Mineral accretion during prenatal growth of cattle

Angus, Hereford and Red Poll crossbred yearling heifers (n = 81) were mated to Brown Swiss bulls. Pregnant heifers were assigned on the bases of weight and breed cross to one of three diets and fed to achieve maternal weight gains of 0, .5 or 1.0 kg/d. Heifers from each treatment group were slaughtered at about 120, 150, 180, 210, 240, or 255 d postmating. Reproductive tracts were recovered at slaughter. Weights of the gravid uterus, fetus, fetal membranes, fetal fluids, uterus, cotyledons and placenta were obtained and have been reported previously. Fetuses were frozen and later ground, mixed and sampled. Dry matter was determined. Samples of each fetus were ashed, and concentrations of Ca, P, Na, K, Mg, Fe and Zn were determined. Relationships between total fetal content of each mineral and day postmating were obtained, and daily fetal accretion rates of each mineral were estimated for several stages of gestation. Daily requirements and allowances of each mineral for pregnancy in beef cows were estimated. Estimated allowances for Ca, P, Na, K, Fe and Zn were small during early gestation but increased rapidly and reached maxima of 8.4 g, 5.2 g, .76 g, .63 g, 65 mg and 21 mg, respectively, at about 250 d postmating.