By Eric van Heugten, Jake Erceg and Mark Knauer, North Carolina State University Department of Animal Science
The use of fibrous ingredients in swine diets has been extensively researched and evaluated by the pork industry because they provide economic alternatives to traditional feedstuffs and may have beneficial effects on intestinal health. As a strategy to enhance the general utilization of fiber, we have initiated genetic selection of pig lines fed high-fiber diets (and concurrent selection of pigs fed standard corn-soybean meal based diets) based on lean growth at the North Carolina Department of Agriculture Tidewater Research Station. Following three generations of selection, we conducted a project to determine growth performance and carcass characteristics of pigs selected on high- or low-fiber diets, when fed either a high- or low-fiber diet.
Pigs were selected using a divergent line selection process. Generation zero consisted of 90 original litters of unselected pigs. All littermates were randomly assigned to a dietary treatment consisting of high-fiber or low-fiber diets, with 50% of pigs from each litter being represented in each dietary treatment. Pigs were fed diets containing 45% fibrous ingredients (high fiber) or control diets consisting of conventional corn and soybean meal based diets (low fiber). From each dietary treatment, 60 gilts and 10 to 15 boars were kept to begin each specific genetic line representing the High-Fiber or Low-Fiber lines. Within each line, pigs were selected based on calculated lean growth and the top 60 gilts and 10 to 15 boars were kept from each line. Lean gain was calculated from initial and final body weights (approximately 40 and 114 kilograms, respectively) and real-time ultrasound measurements using equations from Burson (Fact Sheet, Pork Information Gateway, 12-04-06) and Brannaman et al. (Journal of Animal Science, Vol. 59, No. 4, 1984). Boars were selected within family group. No more than two boars per family were selected and outcrossed with gilts from unrelated family groups to decrease the inbreeding coefficient of each genetic line. In Generation 1, pigs from each line were fed experimental diets representative of their selection criteria (HF or LF). From each line the top 60 gilts and 10 boars were kept, based on highest lean gain per day, to continue selection. This process was repeated for Generation 2 and Generation 3. Pigs used in the current study were in the third generation of selection.
One hundred and seventy-five barrows (initial body weight was 45.8 ± 6.5 kilograms), consisting of 102 HF selected and 73 LF selected genetic line pigs were assigned to four treatments arranged in a 2 x 2 factorial randomized complete block design with genetic line (HF or LF) and dietary treatment (low fiber or high fiber) as factors. Pigs were housed in group pens (7.44 m2) with four to five pigs per pen using a total of 36 pens, resulting in nine replicates per treatment. Pigs were blocked by weight into either a light weight or heavy weight block, to achieve more uniform marketing weights at the end of the study. Pigs were taken off test by weight block to achieve a similar final market weight of 118 kilograms. Heavy weight block pigs were marketed on Day 64 of the study, while light weight pigs were marketed on Day 78. Pigs were provided ad libitum access to feed and water throughout the experimental period.
Diets were manufactured in meal form at the North Carolina State University Feed Mill Educational Unit. Treatment diets were fed in three separate phases, with Phase 1 fed from Day 0 to 25, Phase 2 from Day 26 to 50, and Phase 3 from Day 51 until marketing. High-fiber diets were formulated to contain 45% fibrous byproducts, consisting of 15% distiller’s dried grains with solubles, 15% soybean hulls and 15% wheat middlings and contained 0.82, 0.71 and 0.62% standardized ileal digestible lysine for Phase 1 to 3, respectively. Control diets were formulated as standard corn and soybean meal based diets and contained 0.93, 0.80 and 0.70% SID lysine for Phase 1 to 3, respectively. Within each phase, diets were formulated to an equal lysine:NE ratio (3.74, 3.17 and 2.76 grams per megacalorie lysine:Net Energy for Phase 1 to 3, respectively). Bulk density (grams per liter) of diets was substantially higher for high fiber diets than control diets (607 versus 453; 609 versus 457; and 611 versus 461 for Phase 1 to 3, respectively).
Pens of pigs were weighed and feed intake was determined on Day 0, 25, 50 and at marketing. At the end of the study, pigs were weighed individually and loin eye area and back fat thickness were determined using real-time ultrasound. Post marketing, hot carcass weights were recorded at a commercial abattoir and carcass yield was calculated as hot carcass weight relative to live pig weight.
Data were statistically analyzed using the MIXED procedure of SAS (SAS Institute Inc., Cary, N.C.). The model included diet (low- or high-fiber), genetic line (HF or LF) and diet x genetic line interactions. Marketing block was used as a random variable and Day 0 body weight was included as a covariate for all performance data, except for carcass data.
There were no interactions between diet and genetic line for any of the growth performance or carcass characteristics measurements. Feeding high-fiber diets reduced average daily gain for all phases and overall (Table 1). Feed intake tended to be reduced during Phase 1, was not different for Phase 2, and was increased for Phase 3 for pigs fed high-fiber diets. This indicates that pigs were able to adjust to high-fiber diets over time and could compensate for the lower energy density and bulk density of the high-fiber diet as body weight increased. Feed efficiency was not impacted during Phase 1, but was poorer for Phase 2, 3, and overall for pigs fed high-fiber diets. Genetic selection of pigs had limited impact on growth performance of pigs with the only impact being tendencies for reduced feed efficiency during Phase 1 and increased feed intake during Phase 2 for pigs selected on high-fiber diets.
Pigs fed low-fiber diets had greater loin eye area, back fat depth, hot carcass weight, yield and lean gain per day than pigs fed high-fiber diets (Table 2). There were no differences between HF and LF selected pigs for carcass characteristics, however, HF selected pigs had decreased yield compared to LF selected pigs.
The present study demonstrates that the inclusion of 45% fibrous byproduct ingredients in diets fed to finishing pigs decreased ADG, gain to feed, hot carcass weight, loin eye area, back fat thickness, yield and daily lean gain. Genetic selection of pigs while fed high-fiber diets did not improve performance or carcass characteristics, regardless of whether pigs were fed low- or high-fiber diets. In fact, HF selected pigs had decreased yield. Lack of positive responses may be attributed to the indirect method of selection for lean gain, rather than direct selection for improved energy and nutrient utilization. Selection of pigs for lean gain when fed high-fiber diets should be carefully considered because it reduced yield, which was clearly evident in only three generations of selection.