Dietary phosphorus effects on performance and nutrient balance in feedlots

Phosphorus (P) requirement of yearling steers was evaluated using five dietary levels (.14,.19,.24,.29, and .34% of DM) in a 105 day finishing trial previously summarized (Erickson et al., 1999). Phosphorus content was reduced by blending corn grits and bran (i.e. corn without P) with dry-rolled corn and then supplementing P from mono-sodium phosphate. Dietary P level did not influence dry matter intake, daily gain, feed conversion, bone size, bone mineral, or bone strength (P > .10). Results indicated that P supplementation was not required and P requirements may be lower than previously predicted. In a subsequent 2-year feedlot study, which involved finishing calves in winter(183 d) and yearlings in summer(132 d), the effect of decreasing dietary P on nutrient balance and animal performance was assessed. Diets were a conventional diet (CON, P = .40%) or a diet formulated to not exceed the animal's NRC-predicted requirement (LOWP, P = .22 to .28%). Because P retention by animals was similar (P > .20) between treatments, feeding LOWP decreased (P < .01) P excretion in both yearlings (5.0 or 2.4 kg of P.steer(-1) and calves (5.7 or 3.4 kg of P.steer(-1)) for CON or LOWP, respectively. Dietary P was utilized more efficiently in the LOWP (25.7%) versus CON (15.6%) treatment. Manure corrected for soil P content resulted in recovery of 2.6 or 1.1 kg of P.steer(-1) with yearlings and 4.7 or 3.0 kg of P.steer(-1) with calves for CON or LOWP treatments, respectively. Based on these results, decreasing dietary P From conventional levels improved efficiency of P use, decreased P excretion, and did not affect animal performance. We concluded that supplementing finishing diets with P from added mineral is an unnecessary environmental and monetary cost to producers. Typical grain finishing diets already contain enough P to elicit optimal gains because grains generally contain .25 to .30% P.