Feed is a major variable cost that impacts efficient and economical pork production. Recent expansion of the ethanol industry has created competition for corn used as livestock feed. This expansion has increased water and energy usage, creating a need for greater efficiency in production. As a result, fungal cultivation of thin stillage leftover from ethanol production represents an opportunity for reducing water and energy usage and adding value to the nutrient profile of ethanol by-products. The ensuing fungal biomass (FB) is more nutrient rich in amino acids and lipids than corn or dried distillers grains with solubles (DDGS), and is seen as a potential feedstuff for animal agriculture.

Therefore, the overall objective of this thesis was to evaluate a novel FB (Rhizopus microsporus var. oligosporus grown on thin stillage) in the diet of nursery pigs, and determine its lysine bioavailability, nutrient digestibility, and its effects on growth performance and tissue accretion. To accomplish these objectives, a series of experiments were conducted and are outlined in two research chapters (Chapter 2 and 3).

In Chapter 2, two experiments were conducted using crossbred nursery pigs. The first experiment assessed growth performance parameters and apparent total tract digestibility (ATTD) of the FB incorporated into diets at three concentrations. The second experiment further examined apparent ileal digestibility (AID), lysine bioavailability using slope-ratio assays, tissue accretion rates and intestinal health. The last chapter (chapter 3) was conducted to assess if probiotic (Pr) supplementation would increase nutrient digestibility of the FB and improve growth performance, and tissue accretion of nursery pigs.

Piglets for all experiments were randomly assigned and housed in individual metabolism pens with free access to water. Pigs were fed ad libitum for the first and third experiments, and restricted for the second to ensure appropriate lysine uptake. Pigs were fed a nursery diet consisting of corn, soybean meal (SBM), and whey protein and substituted with or without FB. All feed contained an exogenous digestibility marker, titanium dioxide, for ATTD and AID of the diets. Feces were collected from all three experiments to determine total tract nutrient digestibility and ileal digesta from the second and third for ileal nutrient digestibility. In the second and third experiments, serial slaughter methods were used that contained initial (ISG) and final (FSG) slaughter group animals for whole body composition and tissue accretion rates. Intestinal health was assessed ex vivo using pig ileum and cecum mounted into modified Ussing chambers.

Results from the first chapter (Experiment 1) indicated that 10 and 20% FB inclusion had no negative effects on growth performance parameters of ADG, ADFI and gain:feed. ATTD coefficients for energy, nitrogen, fat, and phosphorus were all reduced in the 20% FB diet compared to the control (P < 0.05). However, compared to the control treatment pigs, digestible energy (DE; 4.01 Mcal/kg) values were significantly improved when feeding 10% FB (4.16 Mcal/kg), but not with 20% FB (3.81 Mcal/kg).

The value of FB as a swine feed stuff is largely dependent on the bioavailability of its lysine to support growth. The lysine bioavailability was assessed in Experiment 2 using slope-ratio assay methods. These results showed that compared to Reference (Ref) diets containing increasing amounts of crystalline lysine that is freely available to support pig growth, when FB had to supply lysine as the first limiting amino acid, pigs had reduced growth performance (P < 0.05). Increasing lysine concentration in the diet did increase lean muscle deposition, but with 100% of the lysine coming from FB, lean and protein accretion rates were lower than the Ref diets (P < 0.05). Lipid accretion was also diminished in FB fed pigs compared to Ref counterparts (P < 0.01). Using these slope-ratio assays, we determined that lysine bioavailability for ADG, G:F, and lean gain to be 54, 61, and 70%, respectively. This is similar to that of DDGS (53-69% available lysine), but lower than corn (74% available lysine). Apparent ileal digestibility and ATTD data followed a similar trend to Experiment 1. No differences were observed between pigs at the 85% lysine requirement, however, there was a 10-20% drop in AID for FB fed pigs compared to the Ref at the 100% lysine requirement (P < 0.05). No differences were observed for lysine, glutamine, and glucose nutrient transport, and ex vivo measures of intestinal health.

Using the data from the previous two experiments, we hypothesized that reduction in lysine bioavailability and digestibility of nutrients in FB diets was due to the high amount of complex polysaccharides such as chitin and chitosan. Therefore, our objective in Chapter 3 (Experiment 3) was to test whether the addition of a Pr would increase the digestibility of FB and improve growth performance of nursery pigs. Results from this experiment showed no significant FB Ã? Pr interaction in any parameters assessed. However, Pr increased ADG (0.47 vs. 0.45 kg/d) and G:F (0.61 vs. 0.57), compared to the no Pr treatments (P < 0.05). Additionally, Pr supplementation increased AID of energy (59 vs. 53%, P < 0.10) and nitrogen (71 vs. 55%, P < 0.01). However, compared to non-FB diets, FB treatments decreased AID of nitrogen (57 vs. 69%, P < 0.01) and ATTD of energy (79 vs. 86%), nitrogen (74 vs. 84%) and DM (82 vs. 88%) (P < 0.01) respectively. Inclusion of Pr to either 0% or 20%FB diets had no effect on ATTD of nutrients and energy.

In conclusion, the FB feedstuff composition is high in fat and energy, protein and essential amino acids such as lysine, representing an attractive option for monogastric nutrition. Further, we have determined its lysine bioavailability to be 54% for ADG, which is similar or lower to reported values for DDGS but decreased compared to corn. We have demonstrated that FB can be used as suitable replacement for corn or SBM at low inclusion levels (< 10%) without influencing nursery pig performance. However, the reduced nutrient digestibility and lysine bioavailability at higher inclusion levels (> 20%) would reduce overall efficiency. The reduced nutrient availability also decreases overall growth and tissue accretion, although no changes to intestinal health were observed. Further investigation is needed to evaluate other methods for increasing digestibility, such as different bacterial species or exogenous enzyme supplementation. Additionally, FB may be suited for formulation in diets of species with greater native chitinase production e.g., poultry. Lastly, given the anti-obesity benefit of chito-oligosaccharides, adaptation of FB for human consumption is an avenue that warrants research.