vitamin D; 17,637 IU vitamin E; 1,764 mg vitamin K; 15.4 mg vitamin B12; 33,069 mg niacin; 11,023 mg pantothenic acid; 3,307 mg riboflavin.

⁴HiPhos 2700 (DSM Nutritional Products, Parsippany, NJ) provided an estimated release of 0.10% STTD P.

⁵Standardized total tract digestible phosphorus.

Table 3.

Phase 3 common diet composition, experiment 1 (as-fed basis)¹

Ingredient	%
Corn	65.55
Soybean meal, 46.5% CP	30.20
Calcium carbonate	1.00
Monocalcium phosphate, 21% P	0.95
Salt	0.60
l-Lysine	0.55
dl-Methionine	0.23
l-Threonine	0.25
l-Tryptophan	0.08
l-Valine	0.15
Trace mineral premix2	0.15
Vitamin premix3	0.25
Phytase4	0.08
Total	100
Calculated analysis
Standardized digestible (SID) amino acids, %
Lysine	1.35
Isoleucine:lysine	54
Leucine:lysine	112
Methionine:lysine	37
Methionine and cysteine:lysine	58
Threonine:lysine	64
Tryptophan:lysine	21.3
Valine:lysine	70
Total lysine, %	1.49
Metabolizable energy, kcal/kg	3,271
Net energy, kcal/kg	2,422
SID lysine:net energy, g/Mcal	5.56
CP, %	20.8
Calcium, %	0.73
Phosphorus, %	0.59
Available phosphorus, %	0.42
STTD P, %5	0.46

¹A common diet were fed from days 21 to 45 (~10.9 to 25.9 kg).

²Provided per kg of premix: 73 g Zn from Zn sulfate; 73 g Fe from iron sulfate; 22 g Mn from manganese oxide; 11 g Cu from copper sulfate; 0.2 g I from calcium iodate; 0.2 g Se from sodium selenite.

³Provided per kilogram of premix: 3,527,399 IU vitamin A; 881,850 IU vitamin D; 17,637 IU vitamin E; 1,764 mg vitamin K; 15.4 mg vitamin B12; 33,069 mg niacin; 11,023 mg pantothenic acid; 3,307 mg riboflavin.

⁴HiPhos 2700 (DSM Nutritional Products, Parsippany, NJ) provided an estimated release of 0.10% STTD P.

⁵Standardized total tract digestible phosphorus.

All dietary treatments were manufactured at the Kansas State University O. H. Kruse Feed Technology Innovation Center in Manhattan, KS and were corn–soybean meal based. The average particle size of the CWB included in the experimental diets was 1,061 µm. Particle size analysis was done using the ANSI/ASAE S319.2 method with a Ro-tap 13-sieve Shaker using flow agent and sieve agitators as recommended by Kalivoda et al. (2017). Samples of complete diets were collected during bagging with a sample collected from every third bag and pooled into 1 homogenized sample per dietary treatment. Samples were stored at −20 °C until they were subsampled and submitted for analysis (Ward Laboratories, Kearney, NE) for dry matter (method 935.29; AOAC International 2019), CP (method 990.03; AOAC International, 2019), crude fiber (ANKOM Technology, 2005), ADF (ANKOM Technology, 1998), NDF (ANKOM Technology, 2017), Ca (method 6.3; Kovar, 2003), P (method 6.3; Kovar, 2003), and Zn (Campbell and Plank, 1991).

Pigs and feeders were weighed every 7 d to determine ADG, ADFI, and G:F. Fecal samples were collected from the same 3 pigs per pen on days 7, 14, 21, and 45 of the study. Fecal samples were collected into clean, single use zipper storage bags and were then stored at −20 °C until fecal dry matter analysis. Equal fecal samples from each pig were pooled by pen respective of day of collection and dried at 55 °C in a forced air oven for 48 hr. Fecal dry matter was determined as follows: (dried sample weight at 48 hr – pan weight)/(initial wet sample weight – pan weight) × 100.

Experiment 2: dietary low CP strategies with CWB

As a follow-up to the results observed in experiment 1, experiment 2 was conducted to determine whether growth performance could be maintained when feeding diets with similar SID Lys levels combined with the supplementation of either essential or nonessential amino acids. To achieve this, 2 groups of 300 pigs (Line 241 × 600, DNA, Columbus, NE, initially 7.0 kg) and 350 pigs (initially 6.2 kg) were used in a 13-d growth phase 2 trial. Pigs were weaned at ~21 d of age and placed in pens of 5 pigs each based on initial BW and gender. The first group within experiment 2 was fed a common pelleted diet with pharmacological levels (3,000 mg/kg Zn from ZnO) for 10 d after weaning, and the second group was fed diets without pharmacological levels of ZnO for 14 d after weaning and prior to the start of the experiment. At 10 or 14 d after weaning, which was considered day 0 of the trial, pens of pigs were randomly allotted to 1 of 5 dietary treatments in a randomized complete block design with BW as the blocking factor. In total, there were 5 pigs per pen and 26 pens per treatment with 12 replicates per treatment in the first group and 14 replicates per treatment in the second group of pigs.

Treatment diets were offered in mash form and in 1 dietary phase (Table 4). All dietary treatments contained 4% CWB and consisted of (1) positive control with ZnO providing 2,000 mg/kg of Zn and 21% CP (1.35% SID Lys); (2) a diet with 110 mg/kg added Zn and 21% CP (1.35% SID Lys); (3) a diet with 110 mg/kg added Zn formulated to 18% CP (1.2% SID Lys); (4) an 18% CP diet with 110 mg/kg added Zn formulated to 1.35% SID Lys by the addition of feed grade amino acids; and (5) treatment 4 with the addition of nonessential amino acids (Gly and Glu). Diet 3 was formulated to 18% CP while adding feed grade amino acids until the minimum Ile:Lys requirement of 52% was met. L-Ile and L-His were added along with other feed grade amino acids to formulate diet 4 to 1.35% SID Lys. The nonessential amino acids (AA) Glu and Gly were added to diet 5 as the combination of Gly and nitrogen from an additional AA has been determined to be required in low CP diets (Powell et al., 2011). A common post-treatment diet was fed from days 13 to 27 to the second group of pigs in experiment 2 (Table 5).

Table 4.

Diet composition, experiment 2 (as-fed basis)¹

	21% CP		18% CP, no ZnO
Ingredient, %	ZnO	No ZnO	1.2% lysine	1.35% lysine	1.35% lysine + NEAA
Corn	53.15	53.45	60.55	60.00	57.60
Soybean meal, 46.5% CP	28.75	28.70	21.35	21.25	21.40
Dried whey	10.00	10.00	10.00	10.00	10.00
CWB	4.00	4.00	4.00	4.00	4.00
Calcium carbonate	0.95	0.95	0.95	0.95	0.95
Monocalcium phosphate, 21% P	0.80	0.80	0.90	0.90	0.90
Salt	0.55	0.55	0.55	0.55	0.55
l-Lysine	0.50	0.50	0.54	0.74	0.74
dl-Methionine	0.20	0.20	0.23	0.31	0.31
l-Threonine	0.24	0.24	0.24	0.34	0.34
l-Tryptophan	0.03	0.03	0.07	0.11	0.11
l-Valine	0.10	0.10	0.17	0.27	0.27
l-Isoleucine	—	—	—	0.13	0.13
l-Histidine	—	—	—	0.04	0.04
Trace mineral premix2	0.15	0.15	0.15	0.15	0.15
Vitamin premix3	0.25	0.25	0.25	0.25	0.25
Phytase4	0.08	0.08	0.08	0.08	0.08
Zinc oxide	0.25	—	—	—	—
Glycine	—	—	—	—	1.10
Glutamic acid	—	—	—	—	1.10
Total	100	100	100	100	100
Calculated analysis
Standardized digestible (SID) amino acids, %
Lysine	1.35	1.35	1.20	1.35	1.35
Isoleucine:lysine	55	55	52	55	55
Leucine:lysine	110	111	110	97	96
Methionine:lysine	35	35	39	40	40
Methionine and cysteine:lysine	57	57	61	60	59
Threonine:lysine	65	65	65	65	65
Tryptophan:lysine	18.9	18.9	21.0	21.1	21.1
Valine:lysine	67	67	70	70	69
Histidine:lysine	35	35	33	32	32
Total lysine, %	1.49	1.49	1.32	1.47	1.47
Metabolizable energy, kcal/kg	3,227	3,236	3,245	3,254	3,247
Net energy, kcal/kg	2,389	2,396	2,440	2,446	2,440
SID lysine:net energy, g/Mcal	5.64	5.62	4.91	5.51	5.52
CP, %	20.7	20.7	17.9	18.3	20.1
Calcium, %	0.75	0.75	0.75	0.75	0.75
Phosphorus, %	0.62	0.62	0.61	0.61	0.60
Available phosphorus, %	0.46	0.46	0.47	0.47	0.47
STTD P, %5	0.51	0.51	0.51	0.51	0.50

¹Experimental diets were fed from ~days 0 to 13 (7.0 to 11.7 kg in experiment 2, and 6.2 to 11.9 kg in experiment 3).

²Provided per kilogram of premix: 73 g Zn from Zn sulfate; 73 g Fe from iron sulfate; 22 g Mn from manganese oxide; 11 g Cu from copper sulfate; 0.2 g I from calcium iodate; 0.2 g Se from sodium selenite.

³Provided per kilogram of premix: 3,527,399 IU vitamin A; 881,850 IU vitamin D; 17,637 IU vitamin E; 1,764 mg vitamin K; 15.4 mg vitamin B12; 33,069 mg niacin; 11,023 mg pantothenic acid; 3,307 mg riboflavin.

⁴HiPhos 2700 (DSM Nutritional Products, Parsippany, NJ) provided an estimated release of 0.10% STTD P.

⁵Standardized total tract digestible phosphorus.

Table 5.

Phase 3 common diet composition, experiment 2 (as-fed basis)¹

Ingredient	%
Corn	65.47
Soybean meal, 46.8% CP	28.30
Choice white grease	2.00
Calcium carbonate	0.75
Monocalcium phosphate, 21% P	1.10
Sodium chloride	0.60
l-Lysine-HCl	0.55
dl-Methionine	0.25
l-Threonine	0.23
l-Tryptophan	0.05
l-Valine	0.16
Trace mineral premix2	0.15
Vitamin premix with phytase3	0.25
Pellet stabilizer4	0.15
Total	100
Calculated analysis
Standardized digestible (SID) amino acids, %
Lysine	1.30
Isoleucine:lysine	53
Leucine:lysine	111
Methionine:lysine	39
Met and cysteine:lysine	60
Threonine:lysine	63
Tryptophan:lysine	19.3
Valine:lysine	70
Histidine:lysine	35
Total lysine, %	1.41
Metabolizable energy, kcal/kg	3,318
Net energy, kcal/kg	2,534
CP, %	19.9
Calcium, %	0.65
Available phosphorus, %	0.44
STTD P5, %	0.48

¹Phase 3 common diets were fed only to group 2 from days 13 to 27.

²Provided per kilogram of premix: 73 g Zn from Zn oxide; 73 g Fe from iron sulfate; 22 g Mn from manganese oxide; 11 g Cu from copper sulfate; 0.2 g I from calcium iodate; 0.2 g Se from sodium selenite.

³Ronozyme HiPhos GT (DSM Nutritional Products, Parsippany, NJ) provided an expected P release of 0.15%. Provided per kilogram of premix: 3,527,399 IU vitamin A; 881,850 IU vitamin D; 17,637 IU vitamin E; 1,764 mg vitamin K; 15.4 mg vitamin B12; 33,069 mg niacin; 11,023 mg pantothenic acid; 3,307 mg riboflavin.

⁴Alltech All-Bind HD (Alltech, Nicholasville, KY).

⁵STTD P = standardized total tract digestible phosphorus.

Fecal samples were collected from the same 3 pigs on days 0, 7, and 13 for group 1, and on days 0, 6, 13, 20, and 27 for the second group of pigs. Feed manufacturing and diet and fecal sample collection and analysis were identical to procedures used in experiment 1.

Statistical analysis

In experiment 1, data were analyzed as a completely randomized design using the lmer function from the lme4 package in R (version 3.6.1 (2019-07-05), R Foundation for Statistical Computing, Vienna, Austria), with pen as the experimental unit and barn as a random effect. Preplanned linear and quadratic contrast statements were used to evaluate decreasing CP level. Contrast statements were used to evaluate the positive and negative control diets, the positive control vs. the 21% CP with CWB diet, and the negative control vs. the 21% CP with CWB diet. A repeated measures statement, with the random effect of barn, was used to analyze fecal dry matter percentages over time. Differences between treatments were considered significant at P ≤ 0.05 and marginally significant at 0.05 < P ≤ 0.10.

Statistical analysis for experiment 2 was performed using the lmer function from the lme4 package in R (version 3.6.1 (2019-07-05), R Foundation for Statistical Computing, Vienna, Austria). Growth performance data were analyzed as a completely randomized block design using pen as the experimental unit and treatment as a fixed effect. Weight block was included in the model as a random effect. A repeated measures statement with a random effect of block was used for analyzing fecal dry matter percentages over time. There were no significant treatment × group interactions, thus data from groups 1 and 2 were combined to evaluate growth performance during the experimental period (days 0 to 13). Differences between treatments were considered significant at P ≤ 0.05 and marginally significant at 0.05 < P ≤ 0.10.

Results

Experiment 1

Results of chemical analysis of diets in experiment 1 were consistent with formulation (Table 6). In phase 1 (days 0 to 7), no evidence for differences were observed for pigs fed the positive control compared with the negative control or compared with pigs fed the 21% CP wheat bran diet (Table 7). For pigs fed diets with decreasing CP, ADG, and G:F decreased (linear, P < 0.001). Pigs fed diets with 21% CP and CWB had increased (linear, P = 0.001) BW on day 7 compared with pigs fed lower CP diets. On day 7, there was marginal evidence for differences for pigs fed the positive control to have increased fecal dry matter compared with pigs fed the 21% CP CWB diet (P = 0.059) without pharmacological levels of ZnO (Table 8). Fecal dry matter percentage increased on day 7 (linear, P = 0.029) for pigs fed diets with decreasing CP level.

Table 6.

Analyzed diet composition experiment 1 (as-fed basis)^1,2

	Control, 21% CP		CP (%) with CWB3
Analyzed composition	Added ZnO	No added ZnO	21	19.5	18	16.5
Phase 1
Dry matter, %	91.1	91.0	91.0	91.1	90.9	90.8
Crude fiber, %	1.6	1.6	2.2	2.1	1.9	2.4
Acid detergent fiber, %	2.1	2.3	2.7	2.8	2.6	2.4
Neutral detergent fiber, %	4.6	4.6	6.0	5.9	5.6	5.6
CP, %	20.3	20.3	20.6	18.6	17.2	15.7
Zn, mg/kg	2,526	164	126	169	125	145
Ca, %	0.89	0.91	0.91	0.86	0.88	0.97
P, %	0.61	0.64	0.63	0.61	0.62	0.60
Phase 2
Dry matter, %	89.2	89.0	89.2	88.9	89.1	89.1
Crude fiber, %	2.1	2.1	2.3	2.1	2.0	2.1
Acid detergent fiber, %	3.1	2.7	3.1	3.3	3.0	2.6
Neutral detergent fiber, %	6.1	6.0	7.2	6.9	6.6	6.2
CP, %	20.8	20.3	21.0	20.3	17.9	15.7
Zn, mg/kg	1,575	105	187	115	153	156
Ca, %	0.90	0.97	0.88	0.79	0.88	0.93
P, %	0.57	0.58	0.61	0.60	0.59	0.58

¹Diets were fed in 2 phases from days 0 to 7 and days 7 to 21, for phases 1 and 2, respectively.

²Complete diet samples were taken at manufacturing. Samples were stored at −20 °C until they were homogenized and subsampled. Duplicate samples per treatment were submitted for analysis (Ward Laboratories, Inc., Kearney, NE).

³CWB was included in the diet at 4% from days 0 to 21.

Table 7.

Effect of CP level with CWB on growth performance of nursery pigs, experiment 1¹

								P-value
	Control, 21% CP		CP (%) with CWB					CP level		ZnO	+ Control vs. 21%	− Control vs. 21%
Item	Added ZnO2	No added ZnO2	21	19.5	18	16.5	SEM	Linear	Quadratic	+ vs. −	CP	CP
BW, kg
Day 0	5.6	5.6	5.6	5.6	5.6	5.6	0.15	0.202	0.461	0.954	0.908	0.862
Day 7	6.9	6.8	6.8	6.7	6.6	6.6	0.18	0.001	0.275	0.256	0.572	0.565
Day 21	12.2	10.8	10.9	10.9	10.3	10.3	0.56	0.003	0.957	<0.001	<0.001	0.709
Day 45	26.9	26.0	26.3	25.9	25.5	25.2	0.66	0.010	0.751	0.056	0.236	0.454
Days 0 to 7
ADG, g	177	165	171	154	135	136	7.4	<0.001	0.226	0.258	0.557	0.585
ADFI, g	171	169	168	169	155	161	8.8	0.158	0.703	0.747	0.653	0.899
G:F, g/kg	1,031	977	1,021	910	868	846	37.9	<0.001	0.141	0.206	0.816	0.300
Days 7 to 21
ADG, g	383	290	294	295	266	262	27.7	0.017	0.823	<0.001	<0.001	0.813
ADFI, g	467	382	395	399	363	377	19.3	0.102	0.705	<0.001	<0.001	0.478
G:F, g/kg	818	753	743	737	732	698	37.2	0.048	0.371	0.005	0.001	0.643
Experimental period (days 0 to 21)
ADG, g	314	248	253	247	222	220	19.3	0.002	0.830	<0.001	<0.001	0.711
ADFI, g	369	311	319	322	293	304	15.2	0.065	0.634	<0.001	<0.001	0.531
G:F, g/kg	851	795	792	767	757	724	27.0	0.002	0.785	0.007	0.005	0.887
Post-test period (days 21 to 45)3
ADG, g	611	624	640	625	632	622	9.2	0.263	0.788	0.338	0.033	0.228
ADFI, g	951	928	964	941	929	924	19.6	0.058	0.565	0.281	0.572	0.103
G:F, g/kg	643	673	664	666	680	674	9.7	0.078	0.491	<0.001	0.007	0.284
Days 0 to 45
ADG, g	473	448	457	448	440	433	11.1	0.012	0.838	0.014	0.124	0.331
ADFI, g	680	639	660	651	631	633	16.6	0.038	0.606	0.008	0.187	0.164
G:F, g/kg	696	701	693	689	697	685	4.8	0.443	0.410	0.468	0.701	0.269

¹A total of 360 pigs (Line 200 × 400, DNA, Columbus, NE initially 5.6 kg) were used in a 45-d growth study with 5 pigs per pen and 12 pens per treatment. ADG = average daily gain. ADFI = average daily feed intake. G:F = feed efficiency.

²ZnO was included in the diet to provide 3,000 mg/kg of Zn from days 0 to 7; 2,000 mg/kg of Zn from days 7 to 21; and no additional Zn other than that from the trace mineral premix (110 mg/kg Zn) from days 21 to 45. The negative control diet contained 110 mg/kg added Zn from the premix for the entire study.

³A common diet was fed from days 21 to 45.

Table 8.

Effect of CP level with CWB in nursery pig diets on fecal dry matter, %, experiment 1¹

								P-value
	Control, 21% CP		CP (%) with CWB					CP level		ZnO
Item	Added ZnO2	No added ZnO 2	21	19.5	18	16.5	SEM	Linear	Quadratic	+ vs −	+ Control vs. 21% CP	− Control vs. 21% CP
Day of collection3
Day 7	28.8	26.8	26.5	27.7	29.3	28.9	0.88	0.029	0.349	0.108	0.059	0.776
Day 21	21.7	20.8	21.3	23.2	24.4	24.4	0.84	0.006	0.282	0.488	0.779	0.679
Day 45	24.1	27.0	26.5	24.9	24.0	23.5	0.70	0.002	0.433	0.004	0.016	0.631

¹Values represent the mean of 3 pigs per pen and 12 pens per treatment. Three pigs per pen were randomly selected and sampled. Fecal samples were then pooled by pen respective of day of collection and dried at 55 °C in a forced air oven.

²ZnO was included in the diet to provide 3,000 mg/kg of Zn from days 0 to 7; 2,000 mg/kg of Zn from days 7 to 21; and no additional Zn other than that from the trace mineral premix (110 mg/kg Zn) from days 21 to 45. The negative control diet contained 110 mg/kg added Zn from the premix for the entire study.

³Experimental diets were fed from days 0 to 21 and a common diet was fed from days 21 to 45.

From days 7 to 21, which correspond to phase 2, decreasing dietary CP resulted in decreased (linear, P = 0.05) ADG, G:F, and BW on day 21. Pigs fed the positive control diet containing pharmacological levels of Zn had increased (P < 0.05) ADG, ADFI, G:F, and BW on day 21 compared with the negative control diet without added ZnO and compared with the 21% CP wheat bran diet.

During the entire experimental diet phase (days 0 to 21), pigs fed the positive control diet containing ZnO had improved (P < 0.05) performance across all growth response criteria measured compared with the negative control and the CWB diet with 21% CP. Decreasing CP in diets containing CWB resulted in decreased (linear, P = 0.002) ADG and G:F, and a marginally significant decrease (linear, P = 0.065) in ADFI. Pigs fed wheat bran diets with lower CP had increased (linear, P = 0.006) fecal dry matter, suggesting more solid feces and lower instances of loose stool.

A post-treatment diet was fed from days 21 to 45. Pigs previously fed the wheat bran diet with 21% CP had an increase (P = 0.033) in ADG compared with pigs fed the positive control diet. Feed efficiency improved for pigs fed the wheat bran diet with high CP (P = 0.007) and for the pigs previously fed the negative control diet (P < 0.001) compared with the positive control diet with pharmacological levels of Zn. A marginal decrease (linear, P = 0.058) in ADFI was observed as CP fed in the previous period decreased; however, a marginal improvement (linear, P = 0.078) in G:F was observed for the pigs previously fed decreasing CP levels. Fecal dry matter percentage (Table 8) on day 45 decreased (linear, P = 0.002) for pigs previously fed diets with decreasing CP, while dry matter was increased for pigs previously fed the negative control compared with the positive control (P = 0.004) and increased for those previously fed the 21% CP wheat bran diet vs. the positive control (P = 0.016).

Overall, from days 0 to 45, decreasing CP resulted in decreased (linear, P < 0.050) ADG, ADFI, and BW on day 45. Pigs fed the positive control diet had increased (P < 0.050) ADG, ADFI, and greater (P = 0.056) BW on day 45 compared with those fed the negative control. Overall, no evidence for differences in any growth criteria measured was observed for pigs fed the positive control vs. pigs fed the 21% CP diet with CWB.

Experiment 2

For experiment 2, chemical analysis of diets (Table 9) resulted in values consistent with diet formulation with analyzed Zn slightly lower than formulated. Analyzed CP was consistent with formulated values; however, for the diet with added nonessential feed grade amino acids, CP analyzed at >18% CP due to the nitrogen from the nonessential amino acids added to the diet. Analysis of crude fiber and NDF resulted in higher values for group 2 than group 1; however, diet formulations and CWB ingredient sources were the same for both groups.

Table 9.

Analyzed diet composition experiment 2 (as-fed basis)^1,2

	21% CP		18% CP, no ZnO
Analyzed composition	ZnO	No ZnO	1.2% Lysine	1.35% Lysine	1.35% Lysine + NEAA
Experiment 2. Group 1
Dry matter, %	92.8	89.9	90.2	90.2	89.9
Crude fiber, %	1.8	2.1	1.5	1.8	1.9
Acid detergent fiber, %	4.1	4.0	3.2	3.4	3.6
Neutral detergent fiber, %	8.7	9.9	7.7	7.9	7.9
CP, %	21.3	21.5	17.6	18.2	19.94
Zn, mg/kg	1,377	130	168	121	100
Ca, %	0.82	0.62	0.76	0.69	0.58
P, %	0.58	0.57	0.53	0.52	0.50
Experiment 2 Group 2
Dry matter, %	90.1	89.6	89.6	89.5	90.0
Crude fiber, %	2.8	2.9	3.0	2.4	2.6
Acid detergent fiber, %	4.5	5.1	4.8	3.6	3.5
Neutral detergent fiber, %	11.4	10.4	12.1	8.9	8.6
CP, %	20.9	21.0	18.2	18.0	20.23
Zn, mg/kg	1,523	140	112	187	113
Ca, %	0.74	0.80	0.68	0.83	0.82
P, %	0.53	0.55	0.60	0.56	0.69

¹In experiment 2 group 1 diets were fed from days 0 to 13, and in experiment 2 group 2, experimental diets were fed from days 0 to 13 and all pigs received the same common diet from days 13 to 27.

²Complete diet samples were taken at manufacture. Samples were stored at −20 °C until they were homogenized and subsampled. Duplicate samples per treatment were submitted for analysis to (Ward Laboratories, Inc., Kearney, NE).

³CP >18% is observed due to the nitrogen from the nonessential amino acids added to the diet.

Pigs in group 1 were fed a common diet which contained pharmacological levels of Zn for 10 d before experimental diets were fed. For group 2, pigs were fed a diet for 14 d containing no pharmacological Zn prior to the start of the trial. During data analysis, no group by treatment interactions were observed, therefore data were combined for both groups.

Pigs fed 21% CP with ZnO had increased (P < 0.05) ADG compared with pigs fed 18% CP with 1.2% Lys and 18% CP with high levels of feed grade amino acids (Table 10). Additionally, pigs fed the 18% CP diet supplemented with high levels of amino acids had decreased (P < 0.05) ADG compared with pigs fed the 21% CP diet with no ZnO. Overall G:F was increased (P < 0.05) for pigs fed 21% CP diets regardless of ZnO inclusion and for pigs fed the 18% CP diet with added nonessential amino acids compared with the other treatments. Final BW on day 13 was heaviest (P < 0.05) for pigs fed 21% CP diets with ZnO compared with pigs fed 18% CP diets. Pigs fed 21% CP without ZnO had increased (P < 0.05) final BW compared with pigs fed 18% CP with high levels of feed grade amino acids. Fecal dry matter percentage (Table 11) on day 6 was increased (P < 0.05) for pigs fed the 1.2% SID Lys diet compared with pigs fed 21% CP diets with other dietary treatments intermediate. Marginal evidence for treatment differences was observed (P = 0.084); however, increased (P < 0.05) fecal dry matter on day 13 was observed for pigs fed the reduced Lys diet (1.2% SID Lys) compared with the pigs fed 21% CP diet without ZnO. In group 2, subsequent growth performance and fecal DM were monitored for an additional 14 d where all pigs were fed a common post-treatment diet. No evidence for differences in any growth or fecal DM responses were observed across dietary treatments.

Table 10.

Effect of CWB and reducing dietary CP content with supplemented essential or nonessential amino acids on nursery pig performance, experiment 2¹

	21% CP2		18% CP, No ZnO
Item	ZnO	No ZnO	1.2% Lysine3	1.35% Lysine4	1.35% Lysine + NEAA5	SEM
BW, kg
d 0	6.6	6.6	6.6	6.6	6.6	0.11
d 13	12.3a	12.0ab	11.7bc	11.6c	11.7bc	0.18
Days 0 to 13 (experimental period)
ADG, g	429a	417ab	386bc	378c	393abc	12.2
ADFI, g	598	586	601	592	565	17.2
G:F, g/kg	719a	713a	642b	640b	695a	10.1
Days 13 to 27 (common period)6
ADG, g	597	618	598	603	594	14.6
ADFI, g	891	925	906	899	904	20.0
G:F, g/kg	668	669	660	673	658	9.3

^abcMeans in the same row with different superscripts differ (P < 0.05).

¹A total of 650 pigs (Line 241 × 600, DNA, Columbus, NE, initial BW of 6.6 kg) were used in a 13-d growth study with 5 pigs per pen and 26 pens per treatment. All diets contained 4% CWB. After weaning, pigs were fed a common starter diet until day 10 or 14 postweaning for groups 1 and 2, respectively, which was considered day 0 of the trial.

²Diets were formulated to 21% CP (1.35% SID Lys) with or without pharmacological levels of ZnO (2,000 mg/kg).

³Treatment diet was formulated to 18% CP and 1.2% SID Lys by adding feed grade amino acids until the minimum Ile:Lys requirement of 52% was met.

⁴Treatment diet was formulated to 18% CP and 1.35% SID Lys with high amounts of feed grade amino acids. l-Ile and l-His were added along with other feed grade amino acids.

⁵Treatment diet was formulated to 18% CP and 1.35% SID Lys with high amounts of feed grade amino acids and with the addition of nonessential amino acids (glycine and glutamic acid).

⁶A common diet was fed only to group 2 with 14 pens per treatment.

Table 11.

Effect of CWB and reducing dietary CP content with supplemented essential or nonessential amino acids in nursery pig diets on fecal dry matter, %, experiment 2¹

	21% CP2		18% CP, no ZnO
Item	ZnO	No ZnO	1.2% Lysine3	1.35% Lysine4	1.35% Lysine + NEAA5	SEM
Day of collection
Day 0	23.1	24.4	23.5	23.2	23.2	0.67
Day 66	16.7b	16.4b	18.9a	17.0ab	18.0ab	0.58
Day 13	18.0ab	17.2b	19.2a	18.1ab	18.2ab	0.51
Day 207	21.9	23.4	21.2	21.3	22.0	0.83
Day 277	20.9	23.0	21.5	22.8	20.8	0.94

^abMeans in the same row with different superscripts differ (P < 0.05).

¹Values represent the mean of 3 pigs per pen and 26 pens per treatment. Three pigs per pen were randomly selected and sampled. Fecal samples were then pooled by pen respective of day of collection and dried at 55 °C in a forced air oven.

²Diets were formulated to 21% CP with or without pharmacological levels of ZnO (2,000 mg/kg).

³Treatment diet was formulated to 18% CP and 1.2% SID lysine by adding feed grade amino acids until the minimum Ile:Lys requirement of 52% was met.

⁴Treatment diet was formulated to 18% CP and 1.35% SID Lys with high amounts of feed grade amino acids. l-Ile and l-His were added along with other feed grade amino acids.

⁵Treatment diet was formulated to 18% CP and 1.35% SID Lys with high amounts of feed grade amino acids and with the addition of nonessential amino acids (glycine and glutamic acid).

⁶Group 1 samples were collected on day 7 and group 2 on collected on day 6 of each trial.

⁷Collection days consist of 3 pigs per pen and 14 pens per treatment. These collection days were only done for group 2 in the post-treatment period from days 13 to 27.

Discussion

Postweaning diarrhea is a common condition in pigs classified by consistent discharge of loose, watery feces during the first 2 wk after weaning (Rhouma et al., 2017). This condition represents a major economic issue for the swine industry and can result in dehydration, reduced feed intake, nutrient digestibility, growth, and, in severe cases, even death (Heo et al., 2013). However, feeding pharmacological levels of Zn (usually around 2,000 to 3,000 mg/kg of Zn) to diminish PWD (Katouli et al., 1999; Højberg et al., 2005) and to promote growth after weaning has been a widely accepted dietary tool (Hill et al., 2000; Case and Carlson, 2002). Zinc is an essential trace mineral with the NRC (2012) recommended level for 5 to 11 kg pigs being 100 mg/kg of Zn. As regulatory restrictions on the use of pharmacological levels of Zn increase, an alternative dietary strategy to minimize PWD while promoting nursery pig growth is needed.

Pigs fed diets supplemented with pharmacological levels of ZnO in experiment 1 had improved performance for all growth response criteria measured during the experimental period compared with pigs fed the negative control with no added ZnO or the high CP diet with CWB. Overall, pigs previously fed diets containing pharmacological levels of ZnO had increased ADG, ADFI, and heavier BW on day 45 compared with pigs fed the negative control. Improvements in performance for pigs fed diets with ZnO in the present research are consistent to results observed in other studies feeding pharmacological levels of ZnO (Smith et al., 1997; Carlson et al., 1999; Case and Carlson, 2002; Molist et al., 2011). High doses of Zn have been proposed to increase feed intake, as Yin et al. (2009) observed increased ghrelin levels, a hormone which stimulates feed intake and muscle growth in early-weaned pigs with ZnO supplementation. Evidence exists that pharmacological Zn reduces intestinal mucosal inflammation, promoting increased nutrient digestibility and improved growth performance (Ou et al., 2007). In experiment 2, the pigs in group 1 received a common starter diet prior to the start of the study that contained pharmacological levels of ZnO; however, performance during the experimental period was similar between pigs fed the 21% CP diet with ZnO or without ZnO. Therefore, pigs in the second group of experiment 2 were fed a common diet with no pharmacological levels of Zn for 14 d prior to the start of the trial to determine if high Zn fed in a starter diet immediately postweaning weakened the response to pharmacological levels of ZnO in the next feeding phase. However, as found in group 1, no responses to pharmacological levels of Zn were observed, therefore because there were no group by treatment interactions, the individual groups were combined. Growth responses to high levels of Zn are not always observed (Schell and Kornegay, 1996; Martínez et al., 2005; Wilt and Carlson, 2009). These authors proposed that the lack of response could be due to low cases of scouring and high initial health status. Burrough et al. (2019) concluded that differences in trial designs, genetics, intestinal microbial populations, as well as environmental factors may attribute to the variable growth response in weaned piglets to pharmacological levels of Zn.

Weanling pigs are commonly offered high protein (high amino acid concentration) diets due to a low feed intake capacity and a high potential for protein deposition (Gloaguen et al., 2014). Excessively high protein diets can initiate digestive issues for nursery pigs as undigested protein contributes to the proliferation of pathogenic bacteria in the GIT (Pluske et al., 2002; Wang et al., 2018). Excess amino acids are deaminated and urea is excreted through urine (van Milgen and Dourmad, 2015). Low CP diets have been shown to reduce inflammatory responses and the proliferation of enterotoxigenic E. coli strains associated with PWD (Opapeju et al., 2009; Kim et al., 2011); however, reductions in dietary CP content by reducing diet amino acid concentrations can result in compromised growth performance. The increased availability of crystalline l-Lys, dl-Met, l-Thr, l-Trp, l-Val, and l-Ile has enabled CP to be practically reduced in swine diets while maintaining high amino acid concentrations. Further reductions in dietary CP may be limited by dietary His levels and other unknown requirements for the next-limiting amino acid.

Lewis (2001) noted that reducing CP content by 2 percentage points allowed pig performance to be maintained, while further reductions in CP with crystalline amino acid supplementation have led to conflicting results. Several researchers have observed no reductions in growth for pigs fed low CP diets supplemented with essential amino acids (Jin et al., 1998; Le Bellego and Noblet, 2002; Heo et al., 2008; 2009). In experiment 1, pigs fed diets with reduced CP and subsequently reduced SID Lys had decreased ADG, feed efficiency, and BW during the experimental period. Wellock et al. (2006) observed growth performance to be diminished when CP content was reduced from 23% to 13% CP and when SID Lys was not balanced. In support of the current findings, dietary CP could not be reduced below 16% CP before the performance of 8 to 25 kg pigs decreased when SID Lys was below the requirement for growth (Jansman et al., 2016). Diets in experiment 1 were formulated to a maximum SID Lys:digestible CP ratio of 6.35% as suggested by Millet et al. (2018) as the authors observed that this ratio was necessary to maintain an adequate amount of nitrogen to enable sufficient synthesis of nonessential amino acids. Thus, SID Lys decreased in the 19.5%, 18%, and 16.5% CP diets to maintain the Lys:digestible CP ratio in experiment 1. The resulting deficiencies in SID Lys likely limited growth performance as dietary CP decreased.

Additionally, reduced growth performance was observed in experiment 2 for pigs fed decreasing CP regardless of whether the SID Lys levels were maintained at similar levels to the control diet or not. The results from the current studies agree with Nyachoti et al. (2006), who fed low CP diets (23% to 17% CP) balanced with essential amino acids and observed reduced final BW and overall ADG and ADFI by feeding diets containing <19% CP. However, Heo et al. (2008) reduced CP, maintained adequate Lys levels, and observed similar ADG and low incidence of diarrhea. Yue and Qiao (2008) observed decreased overall performance and final BW for pigs fed diets with 17.2% CP compared with pigs fed 23.1% CP. SID Lys was balanced in these experiments, similar to experiment 2; however, performance was still reduced. Nyachoti et al. (2006) concluded that decreased performance in low CP diets could be due to other limiting amino acids, as Yue and Qiao (2008) determined that deficiencies in nonessential amino acids such as glutamine (Gln), could be a driving factor in reduced performance. It has been suggested that minimal levels of amino acids classified as nonessential are required by the pig (Wu, 2014).

Therefore, one of the objectives of experiment 2 was to determine whether growth performance could be maintained when feeding diets with similar SID Lys levels combined with the supplementation of either essential or nonessential amino acids. Growth was limited for pigs fed the 1.2% SID Lys diet compared with those fed diets with 1.35% which agreed with our hypothesis and the results of experiment 1 where SID Lys was reduced. In experiment 2, the addition of essential amino acids to achieve 1.35% SID Lys did not improve growth performance. A study conducted by Opapeju et al. (2008) observed decreased growth in pigs fed low CP diets (17% CP) with balanced SID Lys supplemented with essential AA including Val and Ile. The authors claimed a shortage of indispensable AA in the low CP diet initiated poor performance. In the present study, the addition of nonessential amino acids glycine (Gly) and glutamic acid (Glu) was hypothesized to maintain growth or improve performance similar to pigs fed 21% CP diets. In experiment 2, feed efficiency was improved for pigs fed 18% CP diets with the addition of nonessential amino acids. The results from experiment 2 are similar to that of Gloaguen et al. (2014) in that feed efficiency was restored with the addition of l-Glu in a 14% CP diet. However, Silva et al. (2020) conducted a study comparing a control diet to a low CP diet with added Ser and Gly and a low CP diet with added Glu. The authors observed that ADG and final BW were reduced for pigs fed both low CP diets compared with pigs fed a 23.6% CP control diet. These findings are similar to experiment 2 when Gly and Glu were supplemented to an 18% CP diet as final BW was reduced.

A considerable issue with low CP diets is an insufficient supply of nonessential amino acids (Wu et al., 2018). Gloaguen et al. (2014) determined that amino acids that have been traditionally determined to be nonessential may be limiting for growth in low CP diets. Wu (2014) suggested that dispensable AA, such as Glu and Proline (Pro), regulate metabolic pathways and due to catabolism of nonessential AA in the small intestine of the pig, these dispensable AA should be included in diets to support the ideal protein requirement for optimal growth. Glycine has been identified as one of the first nonessential AA that may become limiting in a low CP diet, as Powell et al. (2011) demonstrated that performance was maintained with the supplementation of Gly and Arginine (Arg) or with Gly and Glu in 20- to 50-kg pigs, but not with the supplementation of Arg or Glu individually. Sun et al. (2020) presented that the metabolism of essential AA and nonessential AA such as Glu and Glutamine (Gln) in the liver decreased with the addition of sodium dichloroacetate, the sodium salt of dichloroacetic acid that shifts the metabolism of AA to the oxidation of glucose. Therefore, if nitrogen utilization and efficiency is enhanced, pig performance may be improved when pigs are fed a low CP diet. It can be concluded that perhaps one or more dispensable AA should be added to low CP diets and if the catabolism of AA in the liver is minimized, the growth performance of nursery pigs fed low CP diets could be maintained.

While pigs fed reduced CP diets experienced a linear reduction in performance, these pigs had increased fecal dry matter during the experimental period, suggesting drier and more solid feces. Nyachoti et al. (2006), Wellock et al. (2006, 2008), and Yue and Qiao (2008) concluded improved fecal consistency from a low CP diet was a result of decreased protein fermentation in the large intestine limiting the production of toxic by-products such as ammonia and amines which can increase the occurrence of gastrointestinal upsets like PWD (Pluske et al., 2002). An improvement in fecal dry matter observed in experiment 1 was driven more by reduced CP rather than wheat bran inclusion because we did not observe a difference comparing the negative control 21% CP diet without wheat bran to the 21% CP diet that did contain wheat bran. Molist et al. (2011) observed a decrease in the E. coli and coliform bacteria counts in piglet feces with the addition of 4% dietary wheat bran. Previous work by Molist et al. (2010) concluded that wheat bran decreased the ability of E. coli to attach to the ileum mucosa after experimental infection. Fiber addition to nursery diets has been suggested to increase the fermentation of carbohydrates while increasing the concentration of fermentation products such as butyrate, the main fuel source for colonocytes (Jensen and Jorgensen, 1994). Fiber inclusion decreases protein fermentation (Hermes et al., 2009), while fiber may provide an alternative binding matrix for bacteria due to the carbohydrate composition being similar to intestinal receptors of enteropathogenic bacteria (Becker and Galleti, 2008). However, like the results of our study, Molist et al. (2011) concluded the addition of wheat bran did not improve growth performance.

In conclusion, pigs fed pharmacological levels of Zn had improved growth performance in experiment 1 similar to previous studies. However, improved growth is not always observed for pigs fed high levels of Zn, similar to performance in experiment 2. Reducing CP and subsequently SID Lys in diets with CWB in an attempt to reduce the occurrence of postweaning diarrhea did improve fecal dry matter; however, this did not maintain or improve growth performance. No improvements in growth performance were observed for pigs fed diets supplemented with high levels of feed grade essential amino acids while improvements in feed efficiency were observed with the addition of nonessential amino acids to low CP diets.

Glossary

Abbreviations

ADFI

average daily feed intake

ADG

average daily gain

CP

crude protein

CWB

coarse what bran

G:F

feed efficiency

NEAA

nonessential amino acids

PWD

postweaning diarrhea

SID

standardized ileal digestible

ZnO

zinc oxide

Conflict of Interest Statement

The authors declare no real or perceived conflicts of interest.