Effects of Phosphorus Supplementation on Grazing Beef Cattle

Phosphorus (P) losses due to overfeeding of the mineral to livestock can contribute to surface water degradation. The objective of this study was to examine the impact to supplementing various levels of mineral P to grazing beef cattle. A producer survey and a research trial were conducted to examine the effects of supplementing mineral phosphorus (P) to grazing cattle. In the first study, mineral tags, producer surveys, and fecal, forage, and soil samples were collected from beef cattle operations in Virginia's Chesapeake Bay watershed. Samples (n=166) were collected from 120 producers in 11 counties. Soil test P results were based on Virginia Cooperative Extension soil test guidelines characterized as low (12 %), medium (37 %), high (35 %), and very high (16 %). Pasture grab samples contained 0.34 ± 0.12% P and forage P concentration increased (P < 0.01) across soil P categories going from low to very high. Fecal total phosphorus (TP) was lowly correlated (R2 = 0.18, P < 0.01) to forage P concentration. Mineral supplements were categorized as nil (<1.0% P), low (1.0-<3.0% P), medium (3.0-<6.0% P), and high (>6.0% P). Fecal TP and inorganic phosphorus (Pi) concentration increased (P < 0.01) with mineral P levels. Fecal TP and Pi were lower (P < 0.01) when nil and low P mineral were supplemented as compared to medium and high P mineral. Soluble P (defined as fecal Pi/fecal TP*100) also increased (P < 0.01) with increasing mineral P content going from nil to high. All farms surveyed required little or no P supplementation in regard to beef cattle P requirements. The majority (82%) of producers were receptive to modifying mineral P supplementation practices based on forage P levels.

A 56-d study was also conducted with eight yearling Hereford steers (261±30 kg) grazing cool-season grass fall re-growth to determine the effects of varying levels of P supplementation on fecal P excretion. Treatments consisted of dicalcium phosphate supplemented at 0 (D1), 10.0 (D2), 20.0 (D3), or 30.0 (D4) g/d in a randomized 4x4 replicated Latin square design. These treatments provided an additional 0, 1.9, 3.7, and 5.6 g/d of P respectively. Two esophageally cannulated steers were used to collect forage samples for nutrient analysis. Forage P content was analyzed from hand collected samples. Forage P concentrations averaged 0.49% of dry matter (DM) across all periods. Chromic oxide (Cr2O3) was administered twice daily via gelatin capsule at 0630 and 1830 to serve as an external marker for determination of fecal dry matter excretion (DME). Indigestible NDF (iNDF) was used as an internal marker to determine dry matter intake (DMI). Due to the high forage P content, average P intake was in excess of National Research Council (NRC) requirements for all diets (D1 = 281%; D2 = 297%; D3 = 323%; D4 =348%). Orthogonal contrasts were performed to assess the relationship between treatment and P excretion. A linear response (P < 0.01) in daily inorganic P (Pi) excretion (0.054, 0.052, 0.062 and 0.063 g/kg of BW ± 0.003 for D1, D2, D3 and D4, respectively) was observed across treatments. Daily total P (TP) excretion increased linearly (P < 0.01) across treatments (0.080, 0.079, 0.092 and 0.093 g/kg of BW ± 0.003 for D1, D2, D3 and D4, respectively. When forage P content is sufficient to meet the requirement of grazing cattle, increasing P supplementation results in greater P excretion without additional benefits to growth or nutrient digestibility.