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