Recording and Sample Collection

The daily food intake of sows was recorded throughout the lactation period. The total born litter size, number of piglets born alive, number of piglets stillborn, individual birth weight, and litter birth weight were recorded. To confirm that a piglet is stillborn, animals that appear to be stillborn should be dissected to determine whether their lungs float; this is because stillborn individuals (intrapartum) look the same as their living littermates, but do not breath (Mota-Rojas et al., 2006). The feed intake of each sow during lactation was recorded daily and average daily feed intake was calculated. Within 24 h of farrowing, the litter size of these sows was standardized to 11 piglets per litter within the same dietary treatment group to eliminate the effect of litter size on milk production. Piglets removed and added to sows were randomly selected. The piglets were weaned at 22 d of age, after which the sows were transferred to the breeding facility. Estrous was detected with a boar once daily, and the interval from weaning to first estrus was recorded.

Colostrum samples (20 mL) were collected from the functional glands of each sow within 2 h of the first piglet’s birth before suckling after 20 IU of oxytocin injection (Qilu Limited Company, Shandong, China). Colostrum samples (10 mL) were centrifuged at 4 °C and 3,000 × g for 20 min, and the supernatant was harvested and stored at −80 °C for analysis of immunoglobulin A (IgA), IgG, and IgM. The other 10 mL sample was used to analyze colostrum composition.

The weights of suckling piglets were recorded in the morning at day 22 of lactation. During the lactation period, the fecal consistency score of piglets was determined daily according to the following criteria: 5, normal, feces firm and well formed; 3 to 4, soft consistency, feces soft and formed; 1 to 2, fluid feces, usually yellowish; and 0, feces watery and projectile. When the fecal consistency was scored at 0, 1, or 2, the piglets were considered to have diarrhea. The incidence of diarrhea in piglets (%) was determined as: number of cumulative piglets with diarrhea/(total number of suckling piglets × number of breastfeeding days) × 100%. The preweaning mortality of piglets was recorded, whereby the survival rate of piglets at weaning (%) = number of weaned piglets per litter/the adjusted litter size × 100%.

On day 22 after delivery, 1 piglet from each litter was randomly selected for slaughter. Before slaughter, a 5-mL blood sample was collected via jugular puncture into a 10-mL tube treated with sodium heparin, and centrifuged at 3,000 × g and 4 °C for 20 min. Plasma was harvested and stored at −20 °C. Then, piglets were anaesthetized with an intravenous injection of sodium pentobarbital (50 mg/kg BW) and exsanguinated by severing the carotid artery and jugular vein. The abdominal cavity was opened, and the viscera were removed. Samples of the jejunum mucosa (about 4 cm² size) were taken from the midpoint of the jejunum and placed in a 4% formalin solution. Approximately 20 g of digesta (wet weight) was taken from the caecum. The mucosa of 20 cm jejunum and colon were collected and immediately frozen in liquid nitrogen and stored at −80 °C.