Effect of selection for reduced residual feed intake on composition and quality of fresh pork
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Selection for improved growth efficiency has the potential to alter meat composition and fresh meat quality. In fact, selection for increased lean growth has resulted in meat that is of poorer quality. One efficiency trait that has gained more interest in pork is residual feed intake (RFI). RFI represents the fraction of total feed intake that is unexplained by maintenance and average requirements for growth and backfat. Selection for reduced RFI can significantly decrease the amount of feed needed for a given rate of growth and backfat. There have been few studies that have focused on the influence of reduced RFI on meat quality. The objective of this study was to determine the extent to which selection for reduced RFI affects pork composition and meat quality. In order to achieve this objective, traits were compared across lines and residual correlations were determined between measured RFI and pork composition and quality. The two lines evaluated in this study consisted of a line selected for reduced RFI over five generations (select) and a randomly selected control line (control). Selection for reduced RFI resulted in 0.052 kg lower RFI per day. Yorkshire gilts (select = 80, control = 89) were harvested and the boneless loins were collected at 24 hours postmortem. Back fat and loin eye depth were collected off the midline of the posterior region of the loin using the Fat-O-Meater. Quality attributes were measured at 2 and 3 d postmortem. Drip loss and water holding capacity were measured in duplicate. Hunter L, a, and b values were measured in triplicate on two chops per carcass. Quality scores of color, marbling, firmness and wetness were assigned by a 3 member panel. Intramuscular lipid and moisture content were determined. Desmin degradation was measured at 2 and 7 d postmortem. Purge loss, cook loss, and the sensory traits of juiciness, chewiness, tenderness, pork flavor, off flavor, and star probe texture were measured at 7-10 d postmortem on cooked chops. The model included the fixed effects of line, slaughter date, MC4R genotype, barn group, and line by slaughter date, and genotype by line interactions, off-test weight as a covariate, and sire, pen, and litter as random effects. Compared to the control line, carcasses from the select line tended to have less (P=0.09) backfat (15.2 y 0.9 vs. 17.3 y 0.7 mm), greater (P<0.05) loin depth and greater (P<0.05) calculated percentage of fat free lean (56.5% vs. 54.8%). Select line chops tended to have greater water holding capacity (P=0.07). Loin chops from the select line had less (P<0.01) intramuscular lipid content than control chops (1.14% vs. 1.67%). Select line chops also tended to have lower subjective marbling scores. Select chops had a greater (P<0.01) percentage of moisture than the control chops. There were no differences between lines for hot carcass weight, pH, drip loss, Hunter L and a values, subjective color, firmness, and wetness scores, or amount of intact desmin at 2 or 7 days postmortem. There were also no differences between lines for all sensory traits and star probe values. Residual correlations between individual RFI values and quality traits were investigated. Tenderness was positively correlated (r=0.24, P<0.01) with RFI and star probe was negatively correlated (r= -0.26, P<0.01). Residual correlation of intact desmin at 2 days postmortem was not significant, but the amount of intact desmin at 7 days postmortem was negatively correlated (r= -0.18, P=0.02) with RFI. Chewiness was negatively correlated (r= -0.15, P<0.05) with RFI. Percentage of intramuscular lipid tended to be positively correlated (r=0.15, P=0.06) with RFI. There have been several studies that have concluded that tenderness is significantly correlated to both marbling and degradation of proteins such as desmin. The results from this study suggest that selection for reduced RFI has the potential to improve carcass composition with few effects on selected measures of meat quality such as pH, water holding capacity, and color. However, reduced RFI could negatively affect eating quality due to the decrease in lipid content and desmin degradation.