This study was undertaken following the guidelines of the Research Policy of Dankook University. The protocol for this study (DK-2-2004) was approved by the Animal Care and Use Committees of Dankook University, Cheonan, South, Korea.

A total of 192 crossbred pigs ([Landrace×Yorkshire]×Duroc, 17 weeks old) with an initial BW of 70.6±3.9 kg were used in an 8-wk trial. Pigs were randomly allotted to one of six dietary treatments (each n = 32) based on their initial BW and sex (8 replications; 4 pigs per pen, 2 barrows and 2 gilts). The pigs in the 6 treatments were fed with diets having different Lys:Thr ratios: 1:0.65, 1:0.66, 1:0.67, 1:0.68, 1:0.69, and 1:0.70 by supplementing the diet with increasing levels of synthetic Thr. Threonine used in the present study was purchased from NL FARM, 318, RIT center, Gyeonggi Technopark, Ansan-si, Gyeonggi-do, Republic of Korea.

The diets were formulated to be isoenergetic and isonitrogenous and to meet or exceed the NRC [13] nutrient requirements (Table 1). All pigs were housed in an environmentally controlled room with a slatted plastic floor. Each pen was equipped with a 1-sided self-feeder and a nipple waterer to allow the pigs ad libitum access to feed and water throughout the experimental period.

Feed intake per pen was recorded daily, individual pig BW was measured on the initial day, at the end of week 4 (day 28) and week 8 (day 56) of the experiment, the feed consumption, and pig BW were used to monitor ADG, average daily feed intake (ADFI) and feed conversion ratio (FCR).

The apparent total tract digestibility (ATTD) of nutrients was measured at the end of the experimental period. Chromium oxide (Cr₂O₃, 0.25%), an indigestible marker was added to the diet 7 days prior to fecal collection to calculate the nutrient digestibility. Fecal samples were randomly collected from at least two pigs/pen (n = 16 per treatment) via the rectal massage and were pooled by pen. Based on treatment, all the feed and fecal samples were dried (70°C for 72 h) and finely ground to be able to pass through a 1 mm screen. Then fecal samples were analyzed for dry matter (DM) and N following the procedure outlined by AOAC International [14]. The gross energy was determined by measuring the heat of combustion by Parr 6400 oxygen bomb calorimeter (Parr Instrument Co., Moline, IL, USA). Chromium was analyzed via UV/VIS spectrophotometer (Optizen POP, Daejeon, Korea). For calculating the ATTD of the nutrients, we used the following formula: Digestibility = 1–([Nf×Cd]/[Nd×Cf]) ×100, where Nf = concentration of nutrient in feces (% DM), Nd = concentration of nutrient in the diet, Cd = concentration of chromium in the diet and Cf = concentration of chromium in the feces.

Feed samples were analyzed for crude protein (N×6.25; method 988.05), crude fat (method 954.02), crude fiber (method 962.09), crude ash (method 942.05), calcium (method 984.01), phosphorous (method 965.17), and AAs (method 982.30E) following the procedures established by AOAC International [14].

The back fat thickness and lean percentage of all live pigs (n = 32 per treatment) were measured 5 cm from the right-hand side of the midline from 3 different sites (shoulder, mid back, and loin at a position directly above the point of elbow, last rib, and last lumbar vertebra, respectively) before the commencement and at the end of week 4 and week 8 of the experiment using a real-time ultrasound instrument (Piglog 105; SFK Technology, Herlev, Denmark [15]). The mean value was taken and used for subsequent statistical analysis.

At the end of the experiment, animals were transported to a local abattoir and slaughtered. After slaughter, carcass weight and backfat thickness were measured from all pigs. Ten pigs per treatment (total 60 pigs) were randomly selected prior to the transport to the abattoir for meat quality assessment. After slaughter, carcasses from the selected pigs were placed in a conventional chiller at 4°C. Then carcasses were fabricated into primal cuts after a 24-h chilling period. Meat samples including lean, and fat were taken via perpendicular cut loins into 2-cm-thick chop beginning from the 10th and 11th ribs region and transported to the laboratory. The pH of muscle was measured 24 h after postmortem using a pH meter (Testo 205, Testo, Lenzkirch, Germany). Sensory evaluation was conducted by trained panelists to evaluate the color darkness, firmness and marbling of fresh loin samples using a five-point assessment scheme according to the procedures established by the National Pork Producers Council (NPPC) [16]. Immediately after the subjective tests were conducted, meat color of the longissimus muscle (LM) as lightness (L*), redness (a*), and yellowness (b*), was determined using a Minolta Chroma Meter (CR-210, Minolta, Tokyo, Japan) to evaluate the freshly cut surface after 30 min of blooming at 4°C. Water-holding capacity (WHC) was measured using methods of Kauffman et al [17]. Briefly, a 0.2-g sample was pressed at 3,000 psi for 3 min onto laboratory grade125-mm-diameter filter paper. The areas of pressed sample and expressed moisture were delineated and determined with a digitizing area-line sensor (MT-10S; M. T. Precision Co. Ltd, 123 Tokyo, Japan). A ratio of water area: meat area was calculated to measure WHC, with smaller ratio indicating higher WHC. The LM area was measured by tracing the LM surface at the 10th rib, which also used the mentioned above digitizing area-line sensor. Cook loss was determined as described previously by Sullivan et al [18]. Briefly, 5 g of meat sample were heat-treated in plastic bags separately in a water bath (100°C) for 5 min. Samples were cooled at room temperature. Cooking loss was calculated as (sample weight before cooking - sample weight after cooking)/sample weight before cooking×100. According to the plastic bag method described by Honikel [19], drip loss was measured using ~4 g of meat sample.

Data were analyzed as a completely randomized-block design using the general linear model procedures (SAS Institute Inc., Cary, NC, USA). The pen served as the experimental unit for all response criteria except meat quality wherein individual pigs were the experimental unit. Variability in data is expressed as standard errors of the mean. Linear and quadratic polynomial contrasts were performed to determine the effects of the different SID Lys:Thr ratios in the diet, with p<0.05 indicating significance and p<0.10 indicating trends.

Increasing the SID Lys:Thr ratio of finishing diets did not exert any significant effects in ADG, ADFI, and FCR during day 1 to 28 and in the overall experiment period although numerical increase in ADG and a slight reduction in FCR was observed during these periods. However, increasing the SID Lys:Thr ratio of finishing diets led to a numerical increase in BW during the end of the experiment. In addition, a significant linear increment in ADG (p<0.05) and trends in reduction in FCR but no effect on ADFI was observed during day 29 to 56 (Table 2).

The ATTD of DM tended to increase linearly (p = 0.094) at the end of the experiment (day 56) with the increase in the SID Lys:Thr ratio of finishing diets. The ATTD of N and energy remained unaffected with the increase in SID Lys:Thr ratio although there were numerical increments in digestibility values (Table 3).

The backfat thickness and lean percentage increased significantly (linear effect, p<0.05) with the increase in the SID Lys:Thr ratio of finishing diets on day 28. In addition, at day 56, a trend in a linear increase in backfat thickness (p = 0.073) and a significant linear (p<0.05) increment in lean percentage was observed as the ratio of SID Lys:Thr increased in the diet of finishing pigs (Table 4).

There were no significant effects on carcass weight and backfat thickness of pigs at slaughter by increasing the SID Lys:Thr ratio in finishing diet. Significant quadratic responses (p = 0.02) for pH and drip loss at day 7 (p = 0.02), a significant linear increase (p<0.05) in cooking loss and drip loss at day 7, and a trend in quadratic response (p = 0.07) in the lightness of meat color (L*) were observed, whereas other meat quality indices were unaffected by varying SID Lys:Thr ratio in finishing diet (Table 5).