Background: Objectives were to examine the effects of selenium(Se) supply and maternal nutritional plane during gestation on mammary gland growth, cellular proliferation, and vascularity at parturition and d 20 of lactation. Rambouillet primiparous ewes(n = 84) were allocated to treatments in a 2 x 3 factorial. Factors were dietary Se(adequate Se [ASe, 11.5 μg/kg BW] or high Se [HSe, 77.0 μg/kg BW]) and nutritional plane(60% [RES], 100% [CON], or 140% [EXC]). At parturition, lambs were removed and 42 ewes(7/treatment) were necropsied. Remaining ewes were fed a common diet meeting requirements for lactation and mechanically milked twice daily until necropsy on d 20. At both necropsy periods, mammary glands were dissected and tissues harvested. Samples were analyzed for RNA, DNA, and protein content, cell proliferation, and vascularity. Where interactions were present(P ≤ 0.05), least squares means from the highest-order interaction are presented. Results: Final body weight of ewes was least(P ≤ 0.002) in RES, intermediate for CON, and greatest for EXC, regardless of stage of the ewe at necropsy(parturition or d 20 of lactation). In ewes necropsied at parturition, mammary glands were heavier(P = 0.02) in EXC compared to RES, with CON intermediate. Concentration of RNA(mg/g) was decreased(P = 0.01) in EXC compared to CON at parturition. There was a tendency(P = 0.07) for a Se by nutrition interaction in percentage of cells proliferating where ASe-EXC ewes had greater(P ≤ 0.02) number of proliferating cells then all other treatments. Mammary vascular area tended(P = 0.08) to be affected by a Se by nutrition interaction where ASe-CON had less(P = 0.007) vascular area than HSe-CON ewes. In ewes necropsied at d 20 of lactation, the number of alveoli per area was decreased(P ≤ 0.05) in RES compared to CON and EXC-fed ewes. Conclusions: Results of this study indicate that proper maternal nutritional plane during gestation is important for mammary gland development, even out to d 20 of lactation.
