Abstract This project aimed to determine the impact of combining supplementary dietary lysine (Lys) with injections of porcine somatotropin (pST) in late-pregnancy on mammary development of gilts. On day 85 of gestation, 44 gilts were divided into four groups: 1) fed standardized ileal digestible (SID) Lys at estimated requirement for late-gestating gilts (CTL, control, 16.6% CP, 18.6 g/d SID Lys, n = 10), 2) fed SID Lys 40% above requirement via additional soybean meal (LYS, 21.1% CP, 26.0 g/d SID Lys, n = 10), 3) fed the control diet and receiving daily injections of 5 mg pST (pST, n = 12), or 4) fed the high Lys diet and receiving daily injections of 5 mg pST (LYSpST, n = 12). Both diets were isoenergetic. The pST (Reporcin®) was diluted in 1 mL of sterile water while CTL and LYS gilts received the vehicle. Dietary Lys treatment started on day 85 of gestation and pST treatment started on day 90 of gestation. Both treatments lasted until slaughter on day 110, at which time mammary glands were collected and dissected. All gilts were weighed and had their backfat thickness measured on days 84 and 110 of gestation. Data were analyzed according to a 2 × 2 factorial design. There was a treatment interaction (P 0.05) for body weight gain of gilts between days 84 and 110 of gestation, with the greatest increase for LYSpST gilts. Only the pST gilts lost backfat in the same period (P 0.05). Weight of the extraparenchymal mammary tissue (outer fat) was not affected by treatments (P 0.10), whereas parenchymal weight was increased by 31% following pST treatment (P 0.01) and by 12% following LYS supplementation (P 0.10), with no treatment interaction (P 0.10). Parenchymal weights for CTL, pST, LYS and LYSpST were 1929.2, 2594.6, 2159.4, and 2501.3 ± 176.3 g, respectively. The response to pST in terms of parenchymal weight was more variable than that to LYS (P 0.05). Lactogenic differentiation of the parenchyma was assessed histologically by measuring alveolar size. There was a main effect of pST (P 0.01) associated with a 21% increase in alveolar circumference, while there was no effect of LYS alone, or in combination with pST (P 0.10). Fetal weight was not altered by treatments. In conclusion, pST injections significantly increased mammary parenchymal weight in late-pregnant gilts, without any synergistic response to pST plus supplemental Lys (protein).
Farmer et al. (Wed,) studied this question.