Heterosis and recombination effects in Hampshire and Landrace swine: I. Maternal traits.
Hampshire and Landrace sows and crossbreds of the two breeds were used to determine heterosis and recombination effects for milk production, milk composition, and litter traits at birth and d 21. Twelve mating types were represented in this study: two purebred, two F1, two F2, two F3, and four backcross. Information was gathered on a total of 358 litters over four farrowing seasons. Milk production was measured at d 10 and 20 of litter age according to the weigh-suckle-weigh procedure. Milk samples were collected at d 10 and 20 of litter age and evaluated for percentages of fat (PCFA), protein (PCPR), lactose (PCLA), and solids-not-fat (PCSN). The model used to evaluate litter traits at birth included main effects of mating type, parity, and farrowing season. The model used for milk production and milk composition traits included these main effects and number of pigs nursed as a covariate. Estimates of maternal genetic effects showed that Landrace females were superior to Hampshire females for number born (NB), number born alive (NBA), litter birth weight (LBW), adjusted 21-d litter weight (ALW), and milk production at d 10 of litter age (WT10). Hampshires were superior to Landrace for PCPR at d 10 of litter age and PCSN at d 10 and 20 of litter age. Heterosis effects were significant (P less than .05) for NBA (.97) and LBW (1.46 kg). Maternal heterosis effects were significant for LBW (3.94 kg; P less than .01). Epistatic recombination losses in the offspring were significant for LBW (6.80 kg; P less than .05). Differences in maternal performance of reciprocal F1 dams were generally not significant. Heterosis and recombination effects were not significant for milk production or milk composition.