Animal Industry Report

Extension Number

ASL R2271



Summary and Implications

Phosphorus (P) is an essential nutrient for beef cattle, but over-supplementation of P may have negative environmental and economic impacts. The P concentration in forage selected by grazing cattle may be sufficient to meet the nutritional requirements without supplementation. Similarly, the crude protein (CP) and in vitro digestible dry matter (IVDDM) concentration of forage selected by grazing cattle may be greater than available forage. This study was conducted to determine the P, CP, and IVDDM concentrations of forage selected by beef cattle grazing cool-season grass pastures. Fifteen fall-calving Angus cows were managed by either continuous or rotational stocking in 30-acre smooth bromegrass pastures, replicated twice. In July and August, one ruminally fistulated steer was placed in each pasture for one week. On two consecutive days at the end of each week, fistulated steers were ruminally evacuated, allowed to graze for 2 h, and sampled for forage selected during grazing. Forage samples were hand-clipped at ground level and from the upper half of the forage sward in pasture areas adjacent to the areas being grazed. A subsample of the ground-clipped forage was placed in the rumen and allowed to soak for 2 h following sampling of selected forage to correct the P, CP, and IVDDM concentrations in the selected forage for salivary P, CP, and IVDDM absorbed by selected forage. Hand-clipped and selected forage samples were analyzed for P, CP and IVDDM concentrations. Phosphorus concentrations in the available forage did not differ by month. However, P concentrations of available forage in continuously stocked pastures (0.22%) tended to be greater than in rotationally stocked pastures (0.20%). The P concentration of selected forage (0.39%) was almost twice as high as the available forage. Available forage in pastures managed by continuous stocking had greater CP and IVDDM concentrations than did forage in rotationally stocked pastures in both months. Results of this study show that cattle grazing cool-season grass pastures are able to select forage with an adequate crude protein and P concentrations to meet their nutrient requirement without supplementation

Copyright Holder

Iowa State University