Mammary gland involution and endocrine status in sows: Effects of weaning age and lactation heat stress

Abstract

The effects of lactation length and/or heat stress on endocrine status of sows and indicators of mammary involution at weaning were studied. Twenty-eight first parity Yorkshire × Landrace sows were housed at 21 (CTL) or 29°C (HS) throughout lactation. Within each temperature group, half the sows were randomly assigned to a weaning age of 22 (W22: CTL, n = 7; HS, n = 8) or 44 d (W44: CTL, n = 6; HS, n = 7). Litter size was standardized to 10 or 11 on day 2 and to 9 on day 23 of lactation. Blood and milk samples were collected on days -1, 1, 2 and 3 relative to weaning (day 0). Concentrations of prolactin, IGF-I and lactose were measured in both fluids and concentrations of Na, K and IGFBP-5 were measured in milk. Standard milk composition was also determined. Lactose was lower (P < 0.001) and protein greater (P < 0.05) in all milk samples from W44 compared with W22 sows. Percentages of milk fat and dry matter were lower (P < 0.05) in W44 than in W22 on days 2 and 3 post-weaning. Milk lactose was reduced (P < 0.05) with heat stress on days 2 and 3. The day before weaning, concentrations of prolactin in blood (P < 0.001) and lactoserum (P < 0.01) were lower and those of IGF-I in lactoserum were greater (P = 0.01) in W44 than in W22. Values of IGFBP-5 in milk were drastically lower (P < 0.001, more than a fourfold difference), those of Na greater (P < 0.001) and those of K unchanged (P > 0.1) for W44 compared with W22. Post-weaning, evidence of leaky tight junctions (increased milk Na, decreased K) was evident a day earlier (2 vs. 3) in W44 compared with W22 and was most marked in HS. In support, plasma lactose concentrations increased transiently on the same day in HS W44. The greater Na/K ratio in W44 compared with W22 taken together with increased plasma lactose particularly in HS shows that mammary epithelial cell tight junctions become leaky as lactation advances. Weaning age therefore has an impact on milk variables which are indicative of the status of mammary involution in sows and some of these effects are exacerbated by heat stress.