Associations of acute phase protein levels with growth performance and with selection for growth performance in Large White pigs

Acute phase proteins (APP) are released into the circulation in mammals upon infection and may be used to diagnose the health status of managed populations of animals such as pigs. The current study determines APP levels in a population of apparently healthy Large White pigs from a single farm, to address two questions: (1) whether phenotypic associations can be observed between productivity and APP, indicating the effects of possible subclinical infections and (2) whether previous selection for either food intake or 'lean growth under restricted feeding' influences APP levels. The APP investigated were alpha1- acid glycoprotein (AGP) and haptoglobin. The APP were measured at 18 and 24 weeks of age in pigs previously selected for high lean growth (no. = 31), low lean growth (no. = 38), high daily food intake (no. = 24) and low daily food intake (no. = 26), but performing under ad libitum feeding conditions. Performance traits and APP levels were constant over the experimental period, indicating that the farm health status did not vary over time. Performance traits and APP were recorded on 119 pigs, of which 80 had both APP and performance measurements. Multiple regression analyses were used to investigate phenotypic relationships between performance traits and APP levels. Plasma concentrations of AGP were higher in 18-week-old pigs compared with 24-week-old pigs (P<0.01) whereas haptoglobin levels did not vary according to age. Significant sex differences in APP levels were observed. Females had higher circulating levels of AGP than males at both 18 weeks and 24 weeks. Females also had higher levels of haptoglobin at 18 weeks. Levels of AGP had significant negative correlations with daily weight gain (-0.59, P<0.01 and -0.48, P<0.05 at 18 and 24 weeks respectively) and with daily food intake (-0.53, P<0.01 and -0.38, P<0.05 at 18 and 24 weeks respectively). At age 24 weeks, haptoglobin was negatively correlated with both daily weight gain (-0.35, P<0.05) and food efficiency (-0.34, P<0.05). Pigs selected for high lean growth under restricted feeding had higher AGP levels than pigs selected for low lean growth under restricted feeding at 18 (593 v. 332 microgram/ml, P<0.01) and 24 weeks of age (313 v. 219 microgram/ml, P<0.05). Selection for daily food intake did not consistently affect AGP levels, and neither selection criteria influenced plasma haptoglobin concentrations. To conclude, we have demonstrated that amongst contemporaneous pigs of the same genotype, higher systemic AGP levels and, to a lesser extent, higher haptoglobin levels are associated with decreased performance, and that genetic selection for 'efficient lean growth under restricted feeding' can increase serum AGP levels.