Phosphorus status of grazing beef cattle in Virginia's Chesapeake Bay watershed

Tuesday, July 22, 2014
Exhibit Hall AB (Kansas City Convention Center)
Scott J. Neil , Virginia Polytechnic Institute and State University, Blacksburg, VA
Kayleigh J. Mize , Virginia Polytechnic Institute and State University, Blacksburg, VA
Deidre D. Harmon , Virginia Polytechnic Institute and State University, Blacksburg, VA
Jason K. Smith , Virginia Tech, Blacksburg, VA
Mark A. McCann , Virginia Polytechnic Institute and State University, Blacksburg, VA
Abstract Text:

Phosphorus (P) is one of the nutrients identified in EPA’s TMDL plan for the Chesapeake Bay watershed.  Major research and extension efforts in Virginia have focused on reducing P losses from concentrated animal feeding operations , however approximately 400,000 beef cows graze pastures in Virginia’s Chesapeake Bay watershed. To better characterize farm P status, fecal, forage and soil samples were collected from beef cattle farms in the watershed. One hundred twenty producers from 11 counties cooperated with sample collection (N = 166). Samples were analyzed for total P (TP) and inorganic P (Pi) using the molybdovanadate yellow and blue methods, respectively. Soil test P values were characterized as low (12 %), medium (37 %), high (35 %), and very high (16 %) based on Virginia Cooperative Extension Soil Test guidelines. Phosphorus content of pasture forage grab samples (mean = 0.34 %; SD = 0.12) was lowly correlated with soil P (r = 0.18; P < 0.0001) and fecal TP (r = 0.15; P < 0.0001). Forage TP levels were compared with Beef Cattle NRC (2001) P requirements for a 545-kg cow (peak milk, 13.6 kg per d). All forage samples were sufficient in P content to meet a dry cow’s requirements, while 98 % met the requirements for late gestation and 88 % met the requirements for peak lactation. Farm mineral supplements were categorized into 4 levels of P content (< 1.0 [nil], 1.0 to 2.5 [low], 3.0 to 5.0 [medium] and > 6.0 % [high]). Forage P content did not affect mineral selection by producers. The mean forage P content of the mineral categories was 0.30, 0.36, 0.32, and 0.40 %, respectively. The mean fecal TP (0.56 %, 0.65 %, 0.79 %, and 0.97 %, respectively) and Pi levels (0.33 %, 0.41 %, 0.55 %, and 0.68 %, respectively) significantly differed between producers that fed a nil and low P mineral, and those that fed mineral in the medium and high categories (P= < 0.001 for TP and Pi). Soluble P (defined as Pi/TP*100) tended to increase across mineral categories going from nil to high. All farms surveyed required little or no P supplementation in regard to cow P requirements. These results indicate that reducing mineral P may be capable of limiting soluble P losses from supplementation of beef cattle.

Keywords: phosphorus, beef cattle, Chesapeake Bay