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The Bayes factor (BF) procedure was applied to examine the additive genetic component of several physiological and vitality variables for newborn pigs. Nine variables were studied: heart rate, arterial oxygen saturation, rectal temperature (all at birth and 60 min later), birth weight, interval between birth and first teats contact, and interval between birth and first colostrum intake. The available numbers of data ranged from 288 (heart rate at 60 min) to 839 records (birth weight) from F(2) Iberian x Meishan newborn pigs. We compared a model with zero heritability (nonheritable) with the one where the additive genetic background was included. The BF was used to discriminate between both candidate models. Very strong evidence of genetic background was detected for heart rate 60 min after birth (BF = 48.90), and strong evidence was detected for rectal temperature at birth (BF = 13.82). Posterior modes (means) of heritabilities were 0.29 (0.32) and 0.40 (0.39), respectively. In addition, substantial evidence of absence of genetic background was detected for arterial oxygen saturation at birth.

Eight physiological and vitality variables related to the first minutes of the life of Iberian x Meishan F2 piglets were analyzed. Variables included rectal temperature, arterial oxygen saturation, heart rate (all were monitored at birth and 60 min later), time to reach the udder, and time to the first colostrum intake. Litter effect within boar, a random source of variation, influenced all variables, whereas the random boar effect was not significant for heart rate and arterial oxygen saturation at birth and the time to reach the udder. Birth weight influenced rectal temperature at birth (P < 0.01) and 60 min later (P < 0.001), arterial oxygen saturation 1 h after birth (P < 0.05), heart rate 1 h after birth (P < 0.001), time to reach the udder (P < 0.05), and time to the first suckle (P < 0.01), whereas viability score influenced rectal temperatures (at birth and 60 min later; P < 0.001), and time to reach the udder (P < 0.001) and to suckle (P < 0.001). Finally, the order of birth showed significant effects for rectal temperature 1 h after birth (P < 0.001) and time to first colostrum intake (P < 0.001). Correlation coefficients between physiological variables were generally low, with the exception of the ones for rectal temperature 1 h after birth and arterial oxygen saturation 1 h after birth (0.38; P < 0.001), and for rectal temperature at birth and rectal temperature 1 h after birth (0.34; P < 0.001). Times to reach the udder and to suckle were highly correlated (0.67; P < 0.001) and also were moderately and negatively correlated with rectal temperature 1 h after birth (-0.36 and -0.38 respectively; P < 0.001). Heart rates at birth and 1 h later, as well as arterial oxygen saturation at birth and 1 h later, were not correlated, showing that the values at birth do not necessarily provide information about the physiological status of the pig 1 h after birth. The recording of physiological variables, birth weight, and viability score may be useful to identify weak piglets quickly and to establish palliative measures.

Iberian x Meishan F2 piglet's preweaning survivability was analyzed using categorical data regression procedures within the proportional hazards assumption. A frailty sire model was assumed with the litter effect treated as an additional random source of variation. Moreover, the relative birth weight within litter and the litter effect were considered time-dependent covariates that changed their values in the second day of life due to cross fostering carried out to standardize litters. Six variables had a significant effect on survivability: birth weight (P < 0.01), relative birth weight within litter (P < 0.001), rectal temperature 60 min after birth (P < 0.01), type of presentation at birth (P < 0.05), presence of stillbirths (P < 0.001), and presence of mummified fetuses (P < 0.001). Small piglets (<0.98 kg) suffered a high hazard ratio (6.57; P < 0.001), with this variable being clearly lower for the rest of birth weight categories. Piglets that were small in relation to their siblings (relative birth weight within litter) also suffered an increased death risk, with a hazard ratio of 1.81 (P < 0.05), which was similar to animals with posterior presentations at birth (hazard ratio = 1.80; P < 0.05). Piglets with a rectal temperature lower than 35.4 degrees C 60 min after birth showed the highest hazard ratio (7.18; P < 0.01). Furthermore, the presence of mummified fetuses decreased the survivability of the remaining siblings, with a hazard ratio of 2.03 (P < 0.01), as did the presence of stillbirths (hazard ratio = 3.55; P < 0.001). The inclusion of the two random effects allowed us to estimate the mode of the joint posterior density of the sire variance (0.08) and the litter variance (1.98). The estimated heritability of preweaning survival reached a value of 0.03. We conclude that piglet survival involves several systematic influences related to birth weight, thermoregulatory ability, and injuries suffered during gestation and farrowing. The genetic variance was small compared with those generated by the common environment, for which the genetic improvement of piglet survival seems difficult.