Degree Type


Date of Award


Degree Name

Master of Science


Animal Science

First Advisor

Nicholas K. Gabler


Residual feed intake (RFI) is a unique measure of feed efficiency (FE) and an alternative to traditional measures of gain:feed or feed conversion ratio. Residual feed intake is defined as the difference between the actual feed intake of a pig and its expected feed intake based on a given amount of growth and backfat. Therefore, selecting pigs for a low RFI (LRFI), results in a more feed efficient animal for a given rate of growth. Using lines of Yorkshire pigs divergently selected for RFI provides a unique research model to study the genetic and physiological factors defining FE differences in pigs and other livestock. Therefore, the objective of this dissertation was to partially explain the physiological differences defining FE gains seen in pigs divergently selected for RFI. More specifically, our objectives were to determine the extent to which apparent total tract digestibility (ATTD) of nutrients and energy, and their utilization and retention explain FE differences (Chapter 2). Additionally, we aimed to determine the extent to which whole body composition and tissue accretion rates explain differences in efficiency between pigs divergently selected for low or high RFI (Chapter 3). In both research Chapters, LRFI and high RFI (HRFI) gilts were selected from the 7th generation of the Iowa State University RFI selection project. All gilts were matched by age and live weight for both the Chapter 2 and Chapter 3 objectives (62±3 kg BW and 60±7 kg, respectively). The gilts used in the digestibility study were randomly assigned to metabolism crates, while the gilts in the body composition study were group-housed in pens equipped with FIRE feeders. All gilts had free access to water and a standard diet based on corn and soybean meal, with the feed in the digestibility study containing an exogenous digestibility marker. In the Chapter 2 digestibility study, total urine and feces were collected for 72 h and nutrient and energy digestibility and P and N balance were then measured and calculated to determine differences between the RFI lines. In Chapter 3, whole body compositions of both an initial (ISG) and a final (FSG) slaughter group was analyzed. Backfat ultrasound scans and initial body weights from the ISG and FSG, together with the ISG fat, protein, and ash whole body compositions were used in a regression analysis to estimate the initial body composition of the FSG gilts.

Results from Chapter 2 showed that the digestibility coefficients for DM (87.3 vs. 85.9%), N (88.3 vs. 86.1%), and GE (86.9 vs. 85.4%) were higher (P ≤ 0.003) in the LRFI versus HRFI gilts, respectively. The DE (16.59 vs. 16.32 MJ/kg DM) and ME (15.98 vs. 15.72 MJ/kg DM) values were also significantly greater (P = 0.0006) in the LRFI gilts. When adjusting for ADFI, P digestibility did not differ between the lines. However, the LRFI gilts tended to have improved N retention compared to HRFI gilts (P = 0.08; 36.9 vs. 32.1 g/d). Chapter 3 body composition indicated that the LRFI gilts in the FSG tended to have decreased total visceral weight (6.22 vs. 6.49 kg, P = 0.09) compared to the HRFI gilts. Both ISG and FSG LRFI gilts had decreased whole body fat percentage (P ≤ 0.02) and GE of the body (cal/g, P ≤ 0.0006) compared to the HRFI gilts. The FSG LRFI gilts also had an increase in whole body protein (%, P = 0.07) compared to their HRFI counterparts. LRFI gilts tended to have an increase in protein (P = 0.09) and water (P = 0.06) accretion, g/d, with a significant increase in ash accretion (g/d, P = 0.04) compared to their HRFI counterparts. Interestingly, we reported no differences in fat accretion between lines. In conclusion, the higher energy and nutrient digestibility, utilization, and retention may partially explain the superior FE, while the data indicating differences in body composition and tissue accretion rates may partially explain the genetic variation seen in pigs selected for LRFI.

Copyright Owner

Amanda Jean Harris



File Format


File Size

111 pages