The effect of divergent selection for residual feed intake on digestibility of control and low energy, high fiber diets

Abstract Text: Since 2001, Residual Feed Intake (RFI) lines of Yorkshire swine at Iowa State University have been selected for increased (low RFI) or decreased (high RFI) feed efficiency under a standard corn and soybean-meal based diet. Group-penned pigs from generation 8 of these lines were fed 2 diets varying in energy and fiber content. Diets consisted of a high energy, low fiber diet (Control), similar to the diet fed during genetic selection, and a low energy, high fiber diet (Fiber), which served as a dietary challenge. The Fiber diet contained 2.03 Mcal NE/kg and 25.9% NDF compared to 2.47 Mcal NE/kg and 9.4% NDF for the Control diet, resulting in an 18% NE and 175% fiber difference per kg diet. Both diets met NRC (1998) requirements and contained titanium dioxide (TiO₂) as a digestibility marker. Therefore, our objective was to evaluate potential interactions between genetic line (low RFI: n=59, high RFI: n=58) and diet composition (Control: n=47, Fiber: n=70). Fecal grab samples (n=117) were collected from barrows and gilts during the first growing phase (30-60 kg body weight) and were then analyzed for TiO₂, DM, GE, and N to calculate apparent total tract digestibility (ATTD, %) of DM, GE, and N. There was a significant interaction between RFI line and diet for ATTD of DM, GE, and N (P <0.05). When fed the Control diet, both RFI lines had a similar DM ATTD coefficients (mean 85.2%), but when the Fiber diet was fed, the low RFI line had a DM digestibility of 59.0% compared to 55.5% for the high RFI line. The average ATTD of GE was 84.4% for both lines when fed the Control diet, but was 58.0% and 54.7% for low versus high RFI pigs, respectively, when fed the Fiber diet. When fed the Control diet, ATTD of N was similar between the lines (mean 79.5%), but when fed the Fiber diet, dropped to 61.6% and 56.2% for the low and high RFI lines, respectively. In conclusion, both RFI lines had similar DM, GE, and N ATTD when fed the Control diet. However, when fed a more fibrous diet, the low RFI line had greater ATTD coefficients for DM, GE, and N compared to the high RFI line. Funding provided by AFRI-NIFA grant #2011-68004-30336.

Keywords: apparent total tract digestibility, residual feed intake, swine