|Abstract:||Two subsequent studies were conducted to characterize marbling variability in pigs. A total of 196 pigs (at 10 weeks of age) were raised in a 98 d growth study. Pigs were fed in a university finisher barn in pens of 4 pigs each. Equal numbers of barrows and gilts sired by boars targeting meat quality or lean growth were used. All pigs were fed the same corn-soybean diet formulated to meet or exceed nutrient requirements based on the 2012 National Research Council requirements of swine, for a 3-phase feeding program. All pigs were ultrasonically scanned at wk 10, 15, 20, and 24 of age to estimate marbling, back fat, and loin depth. Two or four contemporary pigs were scanned at each of these time points and then slaughtered on the following day to validate the ultrasound procedure. All trial pigs were slaughtered at 24 weeks of age, and loins were collected. Bone-in loins were sliced into 2.54 cm thick chops. Chops from the 6th rib, 10th rib, last rib and 4th lumbar vertebrae were analyzed for visual color and visual marbling, subjective firmness, extractable lipid (IMF) and moisture content, and Warner-Bratzler shear force (WBSF). Determination of marbling from ultrasound images in this trial was not an accurate indicator of marbling deposition throughout the growing phase. Ultrasonic values overestimated marbling in young pigs and reported a lesser range of values than was actually observed in market weight pigs. This could be due to technician error or use of the technology outside of the recommended scope of use. Data from the serial slaughter of young pigs did not validate ultrasonic marbling estimates, but it did provide valuable information about marbling deposition. The pattern of marbling with maximum values and minimum values observed along the length of the loin at early ages mimicked the pattern observed in market weight pigs. Even though intramuscular fat is often thought of as a tissue that develops later in life, the differences in marbling deposition along a loin were present beginning at the conclusion of the nursery phase. The applied meat quality portion of the study characterized variability of marbling throughout the loin. Intramuscular fat was the greatest at the anterior and posterior chop locations, intermediate at the 10th rib, and the least at the last rib. Visual marbling score was the least at the anterior location and the greatest at the posterior location. Chops from the 6th rib location were the most tender and chops from the last rib area were the least tender. There were also differences in color score, firmness, moisture, and cook loss across the different locations. Therefore, marbling and tenderness varied among locations in a pork loin. Sex, sire line and anatomical location accounted for 47% of the variability in IMF. However, these factors only explained 14% of the total variability in tenderness. From these results the following conclusions can be made. Intramuscular fat is present and variable in pigs from a very young age, however quantification of marbling deposition throughout the growing period is difficult to estimate. Ultrasound imaging of a young pig is not ideal to identify pigs with a potential to be highly marbled. Mean differences for marbling and WBSF existed throughout the loin of market weight pigs, however marbling exhibited differences in variability within each of the annotated groups but WBSF did not. Therefore, marbling is variable, but how this might contribute to variability in eating experience warrants more investigation. As overall quality of a loin improves its variability also increases. As a producer improves meat quality, they will also increase the variability of that quality. This is not necessarily a bad thing if there is incentive for improved meat quality, but is important to be aware of when selecting for increased marbling.