Effects of gestation, lactation, and maternal calcium intake on mechanical strength of equine bone

Skeletal homeostasis during late gestation, lactation, and the post-lactational recovery period is poorly understood. In an experiment using an animal model (the horse), metacarpal breaking strengths (MBS) estimated via transmission ultrasonics were examined during the last 12 weeks of gestation and for 40 weeks after parturition. MBS increased during the last 6-10 weeks of gestation in mares fed amounts of calcium (Ca) recommended by the National Research Council; maximum MBS coincided with the week of parturition. In contrast, MBS in mares fed 20% less Ca remained relatively constant during the last 12 weeks of gestation. In contrast to increases during late gestation, MBS decreased steadily in all mares during the first 12 weeks of lactation. MBS increased after approximately 12 weeks of lactation, but more slowly than they had declined. MBS of the bones of mares fed recommended amounts of Ca were fully restored at 24 weeks post-parturition, but those of Ca-deficient mares had not fully recovered even 20 weeks after milk production had ceased (40 weeks after parturition). Mid-cannon mediolateral diameters of foals born to mares fed Ca-deficient diets were thinner and mechanically weaker at birth (both p < 0.01). These differences in limb bone size and strength persisted during the first 40 weeks of life.