Growth and accretion of energy and protein in the gravid uterus during late pregnancy in Holstein cows.

Multiparous Holstein cows (n = 18) were bred artificially to the same bull and then slaughtered at times ranging from 190 to 270 d postconception to assess accretion of energy, protein, fat, and ash by the conceptus. Wet weights, dry weights, and concentrations of energy, CP, crude fat, and ash were obtained for the following: fetus, combined amniotic and allantoic fluids, fetal membranes, cotyledons, caruncles, and uterine tissues. Rates of accumulation of these components in the gravid uterus (sum of all uterine contents) and fetus were described by linear or quadratic equations. Estimated rates of accretion of energy in the gravid uterus (i.e., conceptus) increased from 567 kcal/d at 190 d of gestation to 821 kcal/d at 270 d of gestation; corresponding rates of accretion of CP were 62 and 117 g/d. These daily rates represent net energy and protein requirements for conceptus growth during late pregnancy in mature Holstein cows. Conversion of predicted net energy to metabolizable energy requirements for conceptus growth, using the accepted efficiency factor of .14, yielded estimates that were consistent with current NRC recommendations. Factorial estimation of absorbed protein requirements is hampered by lack of precise information on the efficiency with which absorbed AA are deposited in conceptus tissues.