Phosphorus metabolism in steers fed high grain diets
Samples were analyzed from three metabolism trials to evaluate the effects of dietary phosphorus (P) intake on P excretion in beef steers fed corn-based finishing diets. In Exp. I, ten steers were fed ten diets over ten periods. Treatments were: two processing methods, whole and rolled corn and five corn sources, commodity, white corn, high oil, high protein, and high oil/high protein corn. In Exp. II, six steers were fed three diets over three time periods. Treatments were three levels of protein to meet 70, 80, and 100% of the degradable intake protein (DIP) requirements for growing steers. In Exp. Ill, ten steers were fed five diets over five time periods. Treatments were: control, 4% or 8% distillers solubles, and 10% or 20% wet distillers grains. For all experiments total urine, feces, feed intake, and orts data were collected during a 5-d collection period, following 9, 10 and 14-d dietary adaptation periods for Exp. I, II, and III, respectively. Dietary P concentration (% DM) and dietary P intake (g/d) ranged from 0.31 to 0.36 and 21.48 to 27.19 and 0.28 to 0.29 and 19.83 to 21.21 and 0.29 to 0.40 and 22.95 to 34.48 in Exp. I, II, and III, respectively. Total P excretion was significantly related to P intake, but there was considerable variation among steers in partitioning excretion of P in urine or feces. Urinary P excretion (g/d) ranged from 3.0 to 7.4, 3.1 to 4.2, and 4.3 to 7.5 for Exp. I, II, and III, respectively. These studies indicate that significant quantities of P are excreted from steers fed corn-based diets and on average, about one-third of P excreted is in the urine. In addition there was significant variation among individual steers in P excretion between urine and feces. A proportion of this variation can be explained by phytate digestibility. A model was developed to help producers determine P excretion in cattle fed high grain or corn processing co-product diets.