<p>Gastrointestinal health in the young pig is a constant challenge for the swine industry. Weaning introduces many stressors such as a new solid diet, being separated from their mother, new pen mates, transportation, and an entirely new environment. Therefore, this dissertation primarily focuses on research aiding the pig immediately post-weaning. In chapter 2, a developmental bacillus direct fed microbial (DFM) was added to nursery pig diets to evaluate its effects on pig growth and health in two 35-d experiments, both randomized complete block designs based on initial BW and sex. Experiment 1 used 376 weaned pigs (17.8 d of age; 5.99±0.18 kg initial BW) allotted to one of four diets. 1) Negative Control (NC; no antibiotics with pharmacological Zn or Cu), 2) NC+DFM 0.275x109 CFU, 3) NC+DFM 0.55x109 CFU, 4) NC+DFM 1.1x109 CFU. Experiment 2 used 420 weaned pigs (20.1 d of age; 6.11±0.34 kg initial BW) allotted to one of four diets: 1) Negative Control (NC; no antibiotics with pharmacological Zn or Cu), 2) NC+DFM 0.55x109 CFU, 3) NC+DFM 1.1x109 CFU, 4) NC+BioPlus 2B 1.1x109 CFU. Experiment 1; during week 1 there were no treatment effects. During week 3 ADFI (P=0.053) quadratically increased while G:F (P=0.028) quadratically decreased as DFM increased in the diet. During d21-35 of experiment 2, ADG (P=0.092) quadratically increased and G:F (P=0.014) quadratically decreased as DFM increased.  Overall, for Experiment 1, ADFI numerically increased (4.3%) with no increase in ADG, resulting in a quadratic decrease in G:F (P=0.010) as the DFM increased in the diet.  For Experiment 2, during week 1, DFM fed pigs tended to have linearly decreased ADFI (P=0.092) and linearly increase G:F (P=0.072). Pigs fed the Bioplus2B bacillus had greater ADFI (P=0.018) than the same dose of the experimental bacillus. During week 2 pigs fed the experimental bacillus at 1.1x109 CFU tended to have improved G:F (P=0.084) than the similar DFM concentration from, the Bioplus 2B. During week 4 ADG tended to linearly (P=0.057) improve with increasing DFM in the diet and the average of all DFMs tended to have improved ADG (P=0.075) over the NC. During week 5 ADFI linearly increased (P=0.029) as DFM increased in the diet and all DFM treatments had greater ADFI (P=0.009) than the NC.  During d 21-35 of experiment 2, DFM fed pigs had linearly increased ADG (P=0.04) and ADFI (0.090). All pigs fed DFM had improved ADG (P=0.068) and ADFI (P=0.032) compared to NC fed pigs. Summarizing these two studies, the DFM product had its greatest effect improving gain feed intake and efficiency during late nursery when simplest diets are fed.</p> <p>In chapter 3, Dacitic tuff breccia (DTB) and poultry by-products (PBP) were added to nursery pig diets to evaluate their effects on growth performance and jejunal characteristics in a 35 d experiment. PBP were added as a replacement for fishmeal and was expected to provide an increased immune challenge to the newly weaned pigs. DTB was added as a functional trace element source and was hypothesized to ameliorate some of the negative impacts of PBP on the gut. A randomized complete block design based on BW and sex was used. Newly weaned pigs (N=564, 20.1±1.2 d of age, 6.18±1.13 kg initial BW). allotted to four diets (15 replicates/treatment; 9 or 10 pigs/pen): 1) Control (C; no DTB or PBP), 2) C+DTB (0.5% inclusion), 3) C+PBP (4% chicken by-product meal + 2% feather meal – replacing fishmeal, corn, and synthetic amino acids in the C), 4) C+DTB+PBP. Phase 1-3 were each 7-d and Phase 4 was from d 21-35. One barrow/pen was harvested 11d post-weaning. Jejunal tissue and mucosa were collected for histological measures and gene expression. In week 2, pigs fed DTB tended to increase ADG (323 vs. 303 g/d; P=0.068) and ADFI (376 vs. 356 g/d; P=0.055) compared to pigs not fed DTB and pigs fed PBP tended to have increased ADG (P=0.093). In week 3 pigs fed PBP had reduced ADG (460 vs 483 g/d; P=0.011) and G:F (0.689 vs. 0.723; P<0.001), and there tended to be an interaction for G:F (P=0.083) with DTB improving G:F when fed in combination with PBP but reducing G:F in the control diet.  In week 4, feeding PBP decreased ADG (384 vs 415 g/d; P<0.01) and ADFI (629 vs 666 g/d; P<0.01) compared to pigs fed no PBP.  For Phase 4 (d21-35) pigs fed PBP had decreased ADG (479 vs 497 g/d; P=0.041) and tended to have reduced ADFI (P=0.092) compared to pigs fed no PBP. Overall (d 0-35), there were no significant differences among treatments in pig growth performance.   Pigs fed PBP had increased jejunal expression of interferon-alpha (P=0.041) and interleukin-10 (P=0.037) and tended to have increased expression of claudin-1 (P=0.076).  Pigs fed DTB tended to have decreased jejunal expression of interferon-gamma (P=0.079).  Jejunal gene expression of tumor necrosis factor-alpha and nuclear factor kappa-beta did not differ among treatments. In conclusion, DTB improved nursery pig growth performance early and pigs fed PBP had decreased ADG and feed efficiency late in the nursery period. Feeding PBP had minimal effects on jejunum architecture but increased the gut immune response.</p> <p>Chapter 4 focuses on weaning age in addition to dietary treatments. Changes in weaning age and diet complexity can impact pig growth post-weaning. 432 weanling gilts and barrows (Topigs Duroc Ⅹ (US York X Landrace)) from the same farrowing group were utilized for a 30 or 35 d growth trial to compare early (EW) versus late weaning (LW) and high or low levels of specialty proteins in nursery diets. 216 pigs were EW (18 or 19 d, avg. age=18.4 days, avg. BW=5.96 kg) and 216 pigs were LW 5 days later (24, 25 or 26 d, avg. age=24.6 d, avg. BW=7.50 kg). At weaning pigs were blocked by weaning age, BW, sex and litter, and randomly allotted to 48 pens with 9 pigs/pen. Pigs at each weaning age were fed a High Complexity (HC) or a Low Complexity (LC) diet. The trial was conducted as a 2X2 factorial design: 1) EW+HC; 2) EW+LC; 3) LW+HC; 4) LW+LC. Pigs and feeders were weighed on d 0, 7, 14, 21, 28, and 35 for EW, and on d 0, 7, 14, 21, 28, and 30 for LW.  LW pigs had improved ADG and ADFI from d 0-7, 7-14, 14-21, and the final week of the nursery (P<0.05). G:F decreased for LW pigs from d 21-28 (P<0.01) compared to EW pigs, however for the overall nursery trial LW pigs had increased G:F (P=0.01). LW pigs had increased overall nursery ADG vs EW pigs (452.5 g/d vs 400 g/d; P<0.01). End of nursery BW was greater for LW vs EW pigs (21.70 kg vs 20.74 kg; P=0.001).  There was a weight block by weaning age interaction, EW heavy pigs had lower ADG compared to LW heavy pigs, however when comparing light weight pigs, the EW light pigs had improved ADG compared to LW light weight pigs (P=0.017). EW pigs were lighter at market (approximately d146 of age) compared to LW pigs (120.78 vs 124.16 kg; P=0.014). An interaction between weaning age, nursery diet, and sex (P<0.07) was observed for market weight, EW barrows fed LC diets were lighter at market (120.77 kg) than EW barrows fed HC diets (126.54 kg).  Whereas for LW barrows fed LC had higher market weights (130.04 kg) than LW barrows fed HC diets (127.59 kg). A similar, but non-significant pattern was observed for gilts. Weaning age and feeding pigs the correct diet for their age in the nursery can have a lifelong effect on the pig. </p> <p>In conclusion, feeding a DFM containing bacillus licheniformis did not impact nursery pig growth performance early in the nursery period. From d21-35 ADG and ADFI was improved in both experiments 1 and experiment 2. While the growth results were sporadically improved throughout the nursery period, the DFM’s greatest effect was it ability to improve ADFI. The improvement in the late nursery phase may be due to the need for the DFM to establish itself in the GIT. DTB tended to improve ADG and ADFI in week 2 of the nursery period, however there were no differences for growth overall. PBP reduced ADG in week 3 of the nursery period, as well as reduced ADG and ADFI in week 4 of the nursery. While there were differences in individual weeks of the trial there were no overall differences in growth performance. However, when looking at gene expression in the jejunum at d11 post-weaning, there was an increase in IFN-α and IL-10 in pigs fed PBP. In pigs fed DTB there was a decrease in IFN-γ jejunal gene expression as well. Feeding a high complexity and low complexity diet had no impact on the nursery pigs growth performance, in hindsight the low complexity diet was perhaps not low enough in specialty proteins. When comparing weaning age, pigs weaned later had increased ADG and ADFI from d0-21 post-weaning. For the overall nursery performance, LW pigs had a higher ADG, and ended the nursery period 0.96 kg heavier than the EW pigs. Comparing pigs of similar bodyweight at weaning, EW pigs were 1.5 kg heavier than LW pigs at the end of the nursery period, but the LW pigs were more feed efficient. There was a weight block by weaning interaction in this trial, as the heaviest LW pigs had higher ADG compared to the heaviest EW pigs. However, when looking at the lightest pigs, the EW pigs outperformed the LW pigs. Looking at market weights of each age group, LW pigs were 3.5 kg heavier on average at d146 of age. Although there was not a difference in diet found in the nursery phase, interestingly there were interactions discovered between treatments at market weight. Barrows are more sensitive to nursery diet compared to gilts, as LW barrows fed the low complexity diet performed better than the LW high complexity fed barrows. EW barrows were heavier at market when fed the high complexity diet, compared to EW barrows fed the low complexity diet. This shows that while the diet didn’t appear to have an impact in the nursery, feeding the pig the proper diet for their age has a lifelong impact on the pig.</p>