Effect of dietary leucine levels on carcass composition, meat quality, and growth performance in finishing pigs 1 Young Hyun, Mike Ellis, Glenn Bressner, and Dave Baker Department of Animal Sciences Introduction Improvements in carcass leanness to meet consumer demands for lean meat has also resulted in a reduction in intramuscular fat which potentially has a negative effect on the palatability of pork. A number of approaches have been investigated to increase intramuscular fat by manipulating nutrients in diets including feeding protein deficient diets (Castell et al., 1994; Blanchard et al., 1999) and high dietary leucine levels (Cisneros et al., 1996). Leucine is a ketogenic amino acid, the carbon skeleton of which is converted to acetyl-coa and acetoacetate in muscle tissue and those intermediates can be used to synthesize fatty acids. In the study of Cisneros et al. (1996), high dietary leucine also resulted in an improvement in muscle color. However, there is evidence that feeding high levels of dietary leucine may have a negative impact on growth performance (Taylor et al., 1984). The objective of this study was to determine the effect of leucine supplementation level on growth performance, carcass characteristics, and pork quality in finishing pigs. Materials and methods Experimental Design The study was carried out with finishing pigs from 78.4 ± 3.46 to 114.8 ± 7.30 kg BW over a 39-day period. A completely randomized design was used with a 2 x 2 factorial arrangement of treatments resulting in 4 treatment combinations [two leucine levels (1.22 and 3.22 % total leucine) and 2 sexes (barrows and gilts)]. A total of 40 individually-fed pigs were used with 10 pigs (five barrows and five gilts) per leucine level by sex treatment subclass. Animals Forty crossbred barrows and gilts (Duroc Yorkshire) were selected at around 70 kg live weight. Pigs were formed into outcome groups of four on the basis of litter of origin and live weight and were randomly allocated from outcome groups to one of 4 pens and pens were randomly allocated to leucine level sex treatment group. Pigs were allotted to treatment one-week prior to the start of the study and allowed a seven-day acclimation period. Housing The study was conducted in the individual pig feeding facility located on the Swine Research Center, University of Illinois. The pens had fully-slatted floors and provided a space allowance of 1.6 m 2 /pig. The temperature within the building was controlled using a thermostat and a combination of fan ventilation and a furnace. Temperature was monitored continuously in the building during the experimental period using Hobo H8 Loggers (Onset Computer Corp., Bourne, MA). The average temperature within the building was 21.1 ± 3.01 o C over the 39-day study period. 1
Diet formulation Diets were based on corn and soybean meal which were analyzed for amino acid content prior to the start of the study. Diets were formulated to meet or exceed the nutrient requirements recommended by NRC (1998) using the analyzed amino acid contents. Duplicated diet samples were taken from each batch of feed and analyzed to determine proximate composition using the methods given by AOAC (1999). The composition of the experimental diets is summarized in Table 1. Feeding and management Pigs were given ad libitum access to feed (via a single-space feeder) and water (from one nipple waterer in each pen). Any health problems and treatments were recorded. Pigs were weighed at the beginning and the end of the experimental period to determine average daily gain. Feed additions were recorded during the experimental period and feeders were weighed at the same time as the pigs. Average daily feed intakes were calculated from feed disappearance. Feed efficiency was determined from live weight and feed intake data. Carcass and meat quality measurements Animals were transported to the Rochelle Foods Inc. (101 South Main, Rochelle, IL) on the day of the last weighing and slaughtered the following morning. Slaughtering and carcass dressing were carried using standard procedures. The head, kidneys and flare fat were removed and the carcass was split down the midline. A hot carcass weight was taken approximately 1-h post mortem and carcasses were held overnight in a chiller (4 o C). At 24-h postmortem, carcass measurements were taken on the left side of the carcass with a metal ruler. Measurements included: carcass length (from the cranial tip of the symphysis pubis to the cranial edge of the first rib adjacent to the thoracic vertebra), mid-line fat thickness at the first and last ribs, and last lumbar vertebra, 10 th rib backfat thickness (3/4 the distance across the longissimus muscle), and longissimus area at the 10th rib. Boneless loin samples were taken from the right side of the carcasses and transported to the Meat Science Laboratory at University of Illinois. Measurements taken on the longissimus muscle included subjective color, firmness, and marbling, evaluated on the cut surface of the muscle at the 10th rib using the procedures of NPPC (1998). Color score was evaluated using six-point scales (1 = pale to 6 = very dark in color), and firmness and marbling were evaluated using five-point scales (1= soft and devoid of marbling to 5 = firm and moderately abundant or greater marbling, respectively). Hunter color (L*, a*, and b*) was also measured on the cut surface of the loin section at the 10 th rib using a Hunter LabScan Spectrocolormeter (Model XE, Hunter Associates Laboratory, Inc., Reston, VA) set at the D65 light setting with a 10 0 angle of reflection. Two chops were cut from the longissimus immediately posterior to the 10 th rib and trimmed of epimysium and external fat. One chop was weighed, placed in a Whirl-pak bag, suspended in a 4 0 C cooler for 48 h, then reweighed and drip loss determined. Ultimate ph was measured on a sample of longissimus homogenized in distilled water using an Orion model 720A ph meter fitted with a Ross sure flow 81-72 electrode (Orion Research, Boston, MA). Proximate analysis: Fat and water contents were determined on the longissimus chop using the procedures described by Novakofski et al. (1989). Samples were oven dried to a constant weight (110 0 C for 48 h), weighed to determine water content, and fat extracted using a mixture of warm chloroform and methanol (4:1). Statistical analysis The animal was considered as the experimental unit. Data from the experiment were analyzed using 2
PROC GLM procedures of SAS (SAS Inst. Inc., Cary, NC) with the model including the effects of leucine level, gender, and interactions between leucine level and gender. Results and Discussion Growth performance Pigs fed high leucine diets were lighter (3.9 %, P < 0.05) at the end of study and grew more slowly (8.7 %, P < 0.05) than pigs fed the diet with the normal leucine level. Other studies have also shown a depression of growth in pigs fed high leucine diets (Taylor et al. 1984). However, Edmonds and Baker (1987) did not find any deleterious effects on growth performance of diets containing 4 % supplemental leucine; however, growth and feed intake were depressed when pigs of 8 kg BW were fed a corn-soybean based diets with 6 % supplemental leucine. Barrows had higher feed intake, faster growth rates, and were heavier at the end of the 39 day test period than gilts (Table 2), results that are similar to most other studies that have compared those sexes (Labroue et al., 1994; Hahn and Baker, 1995). Carcass and meat quality characteristics There was no effect of leucine level and gender on hot carcass weight, dressing percentage, and carcass fat and muscle measurements (Table 3). Pigs fed high leucine diets had a higher marbling score and more intramuscular fat in longissimus muscle than pigs fed control diet (P < 0.05). However, dietary leucine level had no effect (P > 0.05) on longissimus color or drip loss (Table 3). As expected, barrows had greater backfat measurements and reduced longissimus area and depth than gilts (Table 3). In addition, barrows had firmer muscle, more marbling, and greater intramuscular fat than gilts (Table 3). There was only one leucine level by sex interaction for firmness of the longissimus muscle. For pigs fed the high leucine diet, barrows had firmer longissimus muscle than gilts (P < 0.05), however, there was no difference in muscle firmness between the two sexes for the control diet (Table 3). Conclusions The results of the present study suggest that levels of intramuscular fat and marbling of pork can be increased by feeding high levels of leucine. However, high dietary leucine levels can deleteriously affect growth performance. Therefore, further studies are required to determine an optimum level of leucine to improve marbling fat content without negatively affecting growth performance. This study did not validate earlier research of a positive effect of high dietary leucine levels on muscle color (Cisneros et al., 1996). References Blanchard, P. J., M. Ellis, C. C. Warkup, B. Hardy, J. P. Chadwick, and G. A. Deans. 1999. The influence of rate of lean and subcutaneous fat tissue development on pork eating quality. Anim. Sci. 68: 477-485. Castell, A. G., R. L. Cliplef, L. M. Paste-Flynn, and G. Butler. 1994. Performance, carcass, and pork characteristics of castrates and gilts self-fed diets differing in protein content and lysine:energy ratio. Can. J. Anim. Sci. 74: 519-528. 3
Cisneros, F., M. Ellis, D. H. Baker, R. A. Easter, and F. K. McKeith. 1996. The influence of shortterm feeding of amino acid-deficient diets and high dietary leucine levels on the intramuscular fat content of pig muscle. Anim. Sci. 63:517-522. Edmonds, M. S., and D. H. Baker. 1987. Amino acid excesses for young pigs: effects of excess methionine, tryptophan, threonine, or leucine. J. Anim. Sci. 64:1664-1671. Hahn, J., and D. H. Baker. 1995. Optimum ratio to lysine of threonine, tryptophan, and sulfur amino acid for finishing swine. J. Anim. Sci. 73:482-489. Labroue, F., R. Gueblez, P. Sellier, and M. C. Meunier-Salaun. 1994. Feeding behavior of grouphoused Large White and Landrace pigs in French Central Test Stations. Livest. Prod. Sci. 40:303-312. NPPC, 1998. Pork composition and quality assessment procedures. National Pork Producers Council, Des Moines, IA. Novakofski, J., S. Park, P. J. Bechtel, and F. K. McKeith. 1989. Composition of cooked pork chops: effect of removing subcutaneous fat before cooking. J. Food Sci. 54:15-17. Taylor, S. J., D. J. A. Cole, and D. Lewis. 1984. Amino acid requirement of growing pigs. 5. The interaction between isoleucine and leucine. Anim. Prod. 38:257-261. 4
Table 1. Diet formulation and calculated chemical composition Treatment Control diet High leucine diet Ingredient composition, % Corn 79.5 79.5 Corn Starch 2.10 - Soybean meal (dehulled) 15.5 15.5 Dicalcium phosphate 0.55 0.55 Limestone 0.70 0.70 Trace-mineral salt 1 0.35 0.35 Illini vitamin premix 2 0.15 0.15 L-lysine HCL 0.15 0.15 L-Leucine - 2.10 Total 100.0 100.0 Chemical composition, % ME (Kcal/kg) 3,420 3,400 Crude protein 13.2 15.2 Calcium 0.45 0.45 Phosphorus 0.40 0.40 Phosphorus (avail.) 0.15 0.15 Lysine 0.79 0.79 Threonine 0.49 0.49 Met + Cys 0.52 0.52 Isoleucine 0.52 0.52 Leucine 1.22 3.22 Tryptophan 0.16 0.16 1 The trace mineral mix contained Se, 85.7 mg; I, 100 mg; Cu, 2.3g; Mn, 5.7 g; Fe, 25.7 g; Zn, 28.6 g;and NaCl, 855 g per kilogram mixture 2 The vitamin premix contained Vit. A, 3,000,000 IU; Vit. D3, 330,000 IU; Vit. E, 44,000 IU; Vit. K, 2.2 g; Vit. B12, 17.9 mg; Riboflavin, 4.4 mg,; d-pantothenic acid, 12.1 g; Niacin, 16.5 g; and roughage product to 1 kg. 5
Table 2. Effect of dietary leucine level and gender on growth performance Leucine level Gender Probability (Pr > F) Items Normal High SEM Barrow Gilts SEM Leucin Gender Interaction e Initial BW, kg 78.9 79.1 0.79 79.7 78.2 0.79 0.843 0.184 0.643 Final BW, kg 115.4 a 111.1 b 1.39 115.9 a 110.6 b 1.39 0.037 0.012 0.885 ADG, g 930 a 829 b 32.2 928 a 830 b 32.2 0.033 0.041 0.881 ADFI, kg 2.89 2.75 0.072 3.05 a 2.58 b 0.072 0.179 0.0001 0.274 Gain:Feed 0.32 0.30 0.009 0.30 0.32 0.009 0.089 0.140 0.445 ab Within a row and treatment comparison, means without a common superscript letter differ (P < 0.05). 6
Table 3. Effect of dietary leucine and gender on carcass measurements and meat quality characteristics measured on the longissimus. Leucine level Gender Probability (Pr > F) Items Normal High SEM Barrow Gilts SEM Leucin Gender Interaction e Carcass characteristics: Hot carcass wt, kg 83.9 81.1 1.55 82.6 82.4 1.55 0.175 0.922 0.785 Dressing percentage 73.1 73.0 0.08 72.9 73.0 0.08 0.157 0.308 0.876 Carcass length, cm 83.1 82.0 0.65 82.1 83.0 0.65 0.173 0.310 0.525 Midline back fat thickness, mm Last rib 18.5 19.4 1.13 16.4 b 1.15 0.558 0.006 0.146 Last lumbar 12.4 13.6 0.48 13.8 a 12.1 b 0.48 0.062 0.005 0.831 10 th rib 19.2 18.3 1.16 20.9 a 16.7 b 1.13 0.518 0.004 0.368 Longissimus area, cm 2 60.3 57.0 1.50 54.7 a 62.6 b 1.49 0.097 0.0002 0.341 Longissimus depth, cm 6.6 6.5 0.16 6.2 a 6.9 b 0.16 0.950 0.001 0.198 Subjective scores for: Color 3.2 3.5 0.17 3.4 3.3 0.17 0.298 0.565 0.093 Firmness 3.5 a 3.1 b 0.14 0.920 0.015 0.020 Barrows 3.3 b 3.8 c 0.19 0.020 Gilts 3.3 b 2.9 a 0.19 0.020 Marbling 3.2 b 3.9 b 0.24 4.0 a 3.1 b 0.24 0.031 0.008 0.151 Ultimate ph 5.7 5.7 0.06 5.7 5.7 0.05 0.854 0.518 0.305 Hunter L* 46.3 46.6 0.97 47.1 45.8 0.97 0.823 0.262 0.463 Hunter a* 6.3 6.8 0.31 6.6 6.5 0.31 0.170 0.908 0.536 Hunter b* 2.7 3.2 0.27 3.1 2.7 0.27 0.145 0.215 0.313 Drip loss, % 2.6 2.5 0.50 2.0 3.1 0.50 0.988 0.120 0.115 Chemical composition of longissimus muscle, % Moisture 73.7 73.4 0.24 73.1 a 74.0 b 0.24 0.200 0.010 0.562 Fat 2.4 a 3.4 b 0.33 3.7 b 2.2 a 0.33 0.020 0.001 0.670 ab Within a row and treatment comparison, means without a common superscript letter differ (P < 0.05). 7