Milk production, reproductive performance, and fecal excretion of phosphorus by dairy cows fed three amounts of phosphorus

Milk production was measured and phosphorus (P) excretion in feces was esti mated in dairy cows fed three amounts of P. A basal diet was formulated to contain 0.31% P (DM basis). Sodium monophosphate replaced corn in the basal diet to give two additional diets containing 0.40 and 0.49% P. The diets w ere fed to eight, nine, and nine multiparous Holsteins from the beginning t o the end of lactation. Milk yields for the 308-d lactation were 10,790, 11 ,226, and 11,134 kg for the three treatments, respectively. The lowest milk yield resulted from decreased milk production during late lactation with t he 0.31% P group. Reproductive performance of the cows was not related to d ietary P content. Fecal P concentration, determined in wk 2, 4, 6, 8, 23, a nd 40 of lactation, increased as dietary P intake was increased. Cows fed t he lowest P diet conserved P by minimizing P excretion in feces and urine, whereas cows in the other two treatments excreted more P through these rout es. A reduction in dietary P from 0.49 to 0.40% reduced fecal P excretion b y 23%. Apparent P digestibilities of less than 40% are indicative of surplu s dietary P. Feeding 0.40% P appeared sufficient to maintain P balance and the level of milk production achieved in this experiment. An example is giv en which illustrates the relationship between dietary and fecal P.