Impact of prepartum dietary phosphorus intake on calcium homeostasis at parturition.

A study was conducted to determine the impact of dietary P intake on vitamin D metabolism and incidence of parturient paresis in aged dairy cows. Thirty dairy cows (10/group) were fed one of three experimental diets for approximately 28 d precalving. Phosphorus intake was .7, 1, or 3 times daily maintenance requirement and Ca intake was three times daily maintenance requirement for all cows. There was a 20% incidence of parturient paresis in each group. Prepartum dietary P intake had no effect on precalving or calving plasma Ca concentrations. Cows fed the low P (.7 times) diet had higher plasma Ca at 3 and 5 d postcalving than did cows fed P at 1 or 3 times maintenance. Plasma phosphorus concentrations reflected dietary P intake. Dietary P intake had no effect on plasma Mg, free hydroxyproline, 1,25-dihydroxyvitamin D, or 24,25-dihydroxyvitamin D concentrations. The range in dietary P from .7 to 3 times maintenance requirement had no effect on the incidence of parturient paresis. However, it did appear to influence Ca homeostasis during the postpartum period as cows fed the low P diet had higher plasma calcium concentrations postcalving. This may be a result of the low P diet enhancing intestinal C absorption by a vitamin D-mediated transport mechanism.