Effects of relaxin on lactational performance in ovariectomized gilts

In gilts, mammary lobulo-alveolar growth begins on about Day 80 of gestation and continues progressively until term. Relaxin in concert with estrogen plays a major role in promoting this mammary gland growth. The present study was conducted to determine the importance for lactational performance of prepartum relaxin-dependent growth of the mammary glands in ovariectomized gilts given progesterone to maintain pregnancy. Twenty-four gilts were either sham ovariectomized or bilaterally ovariectomized and assigned to four treatment groups: sham-ovariectomized control, ovariectomized progesterone-treated, ovariectomized progesterone- and (starting at Day 80) relaxin-treated, and ovariectomized progesterone- and (starting at Day 100) relaxin-treated. Piglets were delivered by cesarian section, and gilts were given uniform colostrum-replete foster litters (born of untreated mothers) to nurse from Day 1 to Day 28 of lactation. Prepartum mammary development appeared by visual examination to be greatly reduced in relaxin-deficient gilts. Stimulus of the mammary nipples by the nursing piglets, however, appeared to overcome relaxin-dependent differences in mammary development among treatments. There was no effect of treatment on the time piglets spent at the udder, piglet mortality, piglet weight at Day 21 of lactation, milk composition, mammary cross-sectional area, or sow weight change during lactation. We conclude that gilts devoid of circulating luteal relaxin can display normal lactational performance when given colostrum-replete foster litters.