The effect of litter size and day of lactation on amino acid uptake by the porcine mammary glands

Twelve multiparous sows (PIC Camborough 15; parity >2) were used to investigate the relationship between litter size and day of lactation, and plasma amino acid (AA) arteriovenous differences (A-VD), AA uptake, and plasma flow across the mammary glands. Sows were assigned randomly to one of the following litter sizes: 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 pigs per litter by cross fostering on d 2 postpartum. All sows were surgically fitted with catheters in the carotid artery and the main mammary vein. Matched arteriovenous blood samples were obtained on d 9, 12, 15, 18, 21, and 24 postpartum. Daily mammary uptake of AA was based on the product of plasma A-VD and daily mammary plasma flow (MPF). Daily MPF was estimated using the Fick method based on lysine conservation across the gland, and daily milk production. For the majority of AA, as litter size increased, A-VD did not increase, except for alanine (P < 0.05, linear and quadratic) and valine (P < 0.1; trend; linear and quadratic). As day of lactation increased, A-VD for the majority of AA increased (P < 0.05, linear and quadratic) except for arginine, lysine, and phenylalanine. As litter size increased, net daily mammary AA uptake increased for all indispensable AA (P = 0.001 to P < 0.05, linear and quadratic), excepting arginine. Milk production increased with increasing litter size (P < 0.001, linear) and with increasing day of lactation (P < 0.05, quadratic). Daily MPF increased (P < 0.05, linear) with increasing litter size, but did not change during the period measured from d 9 to 24. In conclusion, litter size appears to be a major determinant of net mammary AA uptake with daily mammary plasma flow a driving variable, whereas AA A-VD is a function of day of lactation and a major variable in determining net AA uptake with advancement of lactation.