Animal Industry Report

Extension Number

ASL R1880



Summary and Implications

For environmental reasons, minimizing phosphorus excretion from cattle is of great interest. Current estimates of forage phosphorus digestibility by cattle consider that phosphorus digestibility does not change with composition of the pasture. To better estimate phosphorus (P) excretion, estimates of P digestibility for forages of different compositions are needed. Four crossbred cow/calf pairs were stocked on four pastures managed with grazing (G) or grazing with hay removal (G/H). Forage was maintained in paddocks at 50% removal. Collected pasture samples and fecal samples from cows administered chromic oxide were analyzed for P, NDF, and ADL contents. Rumen evacuations of steers were conducted to evaluate composition of consumed forage for each treatment. Forage analyzed from paddocks where steers grazed demonstrated no grazing management effects on composition, which was evidenced by no differences in composition of rumen contents of the steers. Analysis of the 13th rib bone concludes the cattle were not deficient in phosphorus. Year 1 results suggest that pastures managed under a combination of grazing and initial hay removal resulted in greater P content of the forage and concomitant increased P excretion by cows consuming that forage. There was no treatment effect for P retention, however a there was a date effect with the G treatment having higher P retention in July and September. These results verify that grazing management practices can have a substantial effect on water pollution potential.

Copyright Holder

Iowa State University