### INTRODUCTION

### MATERIAL AND METHODS

### Animals and traits

*ad libitum*access to water and feed using a standard scheme as practiced on most farms in Korea. As no single weaning scheme was adopted, there was some variation in the length of suckling periods. Weaning ages ranged from 17 to 34 d. Individual body weights were measured at birth (B), at weaning (W: mean, 22.9 d; SD, 3.4), at the beginning of a performance test (P: mean, 72.7 d; SD, 4.8), and at the end of the performance test (T: mean, 152.4 d; SD, 7.8). The traits analyzed were average daily gain in body weight (ADG) and back fat thickness (BFAT). The latter was measured at 5-cm from the edge of the split back at the level of the third to fourth last rib using a Piglog 105 device (Carometec A/S, Copenhagen, Denmark) and adjusted for 90 kg of body weight. ADGs were measured for the following periods: from birth to weaning (DGBW), from weaning to the beginning of the performance test (DGWP), from the beginning to the end of the performance test (DGPT), from birth to the beginning of the performance test (DGBP), from birth to the end of the performance test (DGBT), and from weaning to the end of the performance test (DGWT). The distribution and basic statistics of the data are presented in Figure 1 and Table 1.

### Statistical analysis

_{i}is the vector of observations for the

*i*th trait, and b

_{i}is the vector of fixed effects, including the contemporary group, birth year-month, sex, sow parity, and a covariate, individual birth weight. Fixed weaning age effects were included in b

_{i}of the first equation above. For random effects, a

_{i}is the vector of random animal effects, m

_{i}is the vector of random maternal genetic effects, c

_{i}in the second equation is the vector of random weaning age effects, and e

_{i}is the vector of random residuals; X, Z, W, and S are incidence matrices relating records of the traits to fixed and random effects.

*ā*and

*m̄*represent the vectors of direct animal genetic effects and maternal genetic effects for the traits, respectively, and

*c̄*and

*ē*represent the vectors of random weaning age and residual effects, respectively, then the random effects are postulated to follow a normal distribution with zero mean and the following distribution parameters:

_{d}is a matrix with order 7, the covariance matrix of the direct genetic effect between traits, G

_{m}is the covariance matrix of the maternal genetic effect, L is a matrix with order 17 in the current study, P is the covariance matrix of weaning age effects between traits, I is the identity matrix of appropriate dimension, and R is the covariance matrix of the residual effects.