From mating until entry into the farrowing shed, gilts were fed 2.2 kg/d and sows were fed 2.5 kg/d of a dry sow diet (13.5 MJ of DE/kg, 16.0% total protein, 0.80% total lysine) once daily in the morning. After confirmation of pregnancy, gilts and sows were ranked by BW within parity for each replicate week of the study and were allocated alternately to each treatment group (n = 120 per treatment and parity in total). Control gilts and sows were not injected, to enable comparison with performance under current commercial management. Gilts and sows allocated to the pST-injection groups were injected intramuscularly daily with 1 mL of sterile water containing 2.5 or 4.0 mg of pST, respectively, which was calculated to provide a dose of approximately 15 μg of pST/kg per day in both parity groups, based on previous BW data from the herd. Gilts and sows were injected daily with pST from d 25 of pregnancy until d 50 (short-term pST) or d 100 of pregnancy (long-term pST). Dams entered the farrowing house between d 108 and 110 of pregnancy (d 109 ± 1) and were fed 3 kg/d of a lactation diet (14.9 MJ of DE/kg, 19.2% total protein, 0.89% total lysine) until farrowing, and were then fed this lactation diet 3 times per day to appetite until weaning. Gilts and sows farrowed naturally at term (referred to as the treatment pregnancy). Within 24 h of farrowing, a minimal cross-fostering approach was used to equalize the litter size to 10 piglets in gilt litters and 12 piglets in sow litters where possible. Sows were weaned in the fourth week of lactation on a set day each week. Litters were weaned at 26.6 ± 0.1 d after birth, and all dams were mated by AI at the first postweaning estrus. Dam removals and reasons for removal were recorded throughout the treatment pregnancy, throughout lactation, and up to the subsequent farrowing (subsequent pregnancy). Dams were fed 2.6 kg/d of a commercial dry sow diet (12.9 MJ of DE/kg, 14.5% total protein, 0.62% total lysine) throughout the subsequent pregnancy and were transferred to the lactation diet on entry into the farrowing house, as described above.
Maternal and Progeny Measures
Pregnant dams were weighed and backfat depth was measured by ultrasound at the P2 site (65 mm from the midline over the final rib, using a linear-array 3.5-mHz live ultrasound instrument; Noveko, Montréal, Canada) before the beginning of treatments at d 20 to 24 of pregnancy, at d 50 of pregnancy, on transfer to the farrowing shed at d 104 to 111 of pregnancy, and at weaning. A subset (approximately 60% of dams to minimize disturbance of the dams on the day of birth) were weighed and P2 backfat was recorded on the day of farrowing for each replicate week of the study. Numbers of live piglets, stillbirths, and mummified piglets were recorded within 24 h of birth. Total litter weight and number were recorded on the day of delivery, after fostering, at d 14 of lactation, and at weaning. Preweaning piglet losses, reasons for removal, and dates at removal were recorded throughout lactation. Numbers of live piglets, stillbirths, and mummified piglets were recorded within 24 h of birth for the subsequent pregnancy.